Coal seams are dual porosity media containing two systems of fractures and matrix pores. The size of matrix pores is very small while specific surface area is large; a large amount of natural gas is adsorbed on the pore surface. It is generally believed that after the adsorbed gas desorbs, the desorbed gas in matrix pores first diffuses into cleats (fractures) under the driving of concentration difference, and then flows to the wellbore in the form of gas-water tow-phase flow. However, it is found that, predicted gas production based on the diffusion mechanisms of desorbed gas through matrix pores greatly deviate the actual production data. Through the preliminary research it is found that the diffusion theory ignores the existence of water in matrix pores, which makes the diffusion model inapplicable, and the gas production model based on the molecular diffusion has some problems.
Therefore, we start our study on the initial distribution of gas and water in matrix pores, and then construct a more accurate transport model by studying the dynamics and phase characteristic of gas in water bearing matrix pores. Based on the transport model, a corresponding gas production prediction model is established, and the gas production rule of coal seam gas (CSG) is obtained.
Results show that almost all desorbed gas in water bearing matrix pores linearly flows into cleats, the gas production rate can stabilize for a long time at the constant bottom hole flowing pressure condition. Through the application of gas production prediction model into the field gas wells, the predicted gas and water production by model are in good agreement with actual production data.
Our study will provide a more accurate technique for gas production forecasting of CSG wells, and moreover, it will provide scientific evidence to the rational development of CSG reservoirs.