Effect of seepage flow on gas loss during the removal of shale core immersed in a drilling fluid

In the process of shale coring, the drilling fluid and its solid-phase immersion inevitably cause core pollution, thereby increasing the flow resistance of the gas inside the core and leading to inaccurate calculations of the loss of shale gas. To study the changes in the seepage resistance and the amount of gas lost while the shale is immersed in the drilling fluid, an experimental study on the effects of immersion pressure on the permeability of fractured shale and matrix shale was carried out. The changes in the permeability due to reservoir fluid changes and the swelling deformation due to soaking in the drilling fluid significantly affected the shale gas loss. In this study, the shale samples were analyzed by varying the immersion time (12, 24, 48, 72, 96 and 120 h) and pressure (4, 8, 12, and 16 MPa). For each saturation cycle, the permeability of the shale samples was measured using the transient method under various injection pressures (1-6 MPa). The results showed that the permeability of the shale samples decreased significantly after immersing in the drilling fluid, and the decrease rate increased with the increase in the immersion time. Compared with the matrix shale samples, the fractured shale samples exhibited better pore micro-fractures and higher hydrophilicity, so the fractured samples exhibited a greater reduction in the permeability than the matrix shale samples. Shale permeability had a negative exponential relationship with the effective stress, and its stress sensitivity increased with increasing immersion time. Importantly, during the shale coring process, under the actions of the effective stress, drilling fluid, and its solidphase pollution, the gas escape rate on the core surface even changed by an order of magnitude. In the case of serious drilling fluid pollution, the core desorption rate was close to zero, in which case the shale gas loss may be easily underestimated.