The Role of the Geologist in the Assessment of Continuous Gas Accumulations

Abstract

Abstract Finding and developing continuous gas accumulations such as shales, coal seams and tight sands requires the simultaneous development of accurate geological models together with effective engineering technologies. This linkage is well-established in conventional gas reservoirs where several decades of experience have resulted in a clear and important role for geology in exploration and production. Continuous gas accumulations, however, are substantially different, in that they are regional in extent and lack obvious seals or traps. Geologic controls on gas-in-place and production rates are no less important in continuous reservoirs, but these controls operate on a different scale. Moreover, geologic factors are often misunderstood and can be difficult to quantify. This can lead to the erroneous belief that the geology is either too complex to be understood, or that it is of little importance, or that regional variability is effectively unpredictable, or that reservoir behaviour can be sufficiently approximated with simplistic models. In all such cases, the importance of geology is underestimated. Many companies have relied largely upon engineering solutions in developing continuous gas resources, while excluding basic geological data and analysis from the project budget as a low-priority item. This makes little sense, however, particularly for marginally economic projects that cannot sustain a high proportion of sub-economic wells. In addition, this approach introduces a high level of guesswork, and reduces the optimization of drilling and completion strategies to trial-and-error. To alter this approach, and to contribute more effectively to the exploration and development of continuous gas accumulations, geologists must do a better job of defining their responsibilities, demonstrating why geological knowledge is important, and offering specific recommendations that will favourably impact the bottom line. Introduction As compared with conventional gas reservoirs, continuous gas accumulations typically extend over large stratigraphic thicknesses (tens to hundreds of metres) and lateral distances (tens to hundreds of kilometres). Owing to their size, continuous gas accumulations can encompass a variety of rock-types, having widely varying reservoir properties. The three principal types of continuous gas accumulations are:coal seam gas (CSG), also known as coalbed methane;shale gas; andtight sands.