Borehole-Deployed Electric Field Sources and Sensors for Permanent Monitoring of CO2 Sequestration in Deep Reservoirs

Assuring the longterm, safe storage of CO2 requires the development of effective monitoring strategies that have minimal environmental impact. Models show measuring the resistivity contrast between injected CO2 and the host reservoir offers a means to image directly CO2 volume and saturation in situ, with the highest accuracy, while producing no noise or other emissions that impact humans. However, there is presently no cost effective method to acquire such subsurface resistivity data over a large area. The new borehole tosurface electromagnetic (BSEM) survey configuration has been shown to image oil towater contact in a 2000 m deep oil reservoir over an area of 25 km2, thereby extending resistivity methods to deep reservoirs. The resistivity contrast between brine and oil and brine and CO2 is almost the same, and models show that the configuration should work at the Phase II test site to image CO2 over an area of approximately 50 km2. However, the source used in BSEM must be located at the bottom of a borehole, which greatly increases cost and reduces system life and reliability. A new type of EM source that does not require any downhole components was compared to three other potential downhole designs (including BSEM) and found to be comparable or better in all categories. The source is easy to deploy and was tested at a functioning oil field close to, and in the same geology, as the proposed Phase II test site. The measured data agreed very closely with model predictions. A controller will be developed to scale up the prototype source to do a complete survey. Two surveys will be conducted at the Kevin Dome CO2 sequestration site. The first will occur before CO2 injection and the second after 150,000 tons of CO2 has been injected. The resistivity images will be compared to a full suite of seismic, well logs and geochemical data acquired in parallel. These will be the first such deep surveys in the US and a critical first step for CO2 applications and for commercial introduction of this method into the US for mapping the distribution of CO2, oil, water and steam in reservoir formations. Commercial Applications and Other Benefits: Commercial applications exist in the CO2 sequestration industry and the oil extraction industry, where injected fluids (water, CO2, steam) are used for secondary and tertiary oil recovery.