Understanding Compressibility Coefficients in Heterogeneous Fractured Rocks and Implications for CSG Reservoir Simulation

Abstract A variety of different compressibility terms, including cleat compressibility and bulk compressibility, are used when modelling coal seam gas (CSG) reservoirs. The relationship between different compressibility terms is often theoretically straightforward, but in practice may be much more complex, particularly when considering heterogeneous and/or fractured rocks. This paper outlines experimental work measuring different compressibility terms using printed rock samples, and analysis that demonstrates some of the challenges associated with relating these compressibilities. Three-dimensional printed rock samples with heterogeneity (layers of different stiffness), some of which included planar fractures, were created. The compressibility of these samples was measured based on changes in permeability (as might be used to estimate cleat/fracture compressibility) and also based on volumetric strain. Simple models were history matched to estimate the cleat compressibility, which is then used to calculate a bulk compressibility based on theoretical relationships. This is then compared to the bulk compressibility measurement based on volumetric strain. Initial results indicate that the relationship between the different compressibility terms is much more complex than theory suggests. The theoretical relationship of bulk compressibility with pore compressibility yields values up to one order of magnitude different from that of laboratory measurements. Our study highlights the importance of cleat compressibility in modelling CSG reservoirs and the significance of bulk compressibility in estimating deformation associated with CSG production. We believe our findings will contribute to a better understanding of compressibility terms in CSG reservoir modelling and encourage further research in this area.