The effectiveness of 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.
Original articleCarbon sequestration regional partnershipsNETL announces 15 projects aimed at carbon sequestrationSOURCE:
National Energy Technology Laboratory
