Study on the invasion mechanism of liquid phase into shale formation under the effect of fluid-solid coupling

The invasion of liquid phase into shale formation can lead to downhole accidents, e.g. borehole instability. So far, however, the invasion of liquid phase into shale formation and the fluid-solid coupling phenomenon have not been studied deeply. The mathematical model for the invasion of liquid phase into shale formation under the effect of fluid-solid coupling was established. In addition, its solution results were verified and analyzed. It is shown that in the early stage of liquid invasion, its seepage in artificial fractures is dominant. With the extension of invasion time, however, the pressure difference in complex porous medium decreases gradually, and the seepage pressure in the matrix tends to be consistent with that in the fracture system. The effect of fluid-solid coupling in the process of liquid invasion is significant, and the permeability of fracture system and matrix increases under the effect of fluid-solid coupling. And in the same time interval, the seepage pressure under the working condition of coupling is higher than that of non-coupling. In conclusion, the research results which are obtained by taking matrix, natural fracture and artificial fracture as three different pressure systems play a guiding role for understanding the invasion mechanisms of liquid phase into shale formation under the effect of fluid-solid coupling scientifically.