A new technology replaces the water

Decahydron has developed a new technology for storing CO₂ in ultramafic rock. It injects the CO₂ as a dense, supercritical fluid, with no water added. It replaces the older method, which first dissolves the CO₂ in large volumes of water and then injects the carbonated water.

Both methods turn CO₂ into carbonate rock for good. The difference is how much CO₂ each well can carry. We compared the two in the same rock, with the same pressure limits and the same costs: the new technology needs a fraction of the wells and no water at all.

The chemistry: the same for both

CO₂ meets waterCO2 + H2O → H2CO3 → H+ + HCO3−The water turns acidic.
Acid dissolves the rockMg2SiO4 + 4H+ → 2Mg2+ + H4SiO4Olivine, pyroxene and serpentine release magnesium, calcium and iron.
Carbonate formsMg2+ + HCO3− → MgCO3 + H+Magnesite and calcite fill fractures and pores.
Stored as rock0.4–0.6 t CO2 per t of rock reactedSolid carbonate: permanent, nothing left to leak.

The two methods: how the CO₂ reaches the rock

ADecahydron’s technology: dense CO₂, no water

Cutaway: dense CO₂ spreading under a cap rock in fractured peridotite, with carbonate veins forming12345
  1. Wellhead: dense CO₂ at about 100 bar
  2. Cap rock holds the CO₂ in place
  3. Dense CO₂ spreads under the cap
  4. It dissolves into the water already in the rock
  5. and turns into carbonate in the fractures

BThe older method: CO₂ dissolved in water

Cutaway: carbonated water sinking through fractured peridotite, with carbonate veins forming12345
  1. Wellhead: CO₂ dissolved in water at 25-33 bar
  2. No cap rock needed: no free CO₂
  3. Carbonated water is denser, it sinks
  4. The acidic water dissolves the rock
  5. and the CO₂ turns into carbonate

Illustration Both routes end in the same carbonate. They differ in how the CO₂ reaches the rock.

One million tonnes a year: the numbers side by side

240×
More CO₂ per well as a dense fluid than dissolved in water
4 vs 644
Injection wells for one million tonnes of CO₂ a year
41 t
Of seawater to pump for every tonne of CO₂ dissolved
66×
Lower injection cost per tonne with dense CO₂ (relative cost, same assumptions for both)

Why one well does the work of hundreds

Two properties set the gap. Dense CO₂ flows through the rock eight times more easily than water. And every tonne injected is CO₂, while carbonated water carries only 2.4% CO₂: the other 97.6% is water. Together, at the same pressure and in the same fractures, one well takes about 240 times more CO₂ as a dense fluid. Even if two-phase flow cuts the CO₂ rate by 70%, the gap is still about 70 times.

0.00.20.40.6Dense CO₂0.059Carbonated water0.50Viscosity in the rock (mPa s)
It flows 8x more easily
0%25%50%75%100%Dense CO₂100% CO₂Carbonated water2.4% CO₂Share of each tonne injected
It is all CO₂, not 2.4%
1101001,00010,000100,000131030100Rock permeability (millidarcy)CO₂ injected per well (t/day)
So one well takes about 240x more
Dense CO₂Dense CO₂, with flow lost to two phases (relative permeability 0.3)CO₂ dissolved in seawaterPermeability measured in our Sharjah wellWater

The water bill: 36 to 45 tonnes for every tonne of CO₂

CO₂ dissolves less in warm water and in salty water. At the Gulf’s 32 °C and a 25 bar dissolution pressure, one tonne of CO₂ needs 36 tonnes of fresh water, 41 tonnes of seawater or 45 tonnes of desalination brine. Higher pressure helps, but a million tonnes of CO₂ a year still means moving 25 to 45 million tonnes of water.

20304050Fresh water0 g/kgGulf seawater42 g/kgDesalination brine74 g/kgt of water per t CO₂
32 °C, 25 bar: Gulf water temperature25 °C, 25 bar32 °C, 40 bar

Energy is not what separates them

Dense CO₂ must be compressed to about 100 bar. Carbonated water needs less compression but has to pump forty tonnes of water for every tonne of CO₂. Taken from a pipeline, dense CO₂ uses less energy per tonne than the dissolved route; even from the capture plant the two are within a third of each other.

020406080100120140CO₂ at 1 bar, at the capture plantCO₂ from a 30 bar pipelineCO₂ from an 80 bar pipelinekWh per t CO₂ injected
Dense CO₂: compression to 100 barDense CO₂: drying and utilitiesCarbonated water: CO₂ compression to 25 barCarbonated water: pumping 41 t of seawater per t CO₂Carbonated water: utilitiesLeft bar of each pair: dense CO₂. Right bar: carbonated water.

Wells decide the cost

To store a million tonnes of CO₂ a year, dense CO₂ needs about 4 wells. Dissolved in water, the same CO₂ needs about 644, because the water takes up almost all of each well’s injection capacity.

Injection wells needed for one million tonnes of CO₂ a year
1101001,000Dense CO₂Carbonated waterInjection wells for one million tonnes of CO₂ a year (log scale)
Dense CO₂: typical caseCarbonated water: typical caseBar: range from good to poor cases (10th to 90th percentile of 20,000 cases)
Relative injection cost per tonne of CO₂

Fewer wells means a lower cost. The chart below shows relative cost: the typical cost of dense CO₂ is set to 1. The older method typically costs 66 times more. Even its cheapest cases cost more than six times the most expensive cases for dense CO₂.

0.3131030100300Dense CO₂Carbonated waterRelative cost per tonne of CO₂ (typical dense-CO₂ cost = 1, log scale)
Dense CO₂: typical caseCarbonated water: typical caseBar: range from good to poor cases (10th to 90th percentile of 20,000 cases)

Where each route belongs

Dissolving CO₂ in water is chemically sound and mineralises fast. It is the right tool in some rocks. In buried, capped ultramafic rock, dense CO₂ does the same job with a fraction of the wells.

Dense CO₂
  • Buried ultramafic rock under a cap
  • Rock with millidarcy-range permeability, as measured in the ophiolite
  • Millions of tonnes a year from a handful of wells
  • No water to source, pump or dispose of
Carbonated water
  • Shallow or outcropping rock with no cap to hold a dense phase
  • Very permeable rock, far above what ophiolites offer
  • Abundant water next to the site
  • Pilots and small volumes

How we compared them. Steady radial flow into a 100 m interval at a pressure 3 MPa above the formation, the window a cap rock allows. Permeability from fall-off tests in our Sharjah well and from published injection data in the same type of peridotite. Dense CO₂ at 712 kg/m³ and 0.059 mPa s; carbonated seawater at 0.50 mPa s, carrying 1 t of CO₂ per 40.8 t of water. Solubility from PHREEQC (Pitzer database); compression and pumping energy from DWSIM process models; electricity at US$0.10/kWh. Costs cover injection only: site compression, water handling, injection wells and energy. Capture, transport and monitoring are the same for both routes and left out. Monte Carlo: 20,000 draws per route on permeability, interval height, pressure window, skin, two-phase losses, clogging and well cost, for one million tonnes of CO₂ a year delivered by a 30 bar pipeline.

Storing CO₂ in ultramafic rock?

If you inject carbonated water today, talk to us about what dense CO₂ could do in the same rock, with the same chemistry and far fewer wells.

Talk to us