Experimental investigation on permeability, meso-damage and fractal characteristics of limestone caprock under THM coupling based on μCT technology

Abstract

Limestone caprock permeability and damage characteristics have a significant impact on the tightness and stability of salt cavern gas storage. To investigate the gas sealing capacity of limestone as caprock for a gas reservoir at different burial depths, triaxial compression coupled with permeability tests of limestone subjected to different confining pressures and temperatures were carried out using the improved MTS 815 test machine. After the tests, X-ray micro-computed tomography (μCT) is used to perform CT scanning of the limestone sample after seepage failure to estimate the rock meso-damage. The experimental results depicted that: (1) At the confining pressures of 10 MPa, 20 MPa and 30 MPa, the final permeability increases with the increase of temperature, while at the confining pressures of 40 MPa, the evolution of the final permeability is opposite. (2) The pore radius of limestone is concentrated in the range of 0–200 μm, and the pore with a radius greater than 1200 μm is the main permeability channel with the greatest contribution. The final permeability is positively correlated with porosity. (3) The fractal dimension decreases with the increase of confining pressure, and some areas of rock do not have fractal characteristics when the confining pressure reaches 40 MPa. Permeability after rock damage is positively correlated with the fractal dimension and the maximum pore diameter and negatively correlated with the tortuous fractal dimension, according to the fractal permeability model. These experimental results and analyses provide scholars and engineers with a thorough understanding of cap rock tightness in the construction of deep-earth gas storage reservoirs.