Repurposing Idle Wells for Low-Temperature Geothermal Water

Abstract California has more than 40,000 idle oil and gas wells on record and similar data applies to the entire US. Some of these wells are located in areas with relatively high geothermal gradients. The temperature at the depths for many of these wells may be high enough that can provide opportunities for repurposing idle wells and accessing subsurface heat for low-temperature geothermal water before permanent plugging and abandonment of the well. Based on an independent detailed heat map, we identified certain California oilfields that indicated high geothermal gradients. To verify, we also used the temperature data from well logs to establish the presence of high-temperature gradients. To access the heat, we considered the use of a closed-loop water circulation system for heat extraction from subsurface. We tested cases with bottom hole temperatures from 160 to 220 °F, geothermal gradient from 1.58 – 2.58 °F/100 ft, reservoir depths from five to twelve thousand feet, tubing with different insulation thicknesses, water circulation rate from 200 to 1,000 BPD, various temperatures of water sources, imperfect cementation, etc. Cases we considered included plugging the hydrocarbon-bearing formations and within the well circulation of low salinity water through annulus and insulated tubing. In a case, a reservoir with the subsurface temperature of 190 °F maintains its temperature very well after 30 years after heating the circulated water at a rate of 800 BPD and deliver warm water of around 90 °F at the wellhead steadily. Results show it is the geothermal gradient, rather than depth or temperature at well bottomhole which determines heating power. Formation porosity and permeability do not affect the temperature of heated water in closed-loop system. A low circulation rate may increase the wellhead temperature with a condition of using high-grade insulation tubing. When temperature of source water is low, heating effect from wellbore is even more significant. Cement degradation does not hurt heating effect of wellbores that much, but such wells require careful mechanical examination before repurposing. Overall, we conclude that while some idle wells may not be used as conventional high-temperature geothermal wells, but some offer possibilities for space heating and agricultural greenhouse uses. Some other states besides California are also experiencing increasing number of orphaned wells. This paper provides a technical basis for evaluating the feasibility of repurposing some of the idle wells for low-temperature geothermal energy extraction.