Experimental study on production characteristics and enhanced oil recovery during imbibition and huff-n-puff injection in shale reservoir

Summary: Microscopic assessment of oil distribution and imbibition mechanisms within shale formations lays the groundwork for future development strategies. In this regard, the Songliao Basin’s continental shale oil holds immense exploration and development potential. In this study, we focus on shale samples extracted from the first member of the Qingshankou Formation (Q1) within the Songliao Basin. These samples were subjected to a comprehensive analysis, encompassing mercury injection capillary pressure (MICP), porosity, and permeability measurements and detailed monitoring processes. The experimental protocol involved multiple injection cycles, commencing with spontaneous oil imbibition, followed by a series of differential pressurized oil saturation stages (eight pressurization steps ranging from 0.2 MPa to 10 MPa). Subsequently, forced imbibition using slickwater under varying pressures was used, and the process was meticulously monitored via gravimetric and nuclear magnetic resonance (NMR) measurements to deduce relative fractions within distinct pores across the entire experimental process. Notably, the results unveiled that, during oil saturation through spontaneous imbibition, the interbedd-type shale core samples exhibit more efficient oil saturation compared with the organic-rich dark massive type. In the former, clay interlayers predominate in absorbing oil, while the latter showcases preferential saturation of mesopores and macropores. Following the differential pressurized oil saturation phase, clay interlayers continued to play a significant role in both sample types, accounting for 54.2% and 57.0% of the interbed-type and massive shale’s oil intake, respectively. Furthermore, a quantification of the slickwater imbibition recovery originating from pores of varying sizes under distinct pressures revealed that clay interlayers and micropores are the primary contributors to imbibition recovery in both sample types. Collectively, the experimental findings corroborate that shale oil can be displaced from nanopores to larger matrix pores and bedding fractures through imbibition, offering valuable insights for enhancing oil recovery operations in practical field scenarios.

Tight gas reservoirs are rich in microscale pores andfractures.The effect of microscale gas seepage in tight sandstone matrix ongas well productivity cannot be ignored. At present, the effect ofmicroscale gas flow on the well testing model of fractured horizontalwells has not been systematically studied. In this paper, first, thecoupling influence mechanism of "multicomponent gas effect-fracturegeometry characteristics" is expounded, second, a seepage modelcoupling the flow in the fracture and the flow in the gas reservoiris established on the fracture wall surface, and third, an unsteadypressure analysis model for fractured horizontal wells in tight gasreservoirs is established. Results show that (a) when the microscalefracture is at a quite small level, the Knudsen diffusion plays adominant role at a wide range of pressures; (b) the mass transferrate of multicomponent shale gas through macrofractures increaseswith increasing CO2 fraction; (c) the unsteady flow processis divided into six stages in turn: microscale diffusion stage, linearflow stage in fractures, linear flow stage between fractures, earlyradial flow stage in gas reservoirs, linear flow stage in gas reservoirs,and late radial flow stage in gas reservoirs; and (d) with an increaseof dimensionless bottom-hole storage coefficient, the second and thirdstages of flow are gradually covered up.

The influence of different reservoir spaces in shale reservoirs on imbibition recovery is a hot spot for improving shale oil recovery. However, the research on the influence of different chemical reagents on the recovery factor of different scale pores is limited, and the influence mechanism of shale imbibition recovery factor under the action of different media has not been systematically studied. Therefore, this study takes the Gulong shale oil reservoir as the research object, carries out imbibition experiments combined with nuclear magnetic resonance testing under different injection fluid conditions, quantifies the contribution of shale pores of different scales to imbibition recovery under different injection media conditions, and analyzes the influence of injection media types on imbibition recovery. The results show that the average contribution rate of different types of pores was in the order of interlayer clay (47.6%) > mesoscale pores (32.7%) > small pores (17.3%) > large pores (2.6%). The total imbibition recovery of shale with millimeter-scale sandy laminae was ranked as alkali solution (61.50%) > acid solution (60.92%) >GJ surfactant (39.79%)> distilled water (32.92%)>guanidine gum (30.38%) , and the total imbibition recovery of laminated shale was ranked as GJ surfactant (39.1%) > slick water (38.0%) > guanidine gum (34.7%) > distilled water (29.2%). Document Type: Original article Cited as: Wei, J., Wang, A., Wang, R., Yang, Y., Zhao, X., Zhou, X. Contribution of different shale storage spaces to recovery rate and mechanism of oil mobilization during imbibition. Capillarity, 2025, 15(1): 12-24. https://doi.org/10.46690/capi.2025.04.03