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| Fig. 1: Depth versus Temperature graph of 1065 oil wells in Texas. [8] (Image source: A. Kornfein) |
Across the state of Texas, 7,400 documented orphaned oil and gas wells lay unused and unplugged. [1] Approximately 80,000 oil wells are drilled per annum in Texas, making unplugged oil wells an ever growing problem that is both expensive and a risk to the environment. [2] Costs of plugging up these wells range from 76 thousand USD to 1 million USD. [3] Under President Biden's Infrastructure Law, Texas was granted $25 million dollars to plug up 800 of the most dangerous orphan wells, but that is still a fraction of those posing a problem. [1] Another solution is to convert these abandoned oil wells into electricity producing geothermal plants. This essay explores what it would entail to build geothermal plants, how much electricity could be produced from those wells, and whether this is a viable option for electrical energy in Texas.
In order to start evaluating the possibility of geothermal in Texas, one first must calculate the geothermal gradient,the increase of temperature with the increase of depth of the earth, and surface temperature. Using well log data from 2500 wells in Texas, Wight and Bennet calculated the surface temperature and geothermal gradient. With a linear regression model, one can calculate the relationship between the temperature T in °C and earth depth d in m:
with 0.0311°C m-1 being the geothermal gradient of Texas and 23.005°C being the average surface temperature. [4] Therefore for an oil well around the depth of 1000 m we have
Oil wells in Texas run from 1500 m to 6000 m deep, which means that with this geothermal gradient, the geothermal power plants would need to operate with a geothermal resource temperature between 72.6°C and 209.6°C.
There are three major kinds of geothermal plants: dry steam, flash steam, and binary cycle. Binary cycles are the most efficient for mid and lower temperature geothermal plants, ranging from 70°C to 150°C. [4] Binary cycle plants are a closed-loop system that transfers heat between two fluids, one of which turns to vapor and runs the turbine to generate electricity. In a review of over 20 separate case studies completed, 19 of the 20 cases used a binary cycle power plant. [3]
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| Fig. 2: Schematic of proposed closed loop binary cycle plant. [4] (Image source: A. Kornfein) |
With binary power plants, the fluids and steam from a geothermal reservoir never come in contact with generator units. The closed loop system releases virtually nothing into the surrounding environment. [4] A retrospective study from the DOE on Binary Cycle plants showed that binary cycle plants are 15% more efficient than flash plants in the temperature range of 150°C to 190°C. [5] There is already a precedent for binary cycle plants at lower level temperatures, including the Chena binary cycle plant in Alaska operating at 73.33°C and a mass flow rate of 33.39 kg/s. [4] Binary cycle power plants are already successfully used to generate electricity in co-produced geothermal fluids in onshore oil wells. The co-produced method 280 kW is being produced by the Rocky Mountain Oilfield Testing Center and 310 kW at Huabei Oilfield in China, both aircooled systems. [4]
One can use water, instead of co-produced geothermal fluid to cycle, inside the binary cycle power plants. This has multiple advantages including not tying the production of electricity to actively pumping oil wells and mitigating environmental consequences associated with geothermal fluid leakage. [4] The efficiency of binary power plants using water is 9-13%. [4]
Using the geothermal gradient and surface temperature, Wight and Bennet were able to calculate the mass flow rates as well as the expected output of a converted oil well power plant. This is seen in Table 1.
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| Table 1: This table shows the possible net power production, including power draw from pumping the water, possible with the Binary Cycle Geothermal oil well conversion. [4] |
Looking at these figures, in order to have the production potential of 1.086 MW of power, one would need to convert 10 oil wells of a depth of 4200 m. To put that in perspective, the total amount of energy that Texas generates in a year is 525,562,940 MWh. [6] (See Table 2.) This means the average power capacity on the Texas grid is (525,562,940 MWh/year) / (365 days/year × 24 hours/day) = 60 GW. This is about 40% of the maximum summer power of 149 GW. If one wanted to produce 60 GW, one would need 552,486 wells of 4200 m, which is not realistic. Fortunately, with Texas vast solar and wind power production ability, geothermal only needs to be a viable replacement for the baseload capacity to make a significant impact.
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| Table 2: Net Summer Capacity and Net Generation of Energy in Texas in 2011 and 2022. [6] |
Another benefit of converting oil wells to geothermal energy plants is the reduction of risk in investors for starting geothermal projects. A huge obstacle for geothermal expansion is the drilling costs. According to Wight and Bennet, drilling constitutes generally 44% of the total project investment. In geothermal projects like the Geysers in California, it was reported that a typical well doublet that can support only 4.5 MW could cost up to 10 million dollars with a 20% failure rate. [7]
The operating costs for geothermal plants remain rather low, but the investment in drilling new wells for geothermal energy is a significant barrier to the expansion of geothermal energy as a resource. [4] By utilizing orphaned wells, the initial investment for geothermal is significantly reduced because the characteristics of the geothermal resource are already accounted for and the well already drilled.
Utilizing binary cycle geothermal power plants appears to be an innovative and viable way to deal with old and abandoned oil wells. While the power generation is on the smaller scale of generation plants, the cost of plugging old wells in combination with the benefits of utilizing already drilled infrastructure make it a realistic solution. Converting oil wells as geothermal plants could be a good option for Texas in the future to provide a reliable, renewable base load electricity for the grid.
© Adri Kornfein. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] E. Douglas, "Texas Will Plug 800 Abandoned Oil and Gas Wells," The Texas Tribune, 26 Aug 22.
[2] J. Malewitz, "Abandoned Texas Oil Wells Seen as Ticking Time Bombs of Contamination," Texas Tribune, 21 Dec 16.
[3] R. Ashena, "Analysis of Some Case Studies and a Recommended Idea For Geothermal Energy Production From Retrofitting Abandoned Oil and Gas Wells," Geothermics 108, 102634 (2023).
[4] N. M Wight and N. S. Bennet, "Geothermal Energy From Abandoned Oil and Gas Wells Using Water in Combination With a Closed Wellbore," Appl. Therm. Eng. 89, 908 (2015).
[5] M. Gallaher, A. Rogozhin, and J. Petrusa, "Retrospective Benefit-Cost Evaluation of U.S. DOE Geothermal Technologies R and D Program Investments: Impacts of Cluster of Energy Technologies," U.S. Office of Energy Efficiency and Renewable Energy, August 2010.
[6] "State Electricity Profiles 2011," U.S. Energy Information Administration, April 2014.
[7] C. Halász, "The Calpine Geysers," Physics 240, Stanford University, Fall 2011.
[8] L. A. Burke, S.. Kinney, and T. B. Kola-Kehinde, "Digital Archive of Drilling Mud Weight Pressures and Wellbore Temperatures from 49 Regional Cross Sections of 967 Well Logs in Louisiana and Texas, Onshore Gulf of Mexico Basin," U.S. Geological Survey, Open-File Report 2011-1266, 2011.