Underground Energy Storage

The ground beneath us is the largest store we have.

Geological formations and engineered underground spaces can hold hydrogen, compressed air, heat, cold and water at a scale no surface tank can match — often for weeks or a whole season. Salt caverns, rock caverns, depleted gas fields, aquifers, borehole fields and old mines each suit a different energy form. BARREL brings geology, drilling, cavern engineering, surface plant and grid connection together in one project, working with specialist subsurface partners.

Sector hub · 24 sections

Section 01

Underground Storage — Overview

Underground storage uses the earth itself as the container. The rock gives pressure containment, insulation and a natural safety distance; on the surface you see only wellheads and a compact plant of compressors, pumps, heat exchangers and electrical equipment.

Natural gas has been stored underground for about a century, and crude oil and LPG in rock caverns for decades — including India's strategic petroleum reserves. The same skills now serve hydrogen, compressed air, seasonal heat and cold, and pumped hydro or gravity storage in disused mines.

Underground stores are the main option for long-duration and seasonal storage: they hold far more energy per rupee of container than any steel tank, but each depends on finding the right geology and proving it with surveys and test wells.

1.1Formations and what they store

Formations and what they store
Formation	Stores	Strength	Maturity
Salt caverns	Hydrogen, compressed air, natural gas, liquids	Tight, self-sealing, fast cycling	Commercial (gas; H₂ in UK and US since 1970s–80s)
Lined rock caverns (LRC)	Natural gas, hydrogen at high pressure	Works in hard rock where there is no salt	Demonstration (Sweden)
Unlined rock caverns	Crude oil, LPG, products	Very large volume, water-curtain sealing	Commercial (India SPR, LPG caverns)
Depleted gas / oil fields	Natural gas; hydrogen studied	Very large volume, proven seal	Commercial for gas; H₂ pilots
Aquifers (ATES)	Warm and cold groundwater	Seasonal heating and cooling	Commercial (Europe)
Borehole fields (BTES)	Heat in rock or soil	Works almost anywhere	Commercial
Abandoned mines	Water (pumped hydro), weights, air, mine-water heat	Reuses existing shafts and voids	Pilots and studies

1.2Why go underground

Scale

One salt cavern can hold over 100 GWh of hydrogen energy — more than the largest battery parks in the world.

Cost per kWh

The rock is the vessel, so the cost of extra storage capacity is very low once wells and caverns exist.

Safety

Hundreds of metres of rock separate the stored medium from people and buildings; fire and blast risks sit mainly in the surface plant.

Land

A few hectares on the surface for a store that would need a large tank farm above ground.

1.3What it is used for

Seasonal hydrogen buffer
Hydrogen to power for firm supply
Compressed air energy storage
Strategic oil and gas reserves
District heating and cooling
Campus and hospital cooling
Pumped hydro without a new valley
Mine closure and reuse
Industrial waste-heat storage

Suitability is always site-specific and must be proven by surveys and test wells. Figures and maturity notes on this page are typical or indicative ranges from public literature, not project guarantees.

Section 03

Salt Caverns

Caverns are washed out of thick salt layers or salt domes by solution mining: fresh water is pumped down a well, dissolves the salt and returns as brine, leaving a sealed cavity that can be hundreds of metres tall and tens of metres wide.

Salt creeps slowly and heals its own cracks, and it is almost impermeable to gas, so caverns hold hydrogen and compressed air with very low leakage and can be cycled many times a year — far more often than porous reservoirs.

3.1Typical salt cavern parameters

Typical salt cavern parameters
Parameter	Typical range	Comment
Depth of cavern roof	500–1,500 m	Deeper allows higher pressure but more creep
Geometric volume	100,000–1,000,000 m³	Around 500,000 m³ is common for new caverns
Operating pressure	≈ 0.3–0.8 of overburden pressure	Often about 60–180 bar for hydrogen
Cushion gas share	≈ 25–35% of total gas	Stays in the cavern to keep it stable
Working hydrogen per cavern	≈ 2,000–6,000 t	≈ 70–200 GWh (LHV)
Leaching time	≈ 2–4 years per cavern	Depends on water supply and brine disposal
Water used for leaching	≈ 7–10 m³ per m³ of cavity	Returned as saturated brine
Cycles per year	Several to dozens	Rate limited by pressure change per day

3.2Developing a cavern

1Regional salt mapping from old wells and seismic surveys
2Exploration well, salt cores and laboratory creep tests
3Rock-mechanics modelling and cavern shape design
4Drilling and cementing the access well with gas-tight casing
5Solution mining with controlled water injection and blanket fluid
6Brine treatment, sale or disposal
7Sonar surveys of the cavern shape during and after leaching
8Mechanical integrity test of the well and cavern
9Debrining and first fill with gas
10Commissioning and first pressure cycles

3.3Well-known reference sites

Hydrogen

Teesside, UK

Three small salt caverns have stored hydrogen for the chemical industry since the early 1970s.

Hydrogen

US Gulf Coast

Salt-dome caverns in Texas (for example Clemens Dome, in service since the 1980s) supply refinery and pipeline hydrogen.

Compressed air

Huntorf and McIntosh

The two long-running compressed-air plants, both using salt caverns (see the CAES tab).

3.4India and the region

India has limited known thick salt formations; the evaporite layers of the Nagaur–Ganganagar basin in Rajasthan are the main area studied.
Salt depth, thickness, purity and water availability for leaching in a dry region all need investigation before any project.
Where salt is absent, lined rock caverns, depleted fields, pipeline line-pack and above-ground vessels are compared instead.

Cavern figures are typical ranges from public literature on gas and hydrogen storage; every cavern is designed for its own depth, salt quality and stress field.

Section 04

Underground Compressed Air Energy Storage

CAES uses cheap electricity to compress air into a cavern and later releases it through a turbine. In the classic diabatic design the heat of compression is thrown away and natural gas reheats the air before the turbine; newer adiabatic designs store the heat and reuse it, with no fuel.

Because the cavern is large and cheap, CAES suits several hours to a few days of storage at 100 MW scale and above.

4.1Reference plants

Reference plants
Plant	Year	Output	Store	Notes
Huntorf, Germany	1978	290 MW (later uprated to about 320 MW)	Two salt caverns, ≈ 310,000 m³ total	First commercial CAES; diabatic, gas-fired reheat
McIntosh, Alabama, USA	1991	110 MW for up to about 26 h	One salt cavern, ≈ 560,000 m³	Diabatic with recuperator for better efficiency

4.2CAES variants

Diabatic

Heat of compression rejected
Gas burnt to reheat air
Proven since 1978
Round-trip roughly 42–55% (fuel counted separately)

Adiabatic (A-CAES)

Heat stored in a thermal store
No fuel
Round-trip roughly 55–70% (design studies)
Large first plants under development abroad

Isothermal / hydraulic

Near-constant temperature by spray or water
Water-compensated caverns keep pressure steady
Pilot and demonstration stage
Can suit hard-rock caverns and mines

4.3Cavern requirements

Pressure window typically about 40–80 bar
Salt preferred; hard-rock caverns and porous rock studied
Fast daily cycling means careful limits on pressure change rate
Moisture and corrosion control in wells and turbines
Volume of the order of 2–6 kWh of electricity per m³ in existing diabatic plants
Constant-pressure designs use a water column from a surface pond

Efficiencies and energy densities are indicative figures from public literature on Huntorf, McIntosh and design studies; actual values depend on design and operation.

Section 05

Depleted Gas Fields and Porous Rock

Porous rock that once held oil or gas has already proved it can trap gas for millions of years under a cap rock. Re-used as a store, it offers very large volumes — enough for seasonal balancing of a whole region.

Natural gas has been stored this way worldwide for decades. Hydrogen is harder: it is lighter, diffuses more easily, can react with minerals and can be consumed by microbes in the rock, so pilots are still testing how much can be recovered and how pure it comes back.

5.1Strengths and open questions

Strengths

Proven seal and known geology from production years
Existing wells and data may be reusable
Very large working volume (TWh-scale for big fields)
Natural fit for seasonal storage

Open questions for hydrogen

Mixing with residual gas lowers purity
Microbial activity can turn H₂ into methane or H₂S
Large cushion gas — often 40–60% of total
Slower injection and withdrawal than salt caverns
Old well integrity must be checked for hydrogen

5.2Pilots and studies

Hydrogen in a depleted gas field

Pilots in Austria and elsewhere in Europe have injected hydrogen or hydrogen-rich gas into small depleted reservoirs to study recovery and purity.

Town-gas history

Aquifers in Europe once stored town gas containing about half hydrogen, giving early data on hydrogen behaviour underground.

Underground methanation

Some research deliberately lets microbes turn hydrogen and CO₂ into methane in the reservoir.

5.3India context

India's mature oil and gas basins (for example Cambay, Assam–Arakan and Krishna–Godavari) contain depleted fields that could be screened for gas or hydrogen storage.
Field data are held by operators and the Directorate General of Hydrocarbons; access and licensing would be part of any study.
Porous-rock storage could also pair with carbon capture work, which uses similar reservoir skills.

Cushion-gas shares and recovery figures for hydrogen in porous rock are still uncertain and come from pilots and modelling; treat them as indicative.

Section 06

Lined and Unlined Rock Caverns

Where there is no salt, caverns can be excavated in hard rock such as granite or gneiss by drill-and-blast, like a large tunnel. Two families exist: unlined caverns kept below the water table, where groundwater pressure holds liquids in, and lined rock caverns with a thin steel liner backed by concrete for high-pressure gas.

India already has a strong precedent: the strategic petroleum reserves built by Indian Strategic Petroleum Reserves Limited (ISPRL) are unlined rock caverns holding crude oil underground.

6.1India's strategic petroleum reserve caverns

India's strategic petroleum reserve caverns
Site	State	Capacity (crude)	Type
Visakhapatnam	Andhra Pradesh	1.33 million tonnes	Unlined rock cavern
Mangaluru	Karnataka	1.5 million tonnes	Unlined rock cavern
Padur	Karnataka	2.5 million tonnes	Unlined rock cavern
Total (Phase I)		5.33 million tonnes	Further sites announced for Phase II

6.2Two kinds of rock cavern

Unlined (hydrodynamic containment)

Cavern sits below the natural water table
Water curtain boreholes above keep rock saturated
Water pressure stops oil or LPG escaping
Used for crude, products and LPG
Low pressure; not suited to hydrogen

Lined rock cavern (LRC)

Thin steel liner, concrete backing, rock carries the load
Pressures up to about 200–250 bar
Shallower (100–200 m) than salt caverns
Natural gas demonstration at Skallen, Sweden, since early 2000s
Small hydrogen pilot at Luleå, Sweden, since 2022

6.3Design points

Rock mass quality, joints and stress measured by boreholes and test adits
Groundwater flow and water-curtain design
Liner material checked for hydrogen embrittlement
Uplift safety: rock cover must resist maximum pressure
Excavation spoil handling and reuse
Tunnel access, shafts and ventilation during construction

Capacities are as published by ISPRL for Phase I; lined-rock-cavern pressures are from Swedish demonstration reports. Rock caverns for hydrogen remain at demonstration stage worldwide.

Section 07

Aquifer Thermal Energy Storage (ATES)

ATES uses a natural water-bearing sand or gravel layer as a seasonal store. Pairs of wells pump groundwater in opposite directions: in summer, cold water from the cold well cools buildings and is returned warmer to the warm well; in winter the flow reverses and the stored warmth feeds heat pumps.

The Netherlands has thousands of ATES systems serving offices, hospitals and campuses. In India, where cooling dominates, the cold side and waste-heat storage are the main interest.

7.1Typical ATES parameters

Typical ATES parameters
Parameter	Typical range
Aquifer depth	20–300 m (low-temperature systems)
Well flow	20–300 m³/h per well
Temperature in the store	Cold well ≈ 5–10 °C, warm well ≈ 15–25 °C (low-temperature); high-temperature ATES up to 50–90 °C
Thermal power per doublet	≈ 0.5–3 MW(th)
Recovery efficiency	≈ 60–90% when flows are balanced
Well spacing	Often 100–200 m between warm and cold wells

7.2What a site needs

Ground

Permeable aquifer of adequate thickness
Low natural groundwater flow
Water chemistry that will not clog wells

Demand

Both heating and cooling, or cooling plus a heat sink
Balanced energy over the year
Large buildings, campuses, data centres, districts

Permits

Groundwater authority approval
Protection of drinking-water aquifers
Monitoring of temperature and water quality

7.3India considerations

Central Ground Water Authority and state groundwater rules apply to any abstraction and reinjection.
Over-exploited aquifers in many districts call for closed balance: every litre pumped is reinjected.
Alluvial aquifers of the Indo-Gangetic plain may suit ATES; hard-rock areas of the Deccan usually suit borehole fields better.

ATES figures are typical values from European practice and literature; Indian aquifer behaviour must be confirmed by pumping tests.

Section 08

Borehole Thermal Energy Storage (BTES)

BTES passes water through closed plastic loops in dozens to hundreds of boreholes drilled close together. Heat flows into the surrounding rock or soil and is recovered months later. No groundwater is pumped, so permits are simpler and it works almost anywhere drilling is possible.

The same boreholes, used every day rather than seasonally, are the ground loops of a ground-source heat pump.

8.1Typical BTES parameters

Typical BTES parameters
Parameter	Typical range
Borehole depth	30–300 m
Borehole spacing	≈ 2–5 m in a compact field
Number of boreholes	Dozens to several hundred
Storage temperature	Up to about 60–90 °C for seasonal solar heat
Rock volumetric heat capacity	≈ 2.0–2.5 MJ/m³·K
Seasonal efficiency	≈ 40–70%, better for larger, well-insulated fields

8.2Reference and uses

Reference

Drake Landing, Canada

A community of 52 homes in Alberta stores summer solar heat in a field of 144 boreholes about 35 m deep, covering most of its winter space heating.

Industrial waste heat

Heat from furnaces, compressors or data centres stored for later process or space heating.

Cooling-led use

In hot climates the field can be pre-cooled at night or in winter to serve daytime or summer cooling.

8.3Design steps

Thermal response test on a trial borehole
Ground model of conductivity and heat capacity
Field layout: depth, spacing, series and parallel loops
Top insulation and edge losses
Heat pump and buffer tank selection
Multi-year simulation of field temperature

Drake Landing figures are as reported publicly by the project; parameter ranges are typical literature values.

Section 09

Abandoned Mines for Energy Storage

A closed mine already has what is most expensive to build: deep shafts, tunnels and voids. These can be turned into the lower reservoir of a pumped hydro plant, a shaft for heavy weights, a compressed-air store or a heat source for heat pumps.

For coal and metal regions facing mine closure, this offers a second life for the site, the grid connection and local skills.

9.1Options for a mine

Underground pumped hydro

A mine void or new cavern as the lower reservoir and a surface pond as the upper one — no dam in a river valley. Heads of several hundred metres are possible.

Shaft gravity storage

Heavy weights raised and lowered in a deep vertical shaft by winches; fast response, modest energy per shaft.

Compressed air

Sealed galleries or lined drifts as a pressure store where the rock is sound and the mine is tight.

Mine-water heat

Flooded workings as a large, steady-temperature water body for heat pumps and seasonal storage.

Open-pit pumped hydro

A worked-out open-cast pit as one reservoir, with a second pond on the overburden dump or higher ground.

Hybrid site

Solar on dumps and tailings, storage in the mine, and the existing substation for export.

9.2Screening a mine

Screening a mine
Check	Why it matters
Depth and void volume	Sets energy (head × volume) and duration
Rock stability and support	Old workings may collapse under water-level swings
Water chemistry	Acid mine drainage attacks turbines and pipes
Groundwater links	Must not pollute or drain nearby aquifers
Shaft condition	Reuse cuts cost; repair may be needed
Grid and land	Existing connection and surface land for ponds or plant
Ownership and closure plan	Who holds liability, and how storage fits the mine closure plan

9.3India context

India has many closed and soon-to-close coal mines; reusing them for pumped storage and solar is being explored under the Ministry of Coal's mine-closure and just-transition work.
Open-cast pits in Jharkhand, Odisha, Chhattisgarh and Madhya Pradesh are among those discussed for pumped storage.
Mine safety (DGMS) and state mining approvals apply alongside power-sector rules.

Mine storage worldwide is mostly at study and pilot stage; studies in Germany and Finland have looked at underground pumped hydro and shaft gravity storage in former mines.

Section 10

System Types and Technical Architecture

Every underground store has three layers: the subsurface store (cavern, reservoir, aquifer, borehole field or mine), the wells or shafts that connect it to the surface, and the surface plant that converts energy in and out.

The configuration depends on what is stored and who uses it — a hydrogen hub, a power plant, a district heating network or an industrial site.

10.1Common configurations

Common configurations
Configuration	Store	Charge	Discharge
Power-to-hydrogen-to-power	Salt or rock cavern	Electrolyser + compressor	Gas turbine or fuel cell
Hydrogen hub buffer	Salt cavern	Electrolysers and pipeline	Steel, fertiliser, refinery users
Diabatic CAES	Salt cavern	Motor-driven compressors	Gas-fired expander turbine
Adiabatic CAES	Cavern + heat store	Compressors, heat recovered	Air reheated from heat store
ATES campus	Aquifer doublet	Chillers / heat pumps	Direct cooling, heat pumps
BTES district heat	Borehole field	Solar thermal or waste heat	District heating via heat pumps
Mine pumped hydro	Mine void + surface pond	Reversible pump-turbines	Same machines as turbines

10.2Main components

Subsurface

Cavern, reservoir, aquifer or void
Cap rock or seal
Cushion gas or water
Monitoring wells

Wells and wellheads

Casing strings and cement
Production tubing and packers
Subsurface safety valve
Wellhead and Christmas tree
Downhole gauges

Surface plant

Compressors and coolers
Dehydration and purification
Heat exchangers and thermal stores
Pumps, turbines, expanders
Flare or vent stack
Electrical and control systems

10.3Hydrogen surface plant in detail

Inlet filters and metering
Reciprocating or diaphragm compressors, often in stages
Intercoolers and after-coolers
Glycol or adsorption dryers (hydrogen picks up water from salt caverns)
Purification (pressure-swing adsorption) for fuel-cell grade
Pressure letdown with heating to avoid freezing
Emergency shutdown valves and blowdown
Gas detection, fire and gas system

Component lists are typical of gas-storage practice; each project's process design sets the final equipment.

Section 11

Power Conversion, Controls and Grid Integration

Underground storage plants are large electrical loads when charging and, for CAES, hydrogen-to-power and mine pumped hydro, large generators when discharging. The electrical design, controls and grid connection are as important as the geology.

11.1Electrical systems

High voltage

Switchyard and step-up transformers
Bay protection and metering
Line connection to state grid or ISTS
Reactive power support

Drives and machines

Large compressor motors with variable-speed drives
Synchronous or variable-speed generators
Reversible pump-turbines for mine hydro
Rectifiers for electrolysers

Auxiliaries

Medium- and low-voltage switchgear
Emergency diesel or battery supply
UPS for controls and safety systems
Earthing and lightning protection

11.2Controls: EMS, SCADA and safety

Energy management (EMS)

Schedules charge and discharge against tariffs and dispatch
Respects pressure-change limits of the cavern
Forecasts renewable surplus
Coordinates with batteries for fast response

SCADA and DCS

Wellhead, compressor and turbine control
Historian for pressure, temperature, flow
Alarm management
Remote monitoring centre link

Safety instrumented systems

Emergency shutdown on high pressure or gas leak
Subsurface safety valve closure
Fire and gas detection
Independent of the process control system

11.3Grid services by type

Grid services by type
Type	Ramp / response	Typical role
Mine pumped hydro	Minutes	Peak shifting, reserves, inertia
CAES	Minutes	Daily shifting, reserves
Hydrogen cavern + turbine	Minutes to start	Weekly and seasonal firm power
Hydrogen cavern + industry	Continuous	Keeps a steel or ammonia plant running when renewables dip
ATES / BTES	Hours	Cuts summer cooling peak, shifts thermal load

11.4Grid connection in India

Connectivity follows CEA technical standards for grid connection and CERC or state connectivity regulations.
Large plants may connect to ISTS; smaller thermal systems sit behind the meter of the user.
Firm and dispatchable renewable (FDRE) and round-the-clock (RTC) tenders are the main market route for long-duration storage paired with solar and wind.

Section 12

Sizing and Design Calculations

First-pass sizing for underground storage uses a few physical relations: how much gas a volume holds at a given pressure, how much heat water or rock holds per degree, and how much energy water or weights hold per metre of height.

The worked examples and the live estimator below are for early screening only. Detailed design uses real-gas equations of state, reservoir simulation and rock-mechanics models.

12.1Gas held in a cavern

Mass of gas at pressure

m = p × V × M ÷ (Z × R × T)

500,000 m³ at 180 bar, 45 °C, Z ≈ 1.1 → ≈ 12.5 kg/m³ → ≈ 6,240 t of H₂ in the full cavern

The real-gas law gives the mass of gas in a fixed volume. Hydrogen is lighter than ideal at high pressure, so Z is above 1 (about 1.05–1.15 in typical cavern conditions, higher at very high pressure).

Symbol	Meaning
m	Mass of gas, kg
p	Pressure, Pa (1 bar = 100,000 Pa)
V	Cavern geometric volume, m³
M	Molar mass, kg/mol (H₂ = 0.002016; air ≈ 0.029)
Z	Compressibility factor (real-gas correction)
R	Gas constant, 8.314 J/mol·K
T	Gas temperature, K (°C + 273.15)

12.2Working gas and energy

Usable hydrogen between pressure limits

Working gas = m(p max) − m(p min); E = working gas × 33.3 kWh/kg

At 60 bar, Z ≈ 1.04 → ≈ 4.4 kg/m³ → cushion ≈ 2,200 t; working ≈ 4,040 t ≈ 135 GWh (LHV)

Only the gas between the maximum and minimum pressure can be used; the rest stays as cushion gas to support the cavern walls. Multiply the working hydrogen by its lower heating value to get stored chemical energy.

Symbol	Meaning
p max	Highest allowed pressure (from rock mechanics)
p min	Lowest allowed pressure (to limit creep and closure)
33.3 kWh/kg	Lower heating value of hydrogen
E	Stored energy, kWh (÷ 1,000,000 for GWh)

12.3Live estimator

Enter a cavern volume (m³), the maximum and minimum operating pressures (bar) and the gas temperature (°C). The estimator uses the real-gas law with an approximate hydrogen compressibility factor (Z ≈ 1.04–1.15, rising with pressure) to give the working hydrogen in tonnes, the cushion gas left at minimum pressure in tonnes, and the stored energy in GWh at hydrogen's lower heating value of 33.3 kWh/kg. It is a screening figure; real caverns need site rock-mechanics limits and an exact equation of state.

Estimate

Working gas: 3,958 t H₂
Energy (LHV): 131.8 GWh
Cushion gas (stays in): 2,203 t H₂
Working-gas share: 64 %

Real-gas estimate with a simple compressibility fit (Z ≈ 1.04–1.15). Allowed pressures come from depth and rock mechanics; confirm with a site study. Preliminary screening only; BARREL confirms any figure with a site study.

12.4Heat or cold in water or rock

Sensible heat stored

Q = V × ρc × ΔT × η

ATES: 300,000 m³ of water × 1.16 kWh/m³·K × 6 K × 0.8 ≈ 1,670 MWh(th) per season

Water stores about 1.16 kWh per m³ per degree; rock about 0.55–0.70 kWh per m³ per degree. The recovery factor η covers heat lost to the surroundings. The same relation with flow instead of volume gives thermal power: 100 m³/h × 1.16 × 6 K ≈ 0.7 MW(th).

Symbol	Meaning
Q	Recovered heat or cold, kWh(th)
V	Water pumped per season, or rock volume, m³
ρc	Volumetric heat capacity, kWh/m³·K
ΔT	Temperature swing, K
η	Recovery efficiency (ATES ≈ 0.6–0.9, BTES ≈ 0.4–0.7)

12.5Mine pumped hydro and shaft gravity

Energy from height

E = ρ × g × h × V × η ÷ 3,600,000,000 (MWh)

1,000,000 m³ of water × 600 m head × 0.75 ≈ 1,230 MWh; a 1,000 t weight in a 1,000 m shaft × 0.85 ≈ 2.3 MWh

Water stored in a mine void at depth h below a surface pond gives potential energy. For weights, replace ρ × V by the mass in kg. Deep mines give high head, so a small volume of water stores a lot of energy.

Symbol	Meaning
ρ	Water density, 1,000 kg/m³
g	Gravity, 9.81 m/s²
h	Net head or lift height, m
V	Usable water volume, m³
η	Round-trip efficiency (pumped hydro ≈ 0.70–0.80)

12.6Rules of thumb for screening

Rules of thumb for screening
Item	Indicative value
Max cavern pressure	≈ 0.8 × overburden at the cavern roof (≈ 0.023 bar/m × depth × 0.8)
Min cavern pressure	≈ 0.25–0.35 × overburden
Pressure change rate	Often limited to about 10 bar per day in salt caverns
Diabatic CAES cavern	≈ 2–6 kWh(e) per m³
Water for leaching	≈ 7–10 m³ per m³ of cavern
Electrolyser energy	≈ 50–55 kWh per kg H₂

Worked examples use rounded inputs and simplified Z values; results are indicative only. Final design limits come from site-specific tests and modelling.

Section 13

Engineering Deliverables Register

An underground storage project produces a long list of studies and documents across geology, wells, process plant and electrical systems. The register below shows the main items BARREL and its partners would prepare or coordinate.

13.1Main deliverables by stage

Main deliverables by stage
Stage	Deliverable	Discipline
Screening	Desk study of geology, demand and grid	Geology / power
Screening	Long list and ranking of candidate sites	Multi-discipline
Feasibility	Geophysical survey report and interpretation	Geophysics
Feasibility	Exploration well programme and core-test results	Drilling / geology
Feasibility	Geomechanical model and storage capacity estimate	Rock mechanics
Feasibility	Hydrogeological model and pumping-test report (ATES)	Hydrogeology
Feasibility	Thermal response test and field simulation (BTES)	Thermal
Pre-FEED	Concept design, block-flow diagram, cost estimate	Process / cost
FEED	Cavern or well design basis and well schematics	Wells
FEED	Process and instrumentation diagrams (P&IDs)	Process
FEED	Hazard identification (HAZID), HAZOP, SIL study	Safety
FEED	Single-line diagram and grid study	Electrical
Detailed	Equipment data sheets and specifications	All
Detailed	Civil, structural and layout drawings	Civil
Approvals	Environmental impact assessment and management plan	Environment
Construction	Leaching programme and sonar survey plan	Cavern
Handover	Commissioning reports, integrity test records, O&M manuals	All

13.2Models kept through the asset life

3D geological model
Geomechanical model
Reservoir / cavern thermodynamic model
Groundwater model
Process simulation
Grid model
Digital twin

The register is illustrative; scope varies by technology and by what the client or other contractors provide.

Section 14

Site Selection and Feasibility

Site selection for underground storage starts with the rock, not the land. A good site has the right formation at the right depth, close enough to the energy source, the users and the grid.

14.1Screening criteria

Screening criteria
Technology	Key geological criteria	Key surface criteria
Salt cavern	Salt ≥ 200 m thick (bedded) or a dome; depth ≈ 500–1,500 m; few impurity layers	Water for leaching; brine outlet; H₂ users or grid
Lined rock cavern	Sound hard rock at 100–200 m; low fracturing	Access tunnel portal; spoil area
Unlined rock cavern	Hard rock below stable water table	Port or pipeline for liquids
Depleted field	Known seal, good permeability, well records	Pipeline links, field licence
ATES	Permeable aquifer 20–300 m, slow groundwater flow	Balanced heating and cooling demand
BTES	Drillable ground, moderate conductivity, little groundwater flow	Heat source (solar, waste heat) and heat users nearby
Mine storage	Stable voids, depth, tight rock, acceptable water quality	Surface pond site, grid connection

14.2Feasibility path

1Define the energy to store, duration and users
2Desk study of geological maps, old wells and mine records
3Shortlist sites and engage landowners and authorities
4Surface geophysics (seismic, gravity, resistivity)
5Exploration or test well, cores and logs
6Laboratory tests and models
7Capacity estimate, concept design and cost
8Go / no-go decision for FEED and approvals

14.3Reducing subsurface risk early

Spend on data before spending on plant
Use existing well and mine data where possible
Plan stage gates with clear stop criteria
Compare against above-ground alternatives at each gate
Seek government geological data (Geological Survey of India, DGH data repository)
Keep the community informed from the first survey

Section 15

Turnkey EPC and Supply Chain

Underground storage EPC combines two worlds: subsurface contracting (drilling, leaching, tunnelling) and surface plant construction (compressors, process, electrical). BARREL can act as the single point of responsibility for the surface plant and integration, working with specialist drilling and cavern partners.

15.1Project phases

1Screening and feasibility (6–18 months)
2Exploration and test wells (1–2 years)
3FEED and approvals (1–2 years)
4Drilling and cavern leaching, or excavation (2–4 years for salt caverns)
5Surface plant construction in parallel
6Integrity testing, first fill and commissioning
7Handover, O&M and long-term monitoring

15.2Typical work packages

Subsurface

Drilling and well completion
Solution mining and brine handling
Rock excavation and lining
Borehole field drilling
Mine rehabilitation

Process plant

Compressors and coolers
Dehydration and purification
Thermal stores and heat exchangers
Piping, valves, flare

Electrical and civil

Substation and grid line
Motors, drives, generators
Control buildings
Roads, ponds, foundations
Fire-fighting systems

15.3Equipment and supply chain in India

Equipment and supply chain in India
Item	India capability	Comment
Drilling rigs and services	Established (oil, gas, mining sectors)	Deep salt drilling experience limited
Casing, tubing, line pipe	Manufactured in India	Hydrogen-grade steel to be qualified
Wellheads and valves	Partly local, partly imported	API-certified sources
Rock tunnelling and caverns	Strong (hydro, metro, SPR projects)	SPR caverns built in India
Process compressors	Assembled locally; large hydrogen units often imported	Through OEM partners
Motors, transformers, switchgear	Strong domestic base	Make in India preferred
Heat pumps and chillers	Local assembly growing	Large industrial units often imported
HDPE ground loops	Manufactured in India	For BTES and ground-source fields

15.4Delivery models

EPC lump sum (surface)

Fixed price for the surface plant once the subsurface is proven.

EPCM

BARREL manages specialist contractors for wells, caverns and plant on the owner's behalf.

Phased alliance

Stage-gated contract that moves from studies to construction as risk falls.

Durations are indicative; salt-cavern leaching time depends strongly on water supply and brine disposal capacity.

Section 16

Testing and Commissioning

Before any gas or heat goes underground, the wells and the store must be proven tight and stable. Commissioning then brings the surface plant on line step by step, with the store filled slowly to its working pressure.

16.1Key tests

Key tests
Test	What it proves
Cement bond and casing logs	Cement seals the casing to the rock
Mechanical integrity test (MIT)	Well and cavern hold pressure with negligible leak
Sonar survey	Cavern shape and volume match design
Pressure build-up test	Reservoir or aquifer behaves as modelled
Pumping and injection test (ATES)	Well yield, clogging risk, water chemistry
Thermal response test (BTES)	Ground conductivity for field design
Liner leak test (LRC)	Steel liner and welds are gas-tight
Factory and site acceptance tests	Compressors, turbines, controls work as specified

16.2Commissioning sequence

1Mechanical completion and punch-list closure
2Pre-commissioning: flushing, drying, purging with nitrogen
3Loop checks of instruments and safety systems
4Energising electrical systems
5Debrining and first gas fill at controlled rate
6Pressure cycling to working range
7Performance and reliability run
8Handover with records and baseline monitoring data

Test methods follow gas-storage practice (for example API and EN guidance); exact criteria are set in the project's test plan and by the approving authority.

Section 18

Safety and Risk

Underground storage has a good safety record in gas storage, but incidents worldwide have come from well failures, poorly understood geology and surface leaks. Most of the risk is managed through well integrity, conservative pressure limits and monitoring.

18.1Main hazards and controls

Main hazards and controls
Hazard	Control
Well leak or blowout	Multiple casing barriers, subsurface safety valve, well integrity programme
Hydrogen fire or explosion at surface	Hazardous-area design, gas detection, ventilation, separation distances
Cavern instability or collapse	Pressure limits from rock mechanics, sonar surveys, minimum pressure
Ground subsidence	Survey benchmarks, InSAR, conservative cavern spacing
Brine spill (leaching)	Lined ponds, pipeline monitoring, disposal permits
Groundwater contamination	Casing and cement design, monitoring wells
Induced seismicity	Microseismic monitoring and traffic-light rules
Mine flooding or acid water	Water treatment, rock support, closure plan

18.2Hydrogen-specific points

Steel selection to avoid hydrogen embrittlement
Leak-tight connections; hydrogen flames are hard to see
Odourless gas — detection must be instrumented
Wide flammable range in air (about 4–75%)
Purge and vent procedures with nitrogen
Emergency response plan with local authorities

Hazard lists are typical of underground gas storage; each project's HAZID and HAZOP sets the actual risk register.

Section 19

Standards, Regulation and Approvals

Underground storage touches mining, petroleum, groundwater, pressure equipment, electricity and environmental law, so the approval path is planned at the start. India has no single rulebook for hydrogen or air storage underground yet; existing gas, mining and safety rules are applied with the authorities.

19.1International standards and guidance

International standards and guidance
Reference	Covers
EN 1918 (parts 1–5)	Underground gas storage in aquifers, fields, salt caverns, rock caverns and surface facilities
API RP 1170	Design and operation of solution-mined salt caverns for natural gas storage
API RP 1171	Integrity of gas storage in depleted reservoirs and aquifers
ISO 16530	Well integrity in the oil and gas industry
API 5CT / API 6A	Casing and tubing; wellhead equipment
ASME B31.12	Hydrogen piping and pipelines
ISO 14687	Hydrogen fuel quality
IEC 60079 series	Equipment for explosive atmospheres
NFPA 2	Hydrogen technologies code
VDI 4640	Thermal use of the underground (ground-source, BTES, ATES)

19.2India: authorities and rules to check

Gas, pressure and safety

PESO — Petroleum and Explosives Safety Organisation (pressure vessels, compressed gases)
OISD standards for oil and gas installations
PNGRB for pipeline connections
Factories Act and state factory inspectorate

Subsurface and mining

Directorate General of Hydrocarbons for petroleum fields and data
State mining departments and mineral concession rules
DGMS for mine safety
Central Ground Water Authority and state groundwater boards

Power sector

Central Electricity Authority technical standards and safety regulations
CERC and state commission regulations for connectivity and tariffs
Ministry of Power storage guidelines
MNRE schemes for green hydrogen

Environment and BIS

EIA Notification 2006 and environmental clearance
State pollution control board consents
BIS adoptions of IEC and ISO standards (IS/IEC 60079 and others)
Coastal or forest clearances where they apply

Approval requirements differ by state and by stored medium; BARREL confirms them with the authorities for each site. Standard titles are summarised, not quoted.

Section 21

Commercial Models, Costs and Investment Risk

Underground storage has high up-front cost for exploration, wells and surface plant, but very low cost per extra unit of energy stored. It pays off when large amounts of energy must be held for long periods.

21.1Indicative cost drivers

Indicative cost drivers
Item	Indicative range (public literature)	Comment
Salt cavern storage (container only)	≈ US$0.2–2 per kWh of H₂ (LHV)	Lowest of any hydrogen store at large scale
Lined rock cavern for H₂	Several times salt cavern cost	Demonstration stage
Above-ground pressure vessels	≈ US$10–30+ per kWh of H₂	For comparison
Diabatic CAES plant	≈ US$1,000–1,500 per kW	Using an existing salt formation
ATES system	Payback often 3–10 years vs conventional chillers	Depends on balance and energy prices
Underground pumped hydro	Often higher per kW than surface pumped storage	Saved dam costs offset by underground works

21.2Revenue and business models

Storage as a service

Capacity rented to hydrogen producers and users
Fee per tonne stored or per injection/withdrawal right
Like gas storage contracts

Power market

FDRE and RTC tender supply
Energy shifting and peak sales
Ancillary services and reserves

Thermal savings

Lower chiller and boiler energy bills
Cooling-as-a-service contracts
Reduced peak electricity demand charges

21.3Investment risks and mitigations

Investment risks and mitigations
Risk	Mitigation
Geology not as expected	Stage-gated spending, test wells before plant orders
Long development time	Early approvals plan; parallel surface works
Uncertain hydrogen demand	Anchor users, phased caverns
Regulatory gaps	Early engagement with PESO, CEA, MNRE and state bodies
Cost overrun underground	Experienced drilling partners, contingency, risk-sharing contract
Revenue certainty	Long-term storage contracts, tender-backed offtake

Costs are indicative global ranges from public studies (for example IEA, NREL and national laboratory work) and vary widely with geology and scale. They are not BARREL quotations.

Section 23

Frequently Asked Questions and References

Short answers to common questions about underground storage, with links to public reference sources.

23.1Questions and answers

What is underground energy storage?

Using caverns, porous rock, aquifers, boreholes or mines to hold energy as compressed gas, heat, cold or raised water, with only wells and a compact plant on the surface.

Why store hydrogen in salt caverns?

Salt is almost gas-tight, heals its own cracks and allows fast filling and emptying. It is the cheapest way to store large amounts of hydrogen and has been used for decades in the UK and US.

How much hydrogen does one cavern hold?

A large cavern of about 500,000 m³ typically holds a few thousand tonnes of usable hydrogen — of the order of 100–200 GWh of chemical energy. Use the live estimator in the Sizing tab for a first figure.

What is cushion gas?

Gas that always stays in the store to keep the cavern walls or reservoir pressure stable. It is not used, but it is part of the first-fill cost.

Does India have salt caverns?

India has limited known thick salt; Rajasthan's evaporite basins are the main area studied. No hydrogen salt caverns exist in India yet.

What are India's strategic petroleum reserves?

Unlined rock caverns at Visakhapatnam, Mangaluru and Padur holding about 5.33 million tonnes of crude in total. They show India can build large underground caverns.

Can rock caverns store hydrogen?

Yes, with a steel liner (a lined rock cavern). Sweden has a small hydrogen pilot and a larger natural gas demonstration. It costs more than salt but works where there is no salt.

What is CAES?

Compressed air energy storage: air is compressed into a cavern when power is cheap and released through a turbine when it is needed. Huntorf (1978) and McIntosh (1991) are the long-running examples.

What is ATES?

Aquifer thermal energy storage: warm and cold groundwater stored in separate wells and used in the opposite season for heating and cooling.

What is BTES?

Borehole thermal energy storage: heat passed into rock through closed pipe loops in many boreholes and recovered months later.

Is ATES useful in India's climate?

Yes where cooling dominates, especially if there is also a heat use (hot water, process heat) to keep the aquifer in balance. Groundwater permits are essential.

Can old coal mines store energy?

Mines can serve as lower reservoirs for pumped hydro, shafts for gravity storage or heat sources. Most projects worldwide are at study or pilot stage.

How long does a salt cavern project take?

Typically six to ten years from screening to operation, including exploration, approvals and two to four years of leaching.

Is underground hydrogen storage safe?

Gas storage underground has a long safety record. Risks are controlled by well integrity, pressure limits, gas detection and monitoring, as with natural gas storage.

Can hydrogen be stored in old gas fields?

It is being tested in pilots. Questions remain about purity, microbial reactions and how much can be recovered.

How efficient is underground storage?

It depends on type: pumped hydro about 70–80%, CAES about 40–70%, ATES recovery 60–90%, and hydrogen power-to-power about 30–40% overall.

Who approves an underground store in India?

Several bodies, depending on type — PESO, OISD, DGH, state mining and groundwater authorities, DGMS, CEA and environmental regulators.

What does BARREL do in such a project?

BARREL can screen sites, coordinate surveys and test wells through specialist partners, design and build the surface plant, connect it to the grid and run long-term monitoring and O&M.

1.1Formations and what they store

1.2Why go underground

Scale

Cost per kWh

Safety

Land

1.3What it is used for

2.1Three storage principles

Pressure (gas storage)

Temperature (thermal storage)

Height (gravity storage)

2.2Hydrogen cavern: one cycle

2.3Compressed air: one cycle

2.4Aquifer thermal: one year

3.1Typical salt cavern parameters

3.2Developing a cavern

3.3Well-known reference sites

Teesside, UK

US Gulf Coast

Huntorf and McIntosh

3.4India and the region

4.1Reference plants

4.2CAES variants

Diabatic

Adiabatic (A-CAES)

Isothermal / hydraulic

4.3Cavern requirements

5.1Strengths and open questions

Strengths

Open questions for hydrogen

5.2Pilots and studies

Hydrogen in a depleted gas field

Town-gas history

Underground methanation

5.3India context

6.1India's strategic petroleum reserve caverns

6.2Two kinds of rock cavern

Unlined (hydrodynamic containment)

Lined rock cavern (LRC)

6.3Design points

7.1Typical ATES parameters

7.2What a site needs

Ground

Demand

Permits

7.3India considerations

8.1Typical BTES parameters

8.2Reference and uses

Drake Landing, Canada

Industrial waste heat

Cooling-led use

8.3Design steps

9.1Options for a mine

Underground pumped hydro

Shaft gravity storage

Compressed air

Mine-water heat

Open-pit pumped hydro

Hybrid site

9.2Screening a mine

9.3India context

10.1Common configurations

10.2Main components

Subsurface

Wells and wellheads

Surface plant

10.3Hydrogen surface plant in detail

11.1Electrical systems

High voltage

Drives and machines

Auxiliaries

11.2Controls: EMS, SCADA and safety

Energy management (EMS)

SCADA and DCS

Safety instrumented systems

11.3Grid services by type

11.4Grid connection in India

12.1Gas held in a cavern

12.2Working gas and energy