Analytical modelling of coal failure in coal seam gas reservoirs in different stress regimes

Abstract

Coal seam gas (CSG) reservoirs typically have low rock strength. During gas production, pressure depletion and matrix shrinkage may cause the differential stress around the wellbore to exceed the coal mechanical strength and result in rock failure. Coal failure has several detrimental consequences including coal fines production, permeability reduction, wellbore filling and damage to pumps and compressors. The matrix shrinkage causes a unique stress path in CSG reservoirs. However, the details of how matrix shrinkage affects coal failure still have remained uncertain. This paper presents a new workflow to evaluate stress distribution around CSG wells and predicts coal failure by coupling the effects of pressure depletion, matrix shrinkage and wellbore simultaneously. The model calculates Maximum Coal Free Drawdown Pressure (MCFDP) by considering the effects of all contributing parameters and Mogi-Coulomb failure criterion. Data from a vertical well in the San Juan Basin in USA were used to evaluate the validity of the developed model. The developed model was applied to evaluate coal failure under three different stress regimes. The results indicate that pressure depletion and matrix shrinkage have a significant effect on coal failure in all stress regimes. In the case of a normal stress regime, it is found that vertical wellbores are the most stable during the life of a reservoir. However, in the case of strike-slip and reverse stress regimes, pressure depletion and matrix shrinkage could cause the change of stress regime and therefore, the optimum wellbore trajectory could change. Additionally, it is found that in the normal stress regime the depletion and matrix shrinkage reduces the MCFDP of horizontal wellbores more than the vertical wells. However, in the reverse stress regime, depletion and shrinkage cause more reduction of MCFDP in vertical wellbores compared to horizontal wells.