Coalbed methane (CBM) reservoirs are naturally fractured formations with face cleats and butt cleats surrounding the coal matrix. Analyzing and predicting CBM production performance is challenging, especially for early production time, because of the complex fracture networks and gas/water two phase flow. In this study, we develop an efficient semianalytical model to predict gas and water production in CBM reservoirs. The node-analysis method is used to discretize the complex fracture networks into small fracture segments. We use the superposition principle to consider the interactions between fracture segments. In addition, we incorporate the critical gas flow mechanisms such as gas desorption and diffusion in our model. A two-phase flow mechanism is considered by iteratively correcting the relative permeability to gas and water and capillary pressure for each fracture segment. Subsequently, we verify this method against a numerical reservoir simulator for scenarios with bi-wing fractures. Furthermore, we apply the model to quantify the effects of various parameters on the gas and water production performance, including matrix parameters, fracture parameters, gas transport mechanism and fracture network complexity. Modeling result shows that the peak gas value increases with matrix permeability, matrix porosity, fracture conductivity, desorption capacities, and diffusivity coefficient. Furthermore, free gas plays the significant role at early stage, adsorption phase gas becoming the dominant flow behavior at later production time. The improved connections between hydraulic fracture and cleats can enhance the gas production and shorten the dewatering time. Face cleat is the major reason of increasing the cumulative gas production and butt cleat plays important role of increasing the peak gas value and shortening the dewatering time. Due to the intensified fracture interferences in the network with the production time, the advantages of complex fracture network is decreasing. Maximizing the fracture complexity through generating large connected networks is an effective way to increase gas production.
This work, for the first time, presents an accurate and fast semianalytical model to perform the two-phase flow of gas and water in CBM wells with complex fracture networks. The approach is easier to setup and less data-intensive than using a numerical reservoir simulator. To consider the recent interests in development of unconventional gas reservoirs, our approach provides a promising technique for better understanding the stimulation effectiveness in CBM reservoirs with complex fracture networks.
