Time-lapse walkaway VSP acquisitions with optimal repeatability in CO2 EOR project, onshore UAE

Summary In 2016 ADNOC started injecting CO2 into the two onshore fields to boost oil production and replace the use of hydrogen gas. Carbon capture could assist in enhanced oil recovery and in protecting the environment. At the same time, the time-lapse walkaway VSP technique was considered to monitor fluid displacement in the reservoir where CO2 and water were going to be injected. The data acquisition consisted of a walkaway line recorded with a vibroseis source and with geophone tools anchored behind cemented casing near the reservoir level in a CO2 monitor well. The walkaway VSP line should be repeated over time, when changing the injection cycle from water to CO2 and vice versa. This study showed how careful survey design, data acquisition and data processing have allowed meeting the required level of repeatability in a time-lapse walkaway survey monitoring project, where CO2 and water have been injected for EOR purposes. The repeatability metrics improved significantly during the successive data processing steps and they provided more reassurance about the final time-lapse differences observed that should allow detecting the injected fluid effects in the final processing stage. Also, the time-lapse corridor stack result at zero-offset VSP was a particularly interesting calibration point.