BoreTHM: models of circulating temperature and borehole stability for drilling into superhot rock

BoreTHM is an open-source Python model for assessing drilling programmes in superhot geothermal rock. It couples circulating-fluid and downhole-tool temperatures with thermal stress at the borehole wall to calculate the mud-weight window: the range of fluid densities that meets the pressure constraints through drilling, connections and trips.

At the deep granite targets studied at Soultz and United Downs, whether a drilling window exists depends on how quickly breakouts form after rock exposure. The Soultz window first opens between the sampled delays of 15 and 20 minutes; the formation time has not been measured. Both complete programmes remain NO-GO for the stated designs. BoreTHM is for drilling engineers and researchers assessing a well design or reusing the thermal and stability calculations. Its reference cases support checks against field observations and published models.

• Documentation guide: choose a reading path.
• Get started: set up Python and run an assessment.
• Read a result: understand the outputs and status labels.
• Explore the examples: site findings and their controlling constraints.
• Understand the models: methods and field comparisons.

Get started

Install BoreTHM with Python 3.11 or later:

python -m pip install borethm

For figures, install "borethm[plot]". Build the water-property cache and run a first assessment:

borethm-water-table
borethm-evaluate --site newberry --json newberry.json

The IAPWS-95 cache is built once, outside the installed package, in the platform's user cache directory. Set BORETHM_CACHE_DIR to choose another location. The evaluator also builds a missing cache on first use. The assessment command prints its result and saves the inputs and calculation provenance as JSON. Evaluations take several minutes; weighted-fluid cases require repeated coupled solves and can take longer.

The same assessment through the public Python interface:

from borethm import evaluate, export_result, find_site
site = find_site("newberry")
result = evaluate(site)
export_result(result, "newberry.json")
print(result["status"]["label"])

Reproduce the reference calculations

Use Python 3.14.4 and the locked dependencies in a source checkout:

git clone https://github.com/ldocallaghan/BoreTHM.git
cd BoreTHM
python -m venv .venv

Use python3 if your system does not provide python. Activate the environment with .\.venv\Scripts\Activate.ps1 in PowerShell, or source .venv/bin/activate on Linux and macOS, then install:

python -m pip install -r requirements-lock.txt
python -m pip install --no-deps -e .
borethm-water-table

Commands for releases before v1.4.0 are in their release records and run from that release's tag.

Run your own assessment

The built-in site keys are soultz, united-downs, larderello, pannonian and newberry.

borethm-evaluate --site soultz --json soultz.json
borethm-sites

The first command evaluates one site; the second reports all five. Use borethm-evaluate --help for the command-line options.

For a new site, follow the site-input guide and copy the documented template. It records units and sources, and checks the inputs before evaluation. The worked example can validate the template without running the full calculation:

python examples/new_site.py
python examples/new_site.py --evaluate --json result.json

The template starts with Soultz's inputs. Replace them with the new site's measurements or explicitly sourced assumptions.

Reading a result

The output guide maps the JSON fields and explains schedule identity.

Start with status and its reasons, then inspect the components that set it. The report includes the operating programme, thermal and hydraulic outputs, stability in each cycle state, allowable mud densities and equipment checks. The JSON retains every evaluated case and the source of each input.

An overall NO-GO can coexist with an INDETERMINATE coating check. Exceeding a manufacturer's stated coating rating identifies a qualification question. It does not predict a particular failure mode.

A mud-weight window is the allowable range of fluid density: enough pressure to keep the borehole stable while staying below the modeled fracture-pressure limit. BoreTHM calculates the range that satisfies all evaluated depths, operating states and stress/strength cases, separately for drilling and trips.

Two pressure descriptions appear in the outputs:

• Target SG-equivalent expresses pressure as an equivalent static column at the target. It is a local stability diagnostic, with the existing friction/backpressure treatment.
• Surface SG at 20 °C is fluid density divided by 1,000 kg/m³. The combined pressure constraints set the allowable range of that surface density.

A positive target interval does not establish a workable whole-hole fluid. If the required minimum density exceeds the permitted maximum, no mud density satisfies both pressure limits. The densities reported in that situation are calculation inputs and cannot be prescribed.

Example results

The worked assessments target rock at 400 °C. Every status below refers to this programme and these assumptions.

These sites have different evidence quality. Soultz and United Downs use measured or calibrated stresses and strengths, extrapolated to the targets. Larderello and Pannonian retain unsourced strength assumptions. Newberry's well check does not reproduce the observed pattern in every rock type. The input-source records explain those differences.

Local stability and the complete programme

At Soultz, eight of nine selected stress/strength cases have an open target drilling interval; at United Downs, two of four do. Both complete programmes remain NO-GO for this programme and these assumptions because no mud density satisfies the pressure limits throughout the evaluated hole, and tool-temperature limits are exceeded.

Independent programme failures

The bounds below combine the pressure constraints across the evaluated depths, states and cases. They are expressed as surface SG at 20 °C; width is upper minus lower. A negative width means the required minimum density exceeds the permitted maximum. Both sites remain NO-GO for this programme and these assumptions.

Tool peaks, drilling / connection / best attempted run-in: Soultz: 197.0 / 197.3 / 201.2 °C; United Downs: 205.7 / 206.1 / 224.5 °C. The limit is 200 °C. Best attempted means the lowest peak among the schedules searched. A wider search could find a lower one. Soultz's reference FORGE-practice run-in reaches 214.5 °C, separately from the 201.2 °C search result.

Reference-practice coating run-in maxima are 274.3 °C at Soultz and 287.4 °C at United Downs, against the stated 204 °C rating. Both retain INDETERMINATE qualification. The coating histories are not necessarily those of the best attempted tool schedule.

Soultz's 0.017 SG-equivalent target drilling deficit and 1.2 °C tool excess are local facts. They do not measure how close the entire programme is to passing: incompatible density requirements elsewhere in the hole and coating qualification remain independent issues.

Sources: Soultz reference, United Downs reference, and settings-matched L4 base trials for Soultz and United Downs. The summary, also available as a two-page PDF, gives the findings and open questions together.

Breakout timing

Whether breakouts form as soon as the rock is exposed, or take time to form, decides whether a drilling mud weight window exists at Soultz and United Downs. See Breakout timing for the criterion, the results and how to reproduce them.

Design comparisons

No evaluated individual design setting or evaluated pair made either deep site's programme pass. Deeper casing gives Soultz an allowable trip-fluid density range while drilling and tools still fail. Lower viscosity can give United Downs an allowable drilling-fluid density range while other gates still fail. Stronger insulation cools tools but leaves pressure and pump constraints.

Only one pair per site was evaluated, and continuous searches sampled settings. The design study gives the tested settings, remaining failures and equipment-availability evidence. It establishes no complete operating envelope or general impossibility.

Models and evidence

The circulation model solves the downgoing and returning fluid temperatures with frictional heating and heat transfer through the pipe, casing and surrounding rock. Rock conduction carries each depth through its exposure and circulation pauses. Fluid properties and pressure losses feed back into the stability and tool calculations.

The circulation/FORGE comparisons, strength calibration and other checks have distinct scopes. They do not validate a complete 10-13 km drilling programme. Full derivations and diagnostic figures are in the physical-model guide; the bibliography, material properties and site inputs retain their sources.

Scope and limitations

BoreTHM screens a stated drilling programme. A passing result is conditional on the modeled inputs and checks; a failure does not settle whether a different design or existing technology could drill the site. Reservoir production and detailed well-integrity design are outside the assessments.

The deep targets lie well below measured stress data. Strength calibration depends on the assumed stress profile, and the reference mud's rheology and constant barite properties are approximations. Full-cycle qualification of insulated pipe and managed-pressure equipment remains unresolved for the hot conditions studied.

Immediate exposure and the skin heuristic prevent cooling from reducing the initial drilling demand in the base model. The failure-delay experiments test hypotheses that no data yet support. They give no evidence that breakouts wait an hour to form. Near-bit pre-cooling and breakout growth remain separate open questions.

The base deep-site verdicts remain NO-GO for this programme and these assumptions across the A1-A9 comparisons performed. These generally change one approximation at a time and do not cover every joint uncertainty. Coolant delivery, trip timing and fluid displacement are among the deferred approximations. Pannonian's historical 0.041 °C tool threshold remains numerically unresolved.

Tests and reproducibility

Run the fast suite in the activated environment:

python -m pytest

The default excludes physical regressions marked slow. To include the complete suite after building the water-property cache:

python -m pytest -m "slow or not slow"

Full physical runs can take an hour or more. Single-threaded BLAS avoids oversubscription during repeated solves; set OPENBLAS_NUM_THREADS=1, OMP_NUM_THREADS=1 and MKL_NUM_THREADS=1 before starting Python for those runs.

Saved reference outputs and numerical regression tables support comparison with the published calculations. The changelog records changes. Historical diagnostic figures use their stated programmes and should be interpreted with their captions.

Find your way around the repository

How to cite and data credits

Use CITATION.cff for the software citation. The scientific bibliography lists the scientific sources.

The benchmark dataset has a separate citation. Its DOI will be added after deposit on Zenodo.

The FORGE data come from the Geothermal Data Repository: well 16B drilling data and deep-well temperature surveys. The Newberry extracts come from well 55-29 stimulation data. These datasets are used under CC-BY 4.0. The UD-1 image interpretation contains NERC materials ©NERC 2024.

The FORGE and Newberry data guides describe the extracts and how to rebuild them. Other input sources are recorded alongside the corresponding data.

Questions and contributions

Use GitHub issues for questions and problem reports. See SUPPORT.md for what to include and CONTRIBUTING.md for development setup and proposing changes.

Licence

The code in src/, scripts/, tools/, tests/ and examples/ is released under the Apache License 2.0. The documentation, figures, results and summary PDF are released under CC BY 4.0. Data in src/borethm/data/ and benchmarks/data/ keep their source licences, as recorded in their data guides and in NOTICE. Releases up to v1.3.4 were published under the MIT licence.