Modeling and application of coalbed methane recovery performance based on a triple porosity/dual permeability model

• Mathematical descriptions of the TPDP model are given in detail.
• Productions simulated using the DPSP and TPDP model are compared.
• Parameter variations during the CBM drainage process are investigated.

The triple porosity/dual permeability (TPDP) model is an advanced coalbed methane (CBM) model for recovery simulation, which processes CBM flows via desorption and diffusion into permeation pores and moves sequentially via Darcy flow through permeation pores and cleats, and, finally, into the wellbore. Based on this theory, this paper describes the mathematical models of desorption, diffusion and permeation during CBM drainage; conducts the measurements of routine core analyses to obtain fluid characteristics, rock mechanics, coal seam domain, diffusion pore property and fracturing property, and nuclear magnetic resonance to acquire porosity and permeability of permeation pores and cleats; and, finally, takes a typical CBM well named QSDU01, located in the Qinshui basin in China, as an example to simulate its recovery performance. The following conclusions are reached. Compared with a dual porosity/single permeability (DPSP) model, the TPDP model is more advanced and can effectively eliminate the slight nonconformities caused by the DPSP model. Reservoir pressure, gas content, porosity and permeability initially decrease rapidly and subsequently slow during the CBM drainage process. The decreased amplitudes of parameters become larger as the distance to the wellbore decreases. Variations of parameters can be divided into two stages, corresponding with the stages of free gas drainage and desorbed gas drainage. The decreased amplitudes of parameters are quite small in the first stage and relatively larger in the second stage. Gas peaks lead to the turning points of the variation curves and cause the variation curves to exhibit a type of multisection.