Effects of cyclic cryogenic and gas injection treatments on shale rock physical and mechanical properties and their application to improve shale hydrocarbon recovery



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Advanced technologies in drilling and hydraulic fracturing operations have unlocked the hydrocarbon production from shale oil/gas reservoirs at economical rates. However, only a small fraction of hydrocarbon is recovered, leaving behind the massive unrecovered hydrocarbon in the formation. Typically, the hydraulic fracturing operation relies on water-based fracturing fluid (e.g., slick-water), where several risks are imposed to the environment and reservoir, including but not limited to, extensive water consumption and formation damage. To minimize these concerns, waterless stimulation methods are being investigated. One of the most promising waterless stimulation methods is the thermal shock technique, especially cryogenic treatment (e.g., liquid nitrogen (LN2)). The efficiency of a cryogenic treatment depends on several factors, such as rock properties, temperature differential, and the treatment cycles. Additionally, the degree of success of the LN2 depends on the external injection pressure, where the vaporization of LN2, without applying any external pressure, may not provide adequate pressure to extend the induced cracks. Despite the remarkable progress among the researchers, cryogenic treatment needs further attention to understand its potential in the oil and gas industry. This research consists of three major objectives. The first purpose is to investigate the physical and dynamic elastic properties of 24 outcropped Eagle Ford core samples considering some essential experimental parameters, where five different experiments were conducted. The second objective is to experimentally investigate the performance of LN2, as cryogenic fluid, on reservoir-temperature outcropped Eagle Ford and reservoir-depth Wolfcamp core samples in stimulating and inducing fractures. Also, the goal is to investigate the effects of cyclic LN2 treatment on the rock physical and mechanical properties of Eagle Ford samples. Since the degree of success of the LN2 depends on the external injection pressure, the third objective of this research is to use LN2 treatment in conjunction with the cyclic gas injection to provide the external injection pressure and improve the shale oil recovery. For this purpose, initially, outcropped Eagle Ford core samples with synthetic mineral oil were used to examine the performance of combining LN2 treatment with cyclic nitrogen (N2) injection. Next, the feasibility of using LN2 combined with cyclic N2 and/or carbon dioxide (CO2) injections in improving shale oil recovery using downhole core samples and dead crude oil from the Wolfcamp formation were investigated experimentally. The findings of this study provide detailed information about the rock-physical and dynamic-elastic properties of one of the largest unconventional resources in USA, the Eagle Ford formation, where direct measurements may not be cost-effective or feasible. Also, this research showed that cyclic cryogenic treatment increases porosity and permeability and decreases ultrasonic velocities of the samples resulting from inducing new cracks and extending and widening the existing ones. Most of the alterations in the properties (porosity, permeability, and ultrasonic velocities) of the core samples occur after completing the first cycle of the treatment. Additionally, permeability enhancement was noticeable (up to 2 orders of magnitude) compared to the porosity and ultrasonic velocity results. The findings of this study also revealed that LN2-assisted N2 and/or CO2 huff-n-puff improved the Eagle Ford and Wolfcamp shale samples recovery factor (RF), where flow-conductivity enhancement, induced-cracks growth, and oil-gas interaction development throughout injecting-soaking-producing stages enhance the stimulated volumes and improve the oil RF. The results demonstrated that higher cumulative oil RFs with fewer cycles were achieved compared to the previous experimental cyclic gas injection works carried out on Eagle Ford and Wolfcamp core samples. A combination of cryogenic treatment and huff-n-puff techniques could be potentially implemented in shale oil reservoirs, opening the opportunity to improve shale oil RF by enhancing the flow conductivity and increasing hydrocarbon mobility.

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Unconventional Resource, Cyclic Cryogenic Treatment, Thermal Shock Stimulation, Porosity Enhancement, Ultrasonic Wave Velocity, Dynamic Elastic Moduli, Eagle Ford Formation, Wolfcamp Formation, Permian Basin, Shale Oil Recovery, N2/CO2 Huff-n-Puff Injection, Cyclic Gas Injection, Enhanced Oil Recovery, Permeability Enhancement