- Number 319 |
- August 30, 2010
NETL leads science-based risk assessment for carbon sequestration
The effectiveness of CO2 (sequestration storage of CO2) depends greatly on storage permanence. A key goal for DOE’s carbon sequestration research program is at least 99% retention of CO2 in storage reservoirs over a 100-year time period. However, variability in field conditions greatly complicates quantitative predictions of leakage risk. DOE's National Energy Technology Laboratory (NETL) is collaborating with other DOE national labs to integrate scientific insight being developed across the carbon sequestration research community, and ensure development of the science base necessary for appropriate risk assessment (including strategic monitoring) to support large-scale underground carbon storage projects. This National Risk Assessment Program (NRAP) is being led by NETL, but includes researchers from the Los Alamos, Lawrence Berkeley, Lawrence Livermore, and Pacific Northwest National Laboratories.
Ensuring the efficacy of large-scale CO2 storage requires accurate prediction of the movement and reactivity of CO2 in a reservoir, while monitoring each site strategically to verify the predictions of a site’s performance. NRAP scientists have evaluated gaps in current scientific knowledge and targeted specific areas for collaborative research. Thus far, the NRAP working groups have identified five primary focus areas: wellbore integrity, natural seal integrity, groundwater systems, strategic monitoring for risk assessment, and systems modeling for science-based risk assessment.
Although many of the labs are contributing in more than one area, each of these topic areas has one Lab that is primarily responsible for steering those efforts. NETL is responsible for research on risk assessment aspects related to wellbore integrity. LLNL is overseeing research on natural seal integrity. PNNL is coordinating research on risks to groundwater systems. LBNL is overseeing research related to monitoring for risk assessment. Finally, LANL is responsible for coordinating research on systems modeling for risk assessment.
Wellbore systems are obviously potential leakage pathways for buoyant CO2 injected into geologic formations. We are referring not just to wells that have been drilled to inject and monitor the CO2, but also old wells that may exist due to historic oil and natural gas exploration and/or production. Early work focused on the integrity of wellbore cement with CO2 under deep well conditions. Deep wells are typically lined with cement to prevent leakage of fluids (such as saline water, oil, and gases) to the surface or into drinking water resources. Since CO2 dissolved in water is acidic, and cement is alkaline, there is the potential for chemical reactions that could affect seal integrity. NETL research has shown that the reaction with typical wellbore cement is too slow to cause leakage in a properly constructed well that is in good condition. As shown in the micrograph, precipitation of carbonate within the cement pores leads to formation of a barrier, which slows the reaction significantly. However, the cement in old abandoned wells could still be a problem; these will all have to be located and sealed before sequestration begins.
During sequestration, CO2 will be injected under pressure, in a supercritical state. Supercritical CO2 is like a liquid but is more compressible and less viscous than water, ideally allowing it to be injected at high pressures without fracturing the reservoir. However, the geomechanical responses to increased fluid pressures in a fluid-rock system could cause fractures to either open or close, affecting both natural seal integrity and wellbore integrity. So, fluid flow and subsequent chemical reactions in a reservoir are being studied to determine whether a preexisting flow path will open or close over time as a result of changing stress and/or chemical dissolution or precipitation. Laboratory research at NETL and some field observations suggest that flow of CO2-saturated brine along an open pathway in wellbore cement, which one might think would cause cement to dissolve, can actually have the positive impact over time of sealing a pathway as minerals that first dissolve, subsequently re-precipitate. Additional research is being conducted to verify this phenomenon and determine whether there are other conditions under which flow pathways may open instead.
After the NRAP develops findings and recommendations, the Regional Carbon Sequestration Partnership field sites will provide an ideal opportunity for applying and validating the new risk-assessment tools.
Submitted by DOE's National Energy Technology Laboratory