Coal Seam Gas (CSG) has increasingly become an important part of natural gas resources for both domestic use and LNG export in Eastern Australia. The matrix shrinkage during primary production results in a unique stress path and new near-wellbore stress distribution in CSG reservoirs. The knowledge of these stresses is essential for coal shear failure prediction and assessing coal fines production risks, particularly in horizontal wells which nowadays are becoming common in Australian CSG fields such as the Bowen Basin.
In this study, an analytical model is developed to evaluate the stress distribution around both vertical and horizontal wells by coupling the effects of depletion, matrix shrinkage, and wellbore trajectory. Then, the model is applied for coal failure analysis in both vertical and horizontal wells in Moranbah Coal Measures, in the Bowen Basin eastern Australia.
The results of the model reveal that the stress path factor in CSG reservoirs, contrary to conventional reservoirs, is not constant during production and it can even be more than one due to the matrix shrinkage effect of coal seams with depletion. Production from these reservoirs will significantly alter the effective horizontal stresses which result in a considerable change to the near-wellbore stress distribution. The results also indicate that in a normal faulting stress regime, the matrix shrinkage will cause less stress differential on the wellbore wall for vertical wells and it is found that this mechanism reduces the coal failure potential during production. Conversely, for highly deviated and horizontal wells, the matrix shrinkage will cause an extra increase of tangential stress and reduction of radial stress on the wellbore wall. Therefore, it results in higher levels of stress differentials on horizontal wellbore wall and increase of the coal failure risks with depletion.
The results of this study can be used as a guide for assessing the risk of solids production, identifying the critical drawdown and depletion pressure and the effects of wellbore trajectory on optimizing coal fines production in CSG wells.