In numerical studies of coalbed methane stimulation, mechanisms of cavity formation and permeability alteration have been modeled using a fully coupled mechanical - fluid flow approach. The coal is represented as a naturally fractured medium on the scale of the cleat system and larger structures. The apertures and spacing of the cleats are selected to reflect the magnitude and anisotropy of the permeability tensor. The mechanical properties of the coal, the liquid and gaseous injection fluids, the dynamics of fluid injection and blowdown, and the applied boundary conditions are explicitly represented. In the context of experimental observations and measurements, the potential of the explicit approach in modeling stimulation mechanisms of cavity completion is demonstrated.
Stimulation of coalbed methane wells using openhole cavity completion has been highly successful in certain regions of the San Juan Basin, south western USA. In other areas, conventional stimulation by hydraulic fracturing has remained the preferred method. The successful techniques evolved predominantly by experience, but in general have not proved viable when transported to other coal basins. In recent years studies have been undertaken in the US and in Australia with the objective of identifying the fundamental mechanisms and developing methods for application of the technique in a broader range of conditions. The process by which cavity wells are stimulated involves the inducement of pore fluid pressure gradients around the wellbore during "blowdown", following high pressure air injection, or in some cases pressure buildup by natural accumulation of methane. Sudden release of pressure from the wellhead during blowdown may induce the coal to cavitate into the wellbore. Repeated pressure surging, cavitation and flushing may lead to development of cavities 3-4m across, before stability is achieved. Hypotheses for the stimulation mechanisms have been put forward based on field observations, laboratory studies and numerical modeling (Palmer, 1992; Mavor, 1992; Logan et al., 1993; Khodaverdian and McLennan, 1993; Mavor and Logan, 1994; Palmer and Vaziri, 1994; Wold et al., 1995). It is generally recognized that the increased wellbore dimension is of itself a relatively minor contributor to the degree of stimulation achieved in a successful operation. The mechanisms which have been proposed to date can be summarized into two main categories. The first category is based on the geometrical aspect of the borehole and the fracture network. These include creation of new fractures, change in conductivity of existing fractures, and change in connectivity between the enlarged wellbore and the fractures, and between the fractures. The second category is based mainly on permeability enhancement associated with stress relief and redistribution as a result of failure (or yielding) of the coal. The basic assumption is that a relationship between stress and permeability exists, with increase in permeability with a reduction in stress. This was based on explicit representation of coal structure, fully coupled fluid flow - mechanical interaction and dynamic effects. The present paper discusses this modeling approach and presents a number of results in the context of observations across a range of coalbed methane conditions.
