Investigation of the Effect of Pore Size Distribution on the Produced Oil from Surfactant-Assisted Spontaneous Imbibition in ULRs

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Abstract Observations from field applications along with laboratory experiments have revealed the significant potential of the surfactant-assisted spontaneous imbibition (SASI) as an encouraging EOR method in unconventional liquid reservoirs (ULR). This study focuses on unveiling the target pore size range for SASI EOR through a combination of experimental results, computed tomography (CT), Scanning Electron Microscope (SEM) and Nuclear magnetic resonance (NMR) technologies. In addition, laboratory results were upscaled to the field-scale to evaluate the effectiveness of the SASI EOR in production enhancement in the Wolfcamp formation. Eight SASI experiments were performed at reservoir temperature using different surfactants on quartz- and carbonate-rich side-wall core samples obtained from the Wolfcamp formation. Contact angle (CA), interfacial tension (IFT), and zeta potential were measured for the saturated core samples. CT-Scan technology is used to visualize the process of oil expulsion from the core plugs and generate core-scale simulation model to history-match laboratory results. SEM is used to match the NMR Pore Size Distribution (PSD) and obtain the Surface Relaxivity for each core sample. The target pore size range for SASI EOR in ULR is determined from NMR results. In addition, the laboratory results were upscaled to estimate the production enhancement through SASI EOR using the field scale model. The primary production mechanism of SASI EOR is highly influenced by wettability alteration and IFT reduction. SASI experiments showed optimistic oil recovery results in both quartz-rich and carbonate-rich core samples with up to 36% and 17.5% of the Original Oil in Place (OOIP), respectively. The NMR technique is used to determine the pore size range from which the oil is produced during the SASI experiment. NMR results revealed that the pore size distribution plays a significant role in SASI EOR with the majority of the imbibed fluid is observed in smaller pores. The consideration of the pore size distribution has a significant impact on successful surfactant selection and a proper EOR process design in ULR. CT-scan technology is utilized to demonstrate the movement of the fluids inside the cores throughout the experiments. CT-scan technology is also used to validate the NMR results, which revealed a direct relation between CT imaging and NMR results. A CT-generated core-scale model was utilized to history-match laboratory results. The capillary pressure and relative permeability curves for the field-scale model were estimated from scaling group analysis and core-scale simulation. The simulation results indicate that SASI EOR has significant potential of enhancing oil production in ULR. The novelty comes from the insight of the essential role of the pore size distribution in SASI EOR through CT and NMR technologies. Besides, a new workflow for surfactant selection is proposed to unveil the real potential of SASI in ULR.