[Daumic (OP)]

CO2 capture is actually seen as a necessary burden for fighting the climate change. But if we aggregate some technologies, CO2 capture can perhaps become the starter of deep geothermal energy. Geothermal heat is a renewable energy that has a great value because it is not sporadic contrary to wind or sun light but has high investment costs. CO2 capture in ultramafic rocks like peridotite can reduce these investment costs.

a) CO2 capture in ultramafic rocks works by chemical reaction between CO2 and MgO contained in rock. This reaction is exothermic (760 kJ / kg) and thus can produce hot water. The article “In situ carbonation of peridotite for CO2 storage” (1) of Peter B. Kelemen and Jürg Matter permits to imagine the following process:
-   two vertical wells to reach a deep layer of peridotite,
-   horizontal drill between the vertical wells with a hydraulic fracturing,
-   water circulating in the fractured zone reaches a high pH by dissolution of a little part of Mg2+ contained in the rock,
-   this high pH water captures CO2 from air when it is pumped out of the outlet well,
-   injection of this carbonated solution in the inlet well,
-   a great part of the dissolved CO2 reacts with rock in the fractured zone to form solid carbonate ; water becomes again basic before reaching the outlet well for a new cycle of CO2 capture,
-   the reaction between CO2 and MgO in peridotite gives a high temperature to the circulating water, temperature perhaps sufficient for urban heating.

(1) https://www.pnas.org/content/early/2008/10/31/0805794105 [nofollow]

b) Peridotite contains a high percentage of MgO but also a smaller percentage of FeO. As seen before in (a), the injected water reaches high temperature and high pH. High pH and high temperature make possible the oxidation of Fe2+ by water. This reaction produces hydrogen gas H2 (2). If we can produce hydrogen besides CO2 capture, the economical value of this gas can lighten the cost of wells and fracturing.

(2) https://hal.archives-ouvertes.fr/tel-00934238/ [nofollow]

c) USGS and other geological administrations have produced maps where are located deposits of ultramafic rocks (3). These maps show that the CO2 capture and hydrogen production can be made in many countries.

(3) https://pubs.usgs.gov/ds/414/downloads/DS414_text_508.pdf [nofollow]

d) Hydraulic fracturing has benefited recently a major improvement: the refracking (4). This technique consists to fracturing a new zone using an ancient vertical well. The process has permitted the cost reduction of shale gas extraction. How can we apply this for CO2 capture? When the fractured zone is saturated with CO2, we can use the same vertical wells to fracture another zone of peridotite deeper than the saturated zone. Step by step, we can use the same wells to reach deeper and hotter rocks.

(4) https://petroleumconnection.com/refracking-great-expectations-for-the-second-time-around/ [nofollow]

CO2 capture in peridotite can aggregate three productions:
-   hot water for heating,
-   hydrogen gas for industrial use or electric generation,
-   very hot water for electric generation.

The economic value of these productions can facilitate CO2 capture and clean geothermal energy.