Over the past several years, a combination of data from field and laboratory experiments, published data from commercial treatments, and concurrent analysis and model development activities has led to a better understanding of the processes occurring in coal during stimulation operations. At the same time, better modeling capabilities have been developed. For example, a new hydraulic fracture design model that includes pressure-dependent non-linear leakoff, multiple interacting fractures, and failure in the coal near the fracture plane as a result of changes in effective stress, is being developed and is being used to study fracture propagation in coal. In addition, two- and three-dimensional discrete element and hybrid finite element models that couple the mechanical and multi-phase fluid response of the coal have been applied to analysing problems arising in cavity completion operations.
The paper compares data obtained from field trials in coal seams with model simulations and discusses implications for stimulation design and execution.
Introduction
Compared with many other reservoir materials coals are weaker and less stiff, more highly naturally fractured, and chemically more active. Coal seams present thin payzone targets, and exist with adjacent rock layers in a strongly layered structure. All of these factors affect the response of coal to drilling, well testing, stimulation and production operations. For example, cavity completion stimulation depends for its success on the unique properties of coal. Both hydraulic fracture and cavity completion stimulations in coal are not always as effective as anticipated, suggesting that a better understanding of coal and stimulation processes is needed. The physical processes that are most important in determining the effectiveness of the stimulations must be included in the models that are used in analysis and design problems. This paper describes two numerical models, under development, that are being used to study stimulation processes in coal. Both models recognise the importance of coupling the fluid pressure to the mechanical response of the coal seam.
Rock and Reservoir Mechanics of Coal
Coal is formed in marine and non-marine environments through heating and compaction (coalification) of organic material as it is buried by other sediments. Coal seams exist as units in depositional cyclothems consisting of sandstones, siltstones, and claystones with a typical cycle progressing from coarser to finer sediments. Coal is often the top, finest-grained layer. Cyclothems often repeat to form a coal sequence containing several coal seams over a vertical section, presenting multiple stimulation targets. A single coal seam will contain layers of bright and dull coal, stony bands, and crosscutting fractures and cleat. Bright coal sections are usually more highly cleated, fractured and weaker than dull sections. On the scale of the seam, coal is a layered, fractured composite material intersected by faults, shears (which may be horizontal and run for long distances within the seam) and volcanic dikes and sills.
For pore pressure changes that produce large changes in coal permeability, such as pressure changes occurring during hydraulic fracturing and cavity completion operations, the process generating the pressure should be coupled to the reservoir response. In the case of hydraulic fracturing, the pressure is generated by fluid leaking off from inside the main hydraulic fracture channel into the surrounding coal. The pressure changes in cavity completion stimulations arise from the strong injection, shut-in, flow back cycles imposed at the well, involving large flow rates over relatively short time periods. The resulting fluid flow and mechanical deformation processes and material failure are strongly coupled and nonlinear and must be linked during the solution. P. 99^
