Abstract
The development and utilization of coalbed methane is of great significance to reduce carbon dioxide emission. Through the research, this paper presents a fast analytical solution method for the productivity of coalbed methane reservoir with finite-conductivity fractured well and stimulated reservoir volume region. Based on the dual-porosity flowing mechanism, combined with the Langmuir adsorb equation, Fick diffusion law, and Darcy law, a mathematical model considering diffusion in matrix and transport in natural fracture system is established, using spherical matrix to describe the transient steady-state sorption, and using cubic matrix to describe the pseudosteady-state sorption. Then, combined with the inner system and outer system, the analytical solution was obtained. Furthermore, the accuracy of the solution was validated against a numerical simulation. According to the Duhamel principle, the effect of wellbore storage and skin factor was got. Due to the SRV region, the linear flow and radial flow will appear before the pressure wave reach the outer region. And then, based on the pressure analysis result, we will have made the sensitivity analysis with different influence parameter. The result reveals that storage coefficient and conductivity factor mainly influence the early time; the permeability ratio and dimensionless SRV region radius mainly influence the property of SRV region. Finally, the analytical solution of the new model was applied to field history match. This model takes into account the adsorption and desorption characteristics of coalbed methane, as well as the SRV zones generated during fracturing. The calculation speed of the new model is increased while the calculation accuracy is retained, and the intensity of software application is reached. The model achieves the purpose of rapid evaluation and accurate prediction of gas well productivity and obtains a set of productivity evaluation method suitable for coalbed methane reservoir with fractured vertical well, which provides a basis for the development and productivity evaluation of coalbed methane reservoir in domestic and international cooperation.
Highlights
Coalbed methane refers to methane gas in coal seams
The PSS model experiences a shorter radial flow in stimulated reservoir volume (SRV) region compared with the TSS model
This difference directs us to control the decrease speed of pressure when we develop the coalbed methane; this will use the reservoir energy efficiently
Summary
Coalbed methane refers to methane gas in coal seams. There are two main ways of occurrence of coalbed methane in coal seams: free and adsorption. The existence form of free gas is the same as conventional gas reservoir. Adsorbed gas is adsorbed in the matrix of coal seam in a dynamic equilibrium way. In the process of exploitation, with the decline of formation pressure, the adsorbed gas is gradually resolved from the adsorbed state and becomes free gas. When analyzing the pressure change in coal seam, the desorption of adsorbed gas and the migration of free gas must be considered [1, 2]
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