Microorganisms can cause detrimental effects in shale gas production, such as reservoir souring, plugging, equipment corrosion, and a decrease in hydrocarbon production volume and quality, thus representing a multi-billion-dollar problem. Prefracturing fluids, drilling mud, and impoundment water likely introduce deleterious microorganisms into shale gas reservoirs. Conditions within the reservoir generally select for halotolerant anaerobic microorganisms. Microbial abundance and diversity in flowback waters decrease shortly after hydraulic fracturing, with Clostridia, a class that includes spore-forming microorganisms, becoming dominant. The rapid microbial community successions observed suggest biocides are not fully effective, and more targeted treatment strategies are needed. At the impoundment level, microbial control strategies should consider biocide rotation, seasonal loading adjustments, and biocide pulse dosing. In shale plays where souring is common, stable 34 S/ 32 S isotope analysis to identify abiotic H 2 S is recommended to evaluate the merits of biocide application in treating reservoir souring. Overall, an improved understanding of the microbial ecology of shale gas reservoirs is needed to optimize microbial control, maximize well productivity, and reduce environmental and financial burdens associated with the ad hoc misuse and overuse of biocides.

This work reports a reliable and systematic study of barite-nucleation kinetics in the presence of scale inhibitors from 4 to 90 C and at various conditions. In this study, we designed and developed an apparatus to study the nucleation kinetics of barite-scale formation by monitoring the change of photocurrent created by a 5-mW, 635-nm red laser.The photodetector has a wide wavelength range in which sensitivity has a peak at 960 nm. A set of convex and concave lenses was used to control the beam diameter so that it can pass through more particles and increase the sensitivity. Temperature and mixing procedure were precisely controlled by an external waterbath and magnetic stirplate, respectively. The photocurrent output was constant when the laser is shining through a clear solution before scale formation. After scale occurred, laser was scattered by scale particles, which causes the decrease of photocurrent. One can expand this method to study nucleation kinetics of other scales such as carbonates, other sulfates, and sulfide scales. In addition, one can customize it to perform study under high temperature, high pressure, and anoxic conditions.With this newly developed "laser" method, we successfully measured the nucleation kinetics of barite in synthetic brine (1 M NaCl, 0.1 M CaCl 2 ) under various combinations of reaction parameters including temperature (T), pH, saturation index (SI), and Ba 2þ /SO 2À 4 ratio (R). Furthermore, the inhibition efficiency of various scale inhibitors including sulfonated polycarboxylic acid, polyvinyl sulfonate, and inulin on barite precipitation was also investigated. On the basis of the experimental results, the relationship of precipitation kinetics of barite as a function of T, pH, SI, and R was established. Results of this study will be incorporated into scale-prediction software to predict the risk of scale formation and the efficiency of scale inhibitors.

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This work reports a reliable and systematic study of barite nucleation kinetics in the presence of scale inhibitors from 4 °C to 90 °C and at various conditions. In this study, we designed and developed an apparatus to study the nucleation kinetics of barite scale formation by monitoring the change of photo-current created by 5 mW, 635 nm red laser. The photo-detector has a wide wavelength range where sensitivity has a peak at 960 nm. A set of convex and concave lens were used to control the beam diameter so that it can pass through more particles and increase the sensitivity. Temperature and mixing procedure was precisely controlled by an external water bath and magnetic stirplate respectively. The photo-current output was constant when the laser is shining through a clear solution prior to scale formation. Once scale occurred, laser was scattered by scale particles which causes the decrease of photo-current. This method can be expanded to study nucleation kinetics of other scales such as carbonates, other sulfates, and sulfide scales. In addition, it can be customized to perform study under high temperature, high pressure and anoxic conditions. By using this newly developed “laser” method, we successfully measured the nucleation kinetics of barite in synthetic brine (1M NaCl, 0.1 M CaCl2) under various combination of reaction parameters including temperature (T), pH, saturation index (SI), and Ba2+ to SO42- ratio (R). Furthermore, the inhibition efficiency of various scale inhibitors including sulphonated polycarboxylic acid, polyvinyl sulphonate and inulin on barite precipitation has also been investigated. Based on the experimental results, the relationship of precipitation kinetics of barite as a function of T, pH, SI and R was established. Results of this study will be incorporated into the scale prediction software to predict the risk of scale formation and the efficiency of scale inhibitors.

pH is one of the most important parameters for evaluating the scale and corrosion potential of the water during oil and gas production. The effectiveness of chemical treatment can also be influenced by pH in the production tubing and reservoir. Unfortunately, pH is not a conservative parameter that can be determined independently. pH changes from reservoir to surface facilities due to changes in temperature and pressure of the production system and the corresponding G/O/W phase changes. Measured pH value of a produced water sample is often unreliable and influenced by temperature and ionic strength of the solution, degree of degassing and sample preservation. Measuring pH in line or with downhole probe at real field condition can be expensive and difficult. pH can be predicted theoretically from charge balance equations more economically and reliably by assuming a good knowledge of thermodynamic equilibrium constants, activity coefficients, G/O/W flow rates, temperature, pressure, and reliable produced water composition. The author's research group recently developed a new automatic titration method to simultaneously measure total alkalinity and weak organic acids concentrations of brine. The carbonate thermodynamics were evaluated with calcite solubility studies at ultra-high temperature and pressure. The newly developed thermodynamic data and method enable accurate prediction of produced water pH at temperature and pressure typically encountered in deep water production. In this paper, the influences of water chemistry parameters on pH are reviewed. The newly developed automatic alkalinity titration method is discussed. pH calculated by the recently validated thermodynamic constants and activity coefficients are compared with live water pH measurements.

Iron sulfide, as one of the main products of sour corrosion in oil and gas production systems, has become a focal point for flow assurance research. The formation of iron sulfide can cause many production problems such as the malfunction of downhole devices which can lead to a significant decline in oil production. Once iron sulfide forms in the production system, it is difficult or impossible to remove chemically and costly to remove physically. Accurate prediction models for iron sulfide formation at reservoir conditions are currently lacking in the industry and are necessary to help control scale and improve flow assurance. Solubility product (Ksp) of iron sulfide is the key parameter to make accurate scale predictions. However, research towards iron sulfide including precipitation, dissolution, inhibition, and removal are notoriously difficult not only due to the complexity of iron sulfide phases and their transitions but also due to the involvement of hydrogen sulfide in the gas phase. Tomson Technologies has developed new technologies to simulate realistic field downhole conditions for scale research. A reliable flow-through apparatus has been customized to perform mineral solubility studies under xHPHT (up to 1720 bar and 250 °C). In order to simulate the strictly anoxic environment and prevent dissolved ferrous iron from oxidizing, dissolved oxygen in the test solutions has been reduced to far less than 1 ppb. This paper is the first to examine the solubility of iron sulfide under these realistic downhole conditions with temperature up to 250 °C, pressure up to 1720 bar in 1M and 3M ionic strength solutions, under a strictly anoxic environment (<< 1 ppb dissolved oxygen). Under the HPHT and high salinity conditions studied, iron sulfide tends to form pyrrhotite (Fe1-xS) and troilite (FeSt) phases instead of mackinawite, the metastable phase (FeSm), which is most common at lower temperatures. Phase transition between pyrrhotite and troilite at elevated temperatures was observed during the solubility experiments. Solubility of iron sulfide decreases with increasing temperature and increases with increasing pressure which is consistent to previous reported results (Kharaka, et al., 1988). Experimental details and major findings from this research will be discussed.

Polymeric scale inhibitors are widely used in the oil and gas field because of their better thermal stability and improved environmental compatibility. However, the squeeze performance of such inhibitors is typically poorer than that of the phosphonates type scale inhibitors in conventional squeeze treatment. In this research, a new method of delivering chemicals to formation has been developed. Polymeric scale inhibitor nanoparticles were developed for scale control. Boehmite (ɤ-AlO(OH)) nanoparticles (NPs) with particle size from 3 to 10 nm were used to cross-link sulphonated polycarboxylic acid (SPCA). Cross-linked polymeric scale inhibitors were synthesized and developed to increase their retention in formations by converting free flowing water-soluble scale inhibitors into a viscous gel. The viscosity of NPs-polymer gel systems can be manipulated by changing reaction conditions such as NP and polymer concentrations, pH, temperature and ionic strength. Polymer adsorbs to the NP surface via an ion exchange mechanism with hydroxyl group on boehmite. The adsorption and desorption behavior of scale inhibitors onto both nanoparticles and core materials are investigated at different pH and temperature values.

Polymeric scale inhibitors used for scale squeeze treatments to control downhole inorganic scale don't perform well when pumped into the reservoir due to the poor adsorption properties on the rock surface. However polymeric inhibitors are more temperature stable than phosphonates and have higher tolerance to elevated cation compositions in the water. Therefore, a new chemistry composed of metal nanoparticles coupled with a polymeric scale inhibitor was developed to improve the squeeze life. The use of nanoparticles in the oilfield has increased in recent years; this development shows how nanoparticles can be used to increased surface area and retention of scale inhibitor in the reservoir. Metal nanoparticles were selected because of their low environmental toxicity and low formation damage potential during injection and flowback. A fast and efficient synthesis method was developed to create a novel chemistry that couples nanoparticles with polymeric inhibitors to produce a product that it was hoped would have excellent squeeze properties in multiple rock permeabilities and compositions. Core flood experiments were conducted on intact core under onshore Permian conditions of temperature pressure and brine composition as well as conditions simulating an offshore conventional field (results will be reported separately). The experimental results will be presented to show the extended squeeze lifetime of the new product in comparison to a traditional polymeric scale inhibitor retained by adsorption and also will give insight into the mechanisms by which the nanoparticle/scale inhibitor enhances squeeze life, both by increased adsorption as well as prolonging release of scale inhibitor. The product developed is able to significantly increase the squeeze life of polymeric scale inhibitors by up to 10x depending on the minimum inhibitor concentration required. The retention of the inhibitor into the rock is significantly increased, while the release is controlled at above minimum effective concentration for extended periods. The theoretic explanation for this is a metal-inhibitor bond, proprietary to the product that allows for continuous release of inhibitor into the solution, without release from the rock. Traditional squeeze returns have a Freundlich isotherm, this product also follows a similar return curve, however does not suffer from the high concentration release at the beginning of the treatment flowback. These results show that nanoparticles can be used in the oilfield to enhance existing scale inhibitors as well as create new combination products that can improve performance. Use on nanoparticles in the oilfield is an evolving topic that has significant room to grow and expand into multiple areas of oilfield chemistry. This study showcases the application of nanoparticles to enhance performance of polymeric scale inhibitors for squeeze application while maintaining a cost effective product that is environmental responsible.