45/E3 Text Recommended Practice for Analysis of Oilfield Waters RECOMMENDED PRACTICE 45 THIRD EDITION, AUGUST 1998 REAFFIRMED, JANUARY 2012 Recommended Practice for Analysis of Oilfield Waters Explora[.]
Introduction
Since the release of RP 45, Recommended Practice for Analysis of Oilfield Waters, Second Edition, in 1968, there have been significant advancements in the analytical requirements for oilfield waters The rise of computer technology has accelerated progress in both oilfield and analytical chemistry, leading to the integration of oilfield water analysis data into computer programs These programs are now capable of predicting water compatibility, scaling tendencies, and fluid movement within reservoirs.
Since 1968, there has been a significant increase in the availability of field instruments for conducting onsite analytical procedures The advancement of computers has enhanced the analytical sensitivity needed to identify dissolved and dispersed constituents in oilfield water Additionally, growing environmental awareness necessitates the use of advanced technology for various analyses of oilfield water.
3.1.3 The changes since publication of the Second Edition have affected the goals, application, and organization of this document.
Goals
This document aims to inform users about the applications of oilfield water analyses, the correct procedures for collecting, preserving, and labeling field samples, and the various analytical methods available, including details on interferences, precision, accuracy, and detection limits Additionally, it outlines the appropriate reporting formats for presenting analytical results.
Applications
Water analyses play a crucial role in addressing technical challenges across various oilfield applications, particularly in predicting formation damage due to incompatibilities between injection and formation water during waterflood or disposal projects, as well as tracking injection water movement They are essential for forecasting scale formation in both surface and downhole equipment, monitoring and predicting corrosion, assessing the efficiency of water treatment systems, and diagnosing and resolving a range of oilfield issues.
Environmental evaluations and regulatory compliance are discussed in 3.4.
A brief discussion of some examples of oilfield water anal- yses applications follows.
Water injection into underground formations can lead to formation damage if the injected water is incompatible with the connate water To predict potential formation damage, it is essential to analyze both the injection and connate water before starting the injection process One significant incompatibility arises when the two waters contain dissolved salts, which can precipitate solids upon mixing The most detrimental of these precipitated solids is an insoluble scale, such as barium (Ba²⁺), which can clog the formation.
Barium sulfate is formed from SO₄²⁻ ions, and the significant difference in total dissolved solids between the two waters leads to changes in ionic concentration upon mixing This results in the swelling of clay minerals and a reduction in formation permeability.
During an injection project, obtaining a water sample that accurately reflects the in situ composition of the reservoir is crucial for understanding steady state conditions However, acquiring a representative reservoir water sample is challenging, as bottom hole samples are ideal but rarely accessible The process of retrieving a sample from the depths often leads to physical and chemical alterations, complicating the analysis.
For example, decreases in temperature and pressure affect both dissolved gas equilibria and individual ionic species equilibria.
Water samples collected from a producing well over time can be analyzed to identify the presence of injected water in the borehole and to assess the compositional differences between injected and connate waters Notable variations in total salinity or total dissolved solids (TDS) will be evident in the samples, indicating either a dilution or an increase in salinity.
Differences in ionic ratios, specifically Na/Ca, Na/Mg, and Na/K, are evident in waters with significant variations in calcium, magnesium, or potassium levels Additionally, anions like bromide or iodide can be present in much higher concentrations in either injected or connate water, serving as naturally occurring tracers.
Water analyses play a crucial role in forecasting the occurrence, composition, and location of mineral scale deposits Scaling typically occurs due to the deposition of calcium, magnesium, strontium, and barium in either carbonate or sulfate forms Factors such as pressure drops, temperature fluctuations, changes in flow rate, and fluid incompatibilities contribute to the induction of scaling.
Scaling tendencies are forecasted using solubility correlations that consider the ionic content of fluids, including ions such as Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺, HCO₃⁻, and SO₄²⁻, along with physical properties like temperature, pressure, and pH Various scaling-tendency estimation models, both available and proprietary, are employed by service companies, contractors, and oil company laboratories The effectiveness of any specific scaling tendency prediction model should be evaluated based on real-world field experience.
Historically, water analyses have primarily focused on corrosion studies, encompassing both monitoring and prediction Key parameters such as dissolved oxygen, carbon dioxide, iron, manganese, sulfide, sulfate, bicarbonate, chlorides, and pH levels have served as crucial tools for effective monitoring and predictive assessments.
Environmental Concerns
The analysis of environmental pollutants in oilfield waters encompasses a spectrum of techniques, from basic pH measurement to the identification of unknown organic compounds within complex matrices This document specifically focuses on the assessment of oilfield waters, despite the sometimes ambiguous boundaries of the subject.
This document does not cover systems primarily involving suspended nonhydrocarbon materials like soil samples and drilling fluids However, the outlined procedures may still apply to aqueous filtrates or mineral acid digestates derived from these samples For clarity, a list of acronyms is included in Appendix B to help readers navigate the industry terminology.
Due to the ever-changing nature of regulatory requirements, it is challenging to pinpoint all potential water analyses that operators may need to monitor Therefore, it is essential to thoroughly review relevant local, state, and federal regulations to ensure compliance with all required tests, whether in offshore or onshore settings.
This document outlines procedures similar to EPA Methods, emphasizing that laboratories conducting environmental testing must adhere to EPA protocols and obtain the necessary certification for regulatory compliance It serves as an overview of environmental analyses that are beneficial for monitoring clean-up efforts and identifying potential problem areas, while also presenting current representative test procedures.
Oilfield water analyses are primarily driven by three key areas of environmental compliance: the measurement of oil and grease, assessments of surface water and soil for environmental evaluations and pit closures, and the evaluation of radioactivity levels related to Naturally Occurring Radioactive Materials (NORM).
R ECOMMENDED P RACTICE FOR A NALYSIS OF O ILFIELD W ATERS 3
In addition, other environmental analyses are becoming more important Water analysis for toxic heavy metals (Ag, As,
Surface-disposed oilfield waters require monitoring for various contaminants, including heavy metals such as Ba, Cd, Cr, Hg, Pb, and Se, as well as oxygen demand indicators like BOD or COD Additionally, it is essential to assess levels of hydrogen sulfide, total petroleum hydrocarbons, and specific organic compounds, including benzene, phenols, and halocarbons.
A key aspect of environmental oilfield water analysis is measuring the oil and grease content in produced water This analysis has traditionally been utilized to assess the operational efficiency of produced water treatment systems and is now mandated by the EPA under the offshore National Pollutant Discharge Elimination System.
(NPDES) permit on all discharges into the navigable waters of the United States.
Many operators utilize ultraviolet, visible, or infrared techniques to periodically analyze the quality of produced water from black oils and condensates as a screening tool While these tests may not yield identical gravimetric results, they are accurate enough to serve as an index for assessing regulatory compliance and operational efficiency.
3.4.1.3 The organic constituents of crude oil found in pro- duced water contain both insoluble (droplets) and soluble
Non-Hydrocarbon Organic Material (NHOM), also called
Water Soluble Organics (WSO) fractions.
3.4.1.4 In practice, operators frequently add chemicals to enhance the performance of their water treatment systems.
Chemicals can impact soluble and insoluble fractions to different extents However, existing screening methods and the EPA's gravimetric approach combine these fractions, making it impossible to differentiate between them To effectively implement chemical treatments targeting a specific fraction, operators require separate assessments of each Methods for achieving this are detailed in section 5.3.21.
3.4.2.1 Typically, native soil samples, pit wall material, and pit contents are tested for pH, Toxic Characteristic Leach- ing Procedure (TCLP), Total Petroleum Hydrocarbon (TPH),
Electrical Conductivity (EC), Sodium Adsorption Ratio
(SAR), Exchangeable Sodium Percentage (ESP), and Cation
In property acquisition or divestiture, environmental assessments often necessitate contaminated soil sampling for substances such as benzene, toluene, ethylbenzene, and xylenes (TCLP, TPH, and BTEX) If groundwater testing is performed, these tests are typically included alongside a Target Compound List (TCL) of volatile and semi-volatile organic materials While many soil analyses fall outside the scope of this document, the analysis of separated water, including filtrates, may be relevant.
The measurement of radioactivity in water discharges is becoming increasingly important, particularly in the oilfield sector API Bulletin E2 outlines methods for assessing Naturally Occurring Radioactive Material (NORM) In addition to the well-known issue of scale deposition, there are growing environmental concerns regarding the accumulation of NORM in production equipment alongside scale buildup.
4 Sample Collection, Preservation, and Labeling
Introduction
Effective sampling is crucial, as no amount of laboratory analysis can make up for poor sampling practices The goal is to obtain a representative portion of the material, ensuring that the sample accurately reflects the whole To achieve this, it is essential to carefully select sampling points, adhere to strict collection techniques—including the choice of appropriate sample containers—and handle the samples properly, utilizing preservatives when necessary to prevent significant compositional changes before analysis.
4.1.2 If the samples are being collected for regulatory pur- poses, the procedures specified by that authority take prece- dence over the recommendations discussed herein.
Preliminary Considerations
Proper sampling relies on using new or thoroughly cleaned sample containers, with the type of container needed varying based on the planned analyses For detailed container specifications for different analyses, refer to Table 1.
For most physical and chemical analyses, sample volumes ranging from 500 to 1000 mL are typically adequate, although larger or multiple samples may be required in certain cases Refer to Table 1 for the standard sample volume needed for each specific analysis.
To obtain representative samples from certain sources, it may be necessary to create composites from smaller samples In contrast, analyzing multiple individual samples can provide more valuable insights for other sources Regulatory guidelines often dictate the specific sampling methods to be used Given the variability of local conditions, a one-size-fits-all recommendation is not feasible.
A sample reflects the composition of its source at a specific time and location If the source maintains a consistent composition, a single grab sample may suffice However, for sources that fluctuate over time, collecting grab samples at regular intervals and analyzing them separately can effectively capture the variations in extent, frequency, and duration.
4.2.3.1.2 In this case, sample intervals must be chosen to capture the extent of the expected changes in composition.
When the source’s composition varies in space (i.e., by sam- pling location), collect samples from the appropriate loca- tions and analyze individually.
Composite samples consist of grab samples taken from a specific location over various times, allowing for the observation of average analyte concentrations They can significantly reduce laboratory effort and costs when aligned with user requirements However, composite samples are unsuitable for analytes that experience considerable and unavoidable changes during storage.
Sampling
The system being sampled must function under standard conditions of flow rate, pressure, and temperature, unless the sampling is intended for analysis under abnormal conditions Any deviations from these normal operating conditions should be documented on the sample identification form.
To ensure representative sampling in distribution systems, it is essential to flush the lines In the case of wells, samples should be collected only after sufficient pumping to obtain reservoir fluid, avoiding the collection of stagnant fluid from the tubing.
Samples for measuring dissolved components must be field-filtered immediately, including those for in-field analyses The optimal method for filtration is to use a filter holder integrated into the system flow stream, utilizing system pressure If system pressure is unavailable, a large sample should be collected and pressure filtered using an alternative method at the sampling site For detailed membrane filter procedures, refer to NACE TM01-73, which outlines test methods for determining water quality for subsurface injection using membrane filters.
Filtered samples should be collected by filling and overflowing the sample bottle multiple times, but this method is not applicable for containers with preservatives or for samples intended for oil and grease analysis Additionally, it is important not to filter samples for analyses that rely on suspended material, including microbiological tests, turbidity, and oil content.
4.3.1.3 Material removed by the filter may occasionally be used for corrosion and scale product analyses These analyses are not covered here, but guidance may be found in NACE TM01-73.
Immediate analysis of samples is preferred, while storage at low temperatures (4°C) for under 24 hours is a suitable alternative, though not always feasible Consequently, sample preservation is often necessary for analyses conducted at remote locations.
4.3.2.2 Complete preservation of a sample is impossible.
Preservation techniques primarily serve to slow down the chemical and biological changes that happen post-sample collection However, it is important to note that most preservatives can interfere with certain tests, making it challenging to use a single sample for all required analyses.
Certain properties and constituents of oilfield waters, such as temperature, pH, and dissolved gases, can change rapidly and should be measured in the field These fluctuations also influence the determination of calcium, total hardness, and alkalinity, making field tests more effective Additionally, if the speciation of iron or manganese valence states is necessary, conducting these analyses in the field is often the most practical approach.
When collecting an independent water sample for organic constituent analysis, it is crucial to avoid rinsing or overflowing the sample bottle with the fluid before sampling This practice can lead to oil deposition on the bottle's sides, resulting in inaccurately elevated organic content levels.
4.3.2.5 Table 1 lists the preservatives that should be used for samples that are shipped to the laboratory for later analysis.
4.3.3.1 As stated previously, the quicker an analysis is per- formed, the better the result obtained However, some sam- ples must be preserved and sent to a remote laboratory for
For effective analysis of oilfield waters, it is crucial to complete all sample analyses, including preserved samples, within their designated hold time Failure to do so may result in substandard analytical results.
4.3.3.2 It is impossible to state exactly how much elapsed time is allowable between sample collection and its analyses.
The hold time for samples is influenced by their composition, the necessary analyses, and the conditions for storage and shipping While Table 1 offers general guidelines for hold times, it is essential to adhere to the maximum hold times outlined in regulatory documents for compliance purposes.
Sample and System Identification
Sample and system documentation play a crucial role in effective sample tracking Each sample label must include essential information such as sample identification (including company, field, and well name), the name of the person who collected the sample, the date and time of collection, the sample point, the requested analyses, and any additional comments.
4.4.2 Every sample container should be labeled with water- proof ink and the label should be applied to the sample con- tainer before the sample is taken.
Field/Laboratory Analyses
Certain components and properties of the system change quickly over time, making it difficult to preserve them for later laboratory analysis Therefore, it is essential to measure these components in the field as close to the sample points as possible A comprehensive analysis requires both field analyses for some components and laboratory analyses for others, utilizing unpreserved samples as well as several specially-treated samples.
Immediately after sampling and filtration, it is essential to measure the following properties in the field: pH, temperature, turbidity (using an unfiltered sample), alkalinity, dissolved oxygen (O₂), carbon dioxide (CO₂), and hydrogen sulfide (H₂S), with the option to stabilize samples using a basic zinc solution for laboratory analysis Additionally, total and soluble iron (Fe²⁺ and Fe³⁺) should be assessed, along with total suspended solids, where primary filtration and washing with distilled water occur in the field, while further washing and weighing can be done in the laboratory Bacterial analysis involves filtering or culturing samples in the field, followed by incubation and enumeration in the laboratory, although detailed bacterial analyses are referenced in NACE TM0194-94, which focuses on field monitoring of bacterial growth in oilfield systems.
Quality Assurance/Quality Control
Quality assurance (QA) and quality control (QC) are essential components of data acquisition and analysis, ensuring the production of reliable and reproducible results.
Quality assurance (QA) involves clearly defining the analysis requirements for a user application and ensuring that the analyst delivers what is needed For instance, obtaining accurate sodium concentration measurements is crucial, necessitating direct measurement rather than relying on inferred calculations.
The QC process ensures that essential sampling and analysis protocols are adhered to, with the level of QA/QC required varying based on user needs These needs can range from minimal QA/QC for basic field estimations to extensive QA/QC for comprehensive analyses required for regulatory compliance.
To ensure effective quality assurance and quality control (QA/QC) in specific field procedures, it is essential to utilize trip blanks and sample duplicates, decontaminate testing equipment and sampling devices, and calibrate all equipment regularly.
Trip blanks are essential for estimating experimental error caused by interactions between the sample and its container, contaminated laboratory rinse water, and sample handling procedures Including a trip blank in the analysis is recommended to ensure accurate results.
A trip blank is prepared for each type of bottle used in the sample collection, filled with deionized water at the laboratory It is then transported to the sampling location and returned to the laboratory without being opened The analysis of the trip blank helps estimate the background error in the results.
Table 1—Summary of Special Sampling or Handling Requirements a
Ammonia P,G 200 Refrigerate, add HCl to pH