Schlumberger

Case Study: Avoiding Water Zone Saves Well for CACT

Bed boundary mapper used to detect oil/water contact and place horizontal section of sidetrack close to top of mature reservoir in South China Sea

Challenge: Place horizontal section of sidetrack in window 1 m from top of pay zone while avoiding drilling into water zone in mature reservoir where depth of oil/water contact is not known.

Solution: Use EcoScope multifunction LWD service, PeriScope bed boundary mapper, and adnVISION475 azimuthal density neutron service to guide well placement decisions.

Result:

  • Avoided entering water zone.
  • Achieved 100% net to gross.
  • Produced oil at rate of 3,500 bbl/d—190% more than 1,200 bbl/d expected.
  • Saved approximately USD 1.4 million of AFE budget.

Develop mature reservoir

CACT—a joint venture of the Chinese National Offshore Oil Corporation (CNOOC), Chevron, and Eni—planned to drill a sidetrack horizontal well in a South China Sea field. CACT called upon Schlumberger to place the horizontal section in a window 1 m from the top of a mature reservoir along a 0.9° downdip. Initial oil production from the sidetrack was expected to be 1,200 bbl/d.

Two key offset wells in the area indicated that the edgewater reservoir was coning up to a bottomwater drive reservoir, but there was a high degree of uncertainty about the current location of the oil/water contact (OWC). As a result, it was possible that the planned horizontal section length of 400 m might not be achieved, and it would be essential to be able to detect the OWC while drilling to avoid penetrating the water zone. Uncertainties about stratigraphy and the structure, a low relief unfaulted four-way dip closure, added to the challenge.

Detect bed boundaries and oil/water contact

A team of CACT and Schlumberger experts successfully completed the sidetrack, despite the uncertainties about the subsurface structure and the location of the OWC, and achieved higher oil production than expected. Real-time density images from close to the bit, provided by the EcoScope multifunctional LWD service, enabled the team to land the sidetrack smoothly 0.8 m below the top of the reservoir, even though the top was 2.6 m shallower than anticipated. To accurately place the horizontal section, the team used a BHA incorporating the adnVISION475 azimuthal density neutron service and a PeriScope bed boundary mapper that clearly delineated distances to the top of the reservoir, intermediate layers, and the OWC—simultaneously.

The official structure map indicated that the reservoir dipped down 0.9°, but the PeriScope mapper revealed that it was relatively flat, with an internal undulation of 0.6° updip, 0.4° downdip, 0.7° updip, and 0.9° downdip. This allowed the team to drill a trajectory that accessed more oil than the planned trajectory would have. After 250 m of the horizontal section had been drilled, PeriScope mapping detected the OWC approximately 2.5 m TVD below and showed it at a consistent depth as the next 50 m were drilled—a depth much shallower than originally thought. To avoid encroachment, entering the water zone, and losing the well, drilling was stopped at that point, 100 m short of the planned 400 m.

Achieve 100% net to gross

The accurate PeriScope mapping of multiple key boundaries and the OWC helped the team place the horizontal section within a 1-m window below the top of the reservoir, access more attic oil, avoid drilling into the water zone, and achieve 100% net to gross—while saving CACT approximately USD 1.4 million (30%) of the total AFE budget. Initial oil production with natural flow was 3,500 bbl/d—190% more than the 1,200 bbl/d expected. For reservoir management purposes, oil production is being controlled at 2,400 bbl/d, with a water cut of just 1%.


Download: Avoiding Water Zone Saves Well for CACT (0.98 MB PDF)

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"The Schlumberger well placement team used the PeriScope mapper to place the lateral in best sand quality to ensure optimum production and further delineated the reservoir structure and oil/water contact, enabling CACT to make key decisions for completions."
Thanh Binh Tran
CACT Asset Team Coordinator