Shale Pore Structure Characterization Based On High Resolution Imaging Technology

Shale gas is an important unconventional hydrocarbon and has great economic potential for the world. The mechanisms of shale gas reservoiring and production are closely related to pore structure. The majority of gas shale pores is less than1μm, and the microstructure is very complex. Most investigations into shale microstructure have relied on technologies such as Scanning Electron Microscopy (SEM), X-ray imaging, Transmission Electron Microscopy (TEM) or Scanning Acoustic Microscopy (SAM). Compared with conventional analysis methods, there are great advantages provided by visual measures. But how to make use of image analysis to achieve close integration of qualitative description and quantitative characterization of pore structure, classification and evaluation is still a difficult problem need to be solved.The nanometer-scaled pore systems of gas shale reservoir were investigated from the Longmaxi formation of Silurian and Niutitang formation of Cambrian in east Sichuan. To understand the pore system of these rocks, the lithology characteristics, total organic carbon content and vitrinite reflectance, total porosity, gas permeability and image analyses by electron microscopy were performed. The parameters of pore structure in organic matter were measured and the results presented here provided deep insights into the nature of pore structures within gas shales. Finally, a mathematical and physical model of shale gas reservoir space and migration pathways was constructed.Shale lithology characteristics which have effects on pore structure were analyzed. The gas shales investigated are quartz and clay rich, with the illite and mixed-layer of I/S are the main clay mineral types, and carbonate mineral content ranges between0.3%～33%. The content of mud level clastic particles is40%-60%, and total organic carbon content ranges between4%～8%. Besides, vitrinite reflectance ranges between1.6%～3.5%. The difference of shale lithology characteristics leads to significant differences of porosity and permeability.The microstructures of pores and fractures have been depicted by high-resolution imaging technology. Micro-CT found that fractures with the length up to5μm were rare in shale. Scanning electron microscopy (SEM) image analysis showed that porosity is found on the microscale in organics. clay mineral intercrystalline and between grains derived from dissolved minerals. The microfracture mainly located in aligns platy clays and around the boundary of grains. The former fracture length is between1～20μm with the width of less than1μm, and the latter fracture length ranges between5～10μm with the width between0.05to0.2μm. The fracture in clay plate has a good connectivity and is an important gas flow channel due to abundant clay. And the microfracture located in grain boundaries linked with pore cluster of discrete organics can be a preferred path for adsorption and free gas.The pore structure parameters of organic within shale have been quantitatively characterized based on image analysis. Image analysis of Field Emission Scanning Electron Microscopic through the software of ImageJ showed that the value of Ro is the decisive factor that affects the number, size, surface of porosity and average pore diameter. The mean diameter of organic pores from Longmaxi formation shale is30～60nm, and more than90percent of the organic matter pores are less than100nm. And surface porosity of organic matter in Longmaxi formation shale is2.4%～15.6%, and its biggest contribution rate to shale porosity is37%.Mathematical and physical model of shale gas reservoir spaces and migration pathways have been constructed. The multiple linear regression mathematical model of reservoir space is established and three physical models of gas migration pathways were built. Analysis results indicate that the pores in organic matter and clay mineral intercrystalline of mixed-layer of I/S are the main reservoir spaces for adsorbed gas and free gas in Longmaxi formation. There are darcy-flow and diffusion in them. However, organic pores are main reservoir space of free gas and seepage plays a main role in Niutitang formation shale due to high organic matter maturity. The mainstream throat diameter of shale is0.1μm, and the corresponding seepage channel is the fracture between clay tablets and grain boundaries.Shale gas reservoir spaces and migration pathways have been investigated comprehensively based on high-resolution image, and image analysis provides a deep insight into the nature of pores structure within gas shale. The results presented here can be used to analyze the micro-mechanism of reservoiring and darcy-flow or dissfusion.