Effects of Seepage on Gas Loss through Shale Desorption during Shale Core Removal

Abstract

In the process of shale coring, the gas adsorption will increase the flow resistance of gas inside the core, which will inevitably affect the accuracy of shale gas loss. To clarify the underlying effects of seepage flow and related factors during shale desorption, we conducted an experimental study on the influence of methane on seepage resistance of fractured shale and matrix shale under different adsorption pressures. Changes in reservoir fluid and deformation resulting from CH4 saturation adsorption resulted in changes in shale permeability. This study investigated six adsorption durations (2, 4, 6, 12, 18, and 24 hours) under adsorption pressures of 5, 9, and 13 MPa in shale samples. During each cycle, different injection pressures (2 to 6 MPa) were applied, and seepage resistance of shale samples was measured by the transient method. The results showed that the permeation resistance of the sample decreased significantly after adsorption of CH4 reached saturation and decreased with increasing CH4 adsorption duration. Compared with matrix shale samples, fractured shale samples were shown to have more suitable pore microcracks and higher CH4 affinity. Therefore, fractured samples were found to have higher permeability resistance and higher adsorption capacity compared to matrix shale. The permeability flow of a sample had a negative exponential relationship with confining pressure, and stress sensitivity increased with increasing CH4 adsorption time. The model representing gas loss indicated a positive correlation between change in impermeability and the flow of escaped gas on the core surface. A significant reduction in the impermeability of the core will result in a significant reduction in shale gas loss.