An Underground Reservoir Resistivity Monitoring System for CO2 Reservoir Characterization

Carbon capture and storage (CCS) in deep subsurface is a promising way to slow the CO2 accumulation in atmospheric and marine and mitigate global climate change. The sequestrated CO2 needs to be safely stored underground for hundreds of years or longer. Monitoring the saturation of CO2 in subsurface reservoirs is crucial to prevent leakage. As is well known, reservoir resistivity is a key parameter used to compute the saturation of CO2, thus, monitoring the saturation of CO2 is to track changes in reservoir resistivity. The preferred monitoring technology requirements include: • sensors or instruments installed outside of casing, ground,… • No cables connecting instruments mounted outside the casing to equipment on ground to prevent any potential CO2 leakage along the cable pathways. However, real-time monitoring of changes in CO2 reservoir resistivity has not yet been achieved with existing methods. To address this, we propose an innovative "Underground Reservoir Monitoring System" capable of monitoring changes in CO2 reservoir resistivity. The system includes three main components: a resistivity measuring method, a power supply method, and a data transmission method. Measuring Method: The basic system uses three toroidal coils mounted on the exterior of a metal casing. One coil serves as the transmitter and is positioned either above or below the CO2 reservoir, while the other two coils, functioning as receivers, are placed near the top and bottom of the reservoir. When alternating current is supplied to the transmitter, it induces a current along the casing that enters the surrounding formation and completes a loop. The receivers detect current leakage and the phase shift along the casing within the reservoir. These measurements allow for the computation of the reservoir's resistivity, which is directly related to CO2 saturation. Power Supply Method: Rechargeable batteries, along with the necessary electronic circuits to power the antennae, are installed near the toroidal coils. Nuclear- based batteries currently available on the market can last up to 20 years, providing a reliable, long-term power source for the system. By applying current through cables on the surface between the well and a nearby well, these coils can also function as remote battery chargers. Data Transmission Method: An efficient electromagnetic (EM) telemetry method is used to wirelessly transmit the measurements to the surface. The toroidal coils also function as transmitters for low- frequency alternating current signals, allowing the data to be sent to surface devices for analysis. A finite element method code was developed to simulate the current leakage and phase shift between the receivers. The simulation results demonstrate that higher formation resistivity outside the casing results in reduced current leakage and phase shift. This relationship enables the calculation of formation resistivity based on the measured current leakage and phase shift. The radial detection range, determined by the distance between the transmitter and receivers, can extend up to 20 meters with an accuracy greater than 1%. An experiment was conducted to validate the measurement methodology. Three toroidal coils—one serving as a transmitter and the other two as receivers— were mounted on the outside of a metal pipe placed in a water tank. When the transmitter was powered, a measuring instrument, connecting to the receivers, detected the current leakage and phase shift. By adjusting the water's resistivity through the addition of salt, measurements of the current leakage and phase shift between the receivers were taken. The experimental results closely aligned with the characteristics predicted by the simulation data.