Abstract: The shale reservoir is usually characterized as many natural fractures and weak planes. A large amount of self-supporting shear fractures appear in the hydraulic fracturing zones. These self-supporting shear fractures play an important role in the development of shale gas. Thus the evaluation on the flow conductivity of these shear fractures can provide great guidance for the development of the shale gas. However, the common evaluation formula of the flow conductivity for shear fractures, which is based on the Darcy flow equation, is not applicable when the fluid flow rate is high. Therefore, it is necessary to develop a new evaluation method suitable for assessing the flow conductivity of the self-supporting fractures, and provide the support for the accurate evaluation of the flow conductivity of the self-supporting fractures in the shale after hydraulic fracturing activities. The shale samples of Longmaxi formation are selected in the experiments. The shear tests are carried out in the self-developed shear-seepage coupling experimental system and the self-supporting shear fractures are generated. The nitrogen flow test is carried out under the stress-seepage coupling conditions. A set of pressure and gas flow data are obtained. The flow characteristic equation of gas flow in the self-supporting fractures is described based on the non-linear Izbash’s law. Thus the formula for calculating the flow conductivity of self-supporting fractures is established. Based on the test data, the flow conductivity of self-supporting fractures is computed. Furthermore, the theoretical gas flow amount is obtained. It is found that the theoretical flow rates and the measured flow rates are in a good agreement. The flow conductivity of the self-supporting fractures under different shear displacements and confining pressures are tested. The experimental results show that the flow conductivity of self-supporting fractures is affected by the shear displacement, roughness etc. At the confining pressure is in the range of 10-40 MPa, the flow conductivity of self-supporting shear fractures are in the range of 0.1 to 1 D?cm. The new method proposed in this paper can provide an effective way for the evaluation of the flow conductivity of the self-supporting fractures when the flow rate is high.

Key words: shale, self-supporting fracture, flow conductivity of the fracture, non-Darcy flow, Izbash′s equation