FIGURE 3.20 Current options for sequestering CO2 in subsurface geological formations. SOURCE: Reprinted with permission from CO2CRC.

cal characterization and model development, typically by teams of sedimentary geologists, geophysicists, and modelers.

The technology for pumping CO2 underground is well developed, based on over four decades of experience in the CO2-EOR industry. Well drilling and completion technology, pumps, and surface equipment are all available. Rigs and rig crews used for oil and gas exploration and production are appropriate for drilling injection and monitoring wells for geological storage projects. The requirements for cementing injection wells for geological sequestration differ from a typical oil and gas well and special provisions must be made to meet the regulatory requirements to fully cement the well throughout its entire length.

CO2-EOR, the largest utilization option, involves injecting supercritical CO2 into an oil field. When mixed with oil, CO2 lowers the viscosity and reduces the density of the oil, leading to improved recovery. For certain types of oil, at elevated pressures and temperatures, oil and water can become miscible, meaning they become a single fluid. Under these conditions, oil recovery can be improved dramatically. The bank of mobile oil is swept to the extraction well, sometimes involving the injection of water to further increase recovery through the so-called WAG (water-alternating-gas) process (Figure 3.21). Successful CO2-EOR operations require sophisticated reservoir engineering calculations, advanced geological modeling, and well completion technologies.

However, unlike oil and gas production, where monitoring tends to focus on injection and extraction rates and wellhead pressures, CO2 sequestration requires tracking the location of the CO2 plume, ensuring that it is not leaking back to the surface, and quantifying the amount of CO2 sequestered. Monitoring techniques include seismic imaging, wellhead and formation pressure testing, well logging,

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