Develop thin-pay reservoir

CACT—a joint venture of the Chinese National Offshore Oil Corporation (CNOOC), Chevron, and Eni—was planning to drill an extended-reach exploration well from a platform in the South China Sea. The objective was to place a long lateral drain in a thin-pay reservoir in the southern compartment of a complex structure characterized by a three-way dip closure against down-to-the-north faults.

Since the only information available about the compartment was from a single control well, there were entry-point-depth, structural, stratigraphic, and survey uncertainties. Ideally, the drain would be at least 550 m long to explore the compartment’s northeast wing. However, the planned length was limited to 400 m due to expected high drilling torque at 7,750 m TD.

Detect distance to boundary

The CACT-Schlumberger team successfully landed the well 0.4 m below the top of the thin pay zone even though the zone was more than 10 m shallower than indicated by the initial model. Then the team placed a 570-m drain entirely within the pay zone. Landing of the well was guided by real-time images from the EcoScope service, and the drain was placed within 0.5 m of the top of the pay zone using a BHA that included a PeriScope mapper, an adnVISION475 tool, and a rotary steerable system.

Even though the actual structure profile was found to differ widely from the initial model, the PeriScope mapper’s ability to simultaneously map the top and bottom pay zone boundaries enabled the team to accurately place the drain in the sweet spot. This was accomplished without exiting the reservoir, which would have reduced the drainage area and lowered the recovery factor. Accurate boundary mapping also helped the team avoid unnecessary lateral adjustments that might have exceeded the maximum drilling torque limit.

Optimize lateral placement

Accurate placement of the lateral drain made possible by the PeriScope mapper allowed the team to extend the lateral 170 m beyond the planned 400 m without exceeding the maximum drilling torque limit. This extension ensured optimum reserve recovery and permitted exploration of the northeast wing. Exploration revealed that the orientation of the boundary was coming from an azimuth opposite to what was indicated by the initial model. It also provided a better understanding of the structure profile of the southern compartment.

Excellent execution by the CACT-Schlumberger team, in combination with early planning and clear communication, saved approximately USD 2.5 million of the AFE. Net-to-gross in the 570-m lateral drain was 100%, and the 7,940-m extended-reach well produced 3,000 bbl/d of oil with no water cut—50% more than the expected production rate of 2,000 bbl/d of oil or equivalent.