Numerical Simulation Of Resistivity For Tight Sandstone Based On 3-D Multi-mineral Digital Rock

Tight sandstone reservoir features low permeability,low porosity,high abundance of clay,and bimodal pore size distribution,which leads to complicate electrical properties.We need to investigate the electrical resistivity quantitatively in order to improve the accuracy of oil saturation evaluation for tight sandstone.Wetting phase is difficult to drainage in resistivity measurement due to low permeability.The resistivity of tight sandstone with low water saturation cannot be measured in lab.The digital rock technique provides an alternate method to study the bulk properties of rock.However,a binary images consist of only two phases,namely,solid matrix and pore space,is insufficient to simulate electrical conductivity of tight sandstone accurately.In this study,we firstly proposed a method to build multi-mineral digital rock for tight sandstone.Core plug samples of tight sandstone are first scanned by X-ray Micro Computed Tomography(? CT)to obtain 3D gray scale images,from which macro intergranular pores and clasts can be identified.Then,energy-dispersive SEM(QEMSCAN)is performed on one end of the samples to obtain a mineralogy mapping.The range of gray value is correlated with mineral phase by image registration method.Then the 3D gray scale images are divided into multi-mineral digital rock by multi-threshold segmentation method.The volume fraction of each mineral agrees well with the experimental result derived from XRD.More mineral information are considered in than two-phase digital rock.The fine structural property of each mineral phase are revealed by high-resolution SEM technique.The micro pores can be identified by the HD image,which are not be resolved by micro CT.The effective conductivity of each mineral phase is calculated by Archie and Waxman-Smits models.And then the effective electrical conductivity of the full-or partial-water saturated multi-mineral digital rock is predicted by the Finite Element Method(FEM).The simulated results with high water saturation agree well with the measurements in laboratory.The resistivity index with low water saturation is predicted by numerical simulation.The Non-Archie phenomenon is observed in low water saturation,where the saturation exponent varies with the water saturation.The virtual multi-mineral digital rocks with different pore structure are built by process-based method.The effect of the pore structure on saturation exponent is studied by numerical simulation.