Pore-Network Simulations of Gas-Water Two Phases Flow in Organic-Rich and Inorganic-Rich Shale

Abstract

Abstract Focused ion beam scanning electron microscopy (FIB-SEM) is an effective method to investigate the pore structure and gas flow in shale. Insufficient investigations have been carried out with regard to the distinctions between the inorganic-rich and organic-rich samples in the flow simulation. In this work, we selected an organic-rich sample and an inorganic-rich sample for high-resolution three-dimensional imaging, and the organic pores and inorganic pores were effectively extracted using an improved digital rock processing method. Subsequently, the pore network model (PNM) of the two samples were generated separately. Finally, the pore network simulation method was introduced to calculate the flow capacity of the two samples. Data analysis shows that the inorganic-rich sample have a higher pore radius and throat radius and a larger shape factor, with more irregular pores. While the organic-rich sample have a higher average coordination number and better connectivity. The absolute permeability of the inorganic-rich sample is larger, at 1.46 × 10−7μm2, while that of the organic-rich sample is 1.05 × 10−7μm2. The capillary pressure curve of the inorganic-rich sample is lower than that of the organic-rich sample, exhibiting a lower drainage pressure PT and a longer liquid-invasion segment. Compared to previous pore network flow simulation methods, we set a uniform gas-wet surface with an advanced angle of 100-110° for the organic-rich sample, and an advanced angle of 20-30° for the inorganic-rich sample. Forced water injection was used to simulate the water (fracturing fluid) injection process in the organic-rich sample. Due to the hydrophobicity of the pores, the gas flow capacity during water injection was quite poor, but it was improved during drainage process. In contrast, due to the hydrophilic of the inorganic pores, the inorganic-rich sample exhibited spontaneous water imbibition during the water injection process. However, the inorganic-rich sample demonstrated higher residual gas saturation during the drainage process, which indicates that fracturing fluid flowback in inorganic pores is more difficult. This work emphasizes the importance of studying the real pore structure of inorganic pores in FIB-SEM experiments, which provides fundamental insights for the description of shale gas reservoirs and the simulation of gas-water two phase flow.