A novel methodology for assessing the potential for formation bridging in coal seam gas fields: A case study from southern Queensland

Abstract

The substantial growth in the development of coal seam gas (CSG) – also known as coal bed methane – fields in southern Queensland, Australia, over the past decade has been associated with various impacts on the environment and human activities. Of particular concern is the potential impact of CSG-induced subsidence on farming operations. Previous research in underground coal mining has shown that overlying competent formations can function as bridges over mined-out areas, effectively preventing subsurface deformation from fully or partially reaching the ground surface. Formation bridging can also occur in CSG producing areas. Modelling formation bridging is essential for understanding and predicting the historical and future impact of CSG production on the surface, respectively. This paper introduces a novel modelling methodology based on the elastic plate theory that helps in assessing the potential for formation bridging in overlying formations during CSG production. The applicability of this approach is demonstrated through a case study in the Surat Basin of southern Queensland. This study shows that excluding bridging for a particular formation rules it out for that formation when overlain by other formations or when it is thinner. Results of this study indicate that formation bridging in the study area may only occur during the very early phase of a CSG well's life cycle. Where the diffusivity of the coal seam is low, leading to a slower expansion of the influence area, there is also an increased likelihood of bridging taking place and persisting for an extended duration. Although the methodology introduced in this paper is mainly analytical, it offers an initial evaluation of the formation bridging probability across extensive basins where conducting detailed three-dimensional numerical modelling is challenging, if not impractical. This preliminary analysis may assist in deciding whether, and where, more comprehensive and detailed assessment of formation bridging is necessary.