Subsurface Accessibility

Introduction

Subsurface access through drilled and completed wells is required for all forms of geothermal energy exploration, characterization, and development. The costs of accessing the reservoir are an important determinant of the economic viability of geothermal energy projects. Reducing those costs is paramount in achieving the geothermal energy potential across all uses of geothermal energy outlined in the GeoVision analysis and ultimately contributing to a net-zero emission economy by 2050.

Well construction in geothermal environments is often hampered by low drilling rates and, as such, the time it takes to construct a well. This is historically the case of wells drilled for geothermal electricity-generation projects. In the western United States, average daily drilling rates are commonly on the order of 150-250 ft/day, about an order of magnitude lower than rates associated with continental oil and gas development in shale rocks. The rock in western U.S. geothermal projects is hard, fractured, and abrasive, making it a challenge to use advanced fixed-cutter bits (i.e., those with polycrystalline diamond compact [PDC] cutters. Formations are also often underpressurized, with low formation fluid pressures; this leads to circulation loss during drilling and can cause additional costs in terms of flat time (i.e., non-productive time, material costs, and loss of drilling equipment). State-of-the-art drill rigs and associated higher daily rig rental costs are commonly eschewed due to the tight margins associated with many geothermal development projects.

Geothermal wells are commonly much larger in diameter than those drilled for oil and gas—often by a factor of more than two. While slow drilling rates of geothermal wells contribute to their high cost relative to oil and gas wells, the cost of steel and cement associated with larger geothermal well diameters is also a major contributor to the high costs of geothermal wells. These materials (primarily casing and cement) are emplaced during geothermal well construction and account for as much as 50% of the cost of a geothermal well (Lowry et al. 2019).

Enabling technologies—from electronics to elastomers that can survive harsh conditions—are fundamental to addressing the environment associated with geothermal drilling. Technologies that perform in high-temperature, high-pressure, high-shock, corrosive environments are required for a wide range of downhole tools needed for drilling, logging, and monitoring of geothermal wells. Additionally, consumables such as lost-circulation materials are ripe for improvement in geothermal conditions. Developing these base technologies will enable the building of systems needed to construct and operate geothermal wells.

GTO has and will continue to pursue efforts to reduce the time and cost associated with the drilling of geothermal wells. Efforts in this area will continue to include investments in base technologies that enable developing tools that are more resilient in the extreme environments associated with accessing geothermal reservoirs.

Table 2.3 highlights GTO subprogram contributions in Subsurface Accessibility RD&D toward meeting overall GTO program goals. The majority of the planned RD&D on drilling time, well components, and enabling technologies will be through the EGS, Hydrothermal, and Low Temperature subprograms in support of Strategic Goals 1, 2, and 3. The DMA subprogram also enables research insights through secondary contributions to all Strategic Goals.

Table 2.3. GTO Subprogram Contributions in Subsurface Accessibility RD&D for Meeting GTO Strategic Goals

Highlighted Performance Goals

Table 2.4 outlines key GTO performance goals through FY 2026 for enabling better drilling and completion of geothermal wells.

Table 2.4. Subsurface Accessibility Highlighted Performance Goals 

Research and Development Pathways

Drilling Time

The costs associated with well construction are largely dependent on the time required to complete a well and the cost of materials used in construction. Geothermal well-advancement rates are generally slow, so one aspect to reducing costs is reducing the time required. Reducing the time it takes to complete a well to depth can have a substantive effect on the cost of drilling a well, particularly with drilling rates as low as they are for geothermal wells; doubling the average daily drilling rate can have a 10–15% savings on the total cost of a geothermal well. Improving drilling time is a challenge, however, because of difficult drilling conditions and an environment in which the advanced technologies used by other industries are either not available or are too expensive to justify under current geothermal economic conditions. Addressing drilling time requires RD&D in two intricately linked areas: improving the hole advancement rate (rock reduction) and reducing flat time (i.e., the time during which the hole is not being advanced).

Improve rock reduction rate. Currently, most geothermal wells are drilled with hard-rock roller bits—the workhorse of the drilling industry for nearly a century. However, in the 1970s, a synthetic diamond-cutting structure, known as PDC, was invented, with ready application to drill bits. As PDC bit performance has improved, the adoption of PDC bits in the drilling industry has been steady. In fact, PDC bits are now the mainstay of the oil and gas drilling industry because of the higher drilling rate and longer life they exhibit relative to roller bits. While GTO was an early and significant sponsor of RD&D related to PDC drill bits, the geothermal industry has generally not borne the fruits of these advances because of the nature of the rock and temperatures associated with geothermal environments. In general, geothermal reservoir rocks are stronger (harder) and more fractured that those found in oil and gas reservoirs. However, with improvements in these bits and evidence that they can provide a step-change in performance (Hackett et al. 2020), renewed and vigorous efforts to test and deploy advanced drilling structures are justified in geothermal environments.

In addition to fixed-cutter PDC bits, percussive drilling also offers rock reduction rates well in excess of that obtainable from roller bits, particularly in hard-rock environments. There are aspects of “hammer drilling” that were traditionally not compatible with geothermal environments, but advances in GTO-sponsored research in air-driven hammers and commercial advances in water-driven hammers offer opportunities to leverage those capabilities to improve rock-reduction rates for geothermal drilling.

The drill bit is just one portion of the bottomhole assembly, i.e., all the drilling tools that sit below the primary drill string. Bottomhole assembly components comprise motors, steering systems, logging-while-drilling/measurement-while-drilling tools, shock subs, and other tools. These tools are used to improve drilling performance and reduce flat time, and many incorporate seals and electronic and power components that limit the temperature environments in which they can operate. There are limited options for geothermal drilling using a subset of the tools available to the oil and gas industry but for sustained operation at temperature above 150–175°C. Additionally, some potentially useful tools are not compatible with larger-diameter geothermal wells. Opportunities to adapt, modify, or develop geothermal-compatible bottomhole assembly components should be investigated.

Geothermal industry access to advanced, modern drill rigs is limited due to tight margins associated with the low-value resource (compared to hydrocarbon). GTO will seek opportunities to support limited trials with more advanced and modern systems to determine if better and more costly equipment could result in lower overall project costs. Such testing would also better identify the “limiters” in performance that need to be overcome to drive down costs, particularly related to time. An early indicator of the promise in investing further in these opportunities was demonstrated at the Frontier Observatory for Research in Geothermal Energy (FORGE) in late 2020 through the use of a data-driven “physics-based limiter redesign workflow,” coupled with training of all personnel in use of this workflow and the use of PDC bits that reduced anticipated drilling time for the first-of-its-kind highly deviated well in granite by more than half.

Improve decision making while drilling. Equally important to RD&D directed at adopting modern rock-reduction technologies is RD&D to improve the decision-making process during drilling. The use of digitally acquired surface and downhole drilling data to diagnose drilling performance is used throughout the oil and gas industry but has seen little adoption in the U.S. geothermal industry—yet this physics-driven approach to drilling control has been shown to have a major influence on drilling performance. This approach begins with acquiring and acting on the mechanical specific energy (MSE) associated with the drilling process, i.e., the measure of the energy required to remove a unit volume of rock. Dupriest (2011) demonstrated a 40% improvement in average hole-advancement rates by making decisions based on MSE. Adopting a workflow that incorporates decision-making digital data in geothermal drilling is important and is necessary to advance the use of PDC bits. The use of MSE data to control drilling not only improves instantaneous drilling rates, but also reduces flat time caused by minimizing energy that is not directed at breaking rock (e.g., deleterious vibrations that cause tool failure). GTO will pursue RD&D directed at adopting and modifying this workflow in geothermal drilling operations.

Manage lost circulation and drilling fluid. Lost circulation, where fluids are lost to the formation during drilling, remains a critical issue in geothermal development due to the fractures and often underpressurized environments present in the subsurface. Lost circulation is a major cause of flat time (specifically non-productive time) associated with geothermal wells and can lead to wellbore damage, well control issues, and potential environmental impact. As such, GTO will necessarily continue technology development aimed to address lost-circulation prediction and control.

The design and management of drilling fluids can have a dramatic effect on the overall rate of penetration, and—other than mud weight drilling fluids—are often an afterthought in the design and execution of geothermal well construction projects. Drilling fluids are vital; they assist in lost circulation control, cool and clean the bit while drilling, lubricate the drill string, maintain stability of the wellbore, control formation fluid pressures, carry rock cuttings to the surface, and transmit hydraulic horsepower for driving downhole motors and other tools in the bottomhole assembly. There is a need for advances to support design of drilling fluid for geothermal applications and development of high-temperature additives, particularly those supporting lost circulation control. Additionally, advanced fluid management systems (e.g., managed pressure drilling) could play a role in improving hole advancement rates and reducing drilling time.

Improve casing and cementing. The time involved in casing and cementing wells has a dramatic effect on the overall rate in which a well is drilled. There are no obvious solutions to this issue, as it plagues all drilling operations. While additional training is always useful in gaining efficiencies, RD&D is warranted to conduct evaluations of options to reduce casing and cementing time—either through time-saving options such as casing while drilling or through methods to reduce the number of casing strings used to develop a typical geothermal well. Developing partnerships, such as with DOE’s Advanced Manufacturing Office and Office of Science, could bolster RD&D to reduce casing and cementing time. Linked to this effort, particularly for highly deviated wells, is understanding the condition of the well before running casing. Gauge wells with limited tortuosity and doglegs reduce the problems and costs of casing and cementing, ensuring the drilling process is efficient reduces problematic wellbore conditions.

Well Components

The primary component costs for geothermal wells are casing and cement. While bit and drilling fluid costs are also important, reducing the costs associated with the casing and cementing of geothermal wells is paramount to reducing costs of geothermal wells. Actual costs are well dependent, but materials and services associated with casing and cementing can be as high as 50% of the well cost.

Reduce casing costs. Geothermal well casing is fabricated from steel, nickel, or titanium alloys, with steel being the predominant material. Since commodity prices drive the cost of these materials, options to reduce these costs should focus on the use of alternative materials as well as the use of less material in the casing of geothermal wells. Research into reducing material use should investigate leaner casing designs that result in safe, long-term well operations; leaner casing designs would have the advantage of less material as well as less flat time associated with casing and cementing activities. While a less likely solution in the near term, alternative casing materials (or casing fabrication methods) deserve attention given recent developments in advanced manufacturing.

Reduce cement costs. Cement is the primary fluid and chemical barrier between reservoir rock and the cased portion of the well. The short- and long-term integrity of the cement are both imperative for sustained well performance. Reducing the cost of cement materials in geothermal well construction can follow a similar path to that of reducing casing; that is, using less material, developing alternative approaches, and investigating leaner casing designs. Using fewer casing strings or decreasing the annular space between the casing and the rock (e.g., through the use of expandable casing) would reduce well cost. Ordinary Portland Cement (OPC) is the most common cement used in geothermal wells. GTO’s work on chemical resistant, high-temperature, self-healing cements should continue, with a focus on developing tailored cement chemistries that are practically deployable and can be sourced at costs comparable to or less than current OPC solutions. Alternatives to existing cementing materials are discussed in Section 2.2.3.3.

Enabling Technologies

Advancements in enabling technologies are necessary to adapt oil and gas methods and tools to geothermal well construction. Such advances are also the foundation to the development of systems that will lower materials costs in geothermal wells. The potential technology space is broad but can generally be divided between materials and electronics.

Research materials and manufacturing method enhancements. Materials commonly used in geothermal well construction are broad and include items such as elastomers for seals, packers, and motors; metals used in casing and drilling tools; organic and inorganic components in cements; solders; and bearings materials and lubricants. These materials have commonalities in their performance limitations within the high-temperature and harsh conditions of geothermal wells and the costs and availability of suitable materials for those conditions. Efforts to overcome these challenges should focus on improving materials that form key components that do not presently perform adequately in geothermal environments and reducing the costs of materials that do perform (without sacrificing that performance). Three areas that should be prioritized: elastomers, lower-cost and high-performance casing, and lower-cost and high-performance cement.

Elastomers degrade at varying rates, depending on the environment to which they are exposed and the length of exposure. Continued improvement of elastomers or alternatives to the organic elastomers used today is an important enabling technology that will support the development of everything from high-temperature drilling tools to casing and test packers used in drilling operations.

As noted, casing is a significant part of the cost of geothermal wells. Researching effective but lower-cost methods to produce and/or deploy this necessary well-construction component is important. RD&D is needed to explore alternatives to existing materials and alternative manufacturing methods that could reduce the amount of casing material needed or the method in which casing is deployed.

Research to develop cementing materials and additives that will provide the performance needed in a geothermal environment should continue. However, methods and technologies that could alter the traditional geothermal well cementing paradigm should not be ignored (e.g., robust designs that do not require fully cemented casing, converting drilling fluid into cement, eliminating cement with a swellable coating on casing). Such research offers long-term opportunity, but the cost associated with cement (and casing) justifies funding into future, robust, lower-cost solutions.

Conduct high-temperature electronics research. Electronics used in drilling and logging wells and for long-term monitoring of well performance are limited in geothermal applications. The availability of electronics for long-term operation above 225°C is also limited. A broader suite of components is needed to expand drilling-related systems and tools required for extended operations in high-temperature geothermal environments. Two basic approaches are needed. The first is to review existing components that are not rated at the requisite operating temperature but may be qualified through testing. It is not uncommon for some components to be underrated by the manufacturer; a method to qualify and publish results of available components has been shown to be a viable way to bolster the components available to tool builders. The second (and more costly) path is to initiate RD&D to build critical components. Among the critical components are processors, multi-chip modules, higher-bit A/D converters, field-programmable gate arrays/electrically erasable programmable read-only memory, failsafe capacitors, oscillators, large memory arrays, and batteries.