Abstract
Substantial volumes of hydrocarbons are trapped in micro-porous low permeability, transition zone, thin reservoirs in the north of Oman. Developing these resources under depletion or by waterflooding is not economically attractive due to the low resource density and poor water injectivity. Numerical modeling studies showed the potential to unlock these volumes economically through CO2 injection. Therefore, an extended field pilot, an Oman and PDO first, was executed to demonstrate the feasibility of CO2 EOR development in these tight reservoirs.
Detailed modeling work indicated that the injectivity of CO2 is the key risk for the success of CO2 development in these reservoirs. Thus, the field pilot was executed to de-risk the CO2 injectivity, establish communication between injector and producer, and ensure a sustainable production rate. A horizontal CO2 injector was drilled 100 meters away from an existing horizontal producer, which also had a horizontal water injector on the other side. This oil producer lacked pressure support during primary and waterflooding, and thus, it was producing intermittently.
CO2 was supplied to the injector and pumped for six months. A surveillance program was put in place to monitor the CO2 injection and production. Furthermore, Pressure Transient Analysis (PTA) tests and multiple saturation logs were performed. In addition, numerical simulation was used to history match the injection and production data during the pilot and calibrate the models.
The pilot demonstrated good and sustainable injectivity of CO2; on average, 34 tons per day were injected against a target of 30 tons per day, while maintaining the pressure below the fracture pressure at the injector. The bottomhole pressure at the producer during shut-in started to increase after 2-3 months from the start of CO2 injection. This confirmed the communication between the injector and producer and the ability of CO2 injection to build pressure at the producer to levels that were never observed during waterflooding. Furthermore, the producer was able to sustain production rate three times higher than before CO2 injection. The DTS/DAS showed a uniform injection profile, indicating good conformance along the horizontal section. Additionally, the time-lapse saturation logs confirmed the de-saturation of oil at the injector by CO2. The numerical model and PTA analysis of the pilot data improved the understanding of the reservoir and provided additional insights.
The results of this pilot demonstrate the potential of CO2 EOR to economically recover oil reserves from tight reservoirs while simultaneously contributing to CO2 emission reduction. Moreover, the success of this pilot project paves the way for unlocking oil reservoirs facing similar challenges. The findings and insights presented in this paper offer valuable knowledge on and practical guidance of pilot execution for the industry.
