AN EXAMINATION OF MVA TECHNIQUES APPLICABLE FOR CCUS IN THIN, STACKED COALS OF THE CENTRAL APPALACHIAN BASIN

摘要

The goals of monitoring, verification, and accounting (MVA) for carbon capture, utilization, andstorage (CCUS) studies include improved understanding of injection and storage processes, evaluation ofinteractions between CO_2, reservoir fluids, and formation solids, and assessment and minimization ofenvironmental impacts (DOE and NETL, 2009). With these critical objectives in mind, it is important todesign a strong, tailored MVA program that is site-specific and addresses all phases of the projectlifecycle. 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 setting in which they are used—atmosphere, near-surface,or subsurface—although some methods, such as isotopic tracers, apply to multiple settings. Commonatmospheric MVA methods include CO_2 detectors and eddy covariance measurements, which bothquantify atmospheric levels of CO_2 and can, therefore, help to detect a leak or confirm storage. Nearsurfacemethods include groundwater monitoring, which can also indicate leakage or proper containment,and surface deformation meters, such as tiltmeters, which can help define the CO_2 plume by delineatingthe 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 asseismic or electrical conductivity/resistivity surveys. Both logging and imaging techniques are oftenconducted in a time-lapse manner, pre- and post-injection, in attempt to record a change in properties dueto the presence of the CO_2 plume.A small-scale CCUS study in southwest Virginia presents a novel application for several established,effective MVA methods. The study, located in an active coalbed methane (CBM) field in BuchananCounty, will involve injecting 20,000 tonnes of CO_2 into a series of thin, unmineable coal seams. ThreeCBM production wells will be converted for use as injection wells. The goals of the study are to test theinjection 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 1.0foot in thickness and distributed over 1000 feet of section. This reservoir geometry creates an unusualtarget for CO_2 injection and also a challenging one for many monitoring and imaging techniques.A smaller CO_2 injection test was conducted 7.5 miles away in Russell County, VA, in 2009. TheMVA used for the Russell County test produced several important findings that have implications for theBuchanan County test. The MVA plan for the Buchanan County test will use the same suite oftechnologies used at the Russell County test as well as some additional methods. MVA for the BuchananCounty test will include gas content measurements at offset wells, groundwater monitoring, traceranalysis, well logging, microseismic monitoring, surface deformation measurement, and time-lapseseismic imaging (crosswell or vertical seismic profile). Multiple monitoring wells will be drilled in orderto facilitate the MVA efforts. Because the reservoir is composed of several seams, ranging in depth from900-2100 feet, multiple tracers may be used to better understand the injection and storage potential ofspecific zones. Surface deformation meters, microseismic monitoring, and time-lapse seismic imagingare state-of-the art tools that have potential to define the subsurface CO_2 plume beyond the borehole scale. The reservoir of thin, dispersed coals is a unique application for these methods, and their use willprovide a feasibility test that will help improve MVA design for CCUS in similar settings. Assessing theresults of time-lapse seismic surveys will be especially complicated due to the several reservoir processesand factors that may produce detectable changes in seismic properties. These include the potential forCO_2 to exist in gas and/or liquid phase within different seams, the presence and amount of water in theformation, the swelling of the coal matrix in the presence of CO_2, and seasonal or other natural variationin the acoustic properties of the formation. Distinguishing the individual contributions of these processesto the observed change will be a challenge but would provide important insights into reservoir processes.The results of MVA for this study can be used to improve design for potential future studies of CCUS inthin coals.