Schlumberger

Case Study: Angola Water Influx Solution

Increased oil production after PS Platform logging pays back intervention costs in just 8 days

Challenge: Increase oil production and reduce growing water production in Estrela and Tubarao fields offshore Angola.

Solution: Log with the PS Platform production services suite with the RST reservoir saturation tool to pinpoint intervals with maximum water production to shut off with mechanical plugback tools.

Result: Increased oil production offset the cost of logging and plugging in just 8 days.

Overview

Water production had been increasing in several wells in the Estrela and Tubarao fields offshore Angola. The operator ChevronTexaco, along with partners TFE, Braspetro and Sonangol, contracted with Schlumberger to investigate the wells and explore options for increasing oil production and reducing water production.

Schlumberger logged three wells—Estrela A2, Tubarao 1 and Tubarao 10—with the PS Platform production services suite and RST Reservoir Saturation Tool, during February 2001. Following extensive log interpretation, it was decided to use mechanical plugback tools (MPBTs) in all three wells.

After the plugs were set, oil production increased 1100 B/D in Estrela A2, 1000 B/D in Tubarao 1 and 300 B/D in Tubarao 10—a total of 2400 B/D. Water production was reduced a total of 2300 B/D to 1000 B/D in Estrela A2, 600 B/D in Tubarao 1 and 700 B/D in Tubarao 10.

Overall cost for the logging and plug setting was approximately $400,000. It took just 8 days for increased oil production to offset that cost. ChevronTexaco is now defining additional wells that could benefit from similar treatments.

Technical description

The tool strings run in wells Tubarao 10 and Estrela A2 derived mixture velocity with a spinner and measured holdup with a FloView holdup measurement tool and a Gradiomanometer specific gravity profile tool. Gamma ray, casing collar locator, pressure and temperature measurements were also acquired.

In the flow analysis logs, the FloView water holdup measurement (blue curve) and Gradiomanometer density measurement (green curve) were displayed on water-oil compatible scales. That meant the two curves would overlay if only oil and water were present.

In the Tubarao 10 flow analysis log, the overlaid blue and green curves below 8774 ft indicate insignificant quantities of gas are present. Below 8788 ft the oil holdup is virtually zero, indicating a wellbore full of water. When combined with a measurement of the spinner mixture velocity, the logs show a water flow rate of about 1000 B/D at 8788 ft.

Above 8788 ft, mixture velocity increases while water holdup decreases. When combined with an oil-water slip model, this shows an increase in oil flow rate with a relatively constant water flow rate.

Over the interval from 8788 to 8795 ft, the flow rate is constant with maximum water production and negligible oil production. A water shutoff plug was set in this interval.

In the Estrela A2 flow analysis log, the spinner mixture velocity and two holdup measurements show the bottom set of perforations producing nearly 900 BWPD and very little oil—approximately 80 B/D. As perforations farther up are passed, the spinner shows a flow rate increase, which the holdup measurements indicate is due solely to additional oil.

This analysis indicated that isolating the bottom perforations would shut off water production with negligible oil losses. Therefore, a bridge plug was set above the bottom set of perforations in the interval from 8034 to 8117 ft.

The additional production that these projects brought about encouraged the operator to actively seek further wells that may benefit from this form of analysis and treatment.

Benefits

  • Reduced water production
  • Increased oil production
  • Intervention costs paid back in 8 days through increased oil production

Download: Angola Water Influx Solution (1.11 MB PDF)

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