This paper reports the development of a fully coupled rock mechanics and fluid flow finite element model and its application in the analysis of the mechanisms of the openhole cavity completions. Unique to the model is the use of elasto-plastic stress-strain relationships with reference to post-failure residual strength to model the mechanical behaviour of coal seams. Cavity simulations using field tests data indicate that for successful cavitation, the residual cohesion of the coal seams under stimulation needs to be sufficiently small. In a typical case of one-step well blow-down, the model predicts following changes in the formation. Three distinctive zones are formed around the wellbore: a physical cavity; a tensile and/or shear failure zone with enhanced permeability due to stress relief and material failure; and an elastic zone extending beyond the failure zone which is highly stressed at the zone interface.
Due to their low productivities, coalbed methane wells usually require stimulation. In recent years, openhole cavity completions have proved to be remarkably successful in improving the coalbed methane well productivity in the [fairway] region of the San Juan Basin in the US. This technique uses a series of controlled injection-blowout processes to create a cavity and, more importantly, to effectively connect the cavity to the reservoir. The average cavity radius for typical wells in the fairway region has been determined to be roughly 1.5 m. Based on their analyses Palmer et. al (1992) suggested that the permeability enhancement during cavitation occurs as a result of (1) self propped vertical tensile fracturing in the direction of maximum horizontal stress (parallel to the face cleats), and (2) shear failure in the direction of minimum horizontal stress. Research at the COAL research site (Completion Optimisation and Assessment Laboratory) carried out by the Gas Research Institute estimated a tensile fracture half- length of 33 m and a shear zone of 7 m (Mavor et. al 1991). Numerical simulations of cavity completion have recently been carried out by Palmer and Vaziri (1994) and Khodaverdian et. al (1995) using field data from several cavity wells in the San Juan Basin. The numerical model used in these studies was adapted from an unconsolidated sand model which solves a set of fully coupled porous medium deformation and single phase fluid flow equations. In the model the pre-failure stress-strain response of the formation is assumed to be hyperbolic. Shear and tensile failure of coal seams under stimulation is described by the Mohr-Coulomb failure criterion. The model allows for cavity development, which is implied by the occurrence of tensile failure in the formation around the wellbore. Palmer and Vaziri concluded that low material cohesion of coal was essential for successful cavitation. In order to create a cavity of 1.5 m radius, in their study they found it necessary to use cohesion values in the range of 0.001-0.035 MPa. These values are 2-3 orders of magnitude smaller than those normally measured on coal samples in laboratory triaxial compression tests (generally of the order of 1 MPa).
