Stress path of coal seams during depletion: The effect of desorption on coal failure

Unmineable coal seams constitute important domestic sources of natural gas in several countries. Although limited in size, coal seams have the advantage of being naturally fractured, which facilitates drainage upon depletion. Reservoir depletion changes the state of stresses in the reservoir. The change of stress is suggested by order-of-magnitude increases of permeability upon depletion and observation of significant fines production in some mature wells, this latter phenomenon known as "coal failure". Desorption-induced shrinkage of coal appears to cause changes of stress in the formation in addition to those of pore-pressure reduction during depletion. We summarize laboratory data replicating the stress path of coal seams far from the wellbore considering no change of horizontal strain and constant total vertical stress. Experimental results show that desorption-induced shrinkage promotes significant lateral stress relaxation sometimes leading to shear failure. Desorption amplifies the effect of pore pressure reduction and makes shear fracture reactivation more likely to happen than in non-sorbing rocks. This study shows the importance of considering depletion in coal seams as a chemo-geomechanical coupled process and the importance of rock failure in bulk reservoir permeability.