Investigation of enhanced oil recovery through fracturing fluid imbibition in unconventional oil reservoirs

Date

2020-12

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Abstract

Crude oil productions from unconventional reservoirs continue to increase and will remain as the leading source of fuel energy supply in the United States. The oil recovery from this type of reservoirs usually relies on the depletion after horizontal well drilling and multi-stage hydraulic fracturing technology. However, the steep decline rate still constrains the ultimate hydrocarbon recovery. While most current Enhanced Oil Recovery approaches resort to replenish reservoir energies through gas injection or cyclic gas injection after the primary recovery phase, this study focuses on the possibility of enhancing tight oil recovery through fracturing fluid imbibition during the stage of well completion. This dissertation combines the approaches of experimental and numerical simulation to investigate the mechanisms of liquid imbibition in shale matrix with different manners. The experiments firstly simulate the process of spontaneous imbibition, forced imbibition, and imbibition under cyclic pressurizations in tight sand, carbonate, and shale core plugs. Meanwhile, a high-pressure imbibition test set-up is designed to execute the proposed experiments. Numerical simulation approach is used to further probe in the mechanisms of imbibition with both core and field-scale models. Models are tuned with the experimental results based on the recovery factors. The results indicate that capillary pressure is the primary driving force for the water-wet matrix, while the effect of gravity is insignificant in unconventional reservoirs regardless of the wettability. In a hydraulic fracture – matrix system, counter-current flow is the dominant imbibition behavior. The effect of externally applied pressure gradient is nonessential on the core-scale model but negatively impacts the recovery in the reservoir scale water-wet matrix. The effect of pressure is insignificant for oil-wet matrix. Similarly, the effect of cyclic pressurization is minor on the imbibition process itself as well. Shale oil reservoirs are characterized by oil-wet status which further reduces the oil production and complicates the imbibition behaviors. The effect of wettability alteration agents is further studied. Imbibition experiments with the presence of surfactants are conducted in the same manner. A numerical model with phase behavior considered is developed to investigate the tendency of imbibition in initially oil-wet core plugs quantitatively. The experimental results implicate that the wettability of oil-wet shale core can be effectively converted to a more water-wet status with the presence of a nonionic surfactant. Oil recovery is significantly enhanced compared with the cores without wettability alteration agents. It is concluded that the surfactant with the ability to alter the wettability of rock surface to more water-wet status while maintaining high interfacial tension between oleic and aqueous phases is the best candidate to trigger spontaneous imbibition. The effect of pressure is notable from our experimental results and cyclic injection is the most efficient manner as the process of wettability alteration is expedited.

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Keywords

Shale oil, Imbibition, Wettability, Enhanced oil recovery (EOR), Hydraulic fracturing

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