The Effect of Differential Coal Seam Depletion on Coalbed Methane Production
Hoang Tan Nguyen and Zihao Zhao
Engineering Honors Degree, 2015
Australian School of Petroleum
University of Adelaide
Coal Bed Methane has been valued as the new and potential energy resource in Australian. With The development of multi-seam Coaled Bed Methane (CBM) in the Surat and Bowen Basins in Eastern Queensland, has raised the need for the study of commingled production. Heterogeneity in reservoir properties and production rates from coalbed methane (CBM) wells is observed throughout these Basins. Therefore, each reservoir property contributes to the overall production profile and it is of our interest to examine how these parameters interact and affects the multi-seam production.
The objective of this study is to generate production profiles of coal seams with various reservoir parameters. This is achieved by using simulation software named Computer Modelling Group (CMG). CMG features Palmer- Mansoori permeability model and Langmuir isotherm curve, which make it suitable for simulation of CBM. The reservoir simulation mainly focuses on varying three parameters: permeability, sorption time, and Langmuir constants to examine the effect of each parameter on the gas rate when producing from three layers at the same time.
As a result, the bottom seam produces a small amount of gas compared to the other 2 seams on the upper layers, even when they have the same reservoir properties. This phenomenon can be explained by pressure depletion as top layers experience pressure drop first. It is found that permeability is the most important parameter in production profile. An increase in permeability will lead to an increase production rate dramatically and bring forward the peak production rate. Langmuir constants as Langmuir pressure and volume also plays a critical role in producing gas as it governs the curvature of the Langmuir isotherm. An increase in Langmuir volume will result in a decrease in Langmuir pressure, therefore a lower reservoir pressure is required to desorb same amount of gas. Desorption time of coal seems to have a significant influence on the gas production rate in the first few months to the first few years of production. The sorption time controls the rate that coal matrix desorbs gas to fractures hence influencing gas production rates. When the sorption time decreases, the maximum production rate and the steady-state profile would be reached sooner.