Coals in the Sydney Basin contain large amounts of gas ranging in composition from pure methane (CH4) to pure carbon dioxide (CO2). These gases are derived from thermogenic, magmatic and biogenic sources and their present-day distribution is mainly related to geological structure, depth and proximity to igneous intrusions.A coal bed methane (CBM) study of the Camden area of the Sydney Basin has been jointly conducted by Sydney Gas Company NL (SGC) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The delineation of high production fairways is vital for any CBM project development to be commercially successful. An integrated research project employing various methods of reservoir characterisation, including geological, geochemical, geomechanical and gas storage analyses contribute to this delineation for the Camden area, where SGC is currently developing the 300-well Camden Gas Project.In particular, accurate determinations of gas content, saturation levels, composition and origin, as well as interpretations about distribution, are essential for identifying sweet spots for CBM production optimisation. The extent of gas saturation is a function of numerous factors, including amounts of gas generated between the Permian and Late Cretaceous, amounts expelled from the system during Late Cretaceous-Tertiary uplift and amounts of subsequent secondary biogenic methane generated and absorbed in the coals. The extent of this secondary biogenic gas generation appears to be greatest in coals proximal to the basin margins, where meteoric waters carrying bacteria and nutrients had ready access. Significant enhancement of methane content also occurs, however, in deeper parts of the basin where permeable structures exist.The integrated study shows that high production CBM wells drilled to date by SGC are located in zones of enhanced permeability. In these locations original thermogenic wet gases have been removed and additional secondary biogenic methane has been generated due to microbial alteration of coal, hydrocarbons and carbon dioxide. This process has replenished the coals by enhancing the methane contents of the respective seams and this phenomenon can be termed ‘bio-enhancement’ in the context of CBM production.

A thick composite coal seam at Dartbrook Mine has been characterised by stress and permeability testing over several years. Most recently, a four well injection interference test was conducted to measure the overall seam permeability anisotropy, and core samples from various sites near the interference test array have been tested in the laboratory. The magnitudes of horizontal permeability components as measured by the well test and by core tests were found to be similar, but the principal permeability directions were reversed. The interference test measured the permeability anisotropy of the entire composite seam while the core samples came from only one of the four seams making up the composite seam. Structural features and hydraulic fracture results suggest that different seams and even parts of single seams in the composite seam are very likely to have significantly different permeability tensors.

TX 75083-3836, U.S.A., fax 01-214-952-9435.Abstract 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.

No abstract

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^