Gas-water flow mechanism during the closure process of self-supporting fractures in shale and its engineering applications

Taking the underground shale of the Silurian Longmaxi Formation in southern Sichuan Basin as the research object, stress-sensitive experiments on self-supporting fractures and micro-visualization experiments on gas-water flow were conducted under simulated reservoir conditions to study the mechanism of microscopic gas-water flow during the fracture closure process and discuss its engineering applications. The results show that as the effective stress gradually increased from 5 MPa to 60 MPa with an increment of 5 MPa per step, the self-supporting fracture closure exhibited a two-stage characteristic of being fast in the early stage and slow in the later stage, with the inflection point stress ranging from 32 MPa to 35 MPa, and the closure degree of 47%-76%. The effective stress increase gradually rose from 5 MPa per step to 20 MPa per step, and the early fracture closure accelerated, with the maximum closure degree increasing by 8.6%. As the fracture width decreased from 500 μm to 50 μm, the gas-phase shifted from continuous to discontinuous flow, and the proportion of the critical gas-phase flow to maintain the continuous gas-phase flow increased. In the early stage of fracture closure (fracture width greater than 300 μm), the continuous gas-phase flow is controlled by the fracture width - the larger the fracture width, the smaller the proportion of the critical gas-phase flow to maintain the continuous gas-phase flow. In the late stage of fracture closure (fracture width less than 300 μm), as the fractures continue to close, the dominant role of the surface roughness of the fractures becomes stronger, and the proportion of the critical gas-phase flow to maintain the continuous gas-phase flow exceeds 70%. A reasonable pressure control during stable production and pressure reduction in the early stage (the peak pressure drop at the wellhead is less than 32 MPa) to delay the self-supporting fracture closure is conducive to the stable and increased production of gas wells.