New Technique Overcomes Legacy-Field Tests' Shortcomings

Proper characterization of the composition of various waters throughout the upstream oil and gas industry is critical to understanding how that water should be handled and treated. For unconventional operations, source water must be thoroughly characterized for its chemical composition. Dissolved minerals dramatically impact the performance of friction reducers, scale inhibitors, corrosion inhibitors, biocides and other chemical additives that are critical to the performance of a hydraulicfracturing operation.

For example, if the water chemistry is not compatible with the chosen friction reducer, fluid viscosity will be lower than anticipated. The immediate observation would be abnormally high fluid pressure due to increased friction with the wellbore. Operating the pumps at higher pressures causes exponential increase in the rate of pump wear and damage. Furthermore, several other causes for the high pressure may be unnecessarily investigated without knowledge of the water chemistry. Proper water-characterization data can pinpoint the problem in this scenario and show how the issue can be corrected as fast as possible, saving unnecessary downtime and cost.

During production, the chemical composition of the produced water may also cause extensive damage to pipes and valves. For example, production fluids are commonly treated with biocides like chlorine dioxide or ozone to kill harmful organisms such as sulfate-reducing bacteria (SRB). If the amount of biocide added is insufficient, then organisms like SRB survive and produce hydrogen sulfide (H2S), which is toxic and sours hydrocarbon. Overdosing the system with biocide also causes oxidation of pipes and valves leading to premature component failure. Testing the production fluids consistently for biocide residuals allows the operator to optimize treatments, minimize cost and maximize production.

Overdosing the system with biocide also causes oxidation of pipes and valves leading to premature component failure.


HOW CAN I TEST MY WATER?

There are several common methods for analyzing water for its chemical composition, each with its own set of advantages and disadvantages. The most sensitive methods for detecting ions in solution use an inductively coupled plasma (ICP) generator coupled with an optical emission sensor (ICP-OES) or a mass spectrometer (ICP-MS) to gather the data. ICP-OES and ICP-MS are the most common instruments used to characterize water in a laboratory.

For complete analysis, these instruments are often supplemented with the output of an ion chromatograph (IC) to give a full readout of ionic composition. The sensitivity of these instruments permits a very low limit of detection (LOD) for their respective analytes, usually in the part per billion (ppb) or even part per trillion (ppt) range. However, when applied to high-salinity waters like those found in the oil and gas industry, the real-world LODs can be quite high due to the massive dilutions that are often required.

Analytical techniques based on colorimetry have become the go-to method for characterizing samples in the field. Simple colorimetric field kits have been available for decades. Colorimetric tests do not usually require large and expensive equipment, which allows them to be used in the field more effectively than alternative methods.

The development times of colorimetric tests are very rapid, often a matter of seconds. This enables results to be obtained much more rapidly than competing methods. Results are commonly interpreted by eye, making the operation of such tests much cheaper than other methods. This, of course, opens the test to the subjectivity of human observation and error, which leads to systematic errors and an inherently high LOD compared to laboratory methods.


WHY IS ACCURATE WATER ANALYSIS SO DIFFICULT?

Water analysis is often complicated by the nature of the sample itself. High levels of salinity, the presence of interferences and the impact of variable sample handling and preparation all contribute to error in the result. How each of these impacts the results depends on the method being used.


Unfortunately, testing water for its composition is a difficult task even for state-of-the-art equipment, especially for the types of water commonly encountered in the oil and gas industry.


TRADITIONAL LABORATORY INSTRUMENTATION

Unfortunately, testing water for its composition is a difficult task even for state-of-the-art equipment, especially for the types of water commonly encountered in the oil and gas industry.

Both ICP-OES and ICP-MS are very fragile and sensitive to their environment. Vibration or motion will cause these devices not to function appropriately, usually resulting in the machine rapidly losing its calibration. This restricts their application to laboratory environments with tightly controlled conditions. Both instruments will not tolerate high levels of salinity in the samples being run, like those found in the oil and gas industry. While the internal construction of an ICPOES system lends itself to an increased tolerance of salinity versus ICP-MS-based systems, both methods will require substantial dilution if a sample’s salinity is too high.

This required dilution has a multiplicative effect on the limit of detection of the instrument. The LOD as given in the specifications for the instrument only applies to an undiluted sample. If, for example, a sample must be diluted by a factor of 10,000:1 before it can be run, then the effective LOD on that sample will be 10,000 times higher than given in the instrument specifications.

Although this principle applies to any analytical method when running diluted samples, ICP-MS/OES typically require the heaviest dilutions on a sample before it can be successfully analyzed. It is not uncommon for high-salinity samples to require dilution by a factor of 100,000 or even 1 million on these instruments. For example, an ICP-MS may report an LOD of 2 parts per trillion for iron detection. However, if the sample must be diluted by 1 million to run on the machine, the LOD is multiplied by 1 million such that the true LOD is 2 parts per million.


LEGACY COLORIMETRIC FIELD METHODS

Sensors are susceptible to interferences that cause false positives or false negatives. Accounting for these interferences can become time consuming and complicated with the number of tests required to measure the interfering species. Colorimetric tests are often single analyte, meaning that multiple different tests with their own respective procedures must be run to obtain data on multiple analytes in a sample.

Many field methods are easily confounded by variability in sample composition and the unpredictable ways that interferences affect results. For this reason, field assays are often required to be individually calibrated by standard additions for every sample run, thus taking additional time and introducing additional error from the test operator.

Many colorimetric field tests need several specific steps to acquire a result. Each of these steps is a place where error is introduced. Each field test will also typically require different steps to be taken, further complicating the process of gathering results on multiple analytes.


WATER CHEMISTRY IS CONSTANTLY CHANGING

The chemistry of a water sample is constantly changing. Produced water experiences dramatic variations in temperature, pressure and pH as it is brought from deep underground to the surface. These changes continue after the water has arrived at the surface. Dissolved gases begin to move in and out of the sample, insoluble compounds precipitate out of solution, and compounds that were previously insoluble begin to dissolve.

For the results of water characterization to be reflective of the composition of water at its source, time is of the essence. The American Petroleum Institute maintains a document describing recommended practices for produced and flowback-water analysis called RP 45. This document lists proper sample gathering and preparation techniques to ensure that any chemical analysis performed is as relevant to the actual chemistry of the water in the system as possible. However, even with proper sample gathering and preservation, many water-chemistry parameters change very rapidly once the sample is taken.

Many of the most important constituents, such as pH, dissolved gases and bicarbonate, cannot be reliably preserved by any method. Minerals of sulfate such as celestite and barite, which are soluble at high temperatures found in the formation, cannot be preserved in solution once they are allowed to cool. For proper characterization of these parameters, analysis must be performed as rapidly as possible once the sample is taken. The above chart shows how sample concentrations can change dramatically over the course of just a few hours.

spwm garland graph


WHAT IS ACTIONABLE DATA?

For data to be used to make an important change during an operation, it must be of a high-enough quality and provided in a timely manner. Data of this type can be classified as actionable data. Though it is generally possible to achieve data that is either high quality or timely, data that is truly actionable is difficult to acquire. Traditional laboratory instrumentation produces data with high precision, but results are typically only provided after waiting several days or even weeks. By the time the data arrives, it is often too late to use it to make any changes.

Legacy field methods provides data onsite in a fairly timely manner, but uncertainty in the accuracy of the results will give pause to those looking to use the data for more than simple bookkeeping. For these reasons, both field and laboratory data are commonly taken only for reference purposes and are not reliable enough to be used to make changes during an operation.


WHAT IS WATER LENS?

The Water Lens method is an advanced colorimetric technique that works through all the shortcomings of legacy field tests and laboratory instrumentation. Raw data is generated by the same general principle as other colorimetric techniques. Each sensor (as well as all supporting chemicals) are preloaded into a 96-well plate. This format lets multiple analyses for multiple analytes to run simultaneously.

Samples are directly loaded into each well of the 96- well plate. The colorimetric responses are read by a 96- well plate spectrometer. Once the initial data is imported, the effects of any interferences are processed. By running multiple analyses simultaneously, this system can quantify the common interferences for a given sensor at the same time the primary analyte is quantified. This is what allows the system to remove the effects of interferences before reporting the results.

 

For data to be used to make an important change during an operation, it must be of a high-enough quality and provided in a timely manner.

 

The system is designed to be operated by a user of any skill level, with minimal training. It is capable of characterizing water samples of any level of total dissolved solids due to simple and rapid serial dilution.

spwm garland dblimagesThe total time for sample analysis is approximately 10 to 12 minutes, including sample preparation. In this time, the primary components of most water samples can be quantified, including cations, anions and organics, as well as several calculated parameters, before the sample materially degrades.

The ability of the Water Lens system to remove the effects of interferences while running multiple samples simultaneously overcomes all the primary limitations of legacy colorimetric methods while maintaining their advantages. This creates a system that is ideal for applications both in the field and lab.

 

Water Lens boron assay does not require any caustic or toxic reagents, and accounts for interferences and sample variation automatically every time the assay is run.

 

A study by Select Energy Services examined the ability of the Water Lens boron assay to accurately determine boron levels in produced waters. The most common legacy field test for boron quantification is the carmine method. This requires concentrated sulfuric acid and in the case of produced-water analysis, must be calibrated by standard additions for every sample run to get accurate results.

Like all the Water Lens assays, the boron assay does not require any caustic or toxic reagents, and accounts for interferences and sample variation automatically every time the assay is run. In the study, 15 different samples prepared from produced water were run on the Water Lens boron assay, as well as ICP-based instrumentation at a laboratory.

Select Energy Services found that the results obtained from both methods were so close that they were virtually indistinguishable, proving that our system provides a level of accuracy commensurate with that of state-of-the-art laboratory instrumentation. Additional third-party and internal studies have further demonstrated that all Water Lens assays meet the same level of accuracy.spwm garland2

The Water Lens system is currently being used both onshore and offshore and has seen significant results. In 2015, a customer using the Water Lens system during a hydraulic-fracturing operation noticed that fluid viscosity had begun to drop dramatically. While the normal course of action would have been to shut down to determine the problem, our customer had the benefit of accurate water characterization data by using the Water Lens system.

The customer noticed that the chemical composition of the source water had changed unexpectedly. By knowing exactly how the composition had changed, the customer was able to adjust chemical additives to bring the fluid viscosity back within acceptable limits. Considering the potential downtime and workover costs that were avoided, the customer estimated that the our system saved them upward of $200,000 in this instance alone, not including the potential loss of production had they not caught this problem before it was too late.

 

Authored by Adam Garland