SEAL EVALUATION OF THE HENSLEY SHALE FOR CARBON DIOXIDE (CO2) ENHANCED COAL BED METHANE RECOVERY IN THE BREATHITT GROUP, NORA FIELD, VIRGINIA

The US Department of Energy South Eastern Regional Carbon Sequestration Partnership estimates 398 Mt of technically feasible CO₂ Enhanced Coal Bed Methane storage capacity in the Pocahontas Basin, with a potential enhanced recovery of > 2.5 TCF of CBM gas production.  Targeted coals for CO₂ ECBM occur within the Early Pennsylvanian lower Breathitt Group of the Nora Field, Virginia.  Best practices for ECBM injection design of this resource require an increased understanding of the surrounding siliciclastic lithologies.

To evaluate the long term risks of CO₂ leakage, the prospective regional Hensley Shale and other potential seals overlying the target coals were analyzed for effectiveness and seal capacity. Based on subsurface geophysical and field data, the Hensley Shale is thick (> 50 ft), laterally continuous (> 3000 km²) and homogenous and constitutes an effective seal.

Petrophysical and petrographic data from core analyses validate high capacity estimates for the Hensley Shale.  Calculations from a mercury injection capillary porosimetry (MICP) test displacement entry pressure of 207 psia, generates an estimated column of 1365 ft of CO₂ before leakage. Modeled median pore throat size is estimated at ~ 0.26 µm. Scanning electron microscopy verifies a microfabric of narrow pore throats between quartz grains floating in a clay matrix. Additional microprobe element mapping and total organic content tests aid understanding of potential mineral-CO₂ reactions.

Standard measurements of porosity, permeability and petrography of overlying Breathitt Group sandstones can be used to evaluate their potential storage capacity and leakage hazards as well as to illustrate relationships among various laboratory petrophysical properties and mineralogy. Interconnected porosity is impacted by diverse cementation types and compaction. Sandstones sampled are considered tight (< 42 µd), with limited lithologic risks for leakage.

Estimated maximum column heights are far greater than ECBM reservoir thicknesses in the target area and indicate no danger of capillary failure. Local structural integrity will be critical to define secondary leakage risks at ECBM injection sites, as small zones (< 50 ft) of brittle fracture may pose seismically undetectable leakage risks.