Pressure transient and tracer testing are conventional methods employed to investigate physical properties associated with fluid flow and/or storage in subsurface reservoirs or aquifers. These methods have been adopted independently to investigate from different physical aspects. Here, to quantify the heterogeneity of pore distributions containing subsurface fluid, a novel concept, which combines the pressure and tracer concentration responses obtained during pressure transient and tracer tests, respectively, has been proposed and validated. Herein, the key parameter is the difference between the apertures of the equivalent planar fractures estimated from the pressure and tracer concentration responses. In particular, this difference is attributed to the fact that the pressure and tracer concentration responses obey different physical mechanisms, diffusion and advection–dispersion problems, respectively, that generate dissimilar responses to the heterogeneity of pore distribution. The concept was successfully validated using laboratory experiments and reservoir simulations conducted at multiple scales. As observed, the apparent pore volume estimated from the pressure responses tended to be larger than the actual value owing to the delay in pressure responses during propagation through the pore. By quantifying the existence of undiscovered permeable structures in a reservoir simulation model, the proposed concept provides an insightful guide for successful decision-making in explorational and developmental geothermal projects. Furthermore, the concept provides a scale for assessing the accuracy of a reservoir simulation model in expressing an actual heterogeneous permeable structure.

Triaxial compression experiments were carried out on samples of fine-grained Aji granite to measure the evolution of permeability during deformation prior to failure under confining pressures of 20 and 40 MPa. During the initial stages of deformation, a small decrease in permeability was observed, due to the closure of pre-existing microcracks; permeability then increased with increasing differential stress. During deformation, permeability varied by up to two orders of magnitude, and we observed a small pressure dependence, with a larger variation observed at 20 MPa than at 40 MPa. This suggests that more cracks developed during brittle deformation under the lower confining pressure. The observed increase in permeability during our experiments was approximately proportional to inelastic volumetric strain, which corresponded to the volume of dilatant cracks. On the other hand, prior to brittle failure we observed a further increase in permeability that was greater than the inelastic volumetric strain, suggesting crack aperture opening accelerated at this stress level (>~80%).