Shallow Geologic Storage of Carbon to Remove Atmospheric CO2 and Reduce Flood Risk

Abstract

Geologic carbon storage currently implies that CO₂ is injected into reservoirs more than 1 km deep, but this concept of geologic storage can be expanded to include the injection of solid, carbon-bearing particles into geologic formations that are one to two orders of magnitude shallower than conventional storage reservoirs. Wood is half carbon, available in large quantities at a modest cost, and can be milled into particles and injected as a slurry. We demonstrate the feasibility of shallow geologic storage of carbon by a field experiment, and the injection process also raises the ground surface. The resulting CO₂ storage and ground uplift rates upscale to a technique that could contribute to the mitigation of climate change by storing carbon as well as helping to adapt to flooding risks by elevating the ground surface above flood levels. A life-cycle assessment indicates that CO₂ emissions caused by shallow geologic storage of carbon are a small fraction of the injected carbon.

Keywords: carbon removal; climate change adaptation; climate change mitigation; flood protection; geologic storage; global warming; negative emission technology; sea level rise.

Figure 1

Shallow geologic carbon storage (Shallow GCS) by injecting carbon-bearing particles. (a) Present conditions where carbon is removed from the atmosphere by photosynthesis and returned by biomass decay and burning biofuels; flood damage is increasing due to sea-level rise and subsidence (SLR + S), storm intensity, etc. (b) Wood particles injected to store carbon and raise ground elevations. Carbon returned to the atmosphere is reduced, resulting in net atmospheric removal. Flood risk is reduced by increasing elevation by the solid injection to raise ground elevation (SIRGE) method introduced by Germanovich and Murdoch.

Figure 2

Uplift over hydraulic fractures created by injecting wood particles. (a) Contours of uplift in mm near a single injection location. (b) Wood particles used for injection. (c) Uplift and total vertical thickness of layers of wood particles in the core as functions of the distance from the injection casing along different radial lines at the single injection location (symbols). Thickness data were measured at points shown in the map inset. Location of the EW section in (c) is shown in (a). Simulated uplift and thickness using flat, cone, and saucer-like geometries as in Figure S5 (lines). (d) Core sample with a layer of wood particles created by multiple injections. (e) Contours of uplift in mm at 3 × 3 array of injection locations at the end of the experiment. Red circles are injection casings, and red crosses are locations where uplift was measured. (f) Simulated uplift using the numerical model described in the Supporting Information (Section S2.3).

Figure 3

Cross-sections of hydraulic fractures inferred from core measurements and the overlying uplift and total vertical thickness of injected wood particles at the single injection location. (a) and (b) show the EW section looking north based on samples at yellow symbols in (g). (c) and (d) show the NS section looking east based on samples at green symbols in (g). (e) and (f) are sections along the NE-SW (red triangles) and NW-SE sections (pink circles) in (g). Locations of the core holes (colored symbols) used to create the section lines in (g) along with contours of elevation (m) relative to the injection casing (dashed lines). (g) is enlarged in Figure S2.

Figure 4

Uplift (left side) and slope (right side) at the ground surface, and cross-sections of vertical displacement from simulations calibrated to field data. (a) Results for injections at 2.4 m depth, as in the field test. (b) Uplift and slope for injections at 7.5 m depth. Uplift in both models displaces 25 m³.

Figure 5

Mass of CO₂ equivalent stored as a function of time (days when no work occurred are omitted). The ratio of CO₂ equivalent to wood particles by mass was kg CO₂-eq/kg_wood = 1.6 (bulk density of the wood: 340 kg/m³; moisture content of the wood: 0.26; molecular weight ratio C/CO₂: 0.27). Eight to 12 injections occurred on each working day, and they are shown as yellow circles. The overall average daily rate is in red. The daily average rates of CO₂-eq stored are in gray.

Figure 6

Simulated displacement and slope within a scalable design element comprising a 4 × 6 array of injection locations to give approximately 1 m of uplift and 16,000 m³ of displacement. Simulations were done with the model described in Section 2.3. (a) Perspective of 24 injected layers at 30 m depth. (b) Perspective of ground surface with 50× vertical exaggeration, color scale as in (c). (c) Map of vertical displacement (uplift) showing a central zone with uniform uplift and a sloping flank. (d) Map of displacement gradient (slope).