Interlayer interference during coalbed methane coproduction in multilayer superimposed gas-bearing system by 3D monitoring of reservoir pressure: An experimental study

Coproduction is an important measure for improving coalbed methane flow and production efficiency in multilayer superimposed gas-bearing systems. By monitoring the reservoir pressure in 3D spaces, we analyzed the mechanism of fluid migration and interlayer interference during coproduction through a four-layer coproduction experiment with a reservoir pressure gradient of 0.4 MPa. The results revealed the coupling between reservoir pressure gradient, matrix shrinkage, and effective stress, along with their joint effect on the reservoir pressure evolution in stages. The increase in reservoir pressure or reverse gas flow in low-pressure coal seams is the direct manifestation of interlayer interference. This interference increases gradually as the difference between the pressure gradients of the reservoirs increases. The gas elastic energy was modeled considering the free and adsorption states. We found that the difference in gas elastic energies between reservoirs is a major cause of interlayer interference. The wellbore connects coal seams with different reservoir pressures, that is, it connects different energy bodies. This destroys the state of dynamic energy balance between the systems, causing fluid flow from high-energy gas-bearing systems to those with low energy to achieve a new dynamic balance. When the reservoir energy difference is significant, the fluid in higher energy systems shields or inhibits the fluid flow to the wellbore in lower energy systems and may even cause "backflow" to lower energy systems.