Cavity completion stimulations have been trialled at two adjacent sites in the Bowen Basin, Queensland, Australia. The trials were conducted under thoroughly characterised reservoir conditions and were comprehensively monitored using multiple observation wells. Integrated with the trials were laboratory investigations of the fundamental mechanisms of cavitation in similar coals. These studies primarily comprised cavitation of coal blocks under full scale stress and pore pressure conditions, and numerical modeling of both the field and laboratory processes using both continuum and discontinuum models. Stimulation mechanisms are discussed in terms of two-phase flow, coupled mechanical - fluid flow in fractures, and coal failure under dynamic conditions.
The potential for high production performance of cavity completed coalbed methane wells has attracted wide interest over several years. The apparent dependence on seam conditions for the relative success of cavity completion stimulations against hydraulic fracture stimulations became clear from experience in the San Juan Basin. Palmer identified the Fairway region of the San Juan within which cavitated wells often outperformed fractured wells by as much as an order of magnitude, but outside of which fracturing has been found to be generally more effective. By 1994, it was reported that more than 920 openhole cavity wells had been completed, of which more than 600 were currently on production. They comprised 33% of San Juan Basin wells, but produced 73% of total gas. Since 1992, a number of field and laboratory based research studies have been reported in which progress has been made in identifying the conditions and mechanisms associated with successful cavity stimulations. These advances were based primarily on analyses of observations at the COAL site in the San Juan Basin, and of results from laboratory tests on coal blocks.
In Australia during 1992, a research project commenced to assess the suitability of cavity stimulation for coalbed methane reservoirs in the Bowen Basin, Queensland. A systematic exploration drilling program had identified regions in which a reasonable potential for successful stimulation was assessed, based on permeability and gas content. However, the Permian coals of the Bowen Basin tend to be stronger and less permeable than the Cretaceous coals of the San Juan Basin. Whilst the remarkable production rates being achieved from cavity stimulated wells in the San Juan Basin provided a strong impetus for trialling the method, the need was recognised to quantify reservoir and operational conditions, and to understand the stimulation mechanisms. The research outcome sought was a rational basis for use of the technique under Bowen Basin conditions.
The main elements of the research program comprised laboratory studies, an initial field trial with some research content, and a second field trial with a predominant research orientation. The field and laboratory studies were interactively linked; field data and observations were simulated in the laboratory, and results from laboratory studies contributed to the design of field operations.
It is the purpose of this paper to discuss the physical mechanisms and associated reservoir conditions relevant to cavity stimulation in the Bowen Basin environment, in the context of the results from the field, laboratory and numerical studies undertaken.
The stimulation mechanisms investigated relate to cavity formation, permeability alteration, and connectivity of the wellbore to the intrinsic permeability of the reservoir. Four main physical mechanisms were studied as potential contributors to the stimulation. These were:
coal failure induced by sudden reduction in pore fluid pressure i.e. blowdown