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This paper presents an investigation into the suitability of abandoned wells in California for Enhanced Geothermal Systems (EGS) and low temperature deep Borehole Heat Exchanger (BHE) applications. The study identifies three counties characterized by high numbers of abandoned wells, medium to high crustal heat flows (75–100 mW/m2), and suitable sedimentary geology: Santa Clara, Monterey, and Santa Barbara. Thermal gradients range between 4 and 7.3 °C/100 m and enable access to the bottom hole temperatures between 40 and 73 °C for an average 1000 m deep well. These rock temperatures are sufficient for low-temperature direct use EGS such as district heating, greenhouse heating, and aquaculture. Economically, the mitigation of drilling costs and the documented lithology both reduce the risk associated with EGS. However, hydraulic fracturing of loosely to moderately consolidated sedimentary rock in transitional stress regimes remains one limitation to the EGS conversion of these abandoned wells. Alternatively, the feasibility of deep BHE applications within abandoned oil and gas wells is demonstrated here with a mathematical model. Predictions show that outlet fluid temperatures >40 °C can be achieved for 1000 m deep wells in regions with temperature gradients >7 °C/100 m.

A method was developed for experimentally observing the evolution of fracture aperture/permeability within a granite-based Enhanced Geothermal System (EGS) in an attempt to better understand the reduction of fracture permeability due to chemical and mechanical processes. Specimens of granite were subjected to flow-through column-like experiments to characterize the evolution of fracture aperture/permeability in a near-field setting for 20 and 40 days. Near-field experimental conditions approached in-situ shallow EGS conditions: granite rock, tensile fracture, 120 °C temperature, and 25–35 MPa effective stress. Following each 20–40 day experiment, permeability, fracture aperture, and mass of minerals dissolved were computed using pore-pressure observations, effluent chemistry, and non-concurrent X-ray computed tomography (CT) scan imaging. Results showed an increased upstream pore pressure, which could indicate a decline in permeability due to a combination of the dissolution of fracture propping asperities and mechanical creep. Effluent solution composition was in agreement with the dissolution of silicates (most notably feldspars) which points to a geochemically driven decrease in aperture due to mineral dissolution rather than mechanical effects. In comparison, the pore pressure measurements corresponded to a greater aperture change than the effluent chemistry (0.127 vs 0.101 μm, respectively). This discrepancy may be attributed to the contribution of other physical processes to fracture aperture change, including mechanical creep. Similarly, the CT scan showed a decreased fracture aperture, but the low resolution and non-concurrent scans influenced higher aperture closure estimates in comparison to the effluent chemistry and pore pressure measurements. However, the CT-scan confirmed the scale-dependency of the chemical and mechanical processes. Fundamentally, the study demonstrated a method for ensuring near-field conditions and observing the chemical and physical processes associated with fracture permeability decline.