Compressibility of coal and its impact on gas production from coalbed reservoirs

With continued gas production from a coalbed methane reservoir, the reservoir pressure declines resulting in geometric and volumetric changes in the solid coal and pore space. This, in turn, affects the now characteristics of coal and, hence, the overall production. This paper describes appropriate laboratory techniques to estimate different compressibilities of coal: bulk, matrix, pore volume and matrix shrinkage. The impact of compressibility on gas production is also evaluated. The results show that bulk and matrix shrinkage compressibilities are much larger compared to matrix, or grain, compressibility. The simulation results show that including pore volume compressibility results in a decline in production of about 15% over a five-year period. including matrix shrinkage compressibility, on the other hand, increases the production over the same period by almost 50%. Furthermore, reduction in the overall bulk volume of coal as a result of matrix shrinkage compressibility effect may also be resulting in a significant decrease in the effective stress and thus increased permeability.