Permeability Evolution Characteristics of Intact and Fractured Shale Specimens

The permeability of shale reservoirs is an important parameter in evaluating the feasibility of shale gas commercial exploitation. The influence of structural anisotropy, bedding planes, and effective stress is of great significance to the permeability of shale reservoirs; thus, a further study on the permeability of shale rock containing bedding planes and fractures is necessary. In this paper, to investigate the gas conductivity of shale rock in different bedding directions and fracture surfaces, permeability tests were conducted on the intact specimens with different bedding inclinations and specimens containing fractures of Longmaxi shale. The pulse decay method is adopted in the determination of the intact shale specimen, and the steady-state method is adopted to measure the permeability of fractured shale specimen. Combined with experiments and theoretical analysis, the anisotropic characteristics of the permeability of intact shale specimens and the permeability of fractured specimens under the applying of effective stress are studied. The permeability of the two kinds of shale specimens decreased exponentially with the increase in effective stress. The main controlling factors on permeability were studied, besides, the flow characteristic of fluid inside the rock with bedding plane or sandwich structures were described. Furthermore, a new model was proposed to describe the anisotropy of rock permeability characteristics. As for shale specimen contains fracture, the function between its equivalent permeability and fracture permeability was derived, as well as the function between equivalent permeability and effective stress, and the function between fracture permeability and effective stress.