Background
Enhanced or engineered geothermal systems (EGS) extracts energy from untapped and imperfect hot rock which is accessible and abundant in the western US. Therefore, EGS offer tremendous potential as an energy resource supporting the energy security of the United States with estimates exceeding 500 GWe for the western US surpassing the resource base hosted by conventional hydrothermal systems [Williams et al., 2008], to an order of magnitude more [Augustine, 2016] for the entire United States. In order to meet these growing energy needs, further research is needed to improve extraction technologies and reach commercial viability of EGS.
Technological challenges associated with developing EGS include:
- Lack of a thorough understanding of techniques to effectively stimulate fractures in different rock types and under different stress conditions to communicate among multiple wells.
- Inability of techniques to image/monitor permeability enhancement and evolution at the reservoir scale at the resolution of individual fractures
- Limited technologies for effective zonal isolation for multistage stimulations under elevated temperatures
- Lack of technologies to isolate zones for controlling fast flow paths and control early thermal breakthrough
- Lack of scientifically-based long-term EGS reservoir sustainability and management techniques.
To address these challenges, the DOE Geothermal Technologies Office initiated the EGS Collab project. The project focuses on understanding and predicting permeability enhancement and evolution in crystalline rock, including how to create sustained and distributed permeability for heat extraction from the reservoir by generating new fractures that complement existing fractures. The EGS Collab project will involve direct collaboration between the geothermal reservoir modeling community, experimentalists, and geophysicists in developing and implementing well-field characterization and development, monitoring, and stimulation methods.
Objective
The objective of this project is to increase the understanding needed to efficiently implement enhanced geothermal systems (EGS) through creating a collaborative research environment to study stimulation of crystalline rock at the 10 meter scale. Key to this effort is the collection of high quality data during stimulation and flow tests to allow comparison to numerical coupled process models in an effort to build confidence in the codes and modeling techniques used.
Location
Understanding local geology including stress magnitude and orientation; and natural fracture abundance and orientation is critical in performing suitable stimulations. Experiment 1 is being performed on the 4850 (feet deep) level at the Sanford Underground Research Facility (SURF, Figure 1) in Lead, South Dakota [Heise, 2015]. As a former gold mine and current underground laboratory, SURF has been reasonably well characterized (e.g., Hart et al., 2014), and provides infrastructure (e.g., ventilation, power, water and internet) in addition to cost-effective proximal monitoring of a deep crystalline rock mass before, during, and after stimulation through multiple boreholes drilled from an underground tunnel.
A Homestake Mine model with the conceptual designs of testbeds 1 and 2. Data for mine levels were from
http://homestake.sdsmt.edu/DXF/DXF.htm (William Roggenthen, SDSMT). Testbed designs are from Joseph Morris of LLNL. The model was built and animated by Pengcheng Fu of LLNL.
Overall Plan
The project has planned three multi-test experiments to increase understanding of:
- hydraulic fracturing (Experiment 1- now complete)
- shear stimulation (Experiment 2 - design underway)
- other stimulation methods in Experiment 3.
Each series of tests within an experiment begins with modeling to support experiment design, and post-test modeling and analysis are performed to examine the effectiveness of our modeling tools and approaches. By doing this, we can gain confidence in and improve the array of modeling tools in use.
Experiment 1 was intended to establish a fracture network to connect an injection well and a production well using hydraulic fracturing. In Experiment 1, we performed several highly monitored hydraulic fracture stimulations and flow tests, and implemented a suite of rock/reservoir characterization methods potentially useful for EGS systems, as well as other methods available to improve understanding.
Experiment 2, examining shear stimulation, is currently being planned and will be performed in the Yates amphibolite at the 4100ft surface at SURF. A vertical and a short horizontal borehole have been cored to investigate fracture and rock conditions, and a number of mini-frac tests and SIMFIP tests have been performed in the vertical hole to identify the directions of the stresses and the magnitude of the minimum principal stress. A number of fracture sets have been identified in the core and logging. These data were used to optimize the orientation of the injection well to induce a shear stimulation of the rock.
Experiment 3 is yet to be executed but will test other simulation methods under different conditions/environments than experiment 1 and 2.
