Coalbed Methane (CBM) is becoming a significant portion of the US gas resource and is gaining importance in Australia, China, Indonesia and Europe. CBM reserves in the United States are estimated at some 450 Tscf. In Australia, CBM resources exceed 300 Tscf, while China has a resource potential greater than the United States and Australia combined. Recent advances in well design and production technology offer scope to significantly increase the proportion of gas contained in coal that can be commercialized. Generally, initial water saturation is 100% within coal seams and gas can only be found as an adsorbed phase inside the coal matrix, so how much adsorbed gas can be released at an economical rate will determine the ultimate gas recovery. The Langmuir isotherm has been widely used in industry to describe the pressure dependence of adsorbed gas. However, temperature dependent adsorption behavior and its major implications for evaluating thermal stimulation as a recovery method for coalbed methane have not been thoroughly explored. Therefore in order to investigate the feasibility of thermal treatment in coal bed methane reservoir successfully, it is crucial to understand the effects of thermal stimulation on the adsorption and desorption phenomenon, and how can we exploit such effects to enhance coalbed methane recovery from hydraulically fractured reservoirs.
In this study, we propose a method to evaluate desorbed gas as a function of pressure and temperature in coal seams, by regression on Langmuir isotherm data. In addition, a CBM reservoir simulator is developed, which is capable of capturing real gas diffusion in the coal matrix and flow in the hydraulic fractures, as well as the heat transfer process within the matrix. This simulator enables us to investigate various thermal stimulation techniques on the global well performance and recovery.
The results of this study show that by increasing the formation temperature, ultimate gas recovery can be improved in CBM reservoirs. The higher the thermal stimulation treatment temperature, the more extra gas can be recovered. However, the efficiency of thermal stimulation is mostly constrained by how much the formation area/volume that can be stimulated in a reasonable period of time. Due to the low heat conductivity of coal, it is not possible to heat up a large drainage area/volume by heating the surface of vertical hydraulic fractures directly. If the heating source (e.g., electromagnetically excited nano-particles) can be dispersed further into the formation through the cleat system during hydraulic fracture execution, then larger formation area/volume can be heated up, depending on how further the nano-particles can be pushed and the arrangement of production (injection) wells. In the case of horizontal fractures, a large formation volume can be thermally stimulated if the fractures can be placed close enough to cover the whole lateral area. Thermal stimulation by hot water/steam injection can increase formation temperature more rapidly than direct element heating methods, especially when the formation permeability is large. Considering large amount of residual adsorption gas still left behind even with low production pressure, thermal stimulation has the potential to enhance CBM recovery substantially if techniques and designs are tailored to the formation properties appropriately.