Monitoring, Verification, and Accounting Techniques For Carbon Capture, Utilization, and Storage In Thin, Gas-Bearing Coals Of The Central Appalachian Basin

The U.S. Department of Energy and National Energy Technology Laboratory have developed best practices for monitoring, verification, and accounting (MVA) activities associated with carbon, capture, utilization, and storage (CCUS) studies.  The stated objectives for an MVA program include improved understanding of injection and storage processes, evaluation of interactions between CO₂, reservoir fluids, and formation solids, and assessment and minimization of environmental impacts (DOE and NETL, 2009).  With these critical objectives in mind, it is important to design a robust, tailored MVA program that is site-specific and addresses all phases of the project lifecycle.  Site factors to consider include the risk profile, geology and geologic structures, and access.  The project lifecycle consists of pre-operation (characterization, baseline tests, etc.), operation/injection (plume assessment, imaging, etc.), closure and site restoration, and post-closure (long-term monitoring).

MVA methods can be categorized by the earth setting to which they apply—atmosphere, near-surface, or subsurface.  Common atmospheric MVA methods include CO₂ detectors and eddy covariance measurements, which both quantify atmospheric levels of CO₂ and can, therefore, help to detect a leak or confirm storage.  Near-surface methods include groundwater monitoring, which can also indicate leakage or proper containment, and surface deformation meters, such as tiltmeters, which can help define the CO₂ plume by delineating the inflated portion of the reservoir and/or new fractures induced by the pressure of the plume.  Subsurface methods include borehole logging techniques and geophysical imaging techniques, such as seismic or electrical conductivity/resistivity surveys.  Both logging and imaging techniques are often conducted in a time-lapse manner, pre- and post-injection, in attempt to record a change in properties due to the presence of the CO₂ plume.  Some MVA methods, such as isotopic tracer analysis, apply to multiple settings.

Until very recently, most CO₂ storage field tests were conducted in deep saline aquifers, usually sandstones with thicknesses of tens to hundreds of feet.  A small-scale CCUS study in Buchanan County, Virginia, will involve injecting 20,000 tonnes of CO₂ into a series of thin, unmineable coal seams, presenting an unusual target for CO₂ injection and a unique application for MVA methods.  For the study, three coalbed methane production wells will be temporarily converted for use as injection wells.  The goals of the study are to test the injection and storage potential of the coal seams and to assess the potential for enhanced coalbed methane (ECBM) recovery at offset wells.  The reservoir consists of approximately 15 coal seams, averaging 1.0 foot in thickness and distributed over 900 feet of section.  Some MVA methods that have been used successfully for saline aquifer settings, such as geophysical imaging of injected CO₂ plumes with time-lapse crosswell techniques, are not suited for geometry of the coal reservoir.  This consideration led to a selection of other technologies that can be used, and combined, to assess the injected CO₂plume within the coal reservoir.

The MVA plan for the Buchanan County study includes gas content measurements at offset wells, groundwater monitoring, tracer analysis, well logging (reservoir saturation, sonic, gamma, etc.), surface deformation measurement, and deployment of a buried passive microseismic array.  Three monitoring wells will be drilled in order to facilitate the MVA efforts.  One of these wells will also be used to collect core samples, which will be analyzed in the laboratory for rock properties that will help to refine reservoir characterization and modeling efforts.  Because the reservoir is composed of several seams, ranging in depth from 860-2130 feet, multiple tracers will be used to better understand the injection and storage potential of specific seams or zones.  Surface deformation meters and the buried passive microseismic array have potential to define the subsurface CO₂plume beyond the borehole scale.  Both methods can be used to extrapolate observations from well logs, gas content readings, or other borehole measurements across a larger areal extent.  The results of the MVA program used in the Buchanan County study will help to guide MVA efforts for future studies of CCUS in thin coals.

Reference

DOE (Department of Energy) and NETL (National Energy Technology Laboratory), 2009. Best practices for: monitoring, verification, and accounting of CO₂ stored in deep geologic formations. Available online at http://www.netl.doe.gov/technologies/carbon_seq/refshelf/MVA_Document.pdf; accessed April 2012.