三维数字岩心孔喉特征与驱替参数对渗流驱替能力影响——以鄂尔多斯盆地富县地区长8油层组致密油储层为例OA

[Objective]The Chang 8 Member of the Yanchang Formation in the Ordos Basin is a typical ultra-low-porosity and ultra-low-permeability tight sandstone reservoir characterized by severe microscopic pore-throat heterogeneity and complex distribution patterns of residual oil after water flooding.Existing studies utilizing digital core simulation mostly focus on the independent characterization of pore structure or single displacement condition,and few quantitative investigations have systematically revealed the synergistic effects of intrinsic pore-throat properties and displacement conditions.A comprehensive quantitative understanding of seepage and displacement evolution mechanism under the joint control of microscopic pore-throat structure and displacement parameters can provide a microscopic theoretical basis for efficient water-flooding development of tight oil in Fuxian area,Ordos Basin.[Methods]In this study,the tight sandstone reservoir in the Chang 8 Member of the Yanchang Formation in Fuxian area,Ordos Basin was selected as the study object.Two representative tight sandstone core samples(labeled YP1 and YP2)were collected and scanned by micro-CT scan with a spatial resolution of 1 μm.A series of image preprocessing procedures,including grayscale enhancement,non-local mean filtering,and threshold binarization,were performed to eliminate instrument noise,and the maximal ball algorithm was adopted to reconstruct 3D pore-throat network.Representative elementary volume(REV)analysis was carried out to confirm the stable core calculation unit with a side length of 350 μm.Subsequently,grid coarsening and geometric defect repair were conducted to reduce computational cost.A microscopic seepage numerical model coupling Navier-Stokes(N-S)equations with the Cahn–Hilliard(C-H)phase-field method was established to simulate single-phase water seepage and transient oil-water two-phase displacement separately.Two sets of controlled variables—multiple gradient driving pressures(0.25-50 MPa)and eight groups of oil-water viscosity ratios(0.06-3.00)—were designed to quantitatively compare the seepage and displacement response patterns of YP1 and YP2 with distinct pore-throat configurations.[Results]Under single-phase seepage conditions,fluid velocity and differential pressure were concentrated within narrow throats to form high stress concentration zones,while fluid velocity and pressure fluctuated slightly inside large pore cavities.Sample YP1 was characterized by large pores,small throats,and strong microscopic heterogeneity,and its displacement evolution curves showed segmented steep changes.To achieve the same residual oil saturation as YP2,YP1 required remarkably higher driving pressure.For both samples,the calculated absolute permeability declined continuously as driving pressure increased,while absolute permeability remained stable in the low-pressure range.Under oil-water two-phase seepage conditions,stress concentration generated by small throats and intricate pore-throat networks easily caused fluid blockage and displacement stagnation.Throat radius and pore-throat heterogeneity directly controlled the advancing speed and sweep range of water flooding,while the volume fraction of connected pores determined the scale of effectively mobilizable pore space and ultimately controlled final displacement efficiency.Isolated micropores were prone to forming permanent residual oil retention zones.Quantitative simulation results showed that,for cores with a high proportion of throats smaller than 8 μm,increasing driving pressure could greatly improve displacement efficiency,whereas cores dominated by large throats showed limited efficiency improvement after increasing pressure.For samples with abundant pores larger than 18 μm and well-developed preferential seepage channels,reducing the oil-water viscosity ratio could expand the sweep range and improve displacement performance.In contrast,reservoirs dominated by micropores showed higher seepage resistance and lower displacement efficiency when the viscosity ratio decreased.The simulated absolute permeability values(0.32×10−3-0.75×10−3 μm2)were consistent with laboratory core measurements,which verified the reliability of the established simulation model.[Conclusion]This study distinguishes the respective controlling effects of pores and throats on microscopic oil-water displacement and clarifies their coupling response patterns under different water-flooding parameters.The quantitative relationships among pore-throat configuration,driving pressure,and viscosity ratio can provide a basis for optimizing water-flooding parameters and predicting residual oil for Chang 8 tight oil reservoirs in Fuxian area,Ordos Basin.Unlike previous single-factor simulation studies,this study establishes a complete quantitative evaluation framework considering the joint influence of intrinsic pore-throat properties and artificial displacement conditions,which offers new microscopic insights for the efficient exploitation of analogous tight sandstone reservoirs worldwide.