Geophysical Monitoring/Interpretation
Example: Monitoring at a CO2 Storage Site
The Regional Carbon Sequestration Partnerships (RCSP), which consist of seven major regional partnerships in the United States, have been conducting a number of small- and large-scale CO2 injection projects to ensure the reliability of the carbon capture, utilization and storage (CCUS) as mitigation at an industrial scale. The monitoring program at Cranfield, Mississippi, is at the largest CO2 injection projects in the RCSP program; as of July 2013, about 4 million metric tons of CO2 had been injected. Various monitoring strategies targeting different intervals, ranging from the injection to the vadose zones, have been utilized in monitoring response to large-scale CO2 injection. In particular, pressure and temperature monitoring of an above-zone monitoring interval (AZMI) has been attempted for the first time in CO2-injection history to evaluate the effectiveness of this approach for the detection of undesirable migration of fluids from the injection zone. Monitoring of the first high-permeable interval overlying the caprock is advantageous in that signals are least attenuated.
Detailed area of study (DAS) at a Cranfield pilot test site: One injection well (31F-1) and two observation wells (31F-2 and 31F-3) and geometric conditions (Petrel model: courtesy of T. Meckel). Background color represents relative seismic amplitude (scale locates in the right-end). Curves along the wellbores denotes well-log data (GR-left and SP-right), which help to distinguish between sandstone and clay-rock layers. Well-log data is highlighted with color in the IZ and the AZMI.
Interpretation & Analysis
Increase in pore fluid pressure was thought to be minimal if no hydraulic communication were to occur between an injection zone (IZ) and an AZMI. However, measured increase in AZMI pore pressure at Cranfield was not small enough to be neglected. So, we can interpret the field-measurement data from the perspective of geomechanical response. Analytically, the pore pressure increase can be estimated using the approach that combines Green's functions with poroelastic theory. The analytial approach requires simple settings, including a single injection zone surrounded by impermeable layers in an axisymmetric configuration. Numerical simulation can overcome several limiations of the analytical approach. In our study, numerical simulation results for pore-pressure increase at 31F-3 in the AZMI agree well with field data in terms of both evolution trend and amount of pressure increase. Therefore, the small pore-pressure increase in the AZMI can be regarded as a result of poroelastic response, rather than of hydraulic communication between the injection zone and AZMI. For more information, please refer to "Kim, S. and Hosseini, S.A. 2014. Above-zone pressure monitoring and geomechanical analyses for a field-scale CO2 injection project, Cranfield, MS. Greenhouse Gases: Science and Technology, 4, 81-98."
Example: Injection-zone analyses. (a) Comparison of bottom-hole pressure near injection well between field-measurement data and numerical-simulation results. (b) Temperature monitoring (right y-axis) at 31F-1 and injection rate (left y-axis).