Study on the characterization of pore structure and main controlling factors of pore development in gas shale

Abstract

Shale has a complex pore structure, which leads to complex petrophysical characteristics. The porosity, pore type, pore size, pore volume, TOC, and other petrophysical parameters of shale all affect the shale gas content. Therefore, using multiple methods to jointly characterize the pore structure of shale with different gas content and study on the main controlling factors of pore development have an important guiding role in the exploration and development of shale gas. In this study, we carry out gas shale samples in different areas. The samples collected from wells in the Sichuan Basin have high gas content, collected from wells near the Pengshui, and Tongren areas have medium gas content and collected from wells near the Guiyang area have low gas content. Shale in these areas with different gas production has been characterized, by the integration of geophysical and petrophysical data, in terms of pore type and pore connectivity, including X-ray diffraction, low-field Nuclear Magnetic Resonance (NMR), low-pressure N2 and CO2 adsorption isotherms, porosity, permeability, thin-section observations, scanning electronic microscopy (SEM) and mercury injection capillary pressure (MICP) measurement. The results show that the pore types are clay mineral intercrystalline pores and intergranular pores, rock skeleton mineral pores, organic pores, and micro-fractures. The high gas-bearing shale has a small pore size, a large specific surface area, and pore volume, and the T2 distribution is bimodal, with a peak value around 1 ms. It shows that micropores and mesopores are widely developed in high gas-bearing shale, which provides the main place for adsorbing gas, especially shale with high TOC content. During the hydrocarbon generation process, a large number of organic pores favorable for gas adsorption will be produced. The low gas-bearing shale has a large pore size, a small specific surface area, and pore volume, and the T2 distribution presents a bimodal shape with a wider peak and the lowest spectral value. This shows that low gas-bearing shale mainly develops macropores and micro-fractures, and the pore connectivity is good, especially the interconnection between small pores and fractures, which leads to gas escape. In the high gas-bearing samples, bone needles and radiolaria are distributed in large quantities, and the silicon content of biological sources is high, which provides a lot of organic matter. In the low gas-bearing samples, the horizontal layer is not developed and the mineral particles are evenly distributed.