Reuse of abandoned oil and gas wells for geothermal energy production

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.