Pressure-dependent fracture permeability of marine shales in the Northeast Yunnan area, Southern China

Abstract

Abstract A series of pressure–dependent permeability experiments were conducted on the Lower Silurian Longmaxi organic–rich shales in northeast Yunnan area, southern China, to investigate the effects of sedimentary bedding, fracture surface roughness, fracture offset, fracture aperture and effective pressure on shale permeability. The results demonstrate that the sedimentary bedding of shale only has a slight influence on matrix permeability, while the fractures can enhance shale permeability dramatically. Even at a maximum effective pressure of 48\u202fMPa, the permeabilities of aligned fractures (without fracture offset) can increase by about one order of magnitude over the permeabilities of the shale matrix. A power–law relation appears to better describe the pressure–dependency of permeability for the tested shale samples than the exponential relationship. The smoother the fracture surface is, the higher the permeability of aligned fractures will be. The fracture offset can improve fracture permeability distinctly. However, the increasing degree of offset fracture permeability is not proportional to the extent of fracture offset. There is a lower pressure sensitivity for offset fracture than aligned fracture. Fracture aperture is the fundamental factor in controlling the fracture permeability, which shows a positive relationship with the cube of fracture aperture. The pressure–dependence of aligned fractures in shale obeys the Walsh permeability model, but the pressure–dependence of offset fractures in shale does not fit well with this model. By combining the experimental results with the geological characteristics of different types of natural fractures, we conclude that the denser vertical intra–formational tension fractures connect with the bed–parallel slip fractures are favorable for step–like migration and enrichment of shale gas in structural highs. The large–scaled high–angle shear fractures likely damage the preservation conditions of shale gas, which is harmful to shale gas enrichment.