![]() ![]() Once trapped below the seal the CO 2 is expected to remain sequestered permanently unless the CO 2 encounters a permeable fault or fracture in the seal or a leaky wellbore. Seals are composed of shale, anhydrite, or low permeability carbonate rocks. ![]() Because the supercritical CO 2 is less dense than the fluids that initially fill the pore spaces in the rocks, it will rise by buoyancy forces through the reservoir rocks until it encounters a low permeability rock, typically called a reservoir seal. Prospective reservoir rocks include sandstone, limestone, dolomite, or mixtures of these rock types. Suitable geological formations for storing CO 2 comprise a porous and permeable reservoir rock, overlain by an impermeable rock ( Figure 7.1). This is due to the high-density of the fluid (~600 kg/m 3) relative to gaseous CO 2 and the reduced buoyancy forces in water-filled geological formations, although the system maintains a strong buoyant drive between CO 2 and brine ( Benson et al., 2005). Compression of the gas to a supercritical fluid allows more CO 2 to be sequestered. After CO 2 is captured, it is compressed into a supercritical fluid, then injected down a well into a geologic formation that is deep enough for the CO 2 to remain as a supercritical fluid, typically 1 km or more. Geological sequestration is a necessary complement to direct air capture of carbon dioxide (CO 2) (see Chapter 5) and bioenergy with carbon capture and sequestration ( Chapter 4). CHAPTER SEVEN Sequestration of Supercritical CO 2 in Deep Sedimentary Geological Formations INTRODUCTION
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