A numerical simulation study on the characteristics of the gas production profile and its formation mechanisms for different dip angles in coal reservoirs

Abstract

The dip angles of coal reservoirs in China vary greatly, among which the southern margin of the Junggar Basin is generally greater than 50°, rendering it a steeply inclined reservoir (SIR). Within a single well's radius of influence, the different effects of gravity on the up-dip reservoir (UDR) and down-dip reservoir (DDR) of a coalbed methane (CBM) well result from tilting. A large difference exists in reservoir pressure, gas content and permeability levels due to the depth of the UDR and DDR. These differences control the characteristics of the gas production profile (GPP) of SIR wells. In order to study the GPP of CBM wells under different coal reservoir dip angles, ECLIPSE numerical simulation software was used to simulate well production at dip angles of 0°/10°/20°/30°/40°/50°/60°/70°/80°. GPP of the wells in reservoirs with different dip angles in UDR and DDR were compared. The formation mechanism of the GPP was explored by analysing the water saturation, reservoir pressure and gas content characteristics of UDR and DDR. The field case was used to verify the accuracy of the numerical simulation results. From the results of the numerical simulation and field cases, we found a “double-peak” gas production profile (DP-GPP) for the well in SIR. The GPP of UDR is also of a “double peak” type, while that of DDR is of a “single peak” type. These types occur because gravitational forces increase the levels of water migration in UDR and obstruct those of DDR, creating different starting times for methane desorption and different periods of time required to reach peak desorption rates, resulting in a time difference. This time difference in methane desorption observed between UDR and DDR has led to the formation of a DP-GPP. Under a larger drainage area (continuous and homogeneous formation conditions), the continuous outward expansion of the desorption range gradually delays the formation of the DP-GPP and make it appeared later. Since methane supplies are affected by surrounding wells, the stronger well interference is likely to fail to form a DP-GPP. The timing of DP-GPP occurrence is gradually delayed with increasing burial depth due to a decrease in permeability.