| Abstract |
Accurate and robust characterization of the stimulated rock volume around EGS wells is critical to understanding and optimizing reservoir performance. Commercially successful stimulations need to produce a highly connected set of wells to minimize hydraulic losses at commercial rates of circulation, create uniform fracture permeability to avoid thermal short-circuiting, and the created fracture permeability needs to exceed the background (matrix) permeability of the surrounding rock to minimize leak-off. A simple set of post-stimulation well tests are proposed to obtain key reservoir parameters that can be used to quickly build a simple hydraulic circuit model that can be solved analytically. This model can then be used to evaluate important reservoir characteristics. The analysis does not consider thermomechanical and buoyancy effects but provides a good starting point for building more sophisticated reservoir models as more production data becomes available. This approach was applied to the circulation test data for Fervo’s Project Red pilot published in Norbeck and Latimer (2023) to inform fracture and matrix permeabilities, deduce the shape and evolution of the thermal front, and estimate best-case reservoir performance. The transient injectivity and steady-state circulation flow analysis yielded an average stimulated reservoir volume (SRV) permeability of 2.5 – 2.8 md surrounding the injector-producer pair, and a matrix permeability of about 0.3 md in the far field. A simple reservoir model with uniform SRV permeability was built around this data and was subsequently used to analytically estimate long-term performance in steady-state production at a rate of 40 l/s that was demonstrated during the circulation test. The projected thermal breakthrough time is about 5 years, with the flow forming a relatively sharp thermal front between the injector and producer, i.e., with a width that is about one quarter of the front propagation distance. Under these simplifying assumptions, temperature drop of the produced fluid is expected to be minimal during most of the project cycle. A relatively steep temperature drop is expected after about 4 years, when the broadened thermal front approaches the producer. |