Critical conditions for coal wellbore failure during primary coalbed methane recovery: a case study from the San Juan Basin

Wellbore failure/collapse is a common problem encountered during coalbed methane (CBM) production. The wellbore failures lead to significant production of coal fines, reductions in well productivity, and downhole accidents. These negative outcomes make it essential to predict the conditions responsible for the failure of wellbores and to know when control measures are needed. This study proposed a semi-analytical model of coal wellbore stability during primary CBM recovery. The model incorporates the effects of sorption-induced strain, horizontal stress anisotropy, reservoir depletion, and well trajectory. Plane-strain poroelasticity and the Mogi–Coulomb failure criterion were applied to estimate critical drawdown and depletion for coal wellbore failure. A real field case from the San Juan Basin was simulated to validate the applicability of the proposed model. Wellbore failure-free operating envelopes were then generated for the life span of the field. The effects of sorption-induced strain and well trajectory on wellbore stability were analyzed, and the onset conditions for local wellbore failure and reservoir-scale coal failure were compared. Results revealed that the critical reservoir pressure (CRP_1) for local wellbore failure exceeded the pressure (CRP_2) for field-scale failure, i.e., local wellbore failure occurred earlier than reservoir failure. The CRP_1 value predicted using the model aligned well with actual field observations, thus verifying the model. The desorption-induced shrinkage effect inhibited wellbore failure when radial stress was the minimum principal stress; however, the shrinkage effect enhanced wellbore failure when radial stress was the intermediate principal stress. The optimal well trajectory for the San Juan Basin occurred at the inclination angle of approximately 20° and azimuth of 90°; and the CRP_1 had its minimum value at this trajectory. This study can help optimize field production plans, well trajectories, and well completion to mitigate coal wellbore instability.