Quantitative Analysis Of Pore Structure For Tight Sandstone In Pore-scale

Tight sandstone reservoirs,with low porosity and low permeability,have received much interest with increasing demand for fossil fuel.The high heterogeneity pore space structure and high clay content often result in weak correlation between the total porosity of tight sandstone and bulk permeability and resistivity.The relationship is critical for the evaluation of oil and gas saturation.Due to the existence of multiple scales of pore types in tight gas sandstone,ranging from micro pores at nanometer scale to natural fractures at millimeter scale,a variety of new testing techniques have been developed to investigate the pore structure in tight gas sandstone.Constant speed mercury injection pressure(MIP),low pressure gas adsorption,and low field nuclear magnetic resonance(NMR)have been applied to identify pore space of tight sandstone.The measured results provide the information of porosity and pore size distribution,but miss the pore space connectivity and the coupling of different pore type.Digital rock physics enable us to analyze pore structure in a visual way.In the study we perform SEM,MAPS,and X-ray CT to study the pore structure from nm-to mm-scale.A workflow of multi-scale digital rock technique is proposed to character pore geometry and connectivity in order to understand the fluid flow or electric conduct in tight sandstones.A series of SEM images tiles(MAPS technique)with the resolution of 100 nm in a large field of view(FOV)are first recorded on one end surface of sample and then stitched together to reveal fine pore structure,pore type and pore size distribution.Energy-dispersive SEM(EDS-SEM)is performed on one the same end of the samples to obtain a mineralogy mapping which will be further used to identify mineralogy.The samples with the diameter of 25.4 mm are scanned by X-ray Micro Computed Tomography(CT)to obtain 3D gray scale images,on which image registration,segmentation and cluster-labelling algorithms are applied to generate multi-mineral digital rock and investigate pore connectivity in 3D.The results of digital rock analysis demonstrate that the pore space of tight sandstone is classified into three types,residual intergranular pore,dissolved pore and micro pore in clay mineral.The results of pore size distribution for each type of pore indicate that the micro pore in clay dominates pore space in tight sandstone.The original intergranular pores are partly filled by clay minerals,where the micro pores contained in form percolation pathway and control the fluid flow through pore pace.The permeability and electrical properties of tight sandstones are dominated by micro pore and have higher dependence of the pore structure than only on porosity.The multi-mineral digital rock proposed in the study can be further applied in numerical simulation of bulk properties and quantitate analysis in nano-geoscience.