MODELING OF OPENHOLE CAVITY COMPLETIONS IN COALBED METHANE WELLS

The overall success of coalbed methane in the San Juan basin is largel y attributable to the openhole cavity completion technique used in the sweet spot (fairway zone). This report presents the first derailed ro ck-mechanics modeling of the formation of cavities in openhole wells i n coalbeds. The modeling uses an adaptation of a model developed for u nconsolidated oil sands. In a typical case, if cohesion and modulus ar e small enough, the model predicts the following. The formation fails after the well is drilled, even while the well is full of water. Incre ased failure follows blowdown of the well. A tensile failure zone exte nds to 5-ft radius, in agreement with cavity size in the field. A shea r failure zone extends out to 50 ft or more, with enhanced permeabilit y of around 300 md at the cavity surface decreasing to formation perme ability near 50 ft. There is stress relief and lowered fracture gradie nt (0.1 to 0.5 psi/ft) out to a distance of 50 ft. This means tensile fractures are probably induced by the injections of air and water that sometimes precede the blowdown phases of cavity completion. A very ra pid pressure communication observed almost 200 ft away from a well bei ng cavitated at the COAL site is consistent with this picture. An over pressured formation is not required for a successful cavity well. If c ohesion is too high, a well will not cavitate, and a conspicuous stres s concentration develops, compressing the cleats and reducing gas prod uction. According to the model, the closer the cohesion is to zero, th e more extensive is the failure zone, and the greater is the productio n from the reservoir. Next, the cyclic surging process in the field is also modeled, and this exacerbates the failure (e.g., cavity size is increased). Additional conclusions are as follows. Surging exacerbates failure: a 5-ft cavity radius can now be achieved with cohesion of 3 psi (whereas without surging was 0.15 psi). Predicted; gas production rate lies in rough agreement with that observed. In the near part of t he plastic failure zone, permeability is enhanced, while farther away from the well, the permeability is actually reduced below virgin state . A zone of enhanced permeability predicted by the model is in general agreement with such zones inferred independently in two separate pres sure/rate matching studies. Finally, cavity wells have only been comme rcially successful in the fairway of the San Juan basin. The modeling work described here suggests that cavity completion leads to a typical stimulation effect. The reason that openhole cavities are the stimula tion of choice in the fairway of the San Juan basin perhaps has more t o do with hydraulic fractures being so unsuccessful there. Where hydra ulic fractures are successful, they should outperform cavity completio ns.