REPORT: PROPER SITING, MONITORING PARAMOUNT TO SAFE CO2 STORAGE

Tamar Hallerman
GHG Monitor
7/12/13

Thorough site characterization and monitoring is key for the successful and permanent sequestration of carbon dioxide underground, according to a new study that evaluates geomechanical data from three of the world’s pioneering CO2 storage sites at Sleipner, Weyburn and In Salah. The report, compiled by researchers at the University of Bristol, BP Alternative Energy and the British and Canadian geological surveys and published in this week’s edition of Proceedings of the National Academy of Sciences, compares data from the world’s first three major CO2 monitoring projects and concludes that their varying results mean that pre-injection site characterization and consistent monitoring work can do the most to ensure that CO2 stays safely underground and does not leak. “What we’ve seen is a huge amount of variability in the response to CO2 injection and that it is controlled by the geologic variability of sites, so site selection and monitoring so that you know your appraisal is correct is entirely key,” head author James Verdon, a geophysicist at the University of Bristol in the U.K., said in an interview with GHG Monitor this week.

Assessing geomchanical data from all three storage sites, the paper finds that Weyburn, located in southeast Saskatchewan, Canada, has seen the most varied results over time, seeing the frequency of microseismic events wax and wane in “apparently counterintuitive patterns.” “Experience at Weyburn shows that, where both injection and production occur simultaneously, the geomechanical response can be more complicated and sometimes nonintuitive,” says the paper, which notes that Weyburn has undergone enhanced oil recovery operations for nearly 20 years. “These effects will need to be accounted for on a site-specific basis via numerical geomechanical modeling.”

On the other hand, while evaluating data from Statoil’s Sleipner project in the Norwegian portion of the North Sea, the paper says the large, high-permeability saline aquifer used for CO2 sequestration there has proven to be ideal, showing “little possibility of geomechanical deformation.” At BP’s In Salah project in Algeria, which operated between 2004 and 2011 and injected CO2 stripped from a natural gas processing unit into the water leg of the relatively low-permeability Krechba gas reservoir there, researchers ultimately experienced increased pore pressures and “substantial” microseismic activity, which raised concerns about seal integrity, according to the paper.

The paper concludes that the different geomechanical behavior exhibited at each site underscores the need for each reservoir to get thoroughly vetted prior to, and during, injection in order to identify any factors that could lead to the increased potential for microseismic events that could create fractures in the caprock seals and provide a pathway for CO2 leakage. “These sites show very different geomechanical responses, highlighting the importance of systematic geomechanical appraisal prior to injection, and comprehensive, multifaceted monitoring during injection at any future large-scale CCS operations,” the paper says.

Pressure, Volume of CO2 Injection Key

The paper notes that pore pressure and the volume of CO2 injected are likely the most important factors that could impact geomechanical deformation in the subsurface. The paper says, for example, that ongoing EOR operations at Weyburn have helped mitigate the pressure increases caused by CO2 injection, but that at In Salah, where the water leg being injected into did not have good pressure communication with the producing parts of the reservoir, there were clearly issues and the “substantial” geomechanical deformation led to thousands of microseismic events, which appear to have reactivated a fracture network there.

The paper says that understanding geomechanics is the most critical for CCS’ boosters since that is where some of the highest-profile criticism of CCS has come from in recent years. Verdon specifically cited the argument of two Stanford geophysicists in a ‘perspectives’ article published in Proceedings of the National Academy of Sciences last summer that stoked fears in the CCS community after it concluded that there is a “high probability” that small to moderate-sized earthquakes will be triggered by the injection of large volumes of CO2 into the subsurface and could threaten the seal integrity of reservoirs and lead to CO2 leakage. The paper caused CCS supporters to rush to the industry’s defense, worried that the paper could start a public relations battle over the ultimate safety of CO2 storage.

While the media attention ultimately died down last summer, Verdon said data from large ‘megatonne’ storage sites like Sleipner, Weyburn and In Salah could be useful in providing valuable data about the safety of storing CO2. “We know CCS is technically feasible—we’ve put several million tons of carbon dioxide into the ground and it has stayed there fine—but that won’t always be the case. There will be some sites where it doesn’t work very well and it turns out that we can’t inject as much CO2 without raising the pressure and running the risk of crazy fractures in the caprock,” Verdon said. “We have to go look at every individual site and do detailed geomechanical analysis before we can say whether each site is suitable or not.”