ACTIVE MANAGEMENT OF GEOLOGIC RESERVOIRS FOR CO2 SEQUESTRATION: CONSTRAINTS IMPOSED BY BRINE COMPOSITIONS AND TREATABILITY

Active Management (Buscheck et al., 2010) combines brine extraction with desalination and residual brine reinjection as a technique of managing pressure, capacity, and risk in storing CO2 in deep saline formations. Potential benefits include a source of usable water and a reduced Area of Review. A necessary factor (examined here) is that is that the displaced brine must be treatable by desalination. Treatment becomes more difficult with increasing Total Dissolved Solids (TDS), which ranges from a lower (regulatory) limit of 10,000 mg/L up to about 400,000 mg/L. Specific brine composition matters, but is less significant. We calculated the result of removing water from various brine compositions, simulating reverse osmosis (RO), focusing on mineral precipitation and increasing osmotic pressure of the residual brine. The rise in the latter is more limiting on practical water extraction. Brines with TDS in the range 10,000-40,000 mg/L TDS are prime candidates for RO treatment, using a process much like that for seawater. Brines with TDS in the range 40,000-85,000 mg/L TDS may be treatable by RO alone, but with lower recovery. Above 85,000 mg/L, less conventional methods are necessary, such as NF (nanofiltration) + RO or multi-stage RO. Above 300,000 mg/L TDS, brines are probably untreatable. We used the U.S.G.S. Produced Waters Database (Breit, 2002) to look at the distribution of TDS and brine composition with depth over various geographic regions (mainly at the state level). A depth of 800 m (2625 ft.) or more is required to maintain CO2 in the supercritical state. Between that depth and about 11,000 ft., there is in general (at the national level) only a small tendency for TDS to increase with depth. In some locales, TDS decreases. Thus, injecting at greater depth does not generally lead to less treatable brine. Data for various western states (e.g., Colorado, Wyoming, California) show substantial fractions of brine in the 10,000-85,000 mg/L TDS range, indicating that active management is feasible, insofar as the brine composition is concerned. The situation is not so attractive in some other states (e.g., Mississippi, Illinois), where the brines are much saltier. Substantial opportunities remain to analyze subsurface brine data using the U.S.G.S. database and its larger cousin, the NATCARB database.