| Abstract |
It is well known that permeability decline with time is a harsh and inevitable reality when producing fluids from or injecting sub-fracturing pressure fluids into fractured geological networks. Currently, it is often considered that these decline effects are mostly irreversible processes such as particle crushing, fines migration, particle embedment, thermal effects, and chemical precipitation. However, recent laboratory testing performed at the Colorado School of Mines has indicated that a portion of these effects may be reversible with potentially significant implications for engineered improvement of long-term production potential from fractured rock reservoirs. This paper details and compares the results obtained from recent laboratory pressure-flow testing performed on a complex multi-fractured unconfined granite reservoir, a simple bi-wing fractured simulated Enhanced Geothermal Systems (EGS) granite reservoir, and homogeneous unconfined single-wing fractured acrylic glass samples with and without proppant. For this analysis, repeatable pressure-dependent flow response was observed through an extensive series of constant pressure, constant flow, stepped constant pressure, and stepped constant flow testing. For the resulting data, it was discovered that a significant portion of the permeability decline with time was reversible and a qualitative theoretical explanation for this observed behavior is proposed. Additionally, with the benefits of controllable boundary conditions and known stable fracture geometry available with laboratory testing, it has been found that some common interpretations for field pressure data could be inaccurate, especially with respect to the interpretation of the fracture extension condition and estimation of fracture reopening pressure. Ultimately, several potential reservoir improvement techniques are suggested with the hope that they may be used to improve long term production levels for geothermal, petroleum, or even water wells. |