- Number 322 |
- October 11, 2010
Storage retention goals for carbon sequestration
The flow path of an acidic fluid
is discolored by chemical
The effectiveness of carbon sequestration depends greatly on storage permanence, so it’s not surprising that a key goal of the carbon sequestration research program at DOE's National Energy Technology Laboratory is to retain 99 percent of carbon dioxide (CO2) in underground reservoirs over a century. The trouble is that variability in field conditions greatly complicates quantitative predictions of leakage risk. To improve these predictions, NETL is collaborating with four other DOE national laboratories in a new effort, the National Risk Assessment Program (NRAP).
The program’s objectives are to integrate scientific insights from across the sequestration research community and to ensure development of the science base needed for appropriate risk assessment to support large-scale underground carbon storage projects. Although several laboratories are contributing in more than one area, each is taking primary responsibility for steering efforts in a particular area:
• NETL is responsible for research on wellbore risk assessment.
• Lawrence Berkeley is overseeing research related to monitoring for risk assessment.
• Lawrence Livermore is responsible for systems modeling for risk assessment.
• Los Alamos is overseeing research on natural seal integrity.
• Pacific Northwest is coordinating research on risks to groundwater systems.
Wellbores are obvious potential leakage pathways for CO2 injected into geologic formations for storage. This is not just from the wells that have been drilled into the strata to inject and monitor the CO2, but also from old wells that may have been drilled for oil and natural gas exploration and/or production. NETL’s seal integrity research aims to predict leakage rates based on an increased understanding of the chemical and physical processes affecting seals.
Early NETL research focused on the integrity of wellbore cement after CO2 injection. Deep wells are typically lined with cement to prevent leakage of gases and fluids, such as saline or oil, to the surface or into underground drinking water resources. However, because CO2 dissolved in water is acidic, and cement is alkaline, there is the potential for chemical reactions that could adversely affect seal integrity. Recent NETL research shows that the CO2 reaction with typical wellbore cement is too slow to cause leakage in a properly constructed well that is in good condition. But the cement in old or abandoned wells could still be a problem; these would all need to be located and sealed before sequestration at any given site is initiated.
Carbon sequestration calls for CO2 to be injected under pressure in a supercritical state, which means that it is similar to a liquid but is more compressible and less viscous than water. Ideally, this allows it to be injected at higher pressures without fracturing the reservoir. However, the geomechanical responses to increased fluid pressures in a fluid-rock system could cause faults and fractures to either open or close, affecting both natural seal integrity and wellbore integrity. Because of this, fluid flow and subsequent chemical reactions in the reservoir are being studied to determine whether a pre-existing flow path will open or close over time as a result of changing stress on the fractured rocks and/or chemical dissolution or precipitation.
Experiments were conducted in which an acidic fluid was allowed to flow through a fracture in cement while under a confining pressure that simulates deep subsurface conditions. This research suggests that the flow of CO2-saturated brine along an open pathway in wellbore cement, which one might think would cause the cement to dissolve, can actually have a positive impact over time by sealing a pathway as minerals that first dissolve then re-precipitate. Additional research is now 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, field tests being conducted by DOE’s Regional Carbon Sequestration Partnerships, will provide ideal opportunities to apply and validate the new risk-assessment tools.
[Linda Morton, 304.285.4543,