Clean brine and reused produced water
Sourcing water for hydraulic fracturing and disposing of produced water are well-known constraints and significant cost items in the development of shale formations in the Permian Basin. Whether the produced water is to be reused or disposed, it must be treated to some extent.
Biocide and scale inhibitors are two common treatments. Biocide helps reduce corrosion (e.g., microbially induced corrosion), which can lead to loss of containment. It can also reduce and delay souring. Scale inhibitors help maintain pipeline flow. It may prevent injection problems in the well and formation near the wellbore.
In the Permian, it is common to find high concentrations of suspended solids composed of very small particles.
With the use of improved polymer chemistries, water-quality requirements for hydraulic fracturing have relaxed significantly. Salt-tolerant polymers (e.g., HPAM-AMPS) and the migration from gels to salt-tolerant slickwater polymers have made it possible to use high-salinity produced water for some or all the water required in hydraulic fracturing.
Iron removal is usually required to ensure compatibility with the polymer in the presence of oxygen. Prevention of oxygen intrusion will stop iron precipitation and eliminate the requirement for iron removal. This strategy is not widely practiced in the Permian.
Although salinity is no longer a problem, suspended solids can be in some fields. In the Permian, it is common to find high concentrations of suspended solids composed of very small particles. Saltwater disposal well (SDW) injectivity can be a problem with this brine. Remediation of the injection well can range from a simple one-day acid job to a multiday workover. Removal of suspended solids without desalination generates what is generally referred to as a clean brine.
Suspended solids can impact the amount and type of chemicals required for scale inhibition and biological control. High concentrations of small suspended particles introduce a high surface area, which causes excessive consumption of biocide, scale inhibitor and corrosion inhibitor. This can impact the operating cost.
In the case of reuse, a clean brine provides greater uniformity and consistency in the makeup water. This helps the completions engineers obtain consistent fracture-fluid properties such as:
• target viscosity
• low pipe friction
• low gel residue
• good proppant transport
• good dynamic break with breaker chemistry In reusing produced water, it is sometimes desirable to use a clean brine instead of raw water.
A high concentration of fine particles can plug the proppant pore space. If the proppant becomes contaminated with fine particles, the flow rate of hydrocarbons through the proppant will be reduced.
While this is a possible scenario, it is not a widespread problem, and raw produced water has been used with success. This is partly because the makeup of HF water occurs only once in a batch mode. Produced water is introduced only once and not continuously as with a disposal well. It is much more likely that the proppant will become plugged from solids generated in the formation itself.
Removing solids before discharge into a pond greatly reduces the need to clean the pond. Deposited solids form a sludge that becomes highly contaminated with bacteria. This causes objectionable odor and hydrogen sulfide. Due to the presence of liners and leak detection, removing the sludge is costly.
The benefit of removing suspended particles to generate a clean brine is not always easy to determine. It is usually straightforward to assess the cost of treatment to produce a clean brine. Several service companies carry out sampling, analysis and provide cost estimates. But the value is a bit more difficult to assess.
In some fields, the produced water can be used for hydraulic-fracture makeup water with minimal treatment. In other fields, the treatment must remove most of the suspended solids. A similar situation exists for disposal wells. Some fields have minimal well impairment. Other fields require costly and frequent remediation jobs. It helps to have a basic understanding of why this would be the case.
Both reuse and brine disposal are examples of the flow of suspended solids through porous media. In the case of disposal, the porous media is the disposal reservoir, which is almost always a porous sandstone with relatively large grain size, large pore-throat diameters and high permeability. In the case of reuse, the porous media is the proppant that has been injected into the fractures.
Both cases involve a suspension of particles in water that flows through a porous media. As the concentration of solids in the fluid increases, the solids tend to plug pore spaces, and the permeability of the media decreases. Particularly for produced water, the particle size spans an order of magnitude.
The subject of suspended-particle flow through porous media has been studied extensively in the design and operation of filtration beds. In filtration, a fluid contaminated with a suspension of solids passes through a bed of material that is designed to capture the particles. In the case of injection or reuse, particle capture is a problem because the captured particles will impair the water flow.
Although the objectives are different, disposal, reuse and filtration all involve flow of suspended solids through and particle capture in porous media. Filtration is easily studied since accurate test methods can be carried out with simple apparatus. Hence, there is an enormous body of information available about bed filtration that can be applied to disposal and reuse.
From this point onward in the discussion, injection-well impairment will be the focus. The ideas discussed can equally be applied to reuse. From the study of filtration, we know that there are four important variables:
1. pore size of the disposal formation
2. particle size of the suspended solids
3. concentration of the suspended solids
4. composition of solids and formation surfaces (do the particles stick to the formation rock?)
In some fields, the produced water can be used for hydraulic-fracture makeup water with minimal treatment. In other fields, the treatment must remove most of the suspended solids.
It turns out that these factors are interrelated. For example, pore-throat diameter by itself has little relevance. It is only when compared to the suspended-particle diameters that the pore-throat diameter is significant. Obviously, if the particle is large relative to the throat diameter, the particle will get stuck in the pore throat, and impairment will occur.
A similar interrelation holds for concentration where a high concentration tends to promote bridging of small particles across the pores and therefore, higher plugging rates. There are other dependencies discussed later.
For the suspended solids, there are three important size ranges. Large particles get captured by size exclusion. They are simply too big to pass through the pores of the disposal formation. This does not happen often since the pore sizes in disposal reservoirs are generally quite large.
Medium-size particles shoot through the porous rock without getting trapped or stuck to the grains of the formation rock. There are various rules of thumb for this. It is generally believed that particles must be smaller than 1/7 to 1/10 the diameter of the pore throat in order to pass through the pores. This is not always the case, and there are a few infamous project failures due to this assumption.
Particles that fall into the small category have diameters in the range of a few microns and smaller, down to say 0.1 micron or so. This size particle has been found in some Permian shale fields. At this size range, colloidal forces become significant.
Colloidal forces can lead to suspended particles sticking to each other or particles sticking to the surfaces of the porous rock. Sometimes both can occur. The impact of these colloidal forces depends very strongly on the chemistry of the produced water, including the hydraulic-fracturing chemistry and the chemical-treatment program applied in the treatment facility.
Having discussed the details of particle capture, it is now appropriate to step back and take stock of the basic parameters involved. An operator must decide for each field whether suspended solids should be removed and how best to remove them.
Each field, and sometimes various wells within a field, will produce water with different properties and chemistries. Specific recommendations cannot really be made accurately without some kind testing for each field.
Designing a Good Water-System Treatment
The starting point for designing and operating a successful water-treatment system for any application is to understand the characteristics of the water to be treated. Samples of shale produced water from various locations in the Midland/Delaware region were analyzed. The samples were taken at the tail end of the process, just upstream of the injection pumps.
This location represents the point at which additional water treatment could be applied to generate a clean brine. Much of the sampling and analysis work was done at site, rather than at a third-party lab, since samples of produced water can change dramatically with time. Suspended solids were characterized using the NACE standard TMO-173. Some additional methods were used.
The starting point for designing and operating a successful water-treatment system for any application is to understand the characteristics of the water to be treated.
It was found that there are roughly 2 million particles in the size range less than 1.2 microns and greater than 0.45 microns. These are colloidal-size particles that have a tendency to attach to the grains in a disposal formation or in the case of reuse, to stick to the grains in the proppant pack.
Also found was half the particles are smaller than 1.3 micron and half the particles are larger than 1.3 microns. It is interesting that we found this situation in every site that we studied, to greater and lesser extent. Reports from equipment vendors and other water-treatment specialists tell a similar story.
For most of the fields studied, a Jorin Visual Particle Analyzer was also used. It confirmed the presence of a high concentration of very small particles in the produced water.
In addition to particle-size analysis, simple permeability (flow-through) tests were run by comparing permeability using raw produced water, clean brine and distilled water. The permeability of the raw produced water was significantly lower than that of the clean brine and distilled water.
The composition of the solids was determined using onsite chemical spot tests, as well as third-party specialized methods. The following made up the composition of the solids:
• Iron compounds (iron hydroxides, sulfides and oxides)
• Clay fines
• Fine shale particles containing kerogen, clay and silica compounds
• Crushed sand
The smallest particles are composed of iron compounds such as iron oxide, iron hydroxide, iron sulfide and iron carbonate. Corrosion processes could not account for the high concentrations of iron. It was concluded that the iron came from geologic processes. As the dissolved iron contacts air in the facility, it becomes oxidized. It then forms a hydrated iron-oxide floc. This floc is seen throughout the Permian.
Small particles of shale (shale fines) were in most samples. This is not surprising given the tremendous pressures exerted on the shale during the fracturing operation. A high silica-toclay ratio makes the shale more likely to shatter like glass. Likewise, crushed sand particles were found, which were probably the result of proppant sand being crushed by overburden pressure.
Chemical jar testing was also carried out. The greatest clarification in the shortest time was achieved by anionic low molecular weight polyelectrolytes. The resulting floc was tight, tough and had good dewatering characteristics. The composition data of the solids and the jar test results were in good agreement with each other.
Effective Treatment-Method Clues
All of this information provides clues as to the most effective treatment methods. Water treatment in the Permian has benefited from years of trial and error including testing scores of technologies. Much has been learned along the way.
Many different technologies have been proposed and evaluated at both the pilot and full scale. In fact, a number of staff have expressed a certain degree of “technology fatigue” with all the different approaches that have been proposed. However, a convergence of technology has started. The most common form of treatment is some variation of iron oxidation and floc-n-drop (coagulation with filtration, settling and/or flotation). There are a couple variations on this that also have been found to work.
Electrocoagulation applications have been well documented. EC operates on the basis of a coagulation/flocculation mechanism provided by an electrical anode/cathode system. It does require a small amount of chemical to speed the settling and separation of the floc.
However, chemical use compared to conventional coagulation/flocculation is greatly reduced. This could be an important factor when removing small particles that have very high surface area and hence, high chemical demand. The major challenges for EC are capital cost and difficulty of operation. Recently, the industry has made good progress in simplifying and bringing down the cost.
Several operators and service companies have had success with iron oxidation followed by chemical coagulation and flocculation for the removal of total suspended solids. The most effective coagulants have been found to be anionic polyelectrolytes, which agrees with the finding discussed previously. In one full-scale pilot, the following results were obtained.
The treated water was applied to hydraulic-fracturing jobs with success. This treated water was compatible with salt-tolerant friction reducers. The friction reduction using this recycled saline brine was comparable with that found for fracturing solutions made up with fresh water. Also, compatibility with formation water using this hydraulic fracturing fluid is assured since the fluid is made up of produced water.
Summary and Conclusions
This article discusses the problems that can be encountered by suspended solids. Field data were collected using simple methods of sampling and analysis. High concentrations of small particles were found in most of the fields studied. Given the size of the particles, the most practical watertreatment method is the conventional approach of iron oxidation together with clarification.
Electrocoagulation is also an option, but capital cost and complexity of operation must be taken into account. In one application where solids were measured and removed to produce a clean brine, improvement in polymer makeup for reuse and disposal injectivity for excess water were improved.
Authored by John Walsh