California Institute for Energy and Environment (CIEE)

WESTCARB and C6 Resources are partners in a CO2 injection project in the Montezuma Hills, 80 km (50 mi) northeast of San Francisco, CA. Through a phased process that involves drilling an appraisal well and injecting CO2 on a small-scale, along with thorough analysis of data and modeling of the system, the goal of the project is to assess the deep geologic formations in the area for Geologic Carbon Sequestration (GCS), and if favorable, inject CO2 currently emitted to the atmosphere from nearby refinery facilities at industrial scales on the order of 1 million tons of CO2 per year. The deep geology at the site is considered very favorable for GCS by virtue of the numerous sandstone formations which are potentially capable of storing large amounts of CO2 and which are vertically separated by thick shale formations that prevent CO2 from migrating upward. This general geologic environment is a proven trap for natural gas over geologic time as evidenced by the nearby Rio Vista Gas Field. Assuming step-by-step progress through the various stages, the Montezuma Hills project will involve drilling an appraisal well to over 3 km (10,000 ft) depth, carrying out a small-scale evaluation injection of 6,000 tons of CO2, and evaluation of the feasibility of developing the site for a large-scale injection (e.g., 1 million tons of CO2), and further consideration of the site for an industrial-scale GCS operation (e.g., 0.75 million tons CO2/yr for 25 years). Because GCS is not widely carried out either in the U.S. or abroad, there is very little experience upon which to base estimates of performance of GCS systems. In the absence of a long track record, leakage risk assessment methods are needed to address concerns by the various stakeholders about the effectiveness of CO2 trapping and the environmental impacts resulting from CO2 injection. For the last two years, investigators at the Lawrence Berkeley National Laboratory (LBNL), The University of Texas at Austin (UT), and the Texas Bureau of Economic Geology (TBEG) have been developing a framework called the Certification Framework (CF) for estimating CO2-leakage risk for GCS sites (Oldenburg et al., 2009). Risk assessment methods such as the CF rely on site characterization, predictive models, and various methods of addressing the uncertainty inherent in subsurface systems. A brief outline of the methods used in the CF is provided in Appendix A. This report presents a discussion of leakage risk issues for the Montezuma Hills project and an outline of the research that needs to be done to carry out a leakage risk assessment by the CF approach. C6 Resources has already gathered and synthesized a large amount of data and information on the Montezuma Hills site to examine the feasibility of injecting CO2 at the site. In this case study discussion and research outline, we focus on public data and information that are important from the perspective of CO2 and brine leakage risk assessment. For understandability, inevitably some overlap with information already collected will occur, but our emphasis is on data and interpretations relevant to leakage risk assessment that apparently have not previously been considered in detail by C6 or WESTCARB related to vulnerable entities and potential risk mitigation. For example, we discuss the shallow aquifers, surface water, potential for pressure impact on natural gas resources, and the significance of historical natural gas seepage. As for risk mitigation, winds in the area are a favorable mitigating factor relative to surface leakage due to their ability to disperse CO2 ground plumes. Note that some of the information and text presented here is taken directly from an LBNL report two of us (Oldenburg and Jordan) contributed to several years ago (Oldenburg et al., 2003), and will be indicated as such.