Subsurface Enhancement and Sustainability

The heat resource in the United States is vast and greatly exceeds geothermal systems that can currently be used economically through targeted drilling and subsequent production of hot fluids for power generation or direct use. Growing geothermal electricity generation to 60 GW_e by 2050, as outlined in the GeoVision analysis, will require developing these heat resources by improving sub-economic naturally occurring hydrothermal systems or developing fully engineered geothermal reservoirs.

The science and engineering knowledge and technology base must be improved to better understand and predict how a reservoir will respond and evolve when subjected to operations that modify the permeability of the reservoir. Numerical tools exist to support these efforts; however, the subsurface is a complex, heterogeneous, and anisotropic environment, and refinement of these tools is a continuous effort. The complexity is amplified by the coupled nature of physical processes where stress, temperature, hydrology, chemistry, and biology can have marked impacts on system response during efforts to improve heat exchange and long-term operation. Data from laboratory, intermediate, and full-scale testing have been illuminating in supporting development of methods to predict reservoir response; as more data become available, the ability to predict reservoir response will continue to improve.

Technologies to develop and manage an enhanced reservoir are common in the oil and gas industry, but systems and knowledge to support analogous operations in geothermal environments are lacking. Developing these systems and knowledge is essential to successful EGS deployment, including greenfield EGS project development as well as near-field EGS efforts directed at capacity expansion of operating hydrothermal fields. For example, methods to enhance geothermal reservoirs through hydraulic stimulation have largely eschewed zonal isolation technologies, where select sections of the wellbore are isolated from the remainder of the well due to the limited availability and cost of suitable systems. However, it is now understood that targeted zonal isolation will facilitate purposeful enhancement approaches and is required for the development of EGS. In addition to zonal isolation, technologies and techniques for enhancement are often not applied consistently in geothermal environments. Hydraulic and chemical stimulation methods have been used in the past; however, sequences of stimulation operations, pumping rates, and pumped volumes of fluids are, at best, subjective. The chemistry of stimulation fluids and their coupled interaction with the reservoir is a known issue, but the ability to exploit potential benefits and mitigate adverse effects has not been developed to allow engineering control of the operation. Alternative methods such as the use of energetic systems to supplement traditional hydraulic stimulations may have promise but are not adequately understood in a geothermal context. Flow control—other than the running of casing and liners—is almost nonexistent in geothermal production; for greenfield EGS, however, some type of active or passive flow control system will be required.

The ability to characterize the reservoir during operations is key to understanding reservoir performance and, subsequently, the ability to intervene and modify reservoir response. The use of microseismic monitoring to characterize the results of reservoir enhancement operations was pioneered by the Energy Research and Development Administration (which later merged with the Federal Energy Administration to form the DOE organization, including GTO) and remains the primary method for monitoring the evolving permeability (fracturing) in the subsurface. Microseismicity monitoring is not only useful in characterizing the reservoir, but is an essential component to ensuring that reservoir enhancement operations are not a hazard or a nuisance to the public. Other monitoring activities such as ground deformation (e.g., GPS and InSAR) and tracer studies—as well as surface and downhole measurements of production and injection well-flow rates, pressure, temperature, and chemistry—all provide key data whose availability are of great utility to operators. Timely data are critical to the ability to respond to changing reservoir conditions.

At least for the near future, microseismic monitoring will be the primary method to track evolving reservoir conditions. The capability of microseismic monitoring has progressed and improved over the last several decades, and the infusion of fiber-optic methods and high bandwidth digital geophones and accelerometers have improved capabilities. Additional areas for improvement remain, both in terms of sensor capability (namely temperature hardening and sensor placement) and the ability to reduce and interpret the data in near real time. Complementary monitoring systems that provide more direct measurements of fluid flow exist (e.g., tracers and electro-magnetic geophysical tools) but improvements that provide timely, continuous, and real-time data at actionable resolution continues to be a need.

GTO has performed RD&D across the spectrum of subsurface enhancement and sustainability, with significant progress made over the last few decades. Challenges remain, however, and it is important to pursue focused efforts to improve scientific understanding, the tools needed for enhancement, and the ability to understand reservoir evolution.

Table 2.5 highlights GTO subprogram contributions in Subsurface Enhancement and Sustainability RD&D toward meeting overall GTO program goals. The majority of the planned RD&D on reservoir response, reservoir development and management, and reservoir characterization and monitoring will be through the EGS, Hydrothermal, and Low Temperature subprograms. The RD&D in this Research Area as applied to EGS will directly achieve all three Strategic Goals, while as applied to the Hydrothermal and Low Temperature subprograms will directly achieve Goals 2 and 3 only. The DMA subprogram also enables research insights through secondary contributions to all Strategic Goals.

Table 2.5. GTO Subprogram Contributions in Subsurface Enhancement and Sustainability RD&D for Meeting GTO Strategic Goals

Table 2.6 outlines key GTO performance goals through FY 2026 to support the ability to characterize and sustain geothermal reservoirs.

 Table 2.6. Subsurface Enhancement and Sustainability Highlighted Performance Goals

Reservoir Response
An essential component in enhancing and sustaining a geothermal resource is the ability to predict the response of the reservoir to operations in the subsurface. Predicting the response of any system to disturbances requires a model of the system and data to support model development and validation.

Better predict response through laboratory and field testing and observations. The ability to predict reservoir response to enhancement operations (e.g., hydraulic stimulation) and evolution during production is required to effectively design and implement intervention strategies in support of EGS, hydrothermal, and low-temperature reservoirs. Both naturally formed and human-engineered geothermal reservoirs will evolve in response to the perturbations to which they are subjected, from putting drilled wells into production to full-scale stimulations in greenfield EGS projects. Better predicting reservoir response is an important RD&D need and will require testing at full scale with detailed characterization and monitoring to test and validate model capability.

Improve coupling of numerical/analytical modeling and validation. GTO has made significant investments developing analytical and numerical codes for predicting geothermal reservoir evolution. The next need is to develop data to validate these codes and refine constitutive models that represent physical processes in these codes. As noted, thermal, hydrological, mechanical, and chemical (and sometimes biological) processes in the subsurface are complex and highly interdependent. The ability to predict these coupled processes can be derived through assessment and analysis. Related data needs are best defined by integrating experimental activities with the modeling community and at scales ranging from laboratory to full-scale field. GTO research will seek to improve predictive capabilities through integrated testing and simulation, in turn enhancing the ability to predict and constrain reservoir response in difficult environments.

Reservoir Development and Management
Enhancing and operating a geothermal reservoir requires developing technologies and techniques to allow targeted operations that control locations within a wellbore for stimulation activities as well as zones where fluids are injected and withdrawn. In addition to targeted stimulations and flow control within wells, managing geochemical interactions, particularly scale formation in the subsurface and surface equipment, is required for long-term operations.

Investigate advances in stimulation technologies and techniques. Enabling technologies for geothermal reservoir enhancement are required for EGS development. These technologies may necessitate new well designs and completion schemes, new tools, injectate chemistries and materials, pumping schedules, and other changes. Advancing stimulation technologies and techniques is a major hurdle in advancing geothermal development toward the potential 60 GW_e of geothermal electricity-generation capacity and 320 gigawatts-thermal (GW_th) of economic district-heating potential outlined in the GeoVision analysis. Relative to the oil and gas industry, the geothermal space is very small and opportunities to advance stimulation technologies through trial and error are not realistic. Modeling and simulation supported by laboratory and field testing will play a vital role in identifying promising technologies and techniques for reservoir stimulation. Replicability of stimulation methodologies across all potential environments is not likely but finding approaches to identify the stimulation methodologies appropriate to the environment is a reasonable and achievable goal. Modeling and simulation will support this goal but modeling alone will not advance this capability; innovation in stimulation methods is also needed.

These advances will also require improvements in rock-mass characterization before, during, and following reservoir enhancement operations. A holistic approach that includes modeling and simulation to support developing advanced technologies and techniques is vital to find adaptable reservoir-stimulation technologies and techniques.

A crucial issue that must be addressed for long-term geothermal operations is the ability to engineer solutions to control mineral scale in the well, reservoir, and surface equipment. Scaling issues in the subsurface are largely dependent on the chemical makeup of injected fluids and the mineralogy of the reservoir rocks. Testing and reservoir models that incorporate chemistry (thermo-hydraulic-mechanical-chemical models) will be vital to understanding and controlling scaling in the subsurface. Amorphous-silica deposition in surface equipment and injection wells remains a broad issue, particularly in geothermal systems operating above 200°C. Efforts to better define the kinetics and hydrodynamics of silica scaling will be essential to developing engineered controls.

Assess and test zonal isolation and downhole flow control. Efforts to employ open-hole stimulation approaches have thus far had limited success. Investigations over the last decade have confirmed that a robust approach to reservoir enhancement and sustained operations will require the ability to both isolate zones with the subject wellbores during stimulation activities and to control the flow into and out of the isolated sections of the wellbore. There are a range of technologies that support these operations in the oil and gas industry, but temperature limitations and wellbore diameters severely limit the application of these technologies in geothermal environments. Developing zonal-isolation and flow-control technologies is imperative to EGS, and advances in these technologies will provide useful reservoir-control options not currently available to the developers of conventional hydrothermal geothermal systems. Advancements in these critical technologies is a priority.

Reservoir Characterization and Monitoring
Reservoirs evolve over time, and understanding this evolution requires acquiring and assessing site data across all phases of development and operations. The ability to respond to reservoir changes requires that these data be processed and analyzed in a manner that is useful and timely; as data volumes expand, this need is particularly critical. Tools and sensors have advanced, but enhancing measurement capabilities and implementation methods is critical to characterize and monitor geothermal reservoirs.

Conduct real-time data collection, analysis, and response. Understanding geothermal reservoir behavior prior to, during, and after development activities is critical to ensure that reservoir enhancement efforts are implemented effectually. Effective characterization and monitoring help minimize capital and operational risks associated with drilling and stimulation of deep wells and are essential to monitoring reservoir performance and evolution over time. Interrogation methods, systems, and technologies must be tailored to the operational needs of geothermal reservoir; however, the utility of these data is dependent on the ability to assess and act in a timely manner.

Implementing advanced data processing and data analytics is necessary to allow timely responses to characterization and monitoring activities. This includes the integration of orthogonal data sources (e.g., microseismic and EM signals) where real-time integration and fusion of data is currently problematic. These needs are particularly important for reservoir stimulation where the ability to adjust operations in response to real-time monitored data is limited and yet critical for successful reservoir enhancement; it is also imperative for monitoring seismic hazards. As the data volume expands (e.g., distributed acoustic sensing), the need for rapid acquisition and analysis is becoming increasingly important and RD&D to ensure that collected data is useful to operational activities is required.

Develop advanced monitoring and characterization systems. There are numerous measurements that, depending on operational needs, are important for either directly or indirectly assessing reservoir performance. Sensing systems deployed in the subsurface are subject to many of the same temperature limitations as other subsurface technologies; this is a critical issue for all downhole systems, including logging tools, monitoring sensors, power sources, and telemetry systems. Temperature limitations need to be addressed across the board. Of note, and because of the importance of microseismic monitoring, is the lack of accelerometers and geophones that will function long-term at temperatures in excess of 225°C. If such equipment is not developed by industry, GTO will need to support RD&D in that direction. Fiber-optic-based sensing systems for measuring a variety of properties (e.g., temperature, pressure, and strain) are expanding rapidly. These systems can produce enormous amounts of data and methods to effectively handle those data need to be advanced; this includes the application of machine-learning algorithms to process and interpret data. Opportunities to advance the integration of monitoring systems with completion technologies should also be investigated (e.g., instrumented casing). Advanced tracers and associated diagnostics to inform fluid-flow paths and contact time are candidates for additional development. In addition, surface, airborne, and space-based technologies for reservoir monitoring should be assessed and integrated into characterization and monitoring systems.