This paper is the progress report for the last two years of a three-year DOE funded project aimed at developing and constructing cost-effective hydraulic fracturing tools. These tools are intended to enable zonal isolation for subsurface heat exchange in Enhanced Geothermal Systems (EGS) field research at the Utah-FORGE site. The project has focused on the development of an economic, scalable completion method for EGS using long-reach, horizontal geothermal wells. The use of cemented sleeves allows the casing to serve as both the frac string and the injection/production string with conformance control—a critical advantage over the existing ‘Plug and Perf’ completion methods. This report describes the development and testing of a Wellbore Tractor for use in temperatures up to 225 °C (440 °F) to effectively detect and control flow.
The development and testing of the sleeve and tractor required component level testing with separate Seal Drag and Collet Test Fixtures. These fixtures were used to iteratively design and test sleeve seal systems for sealing capabilities and drag forces, as well as a Collet Test Fixture to test ball catch/pass capabilities. Tractor prototype components were tested "dry" and simulated wellbore conditions. Fracture and reservoir modeling were used in the system design process. The tool development was complemented by evaluating the performance of the Utah-FORGE hydraulic fracturing field tests and the published results of the Fervo Project Red geothermal trials at Blue Mountain in Nevada.
Results from testing demonstrated the Collet would always "catch" the ball at 8 bbl/min or less and would always "pass" the ball at 11-12 bbl/min. Testing was highly repeatable over four days of testing with no damage to the ball. Nineteen seal drag tests were performed with final, repeatable drag forces of less than 400 lbs. at 440 °F, 2000 psi differential pressure. Tractor components were successfully tested with a flask in 12-hour duty cycles at 225 °C (440°F) wellbore temperature. Final tractor and sleeve designs were completed with fabrication underway for system integration testing (TRL-6) and field testing (TRL-8). Numerical and analytical modeling of field data established the existence of low permeability in the granitoid and high permeability in the induced hydraulic fracture. Economics modeling demonstrated the importance of conformance control to maintain a balanced flow distribution in all hydraulic fractures, magnitude of the flowrates, and surface area of the heat exchange drainage volume.
This paper outlines the tools and methods needed to achieve an economic EGS. Specifically, a large subsurface heat exchanger can be constructed with multi-stage fracturing using cemented sleeves with the speed of ‘Plug and Perf’ techniques. Unlike horizontal wells completed with ‘Plug and Perf’, the cemented sleeve and tractor results in highly improved conformance control to equalize heat recovery.
