CCUS stands for Carbon Capture, Utilization and Storage. Too often, it is interpreted to mean “Carbon dioxide” Capture, Utilization and Storage; this may lead to a constraint on the imagination, leading to a reluctance to assess other potential pathways to achieve the goals of atmospheric emissions reductions. Several “unconventional” CCUS pathways are discussed here, perhaps helping to trigger ideas that may lie outside of the conventional pathway of “CO2 capture, compression and injection”, whether for EOR or direct sequestration. Furthermore, only direct sequestration will be addressed here.
Cyclic scCO2 Injection: Standard approaches usually involve scCO2 (supercritical CO2) although injection of CO2/H2O mixtures has been studied. For long-term sequestration, all agree that dissolved CO2 is far more secure than accumulations of vast volumes of low-density, low viscosity scCO2, but in the interests of pore volume utility, pure scCO2 injection is usually favored. A transition zone develops (0 < Sw <1) as water is displaced by more buoyant scCO2, and that a broad unsaturated zone is generated with a large internal surface area favoring rapid CO2 dissolving into the aqueous phase. If scCO2 is injected to a particular volume (V is a function of p, T, reservoir geometry and displacement efficacy), it can be followed by injection of waste water, and an “inverse” transitional zone is generated. This displaces and mixes with the scCO2, and with the right water volume injection, the scCO2 is fully dissolved. Waste water and scCO2 injection are alternated, so that the great majority is dissolved, and only limited regions of free scCO2 remain, and the risk of escape is greatly reduced is this dissolved form.
Density Driven Flue Gas Injection: Suppose a vertically extensive or inclined saline aquifer of suitable properties is available. If a horizontal well is placed at the base and CO2-enriched flue gas is injected, it will be buoyant, and a plume of bubbles will rise. However CO2 is perhaps 15× as soluble as N2, so a chromatographic separation takes place, and the N2 will accumulate at the top of the reservoir, where it can be vented to the atmosphere, leaving the CO2 in solution. Buoyant bubble flow generates convective flow cells in the reservoir, with the water phase displaced outward and downward in a large density-driven vortex. The injection process is continued for as long as the conditions allow (recognizing the existence of flow instabilities and channelling…), and is monitored through the CO2 content in the venting N2.
Biosolids Injection: Biosolids (human and animal wastes, organic-rich process liquids such as Kraft Process black liquor, organic garbage, etc.) will decompose at depth (500-2000 m) in a few years through the actions of methanogenic archaebacteria found in all rocks (the process of coalification). Decomposition products are CO2, CH4 and remnant carbon-rich solid. CO2 is much more water-soluble than CH4, and most of the evolved CH4 can migrate to the top of the injection reservoir to be produced for beneficial use. The solid carbon-rich phase is forever entombed with zero escape risk, and most of the CO2 remains is dissolved state. This process is not limited by pore volumes available because the biosolids and waste water slurry must be injected under hydraulic fracturing conditions. If not already in the form of a slurry, biosolids are ground and screened to reduce particle size to less than 2-3 mm, slurried with waste process water, and injected into a vertical wellbore with open perforations in the target formation. Given the reliable supply of biosolids, this secure sequestration process can not only go on indefinitely, it provides a highly effective, economical, and environmentally secure method of complete sewage and organic garbage treatment.
Carbon Disulphide Injection: The buoyancy of scCO2 is of concern to sequestration (and EOR) processes; its accumulation in a large vertical extent means that the pressure differences at the top of the scCO2 column are large, leading to increased risk of caprock breaching. Suppose that a cheap way of combining sulphur-rich waste carbon (petcoke from heavy oil) with surplus waste sulphur can be developed to generate CS2. This dense phase (ρ = 1.54 g/cm3) can be injected at depth into old oil and gas wells, with no realistic chance of escape because the CS2 is negatively buoyant in the presence of all other pore fluids, including saturated brines. Hence, escape risks are realistically zero indefinitely, and surplus sulphur is co-disposed with carbon to achieve excellent use of pore volumes because of the gravity displacement effects. That is, the CS2 will continue downward migration as long as a sufficiently permeable pathway exists in the saline aquifer.
No claims about cost-effectiveness are made here, although assessment of several of these processes suggests they may be economically viable in comparison to other approaches in similar economic regimes. It can be claimed, however, that all of these methods achieve a high degree of security against uncontrolled CO2 escape, and it appears they can be implemented in reservoir conditions far less favorable than those required for continuous pure scCO2 injection. In all cases, only wastes are injected.