CO2 sequestration in coal seams has become an attractive carbon sequestration technology for two reasons. First, the injection of CO2 enhances methane production from the coalbed (ECBM). Second, it reduces the effect of global warming by storing CO2. The performance prediction of ECBM is very complex, as it is highly affected by the complexity of porosity-permeability variation due to matrix shrinkage and swelling effects. An additional complicity is added if the reservoir properties are not available. In this paper an integrated model was developed to simulate the performance of ECBM.
A compositional material balance (CMB) was used to track propagation of CO2 concentration in the reservoir. CMB was combined with the stream tube concept to convert the 2D to 1D problem. An optimization algorithm was also used with the integrated model. The integrated model could be used as a history-matching tool to estimate the initial formation permeability, the initial formation porosity, the matrix shrinkage, and swelling coefficients that reflected the permeability changes.
Coreflood data was used to verify the model as a 1D model assuming that the core was a one-stream tube in a 1D flow problem. The results showed a good match between the experimental data and the integrated model and the coal properties was estimated. The model was then used to analyze the production behavior of ECBM in five-spot pattern at different injection and production conditions. It was used to predict the production gas flow rate, gas composition, CO2 concentration, formation porosity, and permeability distribution across the reservoir. That help to trace the injected CO2 and predict CO2 breakthrough time. The model was also used to examine the effect of the injected gas properties on the ECBM performance.
This model can be used as a tool in ECBM development to optimize the gas injection and production conditions (production and injection pressures, and injected gas properties) to optimize the breakthrough time and the sweep efficiency.