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Liang, Feng (Aramco Services Company: Aramco Research Center — Houston) | Al-Muntasheri, Ghaithan (Aramco Services Company: Aramco Research Center — Houston) | Ow, Hooisweng (Aramco Services Company: Aramco Research Center — Boston) | Cox, Jason (Aramco Services Company: Aramco Research Center — Boston)
Abstract In the quest to discover more natural gas resources, considerable attention has been devoted to finding and extracting gas locked within tight formations with permeability in the nanoto microdarcy range. The main challenges associated with working in such formations are the intrinsically high temperature and high pressure bottom hole conditions. For formations with bottom hole temperatures around 350-400°F, traditional hydraulic fracturing fluids that use crosslinked polysaccharide gels, such as guar and its derivatives, are not suitable because of significant polymer breakdown in this temperature range. Fracturing fluids that can work at these temperatures require thermally stable synthetic polymers such as acrylamide-based polymers. However, such polymers have to be employed at very high concentrations in order to suspend proppants. The high polymer concentrations make it very difficult to completely degrade at the end of a fracturing operation. As a consequence, formation damage by polymer residue can block formation conductivity to gas flow. This paper addresses the shortcomings of the current state-of-the-art high temperature fracturing fluids, and focuses on developing a less damaging, high-temperature stable fluid that can be used at temperatures up to 400°F. A laboratory study was conducted with this novel system which is comprised of a synthetic acrylamide-based copolymer gelling agent and is capable of being crosslinked with a nano-sized particulate crosslinker (nano-crosslinker). The laboratory data has demonstrated that the temperature stability of the crosslinked fluid is much better than a similar fluid lacking the nano-crosslinker. The nano-crosslinker allows the novel fluid system to operate at significantly lower polymer concentrations (25 to 45 pptg) when compared to current commercial fluid systems (50 to 87 pptg) designed for temperatures from 350°F to 400°F. This paper presents results from rheological studies which demonstrate superior crosslinking performance and thermal stability in this temperature range. This fracturing fluid system has sufficient proppant carrying viscosity, and allows for efficient cleanup using an oxidizer-type breaker. Low polymer loading and little or no polymer residue are anticipated to facilitate efficient cleanup, reduced formation damage, better fluid conductivity and enhanced production rates. Laboratory results from proppant-pack regained conductivity tests are also presented.
Liang, Feng (Aramco Services Company: Aramco Research Center—Houston) | Al-Muntasheri, Ghaithan A. (Aramco Services Company: Aramco Research Center—Houston, Saudi Aramco) | Li, Leiming (Aramco Services Company: Aramco Research Center—Houston)
Abstract Polysaccharide-based fluids such as guar fluids are commonly used in hydraulic fracturing operations, primarily because of their abundance, relative low cost, and capability to work at up to 350°F when formulated at high pH. However, one notable disadvantage for most guar is the insoluble residue which tends to cause permeability reduction. Another disadvantage for using guar-based fluids at high pH is the tendency of forming divalent ion scales at high pH. In general, thermally stable synthetic polymers, such as acrylamide based polymers are considered to be residue-free. They can be used for preparing fracturing fluids around 300-450°F or more. However, high dosage of acrylamide based polymers may still cause formation damage due to factors such as incomplete degradation. This paper demonstrates the advantage of using selected nanomaterials to enhance the thermal stability of the crosslinked synthetic fluids and to reduce the polymer loading. With addition of a small amount of nanomaterials (e.g. 0.02wt%), the polymer loading used at temperatures 300-450°F can be reduced dramatically compared to other existing commercial products. This paper presents results from rheological studies which demonstrate superior viscosity performance and excellent thermal stability of this novel fluid system enhanced with the nanomaterials at this high temperature range. For example, at 30 pounds per one thousand gallons (pptg) polymer dose, the fluid viscosity stays above 500cP (at 40 s shear rate) at 350°F for more than 3 hours. This fracturing fluid system has therefore shown sufficient proppant carrying and transporting capability. The fluids can be efficiently broken to allow for good cleanup using oxidative breakers. Proppant-pack conductivity tests for a 300°F fluid formulation gives 83% regained perm proving the low damaging potential of this type of fluids. Using low loading of residue-free polymer therefore results in better cleanup, reduced formation damage, and enhanced production rates. The nanomaterial-enhanced fracturing fluid system based on the synthetic acrylamide copolymers demonstrates the unforeseen combination of a number of advantages including low polymer loading, excellent high-temperature performance, and high retained permeability.
Summary Hydraulic-fracturing fluids are used to break down subterranean formations where oil and gas are trapped. Pad or prepad fluids are first pumped into the formation to generate the fracture geometry. Once the fracture geometry is created, additional fluid containing proppant is used to transport these solid particles into the fractures. Then, the hydraulic pressure is released and the fracture will tend to close. At that stage, the proppant prevents fracture closure and provides a conductive channel for hydrocarbons to flow into the wellbore. Biopolymers, synthetic polymers, foams, viscoelastic surfactant (VES) fluids, and slickwater are all used as fracturing fluids, each with properties that are beneficial under certain conditions. Today, their formulations are well-developed, and more recently, may incorporate small-sized particles in the nanometer size range. Such nanoparticles have addressed certain technological limitations of fracturing fluids. For example, VES fluids were reported to suffer high leakoff rates in moderate permeability reservoirs (200 md) and were limited in temperature (beyond approximately 220°F, viscosity was diminished significantly). Another challenge is the pressure-dependent behavior of borate-crosslinked gels, where the viscosity was found to drop significantly under high pressures. Also, in high-temperature reservoirs (>350°F), it is very challenging to design a fluid that can sustain enough viscosity for a required period of time. Synthetic polymers (mainly acrylamide-based polymers) are commonly used and have been reported to be used at high concentrations. These high-concentration requirements are imposed by the need for a stable viscosity under high-temperature conditions. High polymer loading increases the potential of formation damage caused by the fluid residue. These challenges, which can be addressed by nanotechnology, could have a major impact on hydraulic-fracturing applications. For instance, the working temperature limit of VES-based fluids was improved from 200 to 250°F (93 to 121°C) by adding zinc oxide (ZnO) and magnesium oxide (MgO) nanoparticles. The borate-crosslinked gels were found to maintain their viscosity at pressures up to 20,000 psi when using boronic acid-functionalized nanolatex silica particles as crosslinkers, while under such high pressures, conventional borate crosslinkers showed more than 80% reduction in viscosity. Moreover, the rheological properties of mixed VES/polymer fluids were enhanced when using nanoparticles. Use of foams can reduce the amount of water consumed in hydraulic fracturing. Alfa olefin sulfonate (AOS) surfactant can be foamed by use of carbon dioxide (CO2). Aluminum oxide nanoparticles were found to stabilize the foams created by AOS, VES, and CO2. This has a potential application in waterless fracturing. This review paper will capture all of these aspects and summarize the most recent experience of nanoparticle usage in hydraulic-fracturing fluids design.
Li, Wai (The University of Western Australia) | Liu, Jishan (China University of Petroleum) | Zeng, Jie (Beijing Oilchemleader Science & Technology Development Co., Ltd.) | Tian, Jianwei (The University of Western Australia) | Li, Lin (The University of Western Australia) | Zhang, Min (The University of Western Australia) | Jia, Jia (The University of Western Australia) | Li, Yufei (Shengli Oilfield Exploration and Development Research Institute, SINOPEC) | Peng, Hui (CNOOC EnerTech Drilling & Production Company) | Zhao, Xionghu (CCDC Drilling & Production Engineering Technology Research Institute, PETROCHINA) | Jiang, Jiwei (Beijing Oilchemleader Science & Technology Development Co., Ltd.)
Abstract Nanomaterials have drawn considerable attention of the oil and gas industry due to their peculiar properties and interesting behaviors. Many experiments, trials and practices were conducted by petroleum scientists and engineers in the last two decades to use various novel nanomaterials to improve exploration and production. Based on the published literature, this article comprehensively reviews the strategies and experience of nanomaterial application in frac fluids, the current problems, and relevant challenges. Based on elaborated design, the nanomaterials such as nano-sized metal, metal oxide, silica, carbonate, carbon, polymer, fiber, organic-inorganic hybrid and other composites can be incorporated in frac fluids to greatly enhance or precisely tune the properties and performances. Consequently, nanomaterial-assisted frac fluids perform well in different aspects including density, rheology, stability, heat conductivity, specific heat capacity, fluid loss, breaking, clean up, proppant suspendability and frictional drag. To optimize the performance and cost-effectiveness of nano-frac fluids, advanced principles and theories in physical chemistry, heat and mass transfer, mechanics and rheology along with industrial design philosophy have been considered and applied. According to the investigation of the literature, nanomaterials have successfully fulfilled the following functions in frac fluids: (1) Improving the rheological behavior by intermolecular interactions (e.g., pseudo-crosslinking in frac fluids, or changing the aggregation pattern of surface-active molecules in surfactant based fluids); (2) Increasing the stability of fluids by enhancing the interfacial strength and toughness, especially in foams and emulsions; (3) Forming a low-permeability pseudo-filter cake to lower the fluid loss; (4) Increasing the viscosifying effect of polymers, which dramatically decreases the required loading of polymer in the fluid; (5) Boosting the thermal stability of frac fluids; (6) Improving the regained fracture conductivity; (7) Reducing the frictional drag of frac fluids; (8) Helping self-suspended proppants achieve better performance and (9) Reducing the required displacing pressure for the residual frac fluid by decreasing interfacial tension to help clean up. These achievements, along with the related design ideas, are reviewed. This paper also discusses the major difficulties and challenges for nano-frac fluids including compatibility, cost and HSE issues. Comprehensive laboratory work should be performed before field application to ensure the reliability of nano-assisted fluid formulations. Large-scale industrial production and a steady supply of nanomaterials will promote the application of nano-frac fluids. Exposure risk, eco-toxicity and biodegradability of nanomateials should be paid more attention. Incorporating the attractive, cutting-edged achievements in chemical and material sciences, nano-frac fluid is predicted to be fully accepted by the petroleum industry due to its great potential and the increasingly declining price of nanomaterials.
Abstract Many fracturing fluids are based on guar and guar derivatives, primarily because of their abundance and capability to operate at relatively high temperatures when formulated at high pH. However, insoluble residue in guar can damage permeability especially in unconventional formations. Another issue for applying guar-based fluids at high pH is the tendency to form scales with divalent ions. The fluid cost can also be strongly influenced by the volatility of the guar price. A third disadvantage is their low thermal stability when the temperature exceeds around 350 ° F. To mitigate these operating issues, a low- or non-damaging, high-temperature fluid system without elevated fluid pH is therefore highly desirable. Thermally stable synthetic polymers such as acrylamide-based polymers and copolymers are considered to be low-residue to residue-free. However, acrylamide polymers at high doses may still cause formation damage in circumstances like incomplete degradation. This paper demonstrates the successful application of a specific acrylamide copolymer to formulate a novel low-loading, non-damaging fracturing fluid system that fulfilled high viscosity requirements over a temperature range from 280 to 450°F. The fracturing fluid system based on the novel acrylamide copolymer demonstrated superior viscosity performance and excellent thermal stability at high temperatures at 450°F or higher. In one example, at the polymer loading as low as 20 lbm/1,000 gallons, the fluid viscosity stayed above 500cP (at 40 s shear rate) at 300°F for about 2.5 hours. In another example, at a polymer loading of 30 pptg, the fluid viscosity stayed above 500cP (at 40 s shear rate) at 400°F for about 1.5 hours. This data indicates that the fluid system has sufficient proppant suspension capability. The fluids could be efficiently broken to allow for good cleanup using oxidative breakers. Proppant-pack conductivity tests showed good regained permeability of over 90% at 300°F, proving the low- to non-damaging potential of the fluid system to formations treated. Moreover, the low-loading fluid system also reduced the fluid cost by about 50% when compared with the commercially available systems with similar viscosity performance. Using the novel low-loading, residue-free acrylamide copolymer has therefore rendered better cleanup, reduced formation damage, lowered operating cost, and enhanced production rates. The fracturing fluid system based on the novel acrylamide copolymer has demonstrated the unprecedented combination of a number of advantages including low polymer loading, robust high-temperature performance, high regained permeability, low scaling tendency, and reduced operating cost.