Use of Formation Pressure Data to Determine the Limitations to Storage Volumes and Injectability of CO2 in the Frio Formation of the Texas Gulf Coast
Measurements of formation pressure during the life of wells producing oil and gas has been an important data-resource in development of a methodology to assess the storage capacity for sub-surface geologic sequestration of CO2. This poster will illustrate the use of pressure decline and initial bottom-hole pressure vs depth curves to define some of the limiting factors within a geologic province that is being considered for CO2 sequestration. In the Frio Formation of the Texas Gulf Coast, pressure data provide a means for characterizing the flow properties of a potential storage formation, including evidence of compartmentalization, drive mechanisms, and pressure boundaries. These factors may limit storage volumes and injectability within the target formations. Identification of boundaries, drive mechanisms, capacity, and potential growth factor beyond the volume of net cumulative production in known traps provides better estimation of the overall assessment of the reservoir for CO2 sequestration. A proposed rule by the U.S. Environmental Protection Agency for injection of CO2 during sequestration projects would limit the maximum pressure of CO2 injection and storage to less than 90 percent of the fracture pressure gradient. Overpressured reservoirs that equal or exceed this regulatory requirement limits the overall formation storage potential. In formations where the depth of overpressured is well defined, such as the Frio, the depth to the top of the overpressured zone may define the maximum depth of the assessment unit, limiting the potential volume available for CO2 storage.
Potential constraints on storage volume, particularly injectivity of CO2, can be identified by temporal changes in pressure during the producing life of wells. Pressure decline curves in the Frio presented indicate many individual fields are now underpressured. Initially, overpressured reservoirs show rapid and severe pressure declines suggesting that fluid flow in the Frio Formation is strongly compartmentalized. Descriptions of some fields attribute production pressures to water-drive mechanisms that could limit injectivity of CO2 because pressures necessary to displace water may exceed the fracture pressure gradient. However, sustained low pressures in most reservoirs producing from the Frio Formation suggest that an active water drive, a characteristic of regional hydrologic flow systems, is not a common feature in this potential CO2 storage compartment.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009