Openhole Cavity Completions in Coalbed Methane Wells in the San Juan Basin

Abstract

Summary Coalbed methane wells in the San Juan basin can be highly profitable, with gas production up to about 10 MMcf/D at depths of about 3,000 ft, if the wells are successfully completed with the openhole cavity technique. We report the first measurement of cavity size (radius of about 5 ft) and shape. A cavity can resemble a cylindrical bookcase with shale ledges like shelves. We also report correlations between successful cavity completions and such reservoir/rock parameters as compressive strength, coal rank, permeability, and reservoir pressure (for example, there is no correlation with the minimum coal compressive strength). In this area, wells completed with the openhole cavity technique often produce roughly 10 times more gas than wells completed with hydraulic fracture stimulations. Wellbore mechanics associated with the cavity - e.g., the enlarged wellbore plus enhanced permeability beyond the cavity - does not seem to explain the cavity/fracture production discrepancy. A number of other possibilities are explored, including permeability anisotropy and completion damage to the reservoir or fracture. Severe damage apparently is associated with hydraulic fracture stimulations in the fairway zone, which would explain their poor performance compared with cavity wells. Introduction In 1987, Meridian Inc. began developing properties in Unit 30-6 (Fig. 1) using openhole cavity completions. Sixty-one wells produced gas at an average rate of almost 4 MMcf/D (with some at 10 to 12 MMcf/D) in 1990. In an adjoining unit (NEBU, Fig. 1) Devon Energy Corp. completed almost 100 coalbed methane wells similarly. Again, rates averaged about 4 MMcf/D. A summary of recent cavity completions is given elsewhere. The openhole cavity completion has become the technique of choice in a sweet spot or high-potential fairway region (Fig. 2). Within the boundary of the fairway, wells with openhole cavity completions produce at 1 MMcf/D or more. The fairway is characterized as an overpressured area with generally thicker coal seams and enhanced natural fracturing.