Active CO2 Reservoir Management: A Strategy for Controlling Pressure, CO2 and Brine Migration in Saline-Formation CCS

CO2 capture and sequestration (CCS) in deep geological formations is regarded as a promising means of lowering the amount of CO2 emitted to the atmosphere and thereby mitigate global warming. The most promising systems for CCS are depleted oil reservoirs, particularly those suited to CO2-based Enhanced Oil Recovery (CCS-EOR), and deep saline formations, both of which are well separated from the atmosphere. For conventional, industrial-scale, saline-formation CCS, pressure buildup can have a limiting effect on CO2 storage capacity. To address this concern, we analyze Active CO2 Reservoir Management (ACRM), which combines brine extraction and residual-brine reinjection with CO2 injection, comparing it with conventional saline-formation CCS. We investigate the influence of brine extraction on pressure response and CO2 and brine migration using the NUFT code. By extracting brine from the lower portion of the storage formation, from locations progressively further from the center of injection, we can counteract buoyancy that drives CO2 to the top of the formation, which is useful in dipping formations. Using “push-pull” manipulation of the CO2 plume, we expose less of the caprock seal to CO2 and more of the storage formation to CO2, with more of the formation utilized for trapping mechanisms. Plume manipulation can also counteract the influence of heterogeneity. We consider the impact of extraction ratio, defined as net extracted brine volume (extraction minus reinjection) divided by injected CO2 volume. Pressure buildup is reduced with increasing extraction ratio, which reduces CO2 and brine migration, increases CO2 storage capacity, and reduces other risks, such as leakage up abandoned wells, caprock fracturing, fault activation, and induced seismicity. For a 100-yr injection period, a 10-yr delay in brine extraction does not diminish the magnitude of pressure reduction. Moreover, it is possible to achieve pressure management with just a few brine-extraction wells, located far from the injection zone. For an extraction ratio of 1, pressure buildup is minimized, greatly reducing the Area of Review, as well as the area required for securing mineral rights. For an extraction ratio of 1, CO2 and brine migration are unaffected by neighboring CO2 operations, which allows planning, assessing, and conducting of each operation to be carried out independently; thus, permits could be granted on a single-site basis. Brine-extraction wells will be useful during monitoring, providing information for system calibration and history matching. One of several key aspects that ACRM has in common with CCS-EOR is the possibility of generating revenue from the extracted fluids; namely, fresh water produced via brine desalination, using technologies such as Reverse Osmosis. These benefits can offset brine extraction and treatment costs, streamline permitting, and help gain public acceptance. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.