Effect of matrix shrinkage on wellbore stresses in coal seam gas: An example from Bowen Basin, east Australia

Abstract

Coal matrix is subjected to shrinkage as a result of gas desorption. This can contribute to the stress distribution changes around the wellbore during the exploitation of Coal Seam Gas (CSG) resources. 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 CSG fields.This study introduces an analytical model to estimate the stress distribution around the wellbore by coupling the effects of depletion, matrix shrinkage, and wellbore trajectory. 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 reveal that the stress path value in CSG reservoirs, is not constant during production and it can even be more than one due to the matrix shrinkage. The shrinkage effect can significantly alter the effective horizontal stresses which accordingly can cause a considerable change to the near-wellbore stress distribution. It is shown that the stress differential may increase or decrease, depending on shrinkage/swelling magnitude and wellbore trajectory. In a normal fault stress regime, the matrix shrinkage decreases stress differential on the wellbore wall in vertical wells and consequently, reduces the coal failure potentials. However, in highly deviated and horizontal wells, the matrix shrinkage causes an extra increase of the tangential stress and reduction of radial stress on the wellbore wall (i.e., greater stress differentials on the wellbore wall). This increases the coal failure risks with depletion. The model was verified versus the observed coal failure pressure in the San Juan Basin, USA.