Experimental study on two-phase gas-water flow in self-supported fractures of shale gas reservoir

Shale self-supporting fracture serves as an important junction connecting matrix and artificial fractures, however, the gas-water flow behavior of the production process remains unclear. Experiment on the gas-water seepage behavior of vertical/parallel bedded self-supported fractures under stress was carried out on Longmaxi Formation shale in the Sichuan Basin. The study shows that the height of shale fracture surface convexities conforms to Gaussian distribution. With the increase of the number and height of the fracture surface convexities, the equivalent aperture and absolute gas/water permeability tend to increase. The surface roughness of vertical-bedding fractures was higher than that of parallel- bedding fractures, resulting in a generally higher conductivity of vertical-bedding fractures than that of parallel-laminated fractures. Compared with tensile fractures, the absolute permeability of vertical-bedding shear fractures increased 3-10 times, and the absolute permeability of parallel-bedding shear fractures increased 2-4 times. In the two-phase gas-water flow condition, the relative permeability of water-phase was positively correlated with the fractional flow of water phase. With the higher fractional flow of water, the water phase was affectd more significantly by the confining pressure and the surface morphology than that of gas phase, and the relative permeability of equal permeability point increased with the increase of the confining pressure, and decreased with the increase of the roughness. Compared with the tensile fractures, the two-phase fluid flow resistance decreased, and the relative permeability of the water phase rised more obviously than that of the gas phase, and the fractional flow of water phase at equal permeability point did not change significantly, the relative permeability of equal permeability point of the vertical-bedding shear fracture increased 1.39 to 1.71 times, and that of equal permeability point of the parallel-bedding shear fracture increased 1.02 to 1.39 times. The results are of great significance in guiding the optimization of pressure-controlled production system when considering the protection of conductivity of self-supporting fracture networks in shale gas wells.