CO2 Storage in Saline Aquifers

CO2 Storage in Saline Aquifers — Saline aquifers represent a promising way for CO2 sequestration. Storage capacities of saline aquifers are very important around the world. The Sleipner site in the North Sea is currently the single case world-wide of CO2 storage in a saline aquifer. A general review is given on the specific risks for CO2 storage in saline aquifer. The regional distribution of CO2 storage potential is presented. Finally, the knowledge gaps and the future research in this field are defined. CO2 Capture and Geological Storage: State-of-the-Art Capture et stockage géologique du CO2 : état de l’art D o s s i e r Oil & Gas Science and Technology – Rev. IFP, Vol. 60 (2005), No. 3 1 GENERAL AND SPECIFIC ASPECTS OF THE STORAGE OPTION Saline Aquifers are defined as porous and permeable reservoir rocks that contain saline fluid in the pore spaces between the rock grains. They generally occur at depths greater than aquifers that contain potable water. Usually, due to its high saline proportion and its depth, the water contained cannot be technically and economically exploited for surface uses. There is currently only one CO2 storage site worldwide in a saline aquifer. This is at Sleipner in the North Sea where CO2 is being injected into the Utsira Sandstone Formation. The amount of CO2 that could potentially be stored in saline aquifers, for a reasonable amount of time, is very large. The basic criterion for all potential storage sites are as follows. Potential storage sites should be in a geologically stable area, as tectonic activity could create pathways for the CO2 to migrate out of the reservoir through the cap rock (low permeability seal) into the overburden and potentially to surface. Saline aquifers can be sandstones or limestones, but to be a potential storage reservoir for CO2 they must have the following properties: – Size: the reservoir must be large enough to be able to store the quantities of CO2 planned e.g. the lifetime emissions of one power plant. The capacity of the storage site is the volume of pore spaces in the aquifer that could be occupied by CO2. – Porosity and permeability: These parameters must be sufficiently high to both provide sufficient volume for the CO2, and to allow injection of the CO2. As CO2 is injected into the pore spaces of the reservoir rock, it displaces much of the in situ pore fluid. If the permeability of the rock is low, or there are barriers to fluid flow, such as faults, injection will cause a progressive increase in the fluid pressure centred on the injection point. This will limit the rate at which CO2 can be injected, and may ultimately limit the amount of CO2 that can be practically stored. Highly structurally compartmentalised reservoirs are likely therefore to be less suited to CO2 storage than large unfaulted or high permeability reservoirs. – Depth: Usually only aquifers below 800 m below sea level are considered for CO2 storage. At temperatures and pressures in the subsurface of around 600 to 800 m CO2 exists in its dense phase as a liquid and occupies much less pore volume than in its gaseous phase. 1 t of CO2 occupies 509 m3 at surface conditions of 0°C and 1 bar. The same amount of CO2 occupies only 1.39 m 3 at 1000 m subsurface conditions of 35°C and 102 bar. (see Fig. 1) CO2 density at a geothermal gradient of 30oC/km. In addition to a reservoir rock, an overlying “cap rock” that is impermeable to the passage of CO2 is required. When CO2 is injected into a reservoir it is more buoyant than the reservoir fluid in the porespaces and will rise to the top of the reservoir. The cap rock, an impermeable low porosity layer will prevent the CO2 migrating vertically and so the CO2 becomes trapped at the top of the reservoir underneath the cap rock. The cap rock provides the main trapping mechanism for the long-term security of storage. Cap rocks are usually shales, mudstones or evaporite layers. The cap rock should ideally be unfaulted, as unsealed faults would provide migration pathways for the CO2 out of the reservoir. In some situations, for example in faulted salt layers, faults can become resealed, and therefore do not present a leakage/seepage pathway. However, their sealing nature would need to be confirmed by detailed analysis of the storage site to ensure the integrity of the storage site.