1.2.2 In classical rheological terms, these fluids typically do not exhibit power-law behavior i.e., a graph of log shear stress versus log shear rate plots as a straight line over a wi
Standards
Bulletin on the Rheology of Oil-Well Drill- ing Fluids, second edition, May 1985
Recommended Practice for Standard Eval- uution of Hydraulic Fracturing Fluids, second edition, January 1983
1 Schowaiter, W R., Mechanics of Non-Newtonian Fluid,
3 Krieger, I.M and Elrod, “Direct Determination of the Flow Curves of Non-Newtonian Fluids II Shearing Rte in the Concentric Cylinder Viscometer” J Appl Phys (1953) 24,
4 Cameron, J.R., “Viscometry of Nonhomogeneous Flows and the Behavior of a Titanium-Cross-linked Hydroxypropyl Guar Gel in Couette Flow,” J Of RheoZogy, 33(1), 15-46
5 Krieger, I.M., “Shear Rate in the Couette Viscometer,” Trans Soc ofRheology, 12:11,5-11, 1968
7 Bird, Armstrong, Dynamics of Polymeric Liquids, Vol 1
8 Craigie, L J., “A New Method for Determining the Rheol- ogy of Crosslinked Fracturing Fluids Using Shear History Simulation,” paper SPE 11653,1983
9 Fan, Y and Holditch, S A., “Use of Volumetric-Average Shear Rate to Test Crosslinked Fluids With the Fann 50 Vis- cometer,” SPE Production & Facilities, (Aug 1995) 191-196
10 Economides, M J and N o k , K G., Reservoir Stimula- tion, Second Edition, Prentice Hall, New Jersey (1989)
11 Gidley, J L., Holditch, S A., Nierode, D E., and Veatch,
R W., Ahances in Hydraulic Fracturing, SPE Monograph
Definitions
Absolute viscosity is defined as the ratio of shear stress to shear rate In Newtonian fluids, this viscosity remains constant regardless of shear rate Conversely, for non-Newtonian fluids, absolute viscosity varies with shear rate at the moment shear stress is measured, leading to the term apparent viscosity, which necessitates the inclusion of shear rate in the data set.
A breaker is a chemical additive that facilitates the degradation of a viscous fracturing fluid into a thinner fluid, allowing for its recovery from the fracture Common types of breakers utilized in water-based polymer fluids include acidic breakers, enzyme breakers, and oxidizing breakers.
RECOMMENDED PRACTICES ON MEASURING THE VISCOUS PROPERTIES OF A CROSS-LINKED WATER-BASED FRACTURING FLUID 5
A Couette viscometer is a type of rotational viscometer featuring a concentric cylindrical bob and rotor, with the fluid sample positioned in the annulus between them The rotor is typically rotated to apply a specific shear rate to the fluid near its surface, while the shear stress is determined by measuring the torque on the bob across the gap between the rotor and bob.
A cross-linked fluid is formed when a linear fluid's polymer interacts with metal compounds like borate, titanate, or zirconate This reaction links the long-chain polymer molecules, resulting in three-dimensional structures that significantly enhance the fluid's apparent viscosity and elastic properties.
A linear fluid is defined as a non-Newtonian fluid that is viscosified through the addition of polymers, without the use of cross-linking additives These fluids are typically pseudo-plastic in nature Commonly used polymers in water-based fracturing fluids include guar and its derivatives, such as hydroxypropyl guar (HPG).
CMHF'G), and cellulose derivatives (HEC, CMHEC)
Newtonian fluids are defined as fluids where the shear stress is directly proportional to the shear rate during laminar flow A common example of a Newtonian fluid is water The viscosity of such fluids is determined by the ratio of shear stress to shear rate When modeled as a power-law fluid, the exponent is set to 1, indicating that the viscosity remains constant regardless of the shear rate.
Non-Newtonian fluids are defined as fluids that do not display a linear relationship between shear stress and shear rate during laminar flow Unlike Newtonian fluids, the viscosity of a non-Newtonian fluid, referred to as "apparent viscosity," is determined by dividing shear stress by shear rate This apparent viscosity varies depending on the shear rate at which the shear stress is measured.
3.1.8 pseudo-plastic fluids: Pseudo-plastic fluids exhibit a reversible decrease in apparent viscosity with an increase in shear rate These fluids are also called shear thin- ning fluids
Rheology is the study of how matter deforms and flows, with a key focus on the apparent viscosity of fracturing fluids This viscosity is influenced by various factors, including shear rate, temperature, time, and shear history.
Rheology encompasses the measurement of various properties of fracturing fluids, including dynamic moduli as a function of frequency This document offers guidelines for assessing the viscous properties of cross-linked water-based fracture fluids.
The viscous behavior of a cross-linked, pseudo-plastic, water-based fluid is influenced by the mixing process, the sequence of applied shear rates and temperatures, as well as the duration of shear and heating.
The sequence of shear rates and temperatures applied to a fluid during measurements is known as shear history It is essential for laboratory personnel to accurately document this shear history for each test To achieve reproducible laboratory results, the shear history must be consistent across different samples of the same fluid.
A shear history simulator is a laboratory device designed to precondition fluids by applying specific shear rates, durations, and temperatures This equipment typically includes mixing and pumping systems, along with tubing coils, to replicate the mixing, pumping, and shear conditions encountered during standard fracture treatments.
Shear rate refers to the velocity gradient in a fluid, defined as the rate at which one fluid particle slides past another, divided by the distance between them In Couette viscometers, the shear rate varies with position in the flow stream relative to the viscometer wall The standard unit for measuring shear rate is seconds inverse (sec⁻¹).
Shear stress refers to the force necessary to maintain a specific fluid flow, defined as a tangential force per unit area It can also be viewed as the momentum flux across a designated area The standard unit for measuring shear stress is lb/ft².
Viscoelastic fluids are unique in that they display both viscous and elastic properties Unlike purely viscous fluids, which react immediately to an applied shear field, viscoelastic fluids take time to fully respond to such forces.
Nomenclature
All symbols are listed in the Nomenclature section and upon their first use in the text
H k kf kP k, rea cm2 B1 bob
Diameter of pipe, cm Extended B2 bob
Extended B5 bob Force, dyne Acceleration from gravity, cm/s2 Plate separation, cm
Slot height, crn Geometry-independent consistency index, dyne-sec" icm2 SIot geometry-dependent consistency index, dyne-sec" >cm2
Pipe geometry-dependent consistency index, dyne-sec" ?cm2 Couette geometry-dependent consistency index, dyne-sec" k m 2
STD.API/PETRO R P 39-ENGL 1998 O732290 ObObLO8 038
Length of test section in pipe viscometer, cm Bob length, cm k n g t h of bob i , cm
Length of bob 2, cm Flow behavior index Pressure drop over the length of the test section, dyndcm' Volumetric flow rate, cm3/sec
The article discusses various measurements related to fluid dynamics, including the radius at any point, tubing radius, and the radii of different bobs and cups, all measured in centimeters It also covers time in seconds and minutes, as well as torque values in dyne-cm for both bobs Additionally, it addresses average fluid velocity in feet per second, volume in the Couette region in cubic centimeters, and the velocity of a Newtonian fluid The article further details velocity components in different directions, including radial, x, z, and û directions, along with slip velocity in centimeters per second and slot width in centimeters.
Angular velocity, racüsec Angular velocity, rad/sec Angular velocity, rad/sec Corrected angular velocity, raàisec Constant = a p l a z = A p f L
Corrected shear rate, sec-' Shear rate at bob, sec"
Shear rate at rough bob, sec'' Shear rate at smooth bob, sec-' Newtonian shear rate, sec-'
Newtonian shear rate at bob, sec-' Newtonian shear rate at wall, sec-l Shear rate at wall, sec-'
Shear stress is measured in dyne/cm² and varies at different points, including the bob and the wall Specifically, shear stress at the bob can be categorized into values for rough and smooth surfaces Additionally, the standard deviation in viscosity is expressed in centipoise (cp), while the rotational speeds of the cup and bob are measured in revolutions per minute (rpm).
Cup rotational speed rpm Actual Power-law Viscosity, cp Nominal viscosity for Couette geometry, cp Apparent viscosity in slot geometry, cp
This document outlines recommended practices for testing fluids in two key scenarios: laboratory testing and field testing The focus of this section is on the best practices for conducting fluid tests in a laboratory setting.
To achieve reproducible data, it is essential to utilize a shear history simulator that effectively models the mixing of fracture fluids This simulator also simulates the pumping of these fluids through tubular goods at high shear rates before they enter the hydraulic fracture.
In the laboratory, a coiled capillary tube simulates the shear history within well tubulars, while a Couette viscometer replicates the shear and temperature conditions of fluid in hydraulic fractures To ensure accurate and reproducible results, it is essential to meticulously follow recommended laboratory procedures and utilize well-maintained, calibrated equipment.
4.1 FLUID PREPARATION ANDTESTING 4.1.1 Water-Based Solutions
The preparation of the fracturing fluid polymer solution sample must follow a specified procedure that includes a detailed description of the base fluid's composition, including its pH and salt content It is essential to identify the source of the water used, whether it is deionized, tap water from a specific city, or seawater The procedure should outline the precise amounts of each component to be added, along with the order and method of their addition Additionally, the mixing process must be documented, specifying the temperature, mixing times, and speeds If necessary, an aging or holding time before testing should be included, along with the total volume of fluid required for the tests Finally, key properties of the polymer solution that influence rheology testing outcomes must be measured prior to the actual test.
2 Apparent Viscosity (e.g as per API RP 39)
4.1.2 Test Procedure for Water-Based Gels 4.1.2.1 Shear History Simulation a The fluid pumped through the shear history simulator should experience the following flow conditions:
1 For fluids to be tested at temperatures less than 2009, the fluid shear history is to be performed at 675 sec-' f 67.5 sec-' for a period of 2.5 minutes i 10 seconds: or
2 For fluids to be tested at temperatures greater than or equal to 2ỷỷOF, the fluid shear history is to be perợormed at
1350 sec-' f 135 sec" for a period of 5.0 minutes f
STD-API/PETRO RP 39-ENGL 1998 0732290 060b109 T 7 4
RECOMMENDED PRACTICES ON MEASURING THE VISCOUS PROPERTIES OF A CROSS-LINKED WATER-BASED FRACTURING FLUID 7
7 b Operation of the Shear History Simulator
1 Set the Shear History Simulator exit valves to a waste tank
2 Begin displacing the gel through the shear loop
3 Adjust the volumetric flow rate to correspond with the shear rate conditions specified above for the given test temperature (refer to 4.2.2)
4 When the pressure drop through the capillary tube has stabilized, begin on-the-fly injection of the crosslinker and/or other additives
Continue pumping the cross-linked fluid to the waste tank until the shear loop is thoroughly flushed with a minimum of two loop volumes, ensuring that the pressure drop in each capillary has stabilized.
4.1.2.2 Viscometer Loading a Re-heat the viscometer bath to the desired test temperature b Attach the B5 or extended €35 bob (refer to 4.2.6) to the
To operate a Couette viscometer, first check the zero reading and attach the rotor cup using the quick-connect swivel fitting Purge the viscometer with nitrogen for 5-10 minutes to eliminate oxygen, which can degrade the polymer during high-temperature testing Connect the sample injection line from the shear history simulator to the rotor cup quick-connect and maintain a shear rate of 100 sec⁻¹ by rotating the cup at 118 rpm Use a stopwatch to inject the required fluid volume—40 mL for extended B5 and 45 mL for B5—by turning the 3-way valve at the bottom of the rotor cup After injection, immediately return the 3-way valve to the "waste" position and disconnect the sample injection flowline Finally, pressurize the rotor to 400 psi with nitrogen, and optionally, perform an ambient temperature shear rate ramp.
(see 4.1.2.3~) if needed to characterize early behavior of fluid during heat-up j Raise the heating bath k Turn crosslinker addition pump off
1 Flush the shear history loop with at least 2 loop volumes of water to remove residual cross-linked gel in the loop
To ensure accurate viscometer operation, it is essential to monitor the sample temperature during the heat-up phase and throughout the test, reaching the desired test temperature (t5°F) within 20 minutes and maintaining it for the test duration The first shear rate ramp should commence at 90 percent of the test temperature or after 20 minutes, whichever occurs first Shear rate ramps involve adjusting the rotor speed from the base level of 118 RPM for a duration of 10, 20, or 30 seconds, or longer if necessary, to achieve a stable shear stress It is crucial to perform this ramp quickly, particularly at high temperatures, to minimize fluid degradation This procedure should follow a stepwise schedule for optimal results.
Shear rate ramps are conducted every 30 minutes after completing the initial ramp series at the test temperature, with a duration of 100 seconds at 18 rpm It is essential to record the shear stress corresponding to each shear rate value Once the test concludes, lower the bath and allow the sample to cool to 100°F or below before releasing the pressure Finally, measure the pH and observe the gel condition when removing the sample from the rotor cup.
Equipment Requirements
Effective batch mixing equipment should offer varying levels of mixing intensity Initially, it is crucial to rapidly disperse the polymer in the mixing water while avoiding air entrapment After achieving dispersion, the mixing intensity should be decreased to a gentle stirring motion to prevent the stratification of the hydrating polymer within the mixing vessel.
The volume of polymer solution required for a shear history simulator can vary significantly, reaching up to several liters A high-speed blade mixer, like the Wanng Blender@ Model CB6, can prepare up to four liters of solution, with variable speed control recommended for optimal mixing This control can be achieved by adjusting the supply voltage using a variable auto transformer For larger volumes, up to 16 liters can be mixed in a 5-gallon HDPE open-top container using an air-driven stirrer, such as the Lightning Mixer Model D A 3 3 To meet the demands of larger shear history simulators, multiple batches of polymer solution can be prepared with these mixing devices Once prepared, the polymer solutions are typically transferred to an accumulator or holding tank linked to the preconditioning equipment.
A Shear History Simulator (SHS) is a device that meets the standards outlined in the Recommended Practices Manual, typically classified as a tube or pipe flow device operating within the laminar flow regime In single-pass mode, fresh fluid is continuously introduced at the inlet while being removed at the outlet.
Operating the device in flow modes such as recirculation or oscillatory is not advisable due to the complex flow patterns they create, which can unpredictably affect the final properties of the fluid In recirculation mode, the fluid circulates repeatedly through the pump and SHS, while oscillation mode causes continuous flow reversals Although simulating turbulent flow in the wellbore could enhance accuracy, achieving turbulent conditions in laboratory-scale equipment used by the SHS is generally impractical.
Operating in laminar flow provides distinct benefits for inter-laboratory testing comparisons At a given shear rate, the energy dissipation rate remains consistent across different laboratory simulators, regardless of the variations in tubing or pipe sizes used.
The design and operation of the SHS can be adaptable while still achieving the necessary preconditioning standards To maintain a manageable size for the SHS, the internal diameter of the tubing should be between 0.080 and 0.305 inches (2.0 to 7.7 mm).
For optimal performance, it is recommended to use tubing with an inner diameter (ID) of 8.0 mm After selecting the appropriate ID, one can calculate the necessary flow rate and tubing length to meet specific preconditioning criteria For instance, to maintain a desired shear rate over a specified time, the relationship can be expressed as \$\dot{y} = \frac{96v}{ID}\$ and the flow rate can be determined using \$v = \frac{\dot{j} \cdot ID}{96}\$.
.Y, = Newtonian (nominal) shear rate, sec-’, v = average fluid velocity, ft/sec, d = tubing internal diameter, inches,
To manage long tubing lengths, coiling is often necessary to fit the tubing into limited spaces like laboratory bench tops It is advisable to use large coil diameters to reduce energy dissipation caused by the flow geometry.
A “large increase in the resistance to flow” occurs when the Dean Number is > 10’”
De = Dean number (related viscous, inertial & cen-
Re = Reynolds number, trifugal forces),
R = tube cross-section radius (D = diam), r = radiusofcurvature
To be sure the radius of curvature has no effect, the follow- ing relationship should be honored
The following data were generated by D Lord for various lab-sized pre-shear loops (for a 4W HPG), using the relation- ship:
Diameter (in.) Number (Re) of Curvature (in.)
The pumping requirements for polymer solutions in a Shear History Simulator (SHS) are influenced by the selected tubing size, ranging from less than 0.1 liters per minute for smaller tubing to over 3.0 liters per minute for larger sizes Due to significant frictional pressure losses in the SHS tubing, high-pressure, positive-displacement pumps are essential Suitable options include plunger pumps or plunger-actuated diaphragm pumps, which should feature adjustable stroke lengths and/or variable-speed electric drives Regardless of whether a simplex, duplex, or triplex pump is chosen, it is crucial to attach a pulsation damper to the discharge, specifically one that can be recharged with nitrogen via a regulator.
An alternative method to supply fluid flow for a small tubing SHS (0.125-in OD, 0.081-in ID) involves using an accumulator with a floating piston that separates the polymer solution from a low viscosity working fluid like oil or water Initially, the accumulator is filled with the polymer solution, forcing the floating piston to expel any remaining working fluid During the preconditioning phase, a low-capacity, high-pressure positive-displacement pump injects the working fluid into the accumulator at a controlled rate, enabling the floating piston to displace the polymer solution into the SHS.
High-pressure liquid-chromatography (HPLC) pumps are typically useful in this application because they can handle the clean, low-viscosity working fluids
RECOMMENDED PRACTICES ON MEASURING THE VISCOUS PROPERTIES OF A CROSS-LINKED WATER-BASED FRACTURING FLUID 9
Table 1-Description of Equipment for Laboratory Testing
Crosslink Tubing Diameter Sample Injection
Backwards Tee Forward Tee Backwards Tee kght Angle wlstatic mixer
Right Angle wlstatic mixer Forward Tee
The requirements for crosslinkers vary based on the specific cross-linking process and the chemical composition of the chosen reagent Typically, crosslinker injection rates are minimal and are influenced by the flow rate of the primary polymer solution during the Shear History.
Low-capacity pumps, like syringe pumps, are effective for injecting cross-linking agents into the polymer solution flow stream that enters the Simulator (SHS) An example of a suitable low-pressure syringe pump is the Harvard Model 909, which operates at a flow rate of 0.00764.
38.2 mvminute) or equivalent, can be used to deliver cross- linking agents to the suction side of the high-pressure dis- placement pump A high-pressure syringe pump, such as an
ISCO Model 500D (O to 500 Wminute) or equivalent, can be used to deliver cross-linking agents to either the suction or discharge side of the primary displacement pump
The addition of a crosslinker to the primary flow is usually achieved via a tee fitting located on either the suction or discharge side of the primary displacement pump This entry point can utilize a straight-run or a side port with a small diameter feed tube, known as a stinger Chromatographic "low dead volume" tees are employed to effectively introduce the crosslinker into the high-pressure stream.
To achieve proper mixing of the crosslinker with the polymer solution, an inline mixing device is typically employed downstream of the tee on the high-pressure side Various commercial inline static mixers are offered by companies such as Cheminer, Inc., K O K O Corporation, K O M M Systems, Inc., and TAH Industries, Inc Proper delivery of the crosslinker to a tee on the suction side is essential for effective mixing.
Coil Diameter Crosslink Pump Base Gel Pump (inches) Accumulator
In certain situations, an inline mixer is typically unnecessary, as the plunger motion within the pump cavity, along with the shearing effects from both suction and discharge check valves, effectively facilitates the thorough mixing of crosslinker with the polymer solution.
instrument Calibration
The critical calibration items for a tubing shear history simulator are flow rate and time Secondary calibration items may include pressure and temperature measurements
For fluids to be tested at temperatures less than 200"F, the fluid pre-shear is to be performed at 675 sec-' k67.5 sec-' for a period of 2.5 minutes i 10 seconds
For fluids to be tested at temperatures greater than or equal to 200"F, the fluid pre-shear is to be performed at 1350 sec-'
2 135 sec-' for a period of 5.0 minutes 210 seconds
Before loading the Couette viscometer, it is essential to verify the total flow rate, which includes the primary displacement pump, crosslinker, and any additional additive pumps A reliable method to conduct this check is by timing the fluid as it fills a graduated cylinder with a stopwatch.
The maximum allowable deviation from the desired flow rate is +lo percent If results exceed this value, pumps must be recalibrated or repaired
The duration of shear is determined by the length of tubing extending from the injection point of the crosslinker or activator to the swivel connection that links the flow line to the viscometer cup.
Accurate measurement of the tubing length is essential and should be based on its diameter, ideally before coiling To verify this length, clean and dry the tubing, then pump at rates of 675 sec⁻¹ and 1350 sec⁻¹, recording the time it takes for the fluid to reach the viscometer cup.
This time should be within 110 seconds
Shear history simulators often include pressure transducers to measure the fluid's pressure response during shear conditioning It is essential to regularly calibrate these devices following the manufacturer's guidelines to ensure accurate measurements.
Thermocouples are often used to measure the fluid temperature during the cross-linking process, with a recommended standard testing temperature of 77 ± 2°F It is essential to calibrate temperature measuring devices according to the manufacturer's instructions.
The effective operation of a Couette viscometer relies on the accurate functioning of the torque sensing transducer, precise rpm control of the drive motor, and reliable temperature regulation It is essential to refer to the manufacturer's operating and calibration manual for detailed calibration instructions tailored to the specific instrument Below are general techniques for conducting calibration checks.
Accurate control of the Couette cup's rpm is essential for applying the desired shear to the fluid, as the shear rate depends on the rotorhub configuration A basic calibration procedure involves using a strobe light tachometer to verify the rpm at the shear ramp settings, ensuring that the maximum deviation from the target rpm does not exceed 5.0 percent.
4.3.2.2 Bearing Condition and Transducer Torque
To ensure accurate torque measurements on the bob, the transducer must exhibit a linear response to a Newtonian calibration fluid across the tested shear rates Nonlinear responses may indicate issues such as a poorly calibrated transducer or contaminated bearings Testing this response can be effectively conducted using a NIST-traceable calibration oil The recommended procedure includes placing the calibration oil in the instrument cup and measuring the temperature with a calibrated thermometer Transducer readings should be taken at 0 rpm and at various shear rate steps to determine power-law coefficients It is essential to compare the viscosity at different shear rates with expected Newtonian behavior and to verify the torque at zero shear rate, along with the coefficient of fit for power-law calculations.
The calibration of transducer deflection to torque is achieved by utilizing a series of calibrated weights These weights are suspended on a thread that passes over a low-friction pulley The thread is wrapped 2.5 times around the bob and secured with tape on its surface The torque (\$T\$) applied to the bob can be calculated accordingly.
The torque (\$T\$) can be calculated using the formula \$T = m g Rb\$, where \$m\$ represents the mass in grams and \$Rb\$ is the bob radius in centimeters The gravitational acceleration (\$g\$) is approximately \$980.7 \, \text{cm/s}^2\$ Different bob radii yield varying torque values, as demonstrated in Table 3, which presents results for commonly used bob sizes.
Table 3-Calibration Factors for Dead Weight Testing
RECOMMENDED PRACTICES ON MEASURING THE VISCOUS PROPERTIES OF A CROSS-LINKED WATER-BASED FRACTURING FLUID 11
This document outlines field procedures aimed at assessing the quality of fracture fluids before and during hydraulic fracture treatments It emphasizes the importance of measuring fluid viscosity to evaluate the overall quality of the fluid, its additives, and the anticipated performance throughout the pumping process and the subsequent return to base conditions.
Field measurements are essential for ensuring the quality of the fluid but should not be relied upon to determine the apparent viscosity values used in fracture treatment design The apparent viscosity obtained in the field is unlikely to match laboratory measurements unless identical mixing and testing procedures are meticulously followed, which is rarely the case Therefore, it is inadvisable to compare field data with laboratory results or use field measurements for treatment design Instead, field data should be utilized exclusively for quality control, including adjustments to buffer, breaker, or crosslinker loading.
Equipment Requirements
The equipment described in the following section is rec- ommended for the preparation of cross-linked water-based fracturing fluids for evaluation of the fluids in the field
To effectively mix the base gel and additives, a blender or laboratory mixing device is essential, with a Waring blender commonly used in the field Utilizing a rheostat to control the blender's speed helps minimize air entrapment in the mixture Additionally, polymer solutions can be prepared with laboratory stirring devices, provided that the final viscosity meets the specifications set by the suppliers of the base gelling agent.
Accurate measurement of liquid buffers, crosslinkers, and additives is essential, necessitating the use of syringes, pipettes, or other quantitative measuring devices These tools must provide measurement accuracy as precise as 0.01 mL.
To accurately weigh dry additives, an electronic balance with a measurement precision of 0.001 grams is essential In fracturing fluids, it is common to use additive concentrations of 1 pound per 1000 gallons of fluid, which translates to approximately 0.06 grams of additive in 500 ml of fluid.
Accurate documentation of field conditions necessitates monitoring basic fluid properties like temperature and pH For comprehensive coverage, thermometers that measure temperatures from 0°F to 300°F are essential Additionally, a temperature-compensated pH meter should be utilized for precise pH measurements in the field, although narrow-range pH paper can serve as an alternative for estimation within the required range.
A portable Couette viscometer, such as the Fann Model 35 or an equivalent, is essential for assessing the viscous properties of both linear and cross-linked fluids It must be easily transportable by air, automobile, or van, and should operate on either rig power or battery power.
To effectively test fluids, it is essential to use equipment that can heat them, such as a water bath or a heated cup, which should operate on either rig or battery power Testing both linear and cross-linked fluids at elevated temperatures is crucial for assessing gel stability and breaker properties, influenced by the additives present in the fluid.
5.2 PREPARATION OF LINEAR POLYMER SOLUTIONS
To prepare linear polymer solutions for fracture treatment, follow the supplier's recommended procedures Begin by adding the powder or slurry to a vigorously stirred blender, controlling the rotation with a rheostat to minimize air entrainment Allow the solution to stir and hydrate for the specified time, recording the duration, and add any necessary additives, such as acidic buffers, for proper gel hydration After 10-15 minutes, measure the viscosity at 5 s\(^{-1}\) using a Couette viscometer with a B 1 bob If the viscosity meets the service company's specifications, incorporate additional additives like buffers and surfactants Finally, mix the fluid at moderate speed for 2-3 minutes to ensure even dissolution and distribution of the new additives.
After mixing all chemicals and additives, it is essential to measure the pH of the fluid to ensure it falls within the specified range Once the base gel is prepared and tested with the necessary buffers and surfactants, breakers, gel stabilizers, and crosslinkers should be added to a measured volume of the polymer solution The fluid can then be transferred to a heated cup or a sophisticated viscometer, like the Fann Model 50, to assess its viscous properties It is crucial to minimize the time the fluid remains at zero shear rate after adding the crosslinker to the linear gel For batch mixing in the field, steps a-h are followed, while for continuous mixing procedures, the timing and order of additive addition must align with those used during the actual treatment.
To test a fluid sample after mixing, a Fann Thermos-Cup or an equivalent is necessary It is advisable to use smaller volume heat cups to speed up the heating process once the gel is added The gel should be filled to about an inch below the top of the heat cup before inserting the bob and sleeve of the Fann 35 or equivalent For most tests involving cross-linked water-based polymer fluids, a B2 bob is recommended, although B1 or B5 bobs may be used for certain cross-linked fluids.
To conduct field testing, first transfer the gel with all additives into the heat cup or high-temperature viscometer and initiate heating Ensure that the heat cup or viscometer is preheated to the desired test temperature Finally, set the viscometer to rotate at 100 rpm for accurate measurements.
To evaluate the fluid's properties in a fracture, a rotational speed of B2 bob, equivalent to a shear rate of 37 sec\(^{-1}\), should be maintained The sample must be heated to the test temperature within 15-20 minutes, with continuous recording of apparent viscosity during this period Once the test temperature is achieved, shear stress readings should be taken every 15 minutes for several hours For fluids intended for short pumping treatments (one hour or less), continuous shear rate measurements are essential Additionally, graphs plotting apparent viscosity against time at a constant shear rate of 37 sec\(^{-1}\) should be created to assess the quality of the fluids, additives, and mixing procedures.
5.5 DISCUSSION OF COMMONLY OBSERVED PROBLEMS
In lower temperature testing (150°F and below), high viscosity fluids like borates often escape from the viscometer To address this, one can either push the gel back into the viscometer or use a rubber cover to contain it The goal is not to measure the absolute viscosity of these visco-elastic fluids, but rather to assess their viscous properties over time and temperature in relation to the breakers and other chemicals intended for the actual treatment.
Shear stress values may sometimes not accurately reflect the true viscous properties of a gel, leading to potential issues For instance, visco-elastic gels can climb out of the gap onto the bob's top, or they may slip along the bob's surface, particularly when oil-based fluid loss additives are present In cases where a closed, high-temperature fluid viscometer is utilized, it may be necessary to abort and restart the test to obtain reliable results.
To effectively use a table-top rotational viscometer with a heated cup, lower the heat cup and remove the sleeve to let the gel fall into the cup By moving the heat cup up and down, you can homogenize the sample for an accurate test.
It is important to differentiate between fluid problems and viscometer testing problems when testing fluids in the field
Determining the cause of poor test results often necessitates extensive testing with various additive concentrations and careful visual observations Testing can be challenging due to the visco-elastic properties of the fluid or slip issues in the viscometer Additionally, adjustments in fluid chemicals or mixing procedures may be essential to achieve the desired fluid properties in the field.
6 CALCULATION PROCEDURES FOR VISCOUS PROPERTIES
General Concepts
6.1.1 Major assumption: Homogeneous fluid with power- law behavior where z k = y = n = shear stress, forcdarea, fluid consistency index, force-sec"/area, shear rate, sec-', flow behavior index, dimensionless
RECOMMENDED PRACTICES ON MEASURING THE VISCOUS PROPERTIES OF A CROSS-LINKED WATER-BASED FRACTURING FLUID 13
A power-law fluid's shear rate is influenced by the geometry of the Couette viscometer and the flow behavior index, highlighting the distinction between geometry-independent rheology and nominal rheology.
The nominal Newtonian shear rate approximates the shear rate under Newtonian behavior, while the consistency index, derived from shear stress and the Couette nominal shear rate, is referred to as machine \( k \) To obtain the actual power-law shear rate and geometry-independent consistency index, corrections are made to the nominal shear rate and consistency index using the power-law index and geometry The nominal viscosity, calculated from the nominal shear rate and machine \( k \), differs from the actual power-law viscosity, which is based on the power-law shear rate and geometry-independent \( k \), as indicated in Equations 6.3 and 6.4, except when the fluid behaves as Newtonian (n = 1) This discrepancy is more pronounced in Couette geometries where the ratio of bob to rotor (cup) radii is significantly less than one.
It is advisable to report geometry-independent rheology and utilize actual power-law viscosities The recommended calculation method involves using nominal shear rates for data reduction, followed by adjusting the fluid consistency index \( k \) to achieve a geometry-independent \( k \) This adjusted \( k \) can then be converted to \( k_p \) for applications in fracture (slot) and pipe flows, if needed.
For data reduction purposes, it is assumed that CGS units will be utilized The fluid consistency index will be converted into commonly used English units, ensuring accurate conversions to facilitate understanding and application.
Torque: dyne-cd1.356 x lo7 = ft-lbf Shear Stress: dyne/cm2/478.8 = lbf /fi2 k: dyne-secn/cm2/478.8 = lb, -sec"/ft2
In the context of basic equations using CGS units, shear stress (\$z\$) is measured in dynes per square centimeter (dyn/cm²) The geometry-dependent consistency (\$k\$) is crucial for understanding machine behavior, while the nominal shear rate (\$y\$) is expressed in seconds inverse (sec⁻¹) Additionally, the geometry-independent consistency index (\$k\$) and power-law shear rate (\$y\$) are also measured in dynes-seconds per square centimeter (dyne·sec/cm²) The power-law index (\$n\$) plays a significant role in characterizing fluid behavior under shear.
A = nominal power-law viscosity, Poise or p = actual law viscosity, Poise or dyne-sec/cm2 dyne-sec/cm2,
6.1.5 Calculation of viscosity, using conventional English k units:
/i = 47,880 * k yC"-') (6.5) where p = actual power-law viscosity, centipoise, k = geometry-independent consistency index y
= power-law shear rate, sec",
6.1.6 Limitationdproblems that may produce erroneous results: a b
Non-power-law over-shear measurement range
1 Change in power-law indices vs shear rate
Borate fluids-climbing out of Couette gap
Under- or over-filled Couette viscometer cup.
CouetteGeometry
The user utilizes a computer for data reduction, employing a calibrated conversion factor to accurately relate torque to bob shear stress Additionally, a comprehensive data table has been created, detailing the relationship between shear stress, RPM, and time.
To convert cup RPM into nominal Newtonian shear rate, calculate Factor 1 and use it to determine the nominal shear rate for each RPM during the shear sweeps The nominal shear rate in (sec\(^{-1}\)) is given by the formula: Nominal shear rate = Factor 1 x RPM.
Factor 1 = id15 * 1/( l-(Rb /Rc)2) (6.6) where
Rb = viscometer bob radius, cm,
6.2.3 For each shear sweep time, perform a linear regres- sion of Equation 6.2 in log form or y = m + b
14 RECOMMENDED PRACTICE 39 where x = log (qJ,
Y = 1%