A dynamic prediction model for gas-water effective permeability in unsaturated coalbed methane reservoirs based on production data

Effective permeability of gas and water in coalbed methane (CBM) reservoirs is vital during CBM development. However, few studies have investigated it for unsaturated CBM reservoirs rather than saturated CBM reservoirs. In this work, the dynamic prediction model (PM-Corey model) for average gas-water effective permeability in two-phase flow in saturated CBM reservoirs was improved to describe unsaturated CBM reservoirs. In the improved effective permeability model, Palmer et al. absolute permeability model segmented based on critical desorption pressure and Chen et al. relative permeability model segmented based on critical water saturation were introduced and coupled comprehensively under conditions with the identical reservoir pressures and the identical water saturations through production data and the material balance equations (MBEs) in unsaturated CBM reservoirs. Taking the Hancheng CBM field as an example, the differences between the saturated and unsaturated effective permeability curves were compared. The results illustrate that the new dynamic prediction model could characterize not only the stage of two-phase flow but also the stage of single-phase water drainage. Also, the new model can accurately reflect the comprehensive effects of the positive and negative effects (the matrix shrinking effect and the effective stress effect) and the gas Klinkenberg effect of coal reservoirs, especially for the matrix shrinkage effect and the gas Klinkenberg effect, which can improve the effective permeability of gas production and render the process more economically. The new improved model is more realistic and practical than previous models.