Effect of Shallow Subsurface Heterogeneities in CO2 Storage Monitoring for EOR: Case Studies from the Gulf Coast

Zahid, Khandaker; Wolaver, Brad D.; Ambrose, William A.; Smyth, Rebecca C.

Geological sequestration of anthropogenic CO2 in deep underground oil reservoirs has been under active consideration to promote carbon capture utilization and storage (CCUS) in an effort to reduce the rate of greenhouse gas emissions and achieve commercial incentives from EOR. Two EOR fields from the Texas Gulf Coast are being characterized to support development of a monitoring plan to document retention of large volume of injected CO2. Geophysical logs have been used to characterize the injection zone and the overburden. While there are geological similarities in the Frio injection zones and overlying Anahuac confining unit in terms of facies architecture, lateral continuity and interconnectivity, and reservoir qualities, the overlying Miocene section shows large variability owing to shift in Cenozoic depocenters and the supply of coarse-grained sediments to the Gulf Coast basin. These changes in Miocene stratigraphy have implications for monitoring out-of-pattern CO2 migration. Large-scale growth faults associated with salt movement and basin subsidence compartmentalized the storage reservoirs and overlying formations that influenced fluid movements. Such change in "above-zone" stratigraphy requires field-specific monitoring strategy to determine any possible "out-of-zone" migration of CO2. The first EOR site, located in a fluvio-deltaic, fault-bounded crestal-graben setting in the Houston Embayment, contains more irregular and heterogeneous sand bodies, especially in middle and upper Miocene successions that are frequently offset by large-to-medium throw faults. Pressure response measurement for out-of-zone CO2 migration in this site requires multiple monitoring intervals for "early" as well as "late" detection above the regional confining unit. The second EOR site, located in barrier-island and strandplain deposits, was separated from the Houston Embayment by the San Marcos Arch that inhibited large supply of sand deposition during Oligo-Miocene time. Permeable sand layers are relatively scarce (low net-to-gross ratio) but are more laterally connected in the middle and upper Miocene strata. Owing to the combination of absence of large faults and laterally continuous sand geometry, pressure perturbations may be readily detectable in the above-zone with less monitoring points.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013