2.1 Carbon neutrality in temperate climate, cloudy, heavily populated areas is not easy

Cornell University selected 2035 as the date by which to become carbon-neutral on its Ithaca, New York, campus. The goal was adopted without an operational plan that was feasible using available technologies of the adoption year, 2013. A similar pledge was made in 2019 by the State of New York in the cold-climate northeastern USA on stable North American continental crust. The state pledged to eliminate net greenhouse gas emissions by 2050, also without an existing plan encompassing the removal of carbon from the energy used to heat residential and commercial buildings. In New York State, the contribution of heating to carbon emissions is large: about 35 % of New York's annual primary energy is consumed for heating, using energy sources dominated by the combustion of natural gas, fuel oil and propane. In New York State, on-site fossil fuel combustion in the residential, commercial and industrial sectors – all of which predominantly supply low-temperature space and water heating – contributes 30 % of greenhouse gas emissions, compared to electricity production contributing 13 % and transportation 36 % (EIA, 2018a, b, 2019; McCabe et al., 2016; NYSERDA, 2019). The comparatively low carbon footprint of New York's electricity generation is due to major hydropower and nuclear contributions. It is evident that the eventual successful decarbonization of the heating of homes and commercial buildings in heavily populated regions demands an overhaul of the energy supply for heating, such as by use of geothermal energy.

The concept under evaluation by Cornell University, to utilize geothermal heat by tapping fluids in rock at temperatures in the range of 60–120 ^∘C, could also serve widely in New York State to heat residences and commercial buildings, for certain industrial activities, and to assist controlled agriculture operations for food production. Although New York State has banned high-volume hydraulic fracturing in horizontal wells, as practiced for production of gas or oil from organic-rich shale, the long-established use of hydraulic pressure to stimulate existing fractures remains legal. Hence both the options of natural transmissivity and fluid flow through stimulated fractures are plausible fluid-flow pathways. Nevertheless, the technical uncertainty and investment risks for direct-use geothermal energy projects are dominated by the uncertainty about establishment of fluid flow through a well field that is sufficiently dispersed to sweep heat from rock surfaces in a large volume of rock, rather than to short-circuit through a small set of flow paths. The university's view of the next step to test the opportunity is that there are three intertwined objectives: to develop a demonstration reservoir using a well pair that will deliver 20 % of Cornell's heating demand, to de-risk the deployment of geothermal direct-use heating throughout New York and other states with similar geology and climate, and to engender activities that are in keeping with the core university mission: research advances, education, and outreach with novel solutions for broader societal issues.

2.2 Accessing geothermal resources where continental geology is “normal”: a world class problem at a geologically representative site

The ICDP-supported workshop focused on land owned by Cornell University in Ithaca, NY, USA, where the surface geology consists of Devonian sedimentary rock and the subsurface geology includes 2.80 km ± 0.2 km of Paleozoic sedimentary rocks overlying complex Precambrian-age metamorphic rocks (Fig. 1). Nearby, the sedimentary rocks are documented (Camp et al., 2018; Al Aswad, 2019) and the metamorphic rocks inferred to be of very low permeability. According to Limberger et al. (2018), across 16 % of Earth's land surface there exist generally comparable geothermal reservoir systems: a thick sedimentary column over continental basement. That statistic demonstrates the commonality of Ithaca, NY, with a large fraction of the globe. Nevertheless, for central New York, the paucity of sufficient porosity in all but a tiny fraction of the strata sets the possible geothermal reservoirs apart from the desirable “Hot Sedimentary Aquifers” reservoir category considered to be especially good targets for geothermal heat extraction (Wright and Culver, 1991), like the highly permeable Dogger Limestone of the Paris Basin (Le Brun et al., 2011). Instead, the central New York State data lead to anticipation that the conditions will be more like those documented by Dillinger et al. (2016) in the Cooper Basin of Australia, where complex diagenesis resulted in low permeability and lack of sufficient fluid production at a geothermal test well. For Cornell, it is likely that access to the geothermal heat will require fracture flow, which puts in the spotlight the natural characteristics of fractures in both sedimentary and metamorphic rocks and raises key questions about the behavior of those fractures in the ambient stress field under the perturbed conditions that might be imposed in an effort to hydraulically stimulate flow along fractures or that might develop during long-term geothermal field production.

Figure 1Map of geology and historical earthquakes in New York State. Geological map shows bedrock composition immediately subjacent to soil and Holocene sediments (USGS map source, https://mrdata.usgs.gov/geology/state/map.html?x=-75.6897884252848&y=42.9882351360762&z=7#, last access: 20 November 2020). The Adirondack Mountains in northern New York are enclosed in a subcircular black polygon. For sedimentary rocks of the Appalachian Basin, the geological periods corresponding to some of the mapped colors are labeled on the map. Subsurface basement lithologies are known from petrographic analysis of samples of cores and cuttings that are archived by the New York State Museum (Benjamin R. Valentino, unpublished data, 2016). Positions of major shear zones of the Adirondack crystalline rocks (solid black lines) are from Valentino et al. (2008) and Valentino and Chiarenzelli (2018). Projections of those shear zones beneath the Appalachian strata (dashed black lines) are based on Chiarenzelli and Valentino (2020). The set of blue lines in western New York is the Clarenden–Lynden fault zone, and the set of blue lines adjacent to the southern Adirondacks are faults that cut lower Ordovician, Cambrian and basement units (from Jacobi, 2002). Earthquakes are from USGS compilation span January 1920–20 January 2020 (USGS Earthquake Catalog, https://earthquake.usgs.gov/earthquakes/map/, last access: 20 November 2020; details of search listed among references cited). Induced earthquakes in western New York are attributable to brine mining-related work and to brine injection tests in an attempt to develop new caverns for gas storage (Smith et al., 2005; Horowitz et al., 2017). Polygon surrounding Ithaca delineates Tompkins County. A black line near Ithaca shows the location of a seismic reflection profile leased by Cornell to establish the distribution of faults and folds (Fig. 3).

Large areas of all continents are underlain at several kilometers' depth by mid-grade to high-grade metamorphic complexes that formed in ancient orogenic belts. Like many locations, the basement rocks anticipated in the proposed Cornell borehole will be composed of complex, heterogeneous lithologies, with anisotropic mineralogical, petrological and fabric characteristics, in which occur numerous generations of fractures that formed since peak metamorphism and are filled with multiple generations of minerals. The region is an archetypical domain of a very low level of natural seismic activity (Fig. 1) – in a circular area exceeding 13 000 km² centered on Cornell University, there have been no earthquakes of magnitude > 2.5 in at least a century. A set of scientific experiments designed for and conducted in a deep borehole in central New York State can address questions that are generic and not specific to the eastern United States.

Integration of findings for the continental crust of New York with those expected from a similar set of experiments designed for a geothermal borehole at TU Delft in the Netherlands (Vardon et al., 2020) could, together, illuminate answers to a series of basic questions, in the interest of accelerating the safe use of subsurface thermal energy. Is the continental crust everywhere at a condition of critical stress (Zoback and Gorelick, 2012)? How heterogeneous is the stress field with depth and across lithologic boundaries? How does the subsurface, especially at the sedimentary rock–basement interface, respond to perturbations? And how will potential chemical and mineralogical reactions during operations couple to permeability, pressure transmission, and strain? Answers to these questions can provide broad insight into seismic risks and potentially how to mitigate them. Technical risks can also be addressed by determining whether the sedimentary rock–basement interface is a relevant geothermal development target and under what rock conditions artificial stimulation could improve permeability to achieve adequate fluid flow and prolonged heat transfer to permit direct use of geothermal energy where continental crust is naturally of low permeability.

5.1 Multiple, complementary boreholes improve the scientific value of a project

A strategy for a scientific borehole mission at Cornell University must be placed in the context of coexistence with a pair of boreholes planned to demonstrate the viability of producing geothermal fluid at a sufficient flow to meet 20 % of Cornell's building heat needs. Those demonstration boreholes must be wide enough in diameter to sustain production at a flow rate on the order of 30–70 kg s⁻¹; typically, due to this flow rate, geothermal production wells are wider in diameter than used for the oil and gas industry, groundwater production, or mineral exploration. Hence careful consideration is needed of what types of in-borehole activities and monitors are compatible with the needs of a well that eventually may function as either an injection or withdrawal well. Given that the geothermal demonstration wells would be wide in diameter, cased through intervals that present environmental or borehole management risks, maintained in a disturbed condition if production is initiated, and necessarily that their operations would prioritize the success of the geothermal project, there would be both short-term design and long-term management challenges related to co-located scientific program activities. The design and management challenges for scientific activities include the following general possibilities: interruptions to the higher-priority geothermal-focused activities; costly delays in higher-priority activities; technical incompatibility between methods of anchoring permanent monitoring equipment and casing programs; and the potential for stuck tools or borehole wall damage or collapse, which place future access and use of the borehole at risk of failure.

The scientific and geothermal demonstration boreholes are unambiguously complementary to one another in their missions, data streams, and outcomes. Nevertheless, discussions at the ICDP Workshop of efforts to complete in the same borehole both programs, science and geothermal operations, repeatedly arrived at concerns that workarounds to do both would create a higher-priced, technically more challenging borehole program with higher overall likelihood of technical failure.

An alternative is to design the initial geothermal borehole demonstration project as a three-hole system (Fig. 4). A first stage would include two boreholes. Hole 1A would be a slimmer diameter hole to the top of the basement and a short distance into the basement, preferably 100–200 m, from which an extensive rock core would be extracted. Hole 1B would follow soon thereafter and be a borehole of wide enough diameter to potentially be used for well field extraction or injection and drilled to the intended project total depth, preferably 1–2 km into the crystalline basement. Hole 1A would serve as a dedicated exploratory/observation hole in which investigations would be undertaken to address uncertainties regarding hydrologic properties and conditions, thermal resource potential, coupled thermo–hydro–chemical–mechanical behavior, and chemical reactivity. These studies, complemented by geophysical and geologic investigations and modeling analysis, would provide important insights for designing subsequent production wells and help reduce risk for the project overall.

Figure 4Schematic of the geology beneath Cornell and of the three boreholes recommended by the workshop exercise. An initial borehole is recommended as stage 1A (yellow well), which would be dedicated to collection of samples and data and later be transformed into a long-term observatory. Holes 1B and 2 are the other two wells illustrated. Those pilot demonstration wells would be of wider diameter, adequate for producing or injecting the large-volume flux needed for a successful geothermal heat project. Perturbations of the near-field subsurface by drilling, testing, stimulating, and producing the wide-diameter wells would be monitored by equipment installed in the smaller diameter borehole as well as by seismometers at the surface and in shallow boreholes.

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Table 1Initial borehole logs, tests and samples recommended for Hole 1A and their purposes.

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In the short term, Hole 1A would be characterized with a battery of geophysical wireline logs to directly measure physical properties, characterize fractures, orient core, and indicate stress, and a vertical seismic profile to tie core and reflection seismic depths (Table 1). Prior to completion, quantitative measures of in situ liquid transmissivity and stress at successive depths are necessary to characterize the reservoir and assist in the assessment of seismic risk. During completion, Hole 1A would be equipped with fiber optic cables to allow for short- and long-term monitoring of the vertical profile of temperature and strain, with seismometers and with pressure sensors. Hole 1A would be available for subsequent geophysical investigations intended to assist in planning for the production drilling and for longer-term monitoring of the disturbed rock mass.

By having a dedicated observation hole, borehole investigations (including and beyond those in Table 1) are possible which could provide broadly applicable scientific insights. For example, an understanding of the thermal and mineralogic history of deep geology within the region can be assessed, the deep microbiology can be sampled and evaluated, and the coupled thermo–hydro–chemical–mechanical processes associated with subsurface perturbations can be monitored, analyzed, and compared with other geologic settings around the world.

A key part of the completion of Hole 1A will be maintaining, ideally for many years, some direct access to the reservoir to allow for direct sampling of fluids and the monitoring of pressures and productivity during the later production. The preliminary results from Hole 1A would be used to finalize the drilling plan for the first production-diameter Hole 1B.

The exploratory borehole would be designed to be vertical to facilitate testing and stratigraphic mapping. In contrast, the production demonstration borehole might intentionally deviate from vertical in order to optimally access the reservoir, or it might involve laterals.

Once both Holes 1A and 1B exist, inter-borehole communication of fluids, pressure, and temperature can be investigated. After formation tests and fracture tests are performed using one or both of these boreholes, the need for a stimulation plan to develop sufficient fracture permeability to maintain viable heat production would be re-evaluated and the detailed plan finalized and conducted. Then the second stage would begin, the drilling of another production Hole 2, planned to land within the zone of fractures stimulated from the 1B borehole.

Whichever borehole plan advances, it will be accompanied by monitoring of seismic activity using a local seismic array that has been operational since 2019 and that might be densified during drilling and testing. Furthermore, group discussions revealed that there is considerable value to complementing the borehole observations with advanced seismic probing of the area near the proposed geothermal well field. Seismic fracture mapping (Sicking and Malin, 2019) offers the possibility of independent documentation of the spatial variability of permeability in the rock volume, and a three-dimensional seismic reflection profile would enhance understanding and modeling of the shape of the sedimentary rock–basement interface, with its key mechanical role. Although these advanced seismic experiments might be conducted prior to drilling the first well, alternatively they could be conducted in the months between boring Holes 1A and 1B and therefore take advantage of newly available sonic velocity data from the borehole and of the potential to combine surface seismic nodes with instruments in Hole 1A.

The workshop led to an appreciation that the scientific value of subsurface studies would be greatly enhanced by including both the small diameter Hole 1A and demonstration project Hole 1B. The core scientific problems involve the behavior of a volume of rock: the coupled responses among stress, strain, mineralogy and chemistry of the rock-fluid system in the volume when perturbed. Given a single borehole as the initial experiment, the information derived from tests would inform us of a single vertical column of rock, whose horizontal dimension is limited to a few square meters. If instead there are two initial boreholes (1A, 1B), the logging data set and active tests will document the properties, conditions, and heterogeneity of a more representative rock volume.

Even more importantly, Hole 1A would be a relatively inexpensive investment to reduce subsequent risks during drilling of the more costly production holes. First, documentation of the temperature profile below Cornell would remove the uncertainty on existing temperature estimates (Fig. 2) which have been used as the basis for assessing the economic viability of a geothermal well field. Second, conducting much of the logging and coring in the comparatively slim hole would greatly reduce the risk of damage or borehole loss to the first demonstration well. Third, data and insight from an integrated package of samples and rock property logs and tests in Hole 1A would allow refinement of the drilling and casing plan for the primary, wide borehole (Hole 1B). Fourth, while formation tests and fracture tests are conducted in the broad diameter well, simultaneous observations in the observation Hole 1A would underpin better interpretations on which further major decisions (preparations for stage 2) will rely. In combination with geophysical data collected in paired borehole–surface experiments, the heterogeneity, vertically and horizontally, of the first two wells would lay the foundation for characterizing the anisotropy of rock, fractures and fluids prior to drilling the third well. This 3-D information would improve models of what will occur during stimulation and inform the design of a third borehole. Having a firm basis in advance for those models would improve the value of the exercise of model validation through comparison of fractures and fluids in the third borehole to the models and comparison of modeled stimulation to true stimulation data.

5.2 Unique properties of the proposed borehole(s) location

Counterintuitively, the lack of anything “special” about a central New York State borehole site is one of the most compelling attributes of this project. To date, scientific boreholes in continental crust have been selected to investigate active tectonic (e.g., Wenchuan Fault, Alpine Fault, San Andreas Fault, East African rift zone; Mori and Ellsworth, 2014) or volcanic systems (e.g., Iceland's Krafla volcano, Japan's Unzen volcano, the US's Yellowstone plume; Kukkonen and Fridleifsson, 2014), or very rare but large ancient cataclysmic events like meteorite impacts (e.g., Chicxulub, Chesapeake Bay; Koeberl and Claeys, 2014). Only the hydrocarbon industry, waste disposal industry, and mining industry drill several-kilometer boreholes into “normal” crust, and those wells focus on a commercial purpose with few or no opportunities for experiments to probe basic scientific questions. This leaves the Earth sciences community with large gaps in understanding of the “average” intraplate continental crust. Boring (drilling) and investigating a “boring” location is in fact novel.

The number of boreholes drilled for scientific purposes or used as holes of opportunity in “boring” older cratons remains very small. That said, such boreholes, which include the Kola “superdeep” borehole in the former Soviet Union, the German KTB project, the Finnish Outokumpu drilling, and most recently the Swedish Scientific Drilling Program, have overturned many of the long-standing presumptions of the characteristics of the crust. Specific examples of breakthroughs include the topics of the existence of fluids (e.g., Lodemann et al., 1997; Smithson et al., 2000), fluid chemistry (e.g., Kietäväinen et al., 2013), gas compositions (e.g., Wiersberg et al., 2020) and seismic characterization (e.g., Simon et al., 2017; Schijns et al., 2012). The Swedish program completed two boreholes, COSC-1 to nearly 2500 m depth in late 2014 and COSC-2 to 2276 m in 2020. Although the scientific goals of the Swedish boreholes differ, their experiences will be invaluable in planning and deploying Hole 1A.

A 2014 summary of completed ICDP projects (de Wit, 2014) illuminated that the proposed Cornell borehole, planned to traverse Devonian through Cambrian strata and continue well into rocks that achieved their mid- to high-grade metamorphic status 1000–1350 Ma, also targets an interval of Earth history that has been ignored in all but two ICDP boreholes.

Cornell University is committed to broad and rapid public dissemination of all data extracted from the borehole. Rather than the development of tools or methods hoped to provide industrial and business advantages, as might be common in some boreholes with experimental activities, the university's intent is that the borehole activities and data be widely disseminated. The workshop served as the initial opportunity to consider some models for archiving and distributing samples and data, such as the usual procedures recommended by ICDP and IODP.

Furthermore, the willingness of Cornell to host a scientific borehole at the initiation of the large-scale geothermal energy experiment provides the opportunity for characterization and monitoring of the natural, unperturbed subsurface state over a large depth range and then to continuously measure natural and human-perturbed changes over time. If appropriately designed and installed, the proposed Cornell borehole set would measure field-scale characteristics, and its findings can be integrated with what is learned in mesoscale underground laboratories that enable and host long-term monitoring, such as the Sanford Underground Research Facility (South Dakota, USA) and Bedretto Underground Laboratory (southern Switzerland).

5.3 The natural organization of scientific themes into an integrated borehole program

The workshop participants focused largely on research themes that related to Earth's internal dynamic activities, a major emphasis for the ICDP program. Three major themes are tightly connected: fluids and elemental cycling, seismicity across a range of length scales, and controlling subsurface fractures and fluid flow. These themes would likely be at the heart of a project that is well positioned to garner sufficient funds to carry out a deep borehole program. Two other major themes, of deep life and of the thermal–chemical–mineralogical evolution from Proterozoic through Phanerozoic times, excited many participants. While it is perceived that these two themes are unlikely to independently attract sufficient funds to drill a borehole, the workshop group welcomed the beneficial knowledge of deep life and deep history that can be extracted from samples and conditions at a borehole.