Cavity completions have been successfully performed in the [fairway] region of the San Juan Basin, for coalbed methane production. The cavity wells in this region generally yield production levels several folds greater than hydraulically fractured wells. Earlier analyses of the field data strongly indicated that the size of the physical cavity (i.e., normally between 4 and 5 feet in radius) cannot account for the post-cavity production levels achieved in the fairway. This suggests that cavity completions in coal induce a zone of enhanced permeability beyond the cavity wall which effectively connects the wellbore to the reservoir. In the current work, the cavitation mechanisms have been addressed through numerical modeling, field data evaluation, and laboratory experimentation. Analyses have shown that a plastic failure zone, with a radius limited to 50 ft, is induced beyond the cavity wall. The increased permeability within this zone is due to material plasticity and shear failure induced during the injection/surging cycles, and stress-dependent permeability of naturally fractured (cleated) coal.
Openhole cavity completion techniques for coalbed methane stimulation use a series of air injections (or shut-ins for natural cavitation) and surging cycles to create a cavity and to stimulate the formation. The cavitation cycles are typically performed over a two-week period. During surging, the well is rapidly opened to the atmosphere to create high rate depressurization. With each cavitation cycle, coal fails and sloughs into the wellbore, and is ejected from the well, leading to creation of cavity. Based on previous work (Palmer et al, 1992, 1994 and Khodaverdian, et al, 1993), the post-cavity formation may include three distinct zones, as illustrated in Figure 1. These include the physical cavity, a plastic zone immediately around the cavity, and an intact zone extending beyond the plastic zone. The physical cavity is induced as the drag forces during surging cause material tensile failure at the cavity wall. Repeated surging cycles enhance this process by weakening the coal in the vicinity of the cavity. The plastic zone around the cavity may be generated due to material shear failure. This is expected to result in significant permeability enhancement within the affected region. The formation outside of the plastic zone remains nominally intact; it does however probably experience changes in stress and, therefore, permeability. Cavity completion mechanisms causing formation stimulation were investigated using a synergistic approach involving field data analysis, numerical modeling, and laboratory experimentation. This article primarily addresses cavity operations in the San Juan Basin fairway and the effect of cavitation on formation permeability, as postulated through numerical modeling and laboratory experimentation. The openhole cavity completion has been used with tremendous success in the San Juan Basin (Palmer, et al., 1992, 1994, and Logan, et al., 1993). Some wells produce in excess of 10 MMCFD from only 3,000 ft depth in the fairway region (Figure 2). A typical Amoco cavity operation was described previously (Palmer, et al., 1994). Certain aspects of cavity operations in the San Juan Basin fairway are described below.
