(559e) Monitoring of an Active CO2-EOR Field Using Charged Wellbore Casing Controlled Source Electromagnetics

Injection of carbon dioxide into depleted oil and gas reservoirs during tertiary recovery (CO₂-EOR) offers numerous tangible benefits including long term storage as well as production of otherwise inaccessible hydrocarbons. In the absence of production activities, brownfield sites are often excellent candidates for CO₂ sequestration due to the extensive characterization that typically has occurred in addition to the presence of infrastructure including legacy boreholes and gas processing facilities. Additionally, hydrocarbon production can help to offset storage costs. For these reasons a substantial component of CO₂ sequestration is anticipated to remain in active and former oilfields.

However, in spite of the benefits, CO₂-EOR operations present unique challenges in order to ensure efficiency and efficacy. Three phase reservoir systems are complicated due to variable phase injection schedules and incompletely characterized fluid-fluid and fluid-matrix interactions. Reservoir models are powerful tools for management, but uncertainty regarding these models can be large and difficult to quantify. Subtle changes in properties including relative permeability and capillarity can have profound impacts on the predicted state of the reservoir at any point in time. This ambiguity in reservoir models can have large impacts with respect to optimal management of the field and risk analysis. Improved methods for subsurface monitoring of these dynamic reservoir systems are therefore needed.

Geophysical methods are a natural means by which to achieve reservoir monitoring and model validation. However, in pressure managed fields such as CO₂-EOR sites, time lapse seismic methods often contain very little signal. Gravity anomalies are often below detection limits. However, electrical methods are typically sensitive to the changes in saturation associated with gas and water injection, and are therefore investigated here. As an added constraint, ideal monitoring technologies need to be minimally invasive and also minimally impactful on oilfield activities.

Most CO₂-EOR sites have been in production for many years and contain numerous legacy boreholes which are not actively in use. These offline legacy boreholes can be used as long deep electrodes which penetrate into the reservoir of interest in controlled source electromagnetic (CSEM) surveys. The use of legacy boreholes as electrodes increases the depth of investigation of charged wellbore casing controlled source electromagnetics (CWC-CSEM) in comparison to conventional CSEM surveys with minimal additional cost. Surface monitoring of magnetic and electric fields are achieved using commodity magnetotelluric (MT) sensors. As such, MT data can be acquired in tandem with CWC-CSEM surveys providing additional information.

As part of an ongoing project funded by the Department of Energy and the National Energy Technology Lab, CWC-CSEM monitoring at an active CO₂-EOR site has been underway for the past several years. Interpretation of the geophysical data is constrained by--and also provides feedback to--reservoir models. As such, the CWC-CSEM method has been shown to provide cost-effective monitoring of tertiary recovery fields. This paper will present findings from the project as well as highlight the approach taken in field acquisition and data processing.