Experimental investigation of the stress-dependent permeability in the Longmaxi Formation shale

Abstract

Understanding the stress sensitivity mechanism of the matrix permeability in shale is crucial for reservoir evaluation and gas production forecasting. However, the shale matrix has a heterogeneous pore structure and a complex material composition. Few investigations have been systematically performed on the stress sensitivity mechanism of the matrix permeability. In this study, multiple techniques, including total organic carbon (TOC) analysis, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), high-pressure mercury intrusion (HPMI), low-pressure N2/CO2 adsorption (LP-N2/CO2GA) and overpressure permeability experiments, were applied to quantify shale the reservoir parameters, and we also developed a model to describe the stress-dependent permeability with pore structure and elastic mechanical parameters. Furthermore, we discuss in detail how the stress-dependent permeability properties depend on these shale reservoir parameters. The results show that the permeability is higher in samples tested parallel to the bedding (averages 4.49 × 10−2mD) compared to samples tested perpendicular to the bedding (averages 4.97 × 10−3mD). However, there is no significant difference in the stress sensitivity coefficient of permeability between vertical-to-bedding samples (SVBS averages 0.24) and parallel-to-bedding samples (SPBS averages 0.20). In addition, the TOC content shows a negative correlation with the stress sensitivity coefficient of permeability. Further investigation indicates that the banded organic matter has a higher degree of ductility than the bulk organic matter, which may increase the pore connectivity in a direction parallel to the bedding. Meanwhile, the stress sensitivity coefficient of permeability is positively correlated with the pore aspect ratio and the matrix elastic modulus. With the increase in organic matter content, the stress sensitivity coefficient of permeability decreases under the comprehensive action of a high content of quartz and an increase in the number of organic pores with a low aspect ratio.