Gas-phase production equation for CBM reservoirs: Interaction between hydraulic fracturing and coal orthotropic feature

Abstract

The efficient development of coalbed methane (CBM) reservoirs entails the accurate evaluation of production behavior, particularly for the gas phase performance. However, subjected to complexity arising from the coal inherent heterogeneous properties and dynamic gas-water two-phase flow, pursuit of high prediction accuracy remains challenging. Although massive excellent contributions have been devoted to addressing this issue, several key factors fail to be captured, and in-depth analysis is lacking, involving coal orthotropic feature, as well as associated interaction with hydraulic fractures. In light of the current knowledge status, the research has developed a robust gas-phase productivity equation for CBM wells, putting the emphasis on the coupling influence from coal orthotropic feature as well as hydraulic fracture. Also, gas-water two-phase flow mechanism is captured by incorporating the relationship between pressure and fluid saturation in coal cleat system. Moreover, traditional factors are considered as well, including gas desorption, stress dependence, matrix shrinkage. Numerical simulation is utilized to clarify the reliability of the proposed equation at the stable state, and desirable agreements can be achieved. Results show that (a) Augment effect, induced by enlarging intersection angle, on gas production rate is evident, and the calculation case suggests the optimized angle is 90°, requiring further investigations; (b) The inherent orthotropic feature acts the detrimental role for gas production performance, and fortunately hydraulic fracturing, treated with suitable intersection angle, is able to alleviate the influence; (c) Gas desorption ability lays the profound basis for evaluation of the CBM development, supposed to be selected as the prominent index for the favorable target development area.