The Prospect of Massive Sequestration of Atmospheric CO2 in Deep Formations of Basalt or Peridotite Appraised by Fracture, Diffusion and Osmosis Analysis and Frac Analogy

ABSTRACT One way to mitigate global warming is to withdraw CO2 from the atmosphere and inject it into deep formations of porous basalt to form carbonate minerals that can bind large quantities of CO2. Such formations are found in many places in the world. The process is to some extent similar to frac. It requires hydraulic fracturing of the basalt to create a sufficient crack surface area needed for penetration of CO2 into basalt. The success further depends on the permeability of basalt, possibly assisted or hindered by gradients of osmotic pressure due to gradients of concentration of sodium and other ions. These diffusion and osmotic effects generally decay with the square of spacing of adjacent parallel cracks. Poromechanical effects such as the gradual transfer of tectonic stress from the fluid phase to the solid phase in the basalt enter the picture, too. An analytical continuum model of the combined interacting effects of fracturing, diffusion, osmosis, poromechanics and crystal growth under stress is a big challenge. The purpose of the conference presentation is to briefly review ways to model these problems physically and computationally and to point out the diverse obstacles faced by the new multi-university-lab project. HISTORICAL LESSON Many of the challenges of the Center for Interacting Geoprocesses in Mineral Carbon Storage, a newly funded EFRC collaborative, multi-university-lab project of University of Minnesota, Northwestern University, Los Alamos National Laboratory (LANL), University of Southampton and Georgia Tech, are similar to those of the 1970s project Hot-Dry-Rock Geothermal Energy project funded by RANN (Research Applied to National Needs, 1974-78), The RANN project involved six mechanics faculty members at Northwestern (led by Johannes Weertmann), three senior researchers at LANL (LASL at that time) and a drilling team (the total cost was about $30 million in today's dollars). Unfortunately, the conclusion of that project came out to be negative but the US is currently investing in this concept (now called Enhanced Geothermal Systems or EGS), most prominently at the Frontier Observatory for Research in Geothermal Energy or FORGE in Utah, inspired by successes in Europe at Soultz-sous-Forêts and Rittershoffen Geothermal Plants. In any case, the lessons are today valuable for deep CO2 sequestration (Kelemen & Matter, 2008), as well as for frac (hydraulic fracturing, or fracking) of gas or oil shale (Z. P. Bažant et al., 2014; Rahimi-Aghdam et al., 2019). Modern frac technology, with horizontal drilling and sand proppants, is applied at FORGE. The objective of the EFRC is to use fracture mechanics to enhance the mineralization of injected CO2 in mafic (basalt) and ultramafic (peridotite) rocks. As with EGS and shale gas, these rocks are relatively impermeable and success will require hydraulic fracturing and the development of a dense fracture network that exposes abundant fracture surface area for chemical reactions.