Interactions Between Laminated Shale Oil Reservoir and Fracturing Fluid: A Case Study from the Chang 73 Member of the Triassic Heshui Area in the Ordos Basin, China

This study systematically investigates the reaction characteristics of laminated shale oil reservoirs in the 73 sub-member of the Yanchang Formation, Heshui area, Ordos Basin, under exposure to CNI-I nanoviscous fracturing fluid. The reservoir matrix comprises 84.85% brittle minerals and 15.15% clay minerals. Fluid–rock interactions significantly dissolve calcite and dolomite, releasing Ca2+ and Mg2+ ions, while clay mineral reactions liberate substantial amounts of Na+. Post-reaction, fluid system stability is markedly reduced, elevating the risk of precipitate formation and pore-throat plugging. Exposure to fracturing fluid reduces the T2 cutoff value of core samples from 3.29 ms to 1.72 ms, indicating a densification of the micro-pore-throat network and a decline in mobile fluid saturation, while fracture apertures exhibit widening. Based on empirical data, a discriminant criterion (R value) defined as the ratio of fracture aperture increment rate to pore-throat diameter reduction rate is established at 1.25, confirming that fracture propagation dominates over pore constriction. Dual-medium modeling yields a net permeability enhancement of 19.35%. Fluid–rock interactions induce overall degradation of rock mechanical properties with pronounced anisotropy: rock strength along the direction perpendicular to bedding declines by 37.546%, Young’s modulus decreases by 1.81%, and Poisson’s ratio increases by 0.02%—all significantly exceeding the degree of degradation parallel to bedding. This anisotropic mechanical degradation predisposes the near-wellbore region to shear slip and wall spalling, prompting the development of targeted engineering mitigation strategies.