(772f) Monitoring Carbon Sequestration Using Charged Wellbore Controlled Source Electromagnetics and Integrated Reservoir Models

Carbon capture and sequestration (CCS) can provide an effective means by which to reduce global CO2 levels. To ensure effectiveness and safety, regulatory frameworks mandate 99% storage permanence over the lifetime of a carbon sequestration project. As such, robust monitoring, verification, and accounting (MVA) technologies are needed in order for large scale CCS to be realized. Many existing MVA measurements require the costly drilling of boreholes and installation of downhole sensors. Borehole data provide minimal spatial information and their expense is prohibitive against the installation of dense monitoring grids. Surface based measurements can detect massive leaks but do not respond to changing reservoir conditions

Geophysical imaging techniques could theoretically provide this needed monitoring information if sufficient physical property change can be attributed to CO2 injection. The density of supercritical CO2 is a function of reservoir conditions, but is often similar to brine. As such gravity and 4D seismic monitoring will often fail to detect appreciable signal variation with injection activities. Seismic attributes sensitive to subsurface pressure changes can be utilized, but the expense of repeated seismic data acquisition is prohibitive for long-term monitoring. In most cases, CO2 is injected into deep saline aquifers or depleted oil and gas reservoirs. The supercritical CO2 will be much less electrically conductive than the brine which it displaces and mixes with. Therefore, electrical or electromagnetic methods, which are sensitive to electrical conductivity, are a promising techniques for monitoring CCS activities.

Many options exist for electrical survey configurations. The desired characteristics for this application are low cost—requiring minimal new infrastructure—and sufficient depth of investigation. In offshore exploration controlled source electromagnetics (CSEM) are increasingly relied upon as a de-risking tool. In this case the hydrocarbons are the resistive target within the conductive brine. We leverage this experience by applying CSEM to the monitoring of an active onshore enhanced oil recovery carbon capture and sequestration project (EOR-CCS) project by utilizing well casings as deep-reaching transmitting electrodes. Electrical current can be injected into the formation of interest by utilizing legacy wellbore casings that penetrate the injection reservoir as long deep electrodes. By monitoring electrical and magnetic fields on the surface, information about saturation changes in the subsurface can be determined through inversion and interpretation of the data. Two major advantages of the method is the ease of access on the surface and the ability to inject current to the depth of reservoir formation.

In order to improve the resolution and reliability of the quantitative interpretation, we develop a methodology for tightly integrated multiphase reservoir flow simulations with the geophysical data inversion. Electrical conductivity can be related to phase saturation using relations such as Archie’s Equation, which will be calibrated and verified using auxiliary data including production and injection records. This multiphysical approach is designed to integrate structural, physical property, and reservoir simulations systematically. Structural information is retained from the reservoir models and associated characterization efforts, and incorporated into the inversion of the CSEM data as geometrical constraints. Furthermore, physical property information derived from petrophysical data, reservoir porosity, and reservoir simulation-produced fluid saturations will be incorporated into the inversion as direction constraints.

In this paper, we will present the preliminary study on the site selection, numerical simulations consisting of reservoir simulations, generation of electrical conductivity models, and electromagnetic-modeling based EM survey design, and preliminary analyses of data acquired in the baseline field survey.