Using interference tests during field startup to solve critical reservoir management issues at the Zafiro field, offshore Equatorial Guinea

The Zafiro field, offshore Equatorial Guinea, was discovered in March 1995 by Mobil (75%) and partner United Meridian Corp. (25%). The field is being developed using subsea wells tied back to a floating production, storage, and offtake (FPSO) vessel. Field development used a fast-track process in which less than 18 months elapsed from discovery to the production of first oil. The Zafiro reservoir of the Zafiro field is highly stratigraphic in nature. Reservoir continuity was undefined before production. Seismic, geological, and pressure data gathered during an appraisal program of the main Zafiro field demonstrated the potential for discrete oil sands being in lateral and vertical communication. However, fluid samples indicated that the reservoir could be very compartmentalized. The implications of these two different models of reservoir continuity for reservoir development and, ultimately, reserves were profound. If the reservoir was very compartmentalized, development would require a significant number of wells, with the main recovery mechanism being solution gas drive. However, if the reservoir was reasonably continuous it could be developed using fewer wells, with pressure maintenance by water and gas injection yielding much higher reserves. To monitor reservoir performance, and to allow reservoir description by interference testing, permanently installed bottomhole-pressure gauges were run in all Zafiro wells during completion operations. These gauges were high-accuracy quartz gauges run on the tubing immediately above the reservoir. Gauge signals were transmitted to the subsea tree through cable strapped on the exterior of the production tubing, and then through a combined electrical and hydraulic control umbilical laid along the sea floor, connecting the subsea wells to the FPSO. A sophisticated data-monitoring and recording system was installed on the FPSO with the capability of recording data at a rate of one reading per second per well. An interference test was carried out during field startup to determine the degree of reservoir continuity within the field. This test involved producing a single well at a fixed rate while observing the reservoir pressure response in four offset wells. The test was designed using both analytical and numerical techniques, and included a clearly defined plan for sequencing well production to maximize reservoir data while minimizing well shut-in times. All bottomhole pressure gauges worked on startup. The interference test showed that all wells completed in the main Zafiro sands were in pressure communication. Strong lunar tidal effects were seen in all well responses. The investment of time and money in the installation of bottom-hole pressure gauges and interference testing during field startup allowed rapid acquisition of data critical to future reservoir management plans. Vital concerns over reservoir continuity were answered immediately, allowing implementation of fast-track plans for pressure maintenance to optimize oil recovery.