Session 5C: Reservoir/Production

The Mammoth Geothermal Complex has been in operation since 1984, and during this timeframe it underwent several expansions and repowers prior to the 2022 commissioning of the Casa Diablo IV station (CDIV). Due to its proximity to population centers, groundwater wells, hot springs, and thermal creeks, a fit-for-purpose monitoring network was installed ahead of field expansion to understand potential environmental impacts. Ormat cooperated with the Bureau of Land Management, the U.S. Forest Service, the U.S. Geological Survey, and the Mammoth Community Water District to collect and analyze baseline data prior to drilling new production and injection wells to support the CDIV project. During the CDIV commissioning, this monitoring network was reviewed as output from the new geothermal production wells was ramped up in incremental steps over a four-week period. The combination of strategic downhole pressure monitoring, geochemical monitoring, temperature monitoring, and ground leveling provided confidence that there would be no measurable impacts to nearby water resources, including thermal features and municipal water supply wells from CDIV operations. Now two years into operation, the water monitoring network continues, and no impacts have been identified.

It is essential to optimize the sustainable utilization of geothermal resources, considering the current growth trend of the geothermal energy sector, to achieve long-term sustainability and profitability. This involves aligning with market demands, optimizing field management and increasing efficiency, which includes maintaining reservoir pressure, optimizing well production, and minimizing environmental impacts of the utilization.

This study aims to increase the understanding of the production from geothermal reservoirs by using production history data and well characteristic curves to analyze the impacts on the resource by using stock reservoir modeling. This mathematical modeling technique can predict how the reservoir might behave over time in response to production activities. These models can be useful for optimizing production strategies, such as the need for drilling make-up wells, reinjection, and scheduling to ensure energy production and long-term resource sustainability.

The main goals of the research are 1) to apply stock modeling to a variety of wells with different characteristics and production histories to better understand the method’s versatility and 2) to analyze the applicability of using stock reservoir modeling for wells with interconnected productions. Specifically, wells from the Þeistareykir geothermal field in North Iceland and the Hverahlið geothermal field in Southwest Iceland are used as case studies to gain insights into their unique characteristics and production histories by applying the model to them.

The Salton Sea Geothermal Field (SSGF) reservoir is renowned for its high-temperature, high-salinity brine, the existence of a lower-salinity reservoir above the hypersaline reservoir and complex thermodynamic behavior. While previous conceptual models have assumed a largely homogeneous reservoir with minor variations along known depths (-4,000 to -9,000 ft ASL), this paper presents an alternative view.

This new model incorporates the key elements of a conceptual model of a hydrothermal system; heat source, permeable pathways, fluid recharge, characterized reservoir, and fluid discharge into shallow hydrology. Each of these elements can be identified in the geoscience datasets. The Salton Sea hydrothermal system has an interesting nuance, a double-diffusive convection system (DDCS). The reservoir physics requirements for a DDCS impose an internal constraint, the maintenance of a relatively consistent vertical fluid density of around 1.0 g/cm3 at all conditions of temperature and pressure, which in turn requires an abundant source of chloride salts. The strongly convective system exhibits decreasing temperature and salinity from the bottom to the top and from the side to the center of the anomaly, consistent with a DDCS.

The conceptual hydrothermal model accounts for known geological features such as the active pull-apart basin and sedimentary changes along the Salton Sink, explaining the presence of highly permeable zones in transitional and metamorphic depths of the Salton Sea reservoir, surface manifestations such as mud pots, and the Imperial CO2 field. This study correlates the conceptual model with the known data from what is believed to be the southeasternmost part of the field where the Hudson Ranch Power I project is located.

Geothermal energy, a sustainable and renewable resource, plays a vital role in the global energy supply. To guarantee peak performance and longevity of geothermal wells, it is essential to implement a wellbore operation and maintenance strategy.

A fundamental component of this strategy is scale management attributed to mineral precipitation from the brine during the reinjection and production process. The accumulation of scale deposits significantly impacts the lifespan and efficiency of geothermal wells by restricting fluid flow, thus reducing wellbore performance. To address these impacts in a geothermal district heating (GDH) system, it is important to administer forward-looking maintenance plans before manageable fluids become unmitigated outages.

The GDH system in the Town of Lakeview, Oregon plays a critical role to heat local schools, the community hospital, the emergency services station, and a dental clinic. By using geothermal energy, Lakeview offsets the expense and environmental impact of traditional fuels like propane, ensuring cost-effective and sustainable heating for the community. The functionality of the GDH was impacted by severe iron oxide-hydroxide scale deposits that had developed over time. In recent years, the scaling drastically reduced the flow rate from 160 gallons per minute (gpm) down to 88 gpm. To efficiently meet the needs of these community buildings, a production rate of 130 gpm is required.

To restore these essential services, various methods for scale removal were considered and electro-hydraulic pulse technology was applied. The targeted pulsing technology in Lakeview achieved breakthrough results in scale removal, resulting in an increased production rate of 206 gpm. The Lakeview case study underscores the importance of regular maintenance and innovative solutions to improve the well production and increase the lifespan of geothermal wells.

Atmospheric cleanout flows and flow tests are standard practices at geothermal projects around the world. These tests are typically conducted after new wells are drilled and act both to cleanout drilling debris from permeable zones and provide initial indications of well performance and reservoir characteristics. However, cleanouts and flow tests have not been conducted at the Puna Geothermal Venture field since 2006 due to complex permit requirements and risks associated with hydrogen sulfide gas and related abatement chemicals. This challenge led to the practice of starting new wells directly to the power plant, which resulted in issues with both well performance and plant equipment. In 2022 an upgraded flow test facility was designed and following completion of two new production wells in 2023, atmospheric cleanouts were safely and successfully conducted using the upgraded facility and improved abatement practices. The tests were completed within permit constraints and ultimately led to increased power generation and improved well performance.

Surface thermal features, most notably hot springs and geysers are increasingly being recognized in the U.S. for their importance to ecosystems, indigenous cultures, agriculture, recreation, and tourism. Combined with an unprecedented push to develop renewable energy projects across the U.S., concerns about impacts to these features from geothermal energy projects has resulted in development of tensions, an increase in regulatory scrutiny, and greater stakeholder opposition to geothermal energy development in some locations. Opposition and increased permitting and monitoring requirements have largely resulted from historical examples of negative impacts, most of which were associated with flash steam plants developed prior to 2000 that caused significant fluid/mass and losses from the reservoirs. However, since 2000, nearly all geothermal capacity additions have been binary plants which re-inject 100% of produced geothermal fluids back into the reservoir. With this trend expected to continue, impacts to surface thermal features will be significantly reduced, and possibly eliminated when field development exclusively uses technologies that minimize water losses (e.g., binary cycle power plants), and reinjection strategies that incorporate preservation of surficial flows as a management factor.

Current regulatory requirements for assessing and managing risks to surface thermal features during permitting, exploration, development, and operations are somewhat inconsistent and unpredictable across different geothermal fields. In some cases, potentially excessive or unnecessary monitoring requirements that may be limited in their ability to provide meaningful insights have led to uncertainty and increases in exploration risk for geothermal energy developers including project delays, cancellations, or hesitation to commit. Varying regulatory requirements can also influence public perception and foster confusion, distrust, and opposition to geothermal projects. This comes at a time when there is an increasing urgency for reliable sources of clean energy, and geothermal can provide a net-zero, renewable solution. Continued integration of geothermal energy into the national energy roadmap can be facilitated through consistent and predictable permitting, providing regulators the framework they need, giving developers a clear path forward, and creating the transparency that the public deserves to feel sure that these resources are being protected.

This project, currently in its beginning phases, seeks to address this important issue by providing a technical basis from which to build a preliminary protocol for assessing and managing potential impacts from new or existing geothermal energy projects to surface thermal features and their associated ecosystems. Development of this preliminary protocol will be informed by (1) literature reviews of well-documented case studies in the western US and internationally to understand the range of conditions that exemplify geothermal-surface thermal systems; (2) development of illustrative conceptual numerical models to quantify, understand, and predict the first-order controls (e.g., pressure and permeability) on surface flows; and (3) additional independent case studies using industry-provided and publicly available data to understand and demonstrate the risk profiles and cause-and-effect relationships between geothermal power production and impacts to surface thermal systems.

Learning from the successes of the process used to develop the Induced Seismicity Management Protocol (ISMP) and the example of Geothermal Interagency Task Force, we ultimately aim to use these initial efforts as a springboard for establishing a surface thermal feature management working group that will collaboratively refine the protocol as well as co-develop recommended best practices for implementation. We envision that the working group will be composed of representatives from diverse stakeholder groups (e.g., regulatory entities, industry, government agencies, Tribes, academia, national laboratories), and will include early and regular engagement with community organizations and environmental groups to understand and meaningfully incorporate their input into the final protocol and best practices for implementation. This will help ensure broad acceptance and implementation of the protocol, which will facilitate a more consistent, and efficient regulatory process, and ultimately help to ensure that geothermal energy continues to provide a reliable source of clean energy and a pathway to achieving greater energy equity in the U.S.