API Specification 5B, Specification for Threading, Gauging, and Thread Inspection of Casing, Tubing, and Line Pipe Threads API Specification 5CT/ISO 11960, Specification for Casing and
Protecting Groundwater and the Environment
Oil and natural gas exploration, development, and production are carried out with a strong emphasis on protecting the environment, particularly underground sources of drinking water (USDWs) Each oil and gas producing state in the United States has established statutes and regulations to ensure environmentally responsible operations Although the specifics of these regulations vary by state, their overarching intent and environmental goals remain consistent.
Groundwater protection during drilling, hydraulic fracturing, and production operations is achieved through the use of steel casing, cement sheaths, and mechanical isolation devices These components are essential in the well construction process to prevent fluid migration between hydrocarbon-producing formations and groundwater The impermeable rock formations that have isolated groundwater for millions of years play a crucial role in maintaining this separation.
To protect groundwater during drilling operations, the wellbore is drilled through aquifers, followed by the immediate installation of a steel casing that is cemented in place State drilling regulations mandate that the surface casing must be set below the lowest underground source of drinking water (USDW) This steel casing safeguards the surrounding zones from materials within the wellbore during drilling, and when combined with additional steel casings and cement sheaths, it provides multiple layers of protection for groundwater throughout the well's lifespan.
Well integrity refers to the containment of hydrocarbons within the well from the subsurface zone to the surface Regular monitoring during drilling and production operations is essential to ensure compliance with established parameters, well design, and permit requirements Additionally, the integrity of well construction is periodically tested to maintain its reliability Detailed monitoring activities conducted before, during, and throughout the life of the well will be discussed in Section 10.
Well Design and Construction
Drilling and completing an oil and gas well involves a series of sequential activities, with key processes highlighted in bold.
When building a location and installing fluid handling equipment, it's essential to understand the definition of a USDW (Underground Source of Drinking Water) as outlined in federal statute (40 CFR 144.3) A USDW is classified as any aquifer that either supplies a public water system or contains enough water to do so, while also providing drinking water for human consumption or having fewer than 10,000 mg/L of total dissolved solids Additionally, it is important to note that the aquifer must not be exempted For more information, visit the EPA's glossary at http://www.epa.gov/region5/water/uic/glossary.htm It's also worth mentioning that "groundwater" may encompass other subsurface waters that do not meet these specific criteria.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 3
— setting up the drilling rig and ancillary equipment and testing all equipment,
— logging the hole (running electrical and other instruments in the well) (see note),
— running casing (steel pipe) (see note),
— cementing the casing (see note),
— logging the casing (see note),
— removing the drilling rig and ancillary equipment,
— perforating the casing (depending on completion type),
— hydraulic fracturing or stimulating the well,
— installing artificial lift equipment (if necessary),
— putting the well on production,
— monitoring well performance and integrity,
— reclaiming the parts of the drilling location that are no longer needed and removing equipment no longer used. NOTE These activities may be conducted multiple times while drilling a well.
Production wells must effectively penetrate the sealing formations above hydrocarbon reservoirs, making the analysis and execution of well construction crucial to prevent leaks For 75 years, the industry has successfully utilized modern drilling techniques, with ongoing technological advancements ensuring the integrity and isolation of these wells.
The primary objective of effective well design is to facilitate the safe and environmentally responsible extraction of hydrocarbons This involves containing hydrocarbons within the well, safeguarding groundwater resources, and isolating productive formations from others Proper execution of hydraulic fractures and stimulation operations is essential Additionally, well construction must prevent leaks between casing strings, ensuring that fluids—such as oil, water, and gas—travel directly from the producing zone to the surface through the well conduit.
The design basis for well construction focuses on ensuring effective barrier performance and zonal isolation through wellbore preparation, mud removal, casing running, and cement placement, all aimed at preventing fluid migration While the choice of materials for cementing and casing is significant, the cement placement process takes precedence Ultimately, the barrier system's ability to safeguard groundwater and isolate hydrocarbon-bearing zones is critical.
Effective well designs and plans incorporate contingency planning to address potential unplanned events While these plans are rarely utilized, they are essential for minimizing risks and safeguarding both people and the environment.
The Drilling and Completion Process
Drilling an oil or gas well involves multiple cycles of drilling, casing installation, and cementing to ensure proper isolation Each cycle features the installation of steel casing in progressively smaller sizes within the previously installed casing The final phase of well construction, known as well completion, may involve techniques such as perforating and hydraulic fracturing, tailored to the specific type of well.
Drilling a well involves the use of a drill string, which is made up of a drill bit, drill collars to apply weight, and drill pipe This assembly is lowered into the hole while being suspended from the drilling derrick or mast at the surface The rotation of the drill string is achieved through a turntable (rotary table), a top drive unit, or a downhole motor drive.
During drilling, fluid circulates through the drill string and the annular space, providing lubrication, removing cuttings, maintaining well pressure, and stabilizing the borehole This drilling fluid typically consists of water, clays, and various additives for fluid loss control, density management, and viscosity enhancement It is a precisely formulated mixture aimed at optimizing drilling performance.
The initial step in the drilling process, as illustrated in Figure 1, involves creating a hole for the conductor pipe, which can sometimes be driven into position like a structural piling Following this, deeper holes are drilled to install the surface casing, and, if required, the intermediate casing, along with the production casing Detailed considerations for each casing string are discussed in Section 7 Notably, the upper sections of the well feature several concentric strings of steel casing.
Local environmental and geological conditions may necessitate modifications to the general design depicted in Figure 1, leading to variations in state regulations aimed at ensuring adequate isolation and protection For instance, the geologic conditions encountered during drilling influence the number of intermediate casing strings required in a well.
Horizontal wells are drilled vertically to a certain depth before being redirected to run horizontally within hydrocarbon formations The vertical section is drilled similarly to traditional vertical wells, while the horizontal section typically utilizes a downhole motor that operates on hydraulic pressure from the drilling fluid This steerable downhole motor allows for precise control of the drilling direction from the surface, ensuring the drill bit remains within the targeted formation.
The design and selection of casing are crucial, as it must endure various compressive, tensional, and bending forces encountered while running in the hole Additionally, it must withstand collapse and burst pressures throughout the well's lifecycle For instance, during cementing operations, the casing is required to resist the hydrostatic forces from the cement column, and post-cementation, it must handle the collapsing pressures from certain subsurface formations, which are present regardless of hydrocarbons.
The design of steel casing strings is crucial for well design and success, ensuring zonal isolation and wellbore integrity It is primarily the responsibility of operating companies, drilling contractors, and their engineers to create and review the casing design, as well as to plan its installation during well construction The processes of casing design and installation are executed with careful technical precision.
Casing features threaded ends with couplings to connect to adjacent pipes, creating a continuous "string" that isolates the hole Proper torque application during casing connections is crucial; excessive torque can overstress and compromise the connection, while insufficient torque may lead to leaks.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 5
Casing for oil and gas wells intended for hydraulic fracturing must comply with API standards, particularly API Spec 5CT These specifications encompass the design, manufacturing, testing, and transportation of casing, which must adhere to stringent requirements for compression, tension, collapse, and burst resistance, as well as quality and consistency It is essential that the casing is engineered to endure the expected hydraulic fracturing pressures, production pressures, and corrosive environments Additionally, any used or reconditioned casing installed in such wells must be tested to confirm it meets the API performance standards applicable to new casing.
Casing and coupling threads must comply with API standards, including API Spec 5B, to guarantee performance, quality, and consistency If proprietary threads from specialized suppliers are utilized, they must undergo stringent testing by the supplier and meet relevant API qualification test subsets.
General
Cementing the casing after it has been installed in the drilled hole is a crucial step in well construction, designed to ensure zonal isolation between various formations and complete separation from groundwater This engineered process provides essential structural support for the well and is vital for maintaining its integrity over time, while also offering corrosion protection for the casing.
Cementing involves pumping cement, or slurry, down the casing and circulating it back up the outside to ensure proper sealing To reduce the mixing of cement with drilling fluid during this process, it is essential to use top and bottom rubber wiper plugs A downhole schematic illustrating a cement job in progress is shown in Figure 2.
T yp ic a l O il an d / o r G a s W ell S ch e m a tic
In te rm e d ia te c a s in g
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 7
Cement Selection
Oilfield cements are specialized products governed by API technical specifications and practices, playing a crucial role in well design, construction, and integrity A variety of cements and additives are available, and it is essential to refer to API standards, such as API Spec 10A and API RP 10B-2, when selecting cementing products Prior laboratory testing of chosen cements, additives, and mixing fluids is necessary to confirm their compliance with well design requirements.
API and other organizations have established comprehensive specifications and best practices for cementing operations, which are accessible to all drilling companies It is essential for operators to adhere to these standards in every well A summary of effective cementing practices can be found in section 5.4.
Zone Isolation
Proper placement of cement around the casing and at the correct height above the drilled hole's bottom is crucial for effective zone isolation and integrity Achieving good isolation necessitates complete annular filling and strong cement bonds with both the formation and casing Key elements of well and seal integrity include the complete displacement of drilling fluid by cement and robust bonding at the interfaces above the hydrocarbon formation Ensuring there are no voids and maintaining strong cement bonds at these interfaces are essential to prevent migration paths and establish reliable zone isolation.
Cementing Practices
The following cement practices are recommended in order to ensure that isolation is achieved.
— Prior to drilling, operators should investigate and review the history of nearby wells for cementing problems encountered, e.g lost returns, irregular hole erosion, poor hole cleaning, poor cement displacement, etc.
— Computer simulation and other planning should be carried out in order to optimize cement placement procedures.
— Operators should use established, effective drilling practices to achieve a uniform, stable wellbore with desired hole geometry.
— Operators should ensure that the drilling fluid selection is appropriate for the designed well and the geologic conditions likely to be encountered.
Selecting the appropriate casing hardware, such as float equipment, centralizers, cement baskets, wiper plugs (both top and bottom), and stage tools, is essential in well design This selection process is crucial for achieving cement design objectives, addressing challenges, and ensuring effective isolation.
Selecting the right casing centralizers is essential for centering the casing within the hole, ensuring effective mud removal and optimal cement placement This is particularly crucial in critical areas, including casing shoes, production zones, and groundwater aquifers.
Proper cement testing procedures must be conducted by qualified personnel, as outlined in API RP 10B-2 The design of cement slurry should incorporate testing to evaluate various parameters based on the specific geological conditions of the site.
— Critical Parameters—Recommended for all situations:
> fluid compatibility (cement, mix fluid, mud, spacer).
— Secondary Parameters—Recommended for use as appropriate to address specific well conditions:
> expansion or shrinkage of set cement;
> mechanical properties (Young’s Modulus, Poisson’s Ratio, etc.).
Effective cement job design requires careful consideration of cement spacer design and volume Often, cement placement involves a two-stage process utilizing a lower density "lead" cement followed by a higher density and compressive strength "tail" cement The tail cement is primarily employed to isolate critical intervals within the well.
— The operator should ensure proper wellbore preparation, hole cleaning, and conditioning with wiper trips prior to the cement job.
— Rotation and reciprocation of casing should be considered where appropriate to improve mud removal and cement placement.
— Service providers should ensure proper mixing, blending, and pumping of the cement in the field.
Casing Centralizers
Centralizing the casing is crucial for effective mud removal and proper cement placement, which are essential for a successful cement job Ensuring that the casing is centered within the hole allows it to be fully encased by cement during the cementing process, thereby achieving the necessary isolation.
Casing centralizers are essential devices that ensure the casing remains centered within the wellbore, allowing for a complete cement sheath around the casing during cementing There are three main types of centralizers: bow-spring, rigid blade, and solid designs The placement and quantity of centralizers in vertical and deviated wellbores are guided by API RP 10D-2, while API TR 10TR4 provides selection guidelines for these devices.
Casing centralizers should be used in wells Computer programs are available that can be used to optimize the number of centralizers needed and their placement within a well.
6 Well Logging and Other Testing
General
Well logs are essential tools for gathering data in formation evaluation, well design, and construction Additionally, mechanical integrity and hydraulic pressure tests play a vital role in assessing well integrity during construction This section outlines the different types of well logs and tests, along with the valuable information they provide.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 9
Open-hole Well Logging
After the drilling of a hole is completed and before casing installation and cementing operations commence, well logging is performed using electrical and other instruments on an electric cable This process, known as open-hole logging, is essential for locating and evaluating hydrocarbon-producing formations The selection of logging tools is meticulously determined by geologists during the well design phase, ensuring the appropriate log types are utilized for effective evaluation.
— Gamma Ray—A device that detects naturally occurring gamma radiation.
Resistivity measures the electrical resistance between probes on a logging tool within the wellbore Typically, at least three resistivity logs are conducted, although up to ten may be utilized, depending on the spacing between the probes Increasing the distance between the probes enhances the radius of investigation.
— Density—A device used to measure the bulk density of, and, by inference, the porosity of the formation
A caliper is a tool used to measure the diameter of a wellbore, and a caliper log is essential for calculating the hole size and volume This data is crucial for designing effective cement jobs in well construction.
Logging generates crucial data on subsurface formations, aiding in the optimization of well design and drilling operations It accurately measures the depth and thickness of formations within the drilled hole, ensuring that casing strings are installed precisely where needed This precision is essential for meeting well design objectives and maximizing the isolation benefits provided by casing and cement.
Many other types of logging tools are available and may be run on a case specific basis.
Cement Integrity (Cased-hole) Logging
After the casing is cemented, "cased-hole" logs can be conducted within the casing These logs typically consist of a gamma ray log, a collar locator that identifies casing collars, and a cement bond log (CBL) that assesses the presence and quality of the cement bond between the casing and the formation.
The CBL is an acoustic device designed to detect both cemented and non-cemented casing by transmitting sound or vibration signals and recording their amplitude When casing is not surrounded by cement, it produces a large amplitude acoustic signal due to energy retention within the pipe Conversely, casing with a well-formed cement sheath exhibits a smaller amplitude signal, as the acoustic energy is absorbed through the coupling with the cement and formation This coupling is essential for achieving the desired isolation.
The variable density log (VDL) is a display that is commonly shown with the CBL, and is a display of the wave train of an acoustic signal
An experienced engineer can effectively pinpoint essential aspects of cement operations, including the top of the cement and the positioning of casing collars, by analyzing data from various well logs For instance, during perforation, a gamma-ray detector is utilized alongside perforating guns, allowing for precise identification of the guns' locations in relation to the formations by comparing the gamma-ray response with the open-hole log and the Cement Bond Log (CBL).
The CBL-VDL is the most widely used cement evaluation tool; however, other options exist that should be considered for a thorough cement evaluation program For detailed information on different cement evaluation tools, refer to API TR 10TR1.
The cased-hole logging program is essential for accurately identifying the positions of the casing, casing collars, and the quality of the cement job in relation to subsurface formations This information is crucial for ensuring that the well's drilling construction meets design objectives and is also valuable for ongoing assessments of well integrity and seals throughout the well's productive life.
Other Testing and Information
Evaluating the quality of a cement job requires comprehensive analysis of supporting data, including drilling reports, cement design, and laboratory results Key information such as open-hole logs, cement placement details, and mechanical integrity test results are essential for assessing well integrity Additionally, conducting hydraulic pressure tests is crucial to verify the effectiveness of the cement seal and ensure the overall integrity of the well.
General
Well design and construction typically involve four key components related to casing strings: conductor, surface, intermediate, and production This section highlights essential considerations for each casing string in the context of well design and construction It's crucial to recognize that state regulations vary due to differing geological conditions, ensuring they address specific local needs rather than being uniform across the United States Nevertheless, the overarching principles of groundwater protection through zone isolation remain consistent.
Casing setting depths are predetermined in the drilling plan, playing a crucial role in ensuring isolation, compliance with regulations, and maintaining well integrity to support drilling operations and manage internal pressures The lengths of casing strings are meticulously adjusted during drilling, informed by measurements and data such as log results, drill cuttings analysis, and assessments of pressures and drilling loads.
The necessity of placing cement back to the surface on each casing string is a topic of frequent discussion, as it is only required in certain situations This requirement is thoroughly evaluated during well design and is governed by state regulations Below are specific recommendations for each casing string.
It is essential that after cementing, the cement around the casing shoe reaches a minimum compressive strength of 500 psi before any further drilling or completion activities begin, achieving 1200 psi within 48 hours under bottomhole conditions Additionally, for production casing, it is crucial to test the cement to confirm its ability to endure the expected hydraulic fracturing pressure.
Each casing string, excluding the conductor casing, must undergo a pressure test before the "drill out" process The required test pressure will differ based on the specific casing string, its depth, and various other factors.
Conductor Casing
The conductor casing is the first casing installed in a well, serving as its foundational support It plays a crucial role in holding back unconsolidated surface sediments and isolating shallow groundwater Beneath the conductor casing lies harder, more consolidated rock, ensuring the stability of the well structure.
Hydraulic fracturing operations involve well construction and integrity guidelines that ensure the stability of unconsolidated surface sediment during drilling The conductor casing plays a crucial role by protecting subsequent casing strings from corrosion and providing structural support for the wellhead load It's important to note that the requirements for the conductor hole can differ based on state and regional regulations.
The conductor hole is typically drilled and fitted with steel casing, which is then cemented according to well design and proper practices In some cases, the casing may be driven directly into the ground, similar to structural piles used in construction Drilling should utilize air, freshwater, or freshwater-based fluids, and the setting depth of the conductor casing must take into account the depth of nearby water wells Compliance with state regulations is essential for acceptable practices in the area.
When cementing conductor casing, it is essential to ensure that the cement reaches the surface If traditional pumping methods fail to achieve this, a small diameter pipe can be inserted between the hole and the conductor casing, allowing cement to be pumped around the exterior of the surface pipe This method is commonly referred to as a “top job” or “horse collar.”
Surface Casing
Once the conductor pipe is installed and cemented, the surface hole is drilled, and the surface casing is placed and cemented using proper techniques The primary function of the surface casing is to protect groundwater aquifers through effective isolation It is engineered to meet all regulatory standards for groundwater isolation and to withstand pressures that may arise during the drilling process.
The surface hole is drilled to a specific depth, considering the deepest groundwater resources and pressure control for subsequent drilling, typically ranging from a few hundred feet to over 2000 feet It is essential to use air, freshwater, or freshwater-based drilling fluid for this process To protect groundwater, the surface casing must be set at a depth that complies with state regulations, which generally require it to be placed below the deepest groundwater aquifer As a guideline, the surface casing should be set at least 100 feet below the deepest underground source of drinking water (USDW) encountered during drilling.
To ensure complete isolation of groundwater aquifers, it is advisable to cement the surface casing from the bottom to the top In some instances, a "top job" may be necessary, similar to conductor casing If cementing from bottom to top is not feasible, it is still recommended to cement across all Underground Sources of Drinking Water (USDWs) to achieve the necessary isolation.
In certain geological conditions, it may not be feasible to run the surface casing deep enough to protect the deepest groundwater aquifer, or it may be unnecessary to use surface casing altogether In such situations, effective zone isolation can be accomplished by utilizing additional casing strings or a combination of surface, intermediate, and production casing along with appropriate cementing techniques.
Before drilling out, it is essential to conduct a casing pressure test on the surface casing after the cement has reached the required compressive strength This test evaluates the casing's integrity to ensure it meets the well design and construction objectives.
After drilling through the surface casing and a brief section of the new formation beneath the casing shoe, it is essential to conduct a formation pressure integrity test, commonly referred to as a "shoe test" or "leak-off test." Should the results of this test prove insufficient, appropriate remedial actions must be taken.
Intermediate Casing
Once the surface hole is drilled and the surface casing is securely cemented, the drilling of the intermediate hole can begin This process aims to isolate subsurface formations that could lead to borehole instability and to safeguard against abnormally pressured formations below the surface.
In certain situations, drilling can proceed from the surface casing directly to the total depth without the need for an intermediate casing string This decision is based on the geological conditions assessed before drilling and is integral to the well design, as well as informed by data and measurements collected during the drilling process.
In many situations, cementing the intermediate casing back to the surface is unnecessary for effective isolation, particularly when the surface casing and cement adequately protect groundwater aquifers Additionally, cementing the intermediate casing to the surface can lead to complications such as lost circulation If the intermediate casing remains uncemented to the surface, it is essential that the cement extends above any exposed Underground Sources of Drinking Water (USDW) or hydrocarbon-bearing zones.
To ensure that the cement integrity meets the well design and construction objectives, it may be necessary to utilize a CBL and other diagnostic tools based on the well design.
Before drilling out, it is essential to conduct a casing pressure test on the intermediate casing after the cement has reached the required compressive strength This test evaluates the casing's integrity to ensure it meets the well design and construction objectives.
After drilling out of the intermediate casing and a brief interval of new formation, a formation pressure integrity test, commonly referred to as a "shoe test" or "leak-off test," must be conducted If the test results are insufficient or indicate a failure, appropriate remedial measures should be implemented Specifically, in the event of a failure, remedial cementing operations are essential to ensure well integrity.
Production Casing
The final step in drilling involves creating the production casing hole, which is drilled and logged before the production casing is installed to the well's total depth and cemented securely The primary function of the production casing is to ensure zonal isolation between the producing zone and surrounding formations, facilitating the safe transfer of hydraulic fracturing fluids and other stimulation techniques without impacting other geological layers Additionally, it houses essential downhole production equipment, such as tubing and packers Throughout the well's lifespan, the production casing plays a crucial role in containing hydrocarbon production and acts as a secondary barrier for the production tubing and packer during the final completion phase.
Cementation options vary based on geological conditions, well design, and wellbore circumstances Typically, the production string cement does not need to reach the surface entirely However, if intermediate casing is absent, cementing the production casing to the surface is advisable At a minimum, the tail cement should extend at least 500 feet above the highest formation designated for hydraulic fracturing Ultimately, the primary goal of cementing is to ensure effective subsurface isolation between different zones.
Before conducting perforating and hydraulic fracturing operations, it is essential to perform a casing pressure test to ensure the production casing's integrity meets well design and construction objectives Additionally, a Cement Bond Log (CBL) or other diagnostic tools should be utilized to assess cement integrity If any signs of inadequate cement integrity are detected, remedial cementing operations should be considered.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 13
Horizontal Wells
Drilling and completing horizontal wells is an advancing technology that enhances production performance in specific formations These wells enable operators to access resources with fewer wells compared to vertical drilling, as multiple horizontal wells can be drilled from a single surface location, minimizing the overall surface impact However, the costs associated with horizontal wells are considerably higher, often ranging from two to three times the expense of vertical wells in certain regions.
Horizontal wells begin with a vertical drilling phase until reaching a "kick-off" point, where the drill bit is then gradually turned to a horizontal position Figure 3 provides a comparison between vertical and horizontal wells The guidelines for installing conductor, surface, and intermediate production casing strings remain consistent with those used for vertical wells.
In horizontal wells, an "open-hole" completion offers an alternative to traditional casing methods by allowing the bottom of the production casing to be positioned at the top of the productive formation This approach ensures that the horizontal section of the well remains entirely within the producing formation In some cases, a short section of steel casing may extend into the production casing without reaching the surface, or a slotted or preperforated steel casing can be utilized in the open-hole section These options, known as "production liners," are typically not cemented in place.
In the case of an open-hole completion, tail cement should extend above the top of the confining formation (the formation that limits the vertical growth of the fracture)
Figure 3—Example of a Horizontal and Vertical Well
A perforation is a crucial opening made in the casing or liner that connects to the reservoir, allowing for the extraction of hydrocarbons This opening facilitates communication with the production casing, serving as the pathway for oil or gas production The predominant method for creating these perforations involves the use of jet perforating guns, which are equipped with specialized shaped explosive charges.
The perforation process, as shown in Figure 4, involves detonating a shaped charge that generates a jet of extremely hot, high-pressure gas, which vaporizes the steel pipe, cement, and surrounding formation This creates an isolated tunnel linking the interior of the production casing to the formation, with these tunnels being separated by cement Furthermore, the producing zone is also isolated from the production casing by cement both above and below the zone.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 15
General
Hydraulic fracturing, a technique used in the oil and gas industry since 1947, is essential for stimulating wells in low permeability formations like fine sand and shale These formations have limited porosity and few interconnected pores, making it difficult for fluids to flow Permeability is crucial as it determines how easily fluids can move through porous rock To extract natural gas or oil from these challenging reservoirs, fluid molecules must navigate complex pathways to reach the well Without hydraulic fracturing, the production rates would be insufficient to justify the costs of drilling and completing the well.
In a traditional nonfractured well, fluid flows towards the well as depicted by the red arrows in the upper section of Figure 5 However, the introduction of an artificial fracture allows individual molecules located far from the well to access the fracture, enabling them to travel rapidly to the well, as illustrated in the lower section of Figure 5.
Hydraulic fracturing enhances the exposed area of hydrocarbon formations, creating a highly conductive pathway from the wellbore through the targeted formation, facilitating the flow of hydrocarbons and fluids into the wellbore Designed by specialists using advanced software, hydraulic fracturing treatments are crucial for well design and construction, with rigorous pretreatment quality control and testing ensuring optimal results.
Hydraulic fracturing involves pumping fluid into the production casing and through perforations into the targeted formation at high pressures, causing the rock to fracture This process is commonly referred to in the field as "breaking down" the formation.
High-pressure fluid injection can cause fractures to grow or propagate, requiring a sufficient pumping rate to maintain the necessary propagation pressure As the fracture expands, a proppant like sand is introduced into the fluid When pumping ceases and excess pressure is released, the fracture tries to close, but the proppant keeps it open, enhancing fluid flow through the newly created high-permeability fracture.
Figure 5—Illustration of a Fractured and a Nonfractured Well
During hydraulic fracturing, a portion of the fracturing fluid can escape from the fracture and infiltrate the adjacent untreated formation, a process referred to as fluid leak-off This fluid can permeate the micropores or pore spaces of the formation, enter existing natural fractures, or fill small fractures that have been created and expanded by the pressure from the induced fracture.
Fractures tend to propagate along the path of least resistance, a phenomenon recognized since the inception of hydraulic fracturing in the oilfield in 1947 This article explores the predictable characteristics and physical properties associated with this path of least resistance.
Horizontal Fractures
Hydraulic fractures develop perpendicular to the least principal stress, which is influenced by the overburden pressure from the weight of the earth above In a three-dimensional context, a rock cube experiences equal opposing stresses to maintain its position The least stress typically occurs in the vertical direction, particularly at shallow depths, where it is identified as the overburden pressure Empirical evidence suggests that horizontal fractures are likely to form at depths of less than 2000 feet.
When pressure is exerted on the center of a rock block, it tends to crack or fracture along the horizontal plane, as this direction offers less resistance Typically, these fractures align parallel to the bedding plane of the formation.
Vertical Fractures
As depth increases, the overburden stress in the vertical direction rises by approximately 1 psi per foot, making it the dominant stress at depths exceeding 2000 feet This relationship is illustrated in Figure 7, where smaller red horizontal arrows indicate lower stress levels, and the induced fractures are oriented vertically, perpendicular to the stress.
Since hydraulically induced fractures are formed in the direction perpendicular to the least stress, as depicted in Figure 7, the resulting fracture would be oriented in the vertical direction
The vertical propagation of a created fracture towards an underground source of drinking water (USDW) is influenced by the upper confining zone or formation This zone effectively halts the vertical growth of the fracture due to its adequate strength or elasticity, which can withstand the pressure from the injected fluids.
Hydraulic Fracturing Process
Hydraulic fracturing operations require the injection of fluid into the well's production casing at high pressure, necessitating that the casing is properly installed, cemented, and capable of withstanding such pressure In certain instances, the production casing may only experience high pressure during hydraulic fracturing, leading to the use of a high-pressure "frac string" to pump fluids while isolating the production casing from the treatment pressure After the completion of hydraulic fracturing, the frac string is subsequently removed.
The well operator or their designated representative must be present on-site during the hydraulic fracturing process Before commencing the treatment, it is essential to test all equipment to ensure it is functioning properly High-pressure lines connecting the pump trucks to the wellhead should be pressure tested to the maximum treating pressure, and any leaks must be addressed before starting the hydraulic fracturing Finally, conducting safety and operational meetings is crucial to ensure a smooth process.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 17
Figure 6—Least Principal Stress is in the Vertical Direction Resulting in a Horizontal Fracture
Figure 7—Least Principal Stress in the Horizontal Direction, Vertical Fracture
Once the necessary conditions are satisfied, the well is prepared for hydraulic fracturing, commonly referred to in the field as the "treatment" or "job." This process is executed in specific stages, which may be adjusted based on site-specific conditions or as needed during the treatment Generally, these stages can be outlined as follows.
The pad is the initial phase of the job, where the fracture begins in the targeted formation during the first pumping This stage is crucial for propagating the fracture into the formation Typically, no proppant is used during the pad, although small amounts of sand may be introduced intermittently to enhance perforation opening Additionally, the pad serves to ensure sufficient fluid volume within the fracture to compensate for any fluid leak-off into the targeted formations during the treatment.
After the initial pad is pumped, the subsequent stages involve different concentrations of proppant The most widely used proppant is ordinary sand, carefully sieved to a specific size Additionally, specialized proppants such as sintered bauxite, known for its exceptional crushing strength, and ceramic proppant, which offers intermediate strength, are also utilized.
The displacement stage aims to clear out the sand-laden fluid to a depth just above the perforations, ensuring the pipe is not filled with sand and maximizing the placement of proppant in the fractures of the targeted formation Often referred to as the flush, this stage involves pumping the final fluid into the well, which may be plain water or the same fluid used previously.
In horizontal wells with extended producing intervals, the treatment process is often conducted in multiple stages, progressing from the bottom to the top of the productive zone This staged approach enhances control and monitoring of the fracturing process.
Hydraulic Fracturing Equipment and Materials
The hydraulic fracturing process necessitates a variety of specialized equipment and materials This section outlines the essential tools and substances required for standard hydraulic fracture operations in both vertical and horizontal wells.
Hydraulic fracturing requires essential equipment, including fluid storage tanks, proppant transport systems, blending and pumping equipment, as well as ancillary items like hoses, piping, valves, and manifolds Specialized monitoring and control equipment provided by hydraulic fracturing service companies is crucial for successful treatment A schematic diagram (Figure 8) illustrates how these components work together.
During fracture treatment, data is collected from various units and transmitted to monitoring equipment, often referred to as a "frac van." Key measurements include the fluid rate from storage tanks, the slurry rate delivered to high-pressure pumps, wellhead treatment pressure, slurry density, sand concentration, and chemical rate.
10 Data Collection, Analysis, and Monitoring
General
This section outlines the essential data collection, analysis, and monitoring activities necessary for effective hydraulic fracture treatment while safeguarding groundwater aquifers Hydraulic fracturing is meticulously designed through computer modeling to ensure that fractures do not exceed the upper confining formation Key factors such as pump rate, pressure, volume, and fluid viscosity dictate the dimensions and geometry of the induced fractures Monitoring techniques are crucial for verifying fracture coverage, refining computer models, and improving procedures for future operations.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 19
Figure 8—Schematic of Typical Fracturing Process
A centrifugal pump efficiently draws pre-mixed gel from the frac tank and delivers it to the blender tub The suction rate is accurately measured using a turbine meter, with the data transmitted to computers through a serial cable for monitoring and analysis.
The deck engine provides hydraulic power to the blender tub and sand augers Approximately 500 hp From frac tanks Transducer sends pressure data to computer via serial cable.
A centrifugal pump efficiently draws slurry from the blender tub and transfers it to the triplex pump The slurry rate is accurately measured using a turbine meter, with data transmitted to computers through a serial cable for monitoring at the wellhead.
4.The blender tub mixes the gel and sand The mix is called “slurry.” Tub level sent to computer via serial cable.
The sand augers efficiently transport sand to the blender tub, with their RPM monitored and data transmitted to computers through a serial cable The triplex pump then delivers a high-pressure slurry to the well, boasting a capacity of 1300 hp Additionally, the triplex pump engine provides power to the pump via the transmission, generating approximately 1500 hp.
Data collection, analysis, and monitoring can be divided into the following activities:
— “mini frac” treatment and analysis,
— monitoring during hydraulic fracturing operations,
— post-hydraulic fracturing monitoring techniques,
Baseline Assessment
Before drilling a well, it is essential to collect and test water samples from nearby sources such as rivers, creeks, lakes, ponds, and existing water wells, in compliance with regulatory requirements If testing has not been conducted prior to drilling, it must be performed before the hydraulic fracturing process begins The sampling area should be determined based on the expected fracture length, including an additional safety factor.
This procedure establishes baseline conditions for surface and groundwater before drilling or hydraulic fracturing begins If later tests show changes, this baseline data helps identify potential sources of those changes Since the components of hydraulic fracturing fluid are known, it is possible to trace alterations in groundwater composition back to their source However, it is crucial to recognize that changes in groundwater composition may also arise from other factors unrelated to drilling, hydraulic fracturing, or oil and natural gas development activities.
10.3 “Mini frac” Treatment and Analysis
Before initiating a fracturing job, an extended "pre-pad" stage is often pumped to conduct diagnostic studies, which can influence the execution of the hydraulic fracture treatment This process, referred to as a "mini frac," allows for data collection and analysis, enabling necessary adjustments to the planned job and enhancing the accuracy of computer models.
Monitoring During Hydraulic Fracturing Operations
Effective process monitoring and quality control are crucial for successful hydraulic fracture treatments and groundwater protection Key monitoring parameters must be consistently observed, while others may be tailored to specific site requirements Advanced software is essential for designing hydraulic fracture treatments and should also be utilized during the treatment to monitor real-time progress and fracture geometry Continuous monitoring of critical parameters, including surface injection pressure (psi), slurry rate (bpm), proppant concentration (ppa), fluid rate (bpm), and sand or proppant rate (lb/min), is vital throughout the hydraulic fracture treatment process.
The collected data is essential for enhancing computer models that guide future hydraulic fracture treatments In regions with extensive experience in these procedures, historical data from previous treatments serves as a reliable predictor for expected outcomes during new treatments.
H YDRAULIC F RACTURING O PERATIONS —W ELL C ONSTRUCTION AND I NTEGRITY G UIDELINES 21
Pressure is typically measured at the pump and in the connecting pipe to the wellhead It is crucial to monitor and control the pressure in the annular space if the annulus between the production casing and the intermediate casing is not cemented to the surface Throughout the hydraulic fracture treatment, monitoring pressure behavior is essential to detect and analyze any unexplained deviations from the pretreatment design before continuing operations While variations usually remain within normal ranges, slight adjustments to the original design may be necessary based on real-time data Additionally, the pressure exerted on equipment must not exceed the working pressure rating of the weakest component.
Unusual pressure behavior during hydraulic fracturing may signal potential issues, such as casing string leaks, which can be promptly addressed by halting the treatment To mitigate risks, the intermediate casing annulus must have a properly sized and tested relief valve, set to ensure that the pressure on the casing remains within its working pressure rating Additionally, the flow line from the relief valve should be securely directed to a lined pit or tank for safety.
Fracture monitoring through microseismic and tiltmeter surveys is selectively applied, primarily to assess innovative techniques, enhance the efficiency of fracturing methods in unexplored regions, and calibrate hydraulic fracturing computer models.
Recent advancements in technology have significantly enhanced the monitoring of hydraulic fracturing operations Since the 1980s, hydraulic fracture mapping using tiltmeters has been a key method, with tiltmeters measuring minute changes in the earth's surface inclination Initially focused on tracking the direct propagation of hydraulic fractures, improvements in tiltmeter sensitivity—capable of detecting changes as small as a nanoradian—along with increased computer processing power, now enable real-time monitoring of tiltmeter data.
Microseismic mapping is a groundbreaking technology that enables real-time, three-dimensional monitoring of microseismic events linked to hydraulic fracture growth This technique involves placing a geophone array in an observation well to harness the energy from the fracturing process, allowing for the mapping of microseismic events By analyzing seismic data from a nearby well, operators can accurately determine the location of these events using established seismic methods This monitoring approach is crucial for assessing key hydraulic fracturing parameters, including vertical and lateral extent, azimuth, and fracture complexity Ultimately, microseismic monitoring serves as an essential tool for operators to ensure that fracturing remains within the targeted reservoir unit and to refine computer models that predict hydraulic fracture performance.
The integration of tiltmeter and microseismic technologies enables real-time mapping of hydraulic fracture treatments This allows operators to make informed decisions about when to conclude a fracturing stage and move on to the next For instance, if the microseismic data suggests that the fracture is approaching the boundary of the targeted hydrocarbon formation, the current stage can be halted, and the subsequent stage can commence.
Post-hydraulic Fracturing Monitoring Techniques
Before hydraulic fracturing, proppant, typically sand, is often tagged with a tracer After pumping the proppant into the formation, a cased-hole log that detects the tracer is utilized to verify the intended placement of the proppant Although the radius of investigation for this log is limited to a few feet, it provides valuable insights into which perforations accepted proppant and the immediate growth of the fracture beyond these perforations.
A temperature log is an essential post-fracture cased-hole logging technique that can be utilized alongside the tracer log It measures temperature variations in the area of interest, revealing that hydraulic fracturing fluid is usually at surface ambient temperature, while formation temperatures at depths of 7500 ft can reach up to 200 °F This significant temperature difference indicates that the formation cools considerably during the fracture treatment By analyzing the temperature log, engineers can identify which perforations accepted the fracturing fluid and gain valuable insights into fracture growth just outside the casing.
It is important to note that the use of the post-hydraulic fracturing monitoring techniques described above is declining with the advent of sophisticated computer modeling techniques.
Post-completion Monitoring
Monitoring well conditions continuously is essential for maintaining the integrity of both the well and its equipment Mechanical integrity pressure monitoring plays a crucial role in assessing the mechanical integrity of tubulars and other well components during production and fracturing operations.
During well drilling, positive pressure tests assess the integrity of the casing and casing shoe During well fracturing, casing integrity is monitored by ensuring there is no leakage into the “A” annulus or between the “A” and “B” annuli After fracturing, the integrity of the tubing and packer is confirmed by demonstrating that there is no leakage of injected fluids into the “A” annulus, which would cause pressure buildup.
Monitoring annular pressures during production is crucial for identifying potential leaks Analyzing the gas content in a charged annulus can provide insights into the source and nature of any possible leak.
All annuli must have maximum and minimum allowable annular surface pressures assigned, taking into account the fluid gradient in each These limits define the safe operational pressure range for the well's current service and should be regarded as "do not exceed" thresholds.
Wellhead seal tests assess the mechanical integrity of sealing elements, such as valve gates and seats, to ensure they can effectively seal against well pressure If abnormal pressures are detected in the annulus, conducting a repressure test on the wellhead seal system can identify whether the source of communication originates from the surface of the wellhead system.
When equipment is removed from a well or depressurized for maintenance, it is essential to conduct a thorough breakdown or visual inspection to document its condition after use For instance, inspecting tubing for corrosion or erosion damage is crucial when it is pulled from the well Additionally, while the tubing is out, a casing inspection log should be utilized to assess the condition of the casing.
Regular inspections by lease operators and well pumpers are essential for detecting abnormal well conditions and monitoring well pressures These inspections of casing head equipment and annulus pressures can quickly reveal leaks between casing strings Additionally, consistent monitoring of gas, oil, and water production rates provides valuable data that engineers can analyze to identify any unusual behavior or potential issues.
API RP 90 provides guidelines for monitoring, diagnostic testing, and establishing maximum allowable wellhead operating pressure (MAWOP) While primarily designed for managing annular casing pressure in offshore wells, its recommendations for dry trees are also relevant for onshore wells experiencing annular casing pressure, such as thermal casing pressure, sustained casing pressure (SCP), and operator-imposed pressure.
[1] Interstate Oil and Gas Compact Commission 1 , 2007 Edition, Summary of State Statutes and Regulations for
[2] U.S Department of Energy 2 , Office of Fossil Energy, National Energy Technology Laboratory, State Oil and
Natural Gas Regulations Designed to Protect Water Resources, May 2009
Additional API drilling, completion, and production publications:
[3] API Specification 4F, Drilling and Well Servicing Structures
[4] API Recommended Practice 4G, Recommendation Practice for Use and Procedures for Inspection,
Maintenance, and Repair of Drilling and Well Servicing Structures
[5] API Recommended Practice 5A3/ISO 13678, Recommended Practice on Thread Compounds for Casing,
[6] API Recommended Practice 5A5/ISO 15463, Field Inspection of New Casing, Tubing, and Plain-end Drill Pipe
[7] API Recommended Practice 5B1, Gauging and Inspection of Casing, tubing and Line Pipe Threads
[8] API Recommended Practice 5C1, Recommended Practice for Care and Use of Casing and Tubing
[9] API Technical Report 5C3, Technical Report on Equations and Calculations for Casing, Tubing, and Line Pipe used as Casing or Tubing; and Performance Properties Tables for Casing and Tubing
[10] API Recommended Practice 5C5/ISO 13679, Recommended Practice on Procedures for Testing Casing and
[11] API Recommended Practice 5C6, Welding Connections to Pipe
[12] API Recommended Practice 10B-4/ISO 10426-4, Recommended Practice on Preparation and Testing of
Foamed Cement Slurries at Atmospheric Pressure
[13] API Recommended Practice 10B-5/ISO 10426-5, Recommended Practice on Determination of Shrinkage and
Expansion of Well Cement Formulations at Atmospheric Pressure
[14] API Specification 10D/ISO 10427-1, Specification for Bow-Spring Casing Centralizers
[15] API Recommended Practice 10F/ISO 10427-3, Recommended Practice for Performance Testing of
[16] API Technical Report 10TR2, Shrinkage and Expansion in Oilwell Cements
[17] API Technical Report 10TR3, Temperatures for API Cement Operating Thickening Time Tests
[18] API Technical Report 10TR5, Technical Report on Methods for Testing of Solid and Rigid Centralizers
1 IOGCC, 900 NE 23rd Street, Oklahoma City, Oklahoma 73105, www.iogcc.state.ok.us.
2 U.S Department of Energy, 1000 Independence Avenue, SW, Washington, DC 20585, www.hss.doe.gov.
[19] API Specification 13A /ISO 13500, Specification for Drilling Fluid Materials
[20] API Recommended Practice 13B-1/ISO 10414-1, Recommended Practice for Field Testing Water-Based
[21] API Recommended Practice 13C, Recommended Practice on Drilling Fluid Processing Systems Evaluation
[22] API Recommended Practice 13D, Recommended Practice on the Rheology and Hydraulics of Oil-well Drilling
[23] API Recommended Practice 13I/ISO 10416, Recommended Practice for Laboratory Testing Drilling Fluids
[24] API Recommended Practice 13J/ISO 13503-3, Testing of Heavy Brines
[25] API Recommended Practice 13M/ISO 13503-1, Recommended Practice for the Measurement of Viscous
[26] API Recommended Practice 13M-4/ISO 13503-4, Recommended Practice for Measuring Stimulation and
Gravel-pack Fluid Leakoff Under Static
[27] API Recommended Practice 19B, Evaluation of Well Perforators
[28] API Recommended Practice 19C/ISO 13503-2, Recommended Practice for Measurement of Properties of
Proppants Used in Hydraulic Fracturing and Gravel-packing Operations
[29] API Recommended Practice 19D/ISO 13503-5, Recommended Practice for Measuring the Long-term
[30] API Recommended Practice 49, Recommended Practice for Drilling and Well Service Operations Involving
[31] API API Recommended Practice 51R, Environmental Protection for Onshore Oil and Gas Production
[32] API Recommended Practice 53, Blowout Prevention Equipment Systems for Drilling Operations
[33] API Recommended Practice 54, Occupational Safety for Oil and Gas Well Drilling and Servicing Operations
[34] API Recommended Practice 59, Recommended Practice for Well Control Operations
[35] API Recommended Practice 67, Recommended Practice for Oilfield Explosives Safety
[36] API Recommended Practice 74, Occupational Safety for Oil and Gas Well Drilling and Servicing Operations
[37] API Recommended Practice 75L, Guidance Document for the Development of a Safety and Environmental
Management System for Onshore Oil and Natural Gas Production Operation and Associated Activities
[38] API Recommended Practice 76, Contractor Safety Management for Oil and Gas Drilling and Production
Invoice To(❏Check here if same as “Ship To”)
❏Payment Enclosed ❏P.O No (Enclose Copy)
❏Charge My IHS Account No.
Print Name (As It Appears on Card):
Subtotal Applicable Sales Tax (see below)
Rush Shipping Fee (see below)
Shipping and Handling (see below)
★To be placed on Standing Order for future editions of this publication, place a check mark in the SO column and sign here:
Pricing and availability subject to change without notice.
❏ API Member(Check if Yes)
Ship To(UPS will not deliver to a P.O Box) Name:
Mail orders must be paid for with a check or money order in U.S dollars, unless you have an established account Please include state and local taxes, a $10 processing fee, and 5% for shipping Send your mail orders to API Publications, IHS, 15 Inverness Way East, c/o Retail Sales, Englewood, CO 80112-5776, USA.
Purchase Orders – Purchase orders are accepted from established accounts Invoice will include actual freight cost, a $10 processing fee, plus state and local taxes.
Telephone Orders – If ordering by telephone, a $10 processing fee and actual freight costs will be added to the order.
Sales Tax – All U.S purchases must include applicable state and local sales tax Customers claiming tax-exempt status must provide IHS with a copy of their exemption certificate.
U.S orders are shipped using traceable methods, with most dispatched on the same day Subscription updates are delivered via First-Class Mail Additional shipping options, such as next-day service and air service, are available for an extra fee For more details, please call 1-800-854-7179.
Shipping (International Orders) – Standard international shipping is by air express courier service Subscription updates are sent by World Mail Normal delivery is 3-4 days from shipping date.
Rush Shipping Fee – Next Day Delivery orders charge is $20 in addition to the carrier charges Next Day Delivery orders must be placed by 2:00 p.m MST to ensure overnight delivery.
All returns require prior approval by contacting the IHS Customer Service Department at 1-800-624-3974 for assistance Please note that a 15% restocking fee may apply, and special order items, electronic documents, and age-dated materials are non-returnable.
API Members receive a 30% discount where applicable.
The member discount does not apply to purchases made for the purpose of resale or for incorporation into commercial products, training courses, workshops, or other commercial enterprises.
Phone Orders: 1-800-854-7179 (Toll-free in the U.S and Canada)
303-397-7956 (Local and International) Fax Orders: 303-397-2740
Online Orders: global.ihs.com
RP 49, Recommended Practice for Drilling and Well Service Operations Involving Hydrogen Sulfide
RP 54, Occupational Safety for Oil and Gas Well Drilling and Servicing Operations
RP 59, Recommended Practice for Well Control Operations
RP 67, Recommended Practice for Oilfield Explosives Safety
RP 74, Occupational Safety for Oil and Gas Well Drilling and Servicing Operations
RP 75L, Guidance Document for the Development of a Safety and Environmental Management System for Onshore Oil and Natural Gas
Production Operation and Associated Activities
RP 76, Contractor Safety Management for Oil and Gas Drilling and Production Operations
API provides additional resources and programs to the oil and natural gas industry which are based on API Standards For more information, contact:
API INDIVIDUAL CERTIFICATION PROGRAMS (ICP ® )
Phone: 202-682-8064 Fax: 202-682-8348 Email: icp@api.org
API ENGINE OIL LICENSING AND CERTIFICATION SYSTEM (EOLCS)
Phone: 202-682-8516 Fax: 202-962-4739 Email: eolcs@api.org
API PETROTEAM (TRAINING, EDUCATION AND MEETINGS)
Phone: 202-682-8195 Fax: 202-682-8222 Email: petroteam@api.org
Phone: 202-682-8195 Fax: 202-682-8222 Email: training@api.org
Check out the API Publications, Programs, and Services Catalog online at www.api.org.
API and its associated trademarks, including the API monogram, APIQR, API Spec Q1, API TPCP, ICP, API University, and the API logo, are protected under copyright law and are registered in the United States and other countries.