Experimental Study on Dynamic Changes of Physical Properties in Deep Coalbed Methane Reservoirs

ABSTRACT There are abundant deep coalbed methane (CBM) resources in the Ordos Basin, which have great exploration and development value. However, the special geological environment caused by deep burial of resources, such as high temperature, high reservoir pressure, and high stress, leads to significant differences in the physical properties of deep coal reservoirs, seriously restricting the development effect of deep CBM. This study takes the deep coal reservoirs in the Linxing–Shenfu block as the research object, and combines coal sample overburden pressure pore permeability and overburden pressure nuclear magnetic resonance (NMR) experiments to deeply study the physical property changes of coal reservoirs under different pressure conditions. The research results indicate that there is a negative exponential relationship between confining pressure, porosity, and permeability, indicating strong stress sensitivity. In the low confining pressure stage (< 10 MPa), the closure of macropores and fractures leads to a significant decrease in permeability and porosity with increasing confining pressure. The permeability loss reaches 90%, and the porosity loss rate is between 20% and 30%. In the high confining pressure stage (> 10 MPa), the impact of confining pressure on permeability reduction and fractal dimension of seepage space weakens, porosity changes tend to flatten, and permeability loss rate is between 50% and 70%. The experimental results on the physical properties of deep coal reservoirs in the research area clearly demonstrate the changing trends of pore structure, pore space and fractal dimension under confining pressure changes, providing a reliable theoretical basis for revealing the dynamic changes of coal reservoir properties and exploration and development.