Quantifying leakage from CO<sub>2</sub> reservoirs
Jerome Neufeld, Dominic Vella, Herpert Huppert, and John Lister
ITG, DAMTP, University of Cambridge, England
Lay-language version of "Localized leakage from porous and viscous gravity currents"
Concern for the long-term fate of the Earth’s climate has increased in recent years, with many studies linking the increased atmospheric concentrations of carbon dioxide (CO2) with the rise in global mean annual temperature. One route to significant reductions in the CO2 emissions from stationary sources, such as power plants, is the capture and storage of large volumes of CO2 in the subsurface: so-called geological carbon sequestration. If geological carbon sequestration is to be implemented on a large scale, one key question is will it leak back to the surface and, if so, how quickly?
In several current implementations of carbon capture and storage (CCS), carbon dioxide emitted from stationary sources is collected and compressed before being injected into porous rock at depths of 1 km or more beneath the Earth’s surface. There the CO2, which is less dense than the salty water or brine it displaces, rises until it is stopped by a relatively impermeable cap rock such as a mudstone or clay. Its rise impeded, the injected CO2 then spreads laterally driven by its own buoyancy, much as honey spreads when poured onto toast.
As it spreads laterally, the CO2 current will come into contact with a large area of the overlying cap rock; any flaws in the cap rock are thus likely to be discovered by the current and could lead to leakage. These flaws may be natural, such as faults or fractures in the cap rock, or man-made, such as abandoned drilling wells. The magnitude and speed of leakage depend both on the geometry of the fault and on the characteristics of the reservoir.
Our work aims to study leakage in different scenarios (e.g., from a well-bore, or fault) by using analogue laboratory experiments combined with theoretical models to describe the flow of the CO2 current. A useful measure of the long term effectiveness of a geological storage site is the efficiency of storage, which we define as the ratio of permanently stored to injected CO2. If fluid never leaks from the reservoir the efficiency of storage is 100%. However, if fluid leaks from the reservoir the efficiency of storage decreases with time. Our models determine the dependence of the efficiency of storage on the geometry of a single leak and also on the time since the spreading CO2 current ‘discovered’ the leakage site. In each of the scenarios we have considered to date, we find that the efficiency of storage decreases indefinitely: ultimately a state is reached in which CO2 leaks at exactly the same rate as it is pumped in! Though this may sound like bad news for CCS, the crucial factor is the time taken for the efficiency of storage to decay to, say, 50%. Our study also predicts this time scale for several different scenarios.
What then are the implications for CO2 sequestration? The time scales involved in leakage are large – large enough that a further series of processes by which the CO2 may be permanently stored may have time to become significant. For example, as the CO2 current spreads a portion of the CO2 is trapped within the small pores of the rock, much as water is retained within a sponge. In addition, as the CO2 current spreads it dissolves into the brine already within the rock forming a mixture that is more dense than either the CO2 or the brine. This mixture therefore sinks to the base of the reservoir where it is stably sequestered. On even longer time scales CO2 may react with the host rock, forming mineral deposits, immobilising the CO2 permanently. In conclusion, while leakage may occur, the rates involved may be slow enough that the majority of injected CO2 will stay securely trapped beneath the surface.