CO2 injection is a controlled process involving the delivery of carbon dioxide into underground geological formations or industrial systems. This technique aims to achieve specific outcomes, ranging from increasing the recovery of petroleum resources to mitigating greenhouse gas emissions. Precise engineering ensures the CO2 reaches its intended subterranean target, interacting with existing fluids or rock structures. It has applications across different industrial and environmental sectors.
Understanding CO2 Injection
CO2 injection introduces carbon dioxide into subsurface environments. Carbon dioxide is injected in a supercritical state, a phase exhibiting properties of both a liquid and a gas, allowing for high density and diffusivity. This unique state makes it effective for displacing fluids or reacting with geological materials deep underground. The injection process is managed to control pressure and flow, ensuring the CO2 behaves as intended within geological structures.
CO2 Injection for Enhanced Oil Recovery
CO2 injection is used in Enhanced Oil Recovery (EOR), a technique to extract additional crude oil from mature reservoirs after conventional methods become less effective. In this process, CO2 is injected into oil reservoirs to improve oil mobility and sweep efficiency. The injected CO2 mixes with the crude oil, which causes the oil to swell, reduces its viscosity, and lowers the interfacial tension, making it easier for the oil to flow towards production wells. This interaction helps to mobilize residual oil that would otherwise remain trapped.
CO2-EOR employs a water-alternating-gas (WAG) process, where CO2 is injected alternately with water to improve sweep efficiency and reduce CO2 channeling. This method can increase oil recovery rates from 20-40% to 30-60% of the original oil in place. Beyond increasing oil production, CO2-EOR offers an economic incentive, as it can revitalize aging oil fields and generate substantial revenue through increased oil output, taxes, and royalties. The technique also provides an opportunity for CO2 storage, as a portion of the injected CO2 remains trapped underground, offering a dual benefit of oil production and carbon management.
CO2 Injection for Carbon Capture and Storage
CO2 injection also serves as a climate mitigation strategy within Carbon Capture and Storage (CCS) initiatives. This involves capturing CO2 from large industrial sources, such as power plants and factories, and then transporting it for injection into deep geological formations for long-term containment. This application aims to reduce greenhouse gas concentrations in the atmosphere, addressing climate change concerns. The captured CO2 is compressed into a supercritical fluid before being injected deep underground to ensure it remains in this dense state.
The primary goal of CCS is to prevent large volumes of CO2 from entering the atmosphere, offering a pathway to decarbonize industrial processes. Suitable geological formations for storage include saline aquifers, depleted oil and gas reservoirs, and unmineable coal seams. These formations possess the necessary porosity to accommodate the CO2 and are overlain by impermeable caprock layers that prevent upward migration.
The Injection and Storage Process
The process of injecting and storing CO2 begins with site selection. Geological formations chosen for CO2 injection must have sufficient porosity and permeability in the reservoir rock to allow CO2 storage, and an overlying impermeable caprock layer to prevent its escape. Common reservoir rocks include sandstone, limestone, and dolomite, while seals are composed of shale, anhydrite, or low-permeability carbonates.
Once a suitable site is identified, injection wells are drilled into the chosen geological formation. CO2, in its supercritical state, is then pumped down these wells at high pressure. As the CO2 enters the porous rock, it displaces existing fluids like saline water. Over time, the injected CO2 becomes trapped through several mechanisms:
Structural trapping occurs when buoyant CO2 rises until it encounters the impermeable caprock, where it is held in place.
Residual trapping involves CO2 being immobilized by capillary forces within the pore spaces of the rock after the initial displacement of fluids.
Solubility trapping happens as CO2 dissolves into the formation water, forming a denser, CO2-rich brine that can sink within the reservoir.
Mineral trapping involves the slow geochemical reaction of dissolved CO2 with minerals in the rock, forming stable carbonate minerals that permanently bind the CO2 within the geological structure.
Monitoring and Safety Measures
Ensuring the safe and effective operation of CO2 injection sites requires comprehensive monitoring and safety measures. Various techniques track the movement of CO2 underground and detect any leaks. Seismic surveys map the subsurface and observe changes in rock properties indicating CO2 migration. Wellbore pressure monitoring measures pressure within injection and observation wells, providing insights into CO2 distribution and containment. Surface measurements, such as eddy-covariance towers, can detect CO2 fluxes above the injection site, indicating any leakage to the atmosphere.
Regulatory oversight establishes guidelines and requirements for CO2 injection projects, ensuring adherence to environmental and safety standards. Contingency plans are developed to address unforeseen events, such as unexpected CO2 migration or well integrity issues. These measures aim to provide confidence in the long-term security of CO2 storage, protecting surrounding environments and communities.