Environmentally Preferable Smart Chemicals Improve Production, Performance
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The oil and gas industry uses acids and oxidizers as polymer breakers in hydraulic fracturing, and triazene and glyoxal for hydrogen sulfide (H2S) mitigation. In addition to serving their intended purpose, however, these chemicals cause secondary effects such as nonspecific oxidation, acid corrosion, precipitation, and danger to oilfield personnel. This paper describes studies that confirm that enzyme-based, environmentally preferable chemistries can be used to break polymers and mitigate H2S in various systems. The biotechnology-based solutions also offer advantages such as meeting environmental regulations and reducing or eliminating secondary effects and health hazards associated with current chemical treatments.
Complexities of Current Chemical Treatments
Fluid for hydraulic fracturing operations usually consists of 99% water thickened with guar or derivatized guar polymers. After the highly viscous fracture fluid is placed into the fracture, the fracture fluid needs to be brought back from the proppant pack, leaving the proppant without damaging the conductivity. This is accomplished by thinning the viscous fluid pumped into the fracture into a Newtonian fluid with a very low viscosity. Chemical breakers reduce the molecular weight of guar polymer by cutting the long polymer chain. As the polymer chain is cut, the fluid’s viscosity is reduced.
Oxidative breakers react rapidly at elevated temperatures and cause the polymer to break prematurely, leading to loss of viscosity and reduction in proppant transport. Encapsulated oxidative breakers have seen limited use because of rapid degradation that causes premature reduction in fluid viscosity. Additionally, oxidizers react nonspecifically with any oxidizable material, including metals and formations. Oxidizers also pose a significant safety hazard.
Reservoir souring is another major problem in oil and gas operations and is estimated to cost $120 billion per year. Reservoir souring occurs as a result of injecting sea water into hydrocarbon reservoirs, causing contamination of fluids with microbial population. Souring decreases production asset value and increases cost. In worst-case scenarios, it necessitates shutting off the wells.
H2S scavengers provide a cost-effective alternative for removing H2S when amine treatment is not possible or cost-prohibitive. Triazines are water-soluble H2S scavengers that have been used successfully for many years. They add a minimum amount of nitrogen and reduce H2S instantly. Recent advances include development of greener quaternary amine compounds (quat) and non-quat products, better delineation of a triazine-based scavenger mechanism, formulation of newer combinations, and the invention of newer classes of scavengers such as unsaturated aldehydes, hydroxy alkyl or alkyl oxides, or azodicarbonamides that are resistant to pH and other changes in the reservoir.
Meeting the Challenge
Expanding environmental guidelines warrant the use of products that have maximum efficiency with no toxicity, cause no harm to the environment or human health, use raw materials that are renewable, and do not generate toxic waste. Amino acids and plant extracts have been shown to work as potential green corrosion inhibitors to protect mild steel in acidic conditions. Several industries have conducted research on aloe extracts and introduced greener products for scale or corrosion inhibition, polymers or gels for enhanced oil recovery, and others.
The complete paper presents details of recent studies in which the authors used novel enzyme β-mannanase HT, cloned from a thermotolerant extremophilic organism, as a robust polymer breaker capable of degrading guar at elevated pH and temperatures. The studies showed that the enzyme is stable in the presence of common fracturing fluid additives. The authors also showed that chlorophyll can serve as a naturally occurring green breaker of guar and crosslinked polymers under alkaline pH conditions.
Additional laboratory and field trial data showed that the recombinant sulfide quinone reductase (SQR) enzyme can be used successfully to mitigate H2S. An enzyme-based scavenger has several advantages such as meeting environmental regulations and reducing or eliminating secondary effects such as solids formation, corrosion, and scaling.
Enzymes’ inherent substrate specificity and infinite catalytic potential contribute to superior performance. Nonchemical, biomolecular enzymes offer several advantages over conventional chemical breakers and H2S scavengers in that they do not damage equipment, are miscible, and do not cause environmental hazards or pose significant health risks to oilfield personnel.
The complete paper discusses the materials and methods used in the studies. Materials include β-mannanase; crude enzymes from cell-free lysates; SQR or biomolecular scavengers; chlorophyll powders of barley, spinach, and wheat; chlorophyll liquid; and high-yield guar polymer, borate crosslinker, and buffers. The complete paper also discusses the methods for determining the viscosity of crosslinked and linear polymers, conductivity measurements, materials compatibility, and measurements of H2S in head space.
The complete paper presents detailed discussion of the results, illustrated with charts and graphs. Included in the discussion are β-mannanase HT enzyme as a nonchemical-based polymer breaker; chlorophyll as a green polymer breaker; enzyme-based or biomolecular scavengers for H2S mitigation; and enzyme scavengers’ compatibility with oilfield metals, plastics, and elastomers.
- Recombinant proteins and enzymes can be produced in unlimited quantities by growing the bacteria in other host systems by microbial fermentation that produces the product using inexpensive sources of carbon, nitrogen, trace elements from yeast extract, and peptone. The bioprocess methods also use low energy and are not hazardous to the environment.
- Recent advances in molecular microbiology and genomics are influencing the industry. The studies conducted and data shown in this paper confirm the adaptation of novel biobased products to oil and gas applications. Metagenomics is used widely to identify microbial population in oil wells and design custom biocides. Several enzymes are used as polymer breakers in hydraulic fracturing. The evolution of chemicals, biologics, and polymers is trending for various applications in upstream oil and gas to improve well performance, integrity, and production.
- Enzymes show better performance because of their inherent substrate specificity and infinite catalytic potential. Enzymes have several advantages over conventional chemical breakers and H2S scavengers because they do not damage equipment, they are miscible, pose no significant health risks to oilfield personnel, and cause no environmental hazards.
- The studies using β-mannanase HT enzyme as polymer breaker showed breaking of crosslinked guar polymers at elevated pH (7 to 12) and temperature (greater than or equal to 175°F) ranges (Fig. 1).
- Chlorophyll-treated crosslinked fluids showed greater than 90% viscosity reduction. The chlorophyll worked efficiently up to 250°F, but the optimal temperature range is 175 to 200°F. The chlorophyll treatment also showed a reduction of molecular weights of a linear polymer from 1,472,000 to 6,000.
- H2S mitigation was addressed using a novel recombinant SQR enzyme. Functional studies conducted by treatment of soured brine and oil revealed 72 and 90% reduction in H2S concentration, respectively (Fig. 2). The scavenger showed 75% reduction of sulfide in simulated mixed production samples containing a 30:70 ratio of brine to oil. Field testing of SQR showed a reduction of head-space sulfide from 400 to 2 ppm and basic sediment and water values of less than 0.5%.
Environmentally Preferable Smart Chemicals Improve Production, Performance
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