Schlumberger

Case Study: Evaluate Gravel-Pack Condition for Sidetrack Potential

RSTPro diagnosis of gravel-pack quality also improves saturation evaluation in the offshore Niger Delta

Challenge: Logging for gravel-pack quality and saturation evaluation is difficult with conventional methods because the amount of gravel may not be uniform across the pack.

Solution: Specification of the RSTPro reservoir saturation tool with an extended characterization database and response model enables accurate determination of gravel-pack quality and hydrocarbon saturation.

Result: The resulting reliable indication of pack quality improved the definition of the borehole environment and increased the accuracy of the saturation evaluation.

Complex completions

The initial completion in a well drilled in a shaly sand in the offshore Niger Delta consisted of an internal gravel pack in a 12 1/4-in hole with 9 5/8-in cemented casing. The pack included standard 5 1/2-in wire-wrapped screens with 20/40-mesh gravel. Conventional reservoir monitoring methods could not reliably evaluate the saturation or the quality of the gravel pack.

Custom characterization answers

After the well had been shut in for over 6 h, the RSTPro tool was run in inelastic mode to acquire carbon/oxygen (C/O) ratio data. The gravel-pack quality diagnostic in Track 2 on the log is based on the near-detector inelastic silicon yield. Good pack quality is shown across the perforated interval and reduced quality above the top perforation.

The crossplot shows the far- to near-detector C/O ratios overlain with the gravel-pack characterization for the wellbore. The two clusters of C/O points indicate the water- and oil-filled sections of the pack. Most of the points in the water-filled section plot within a 0% limit for borehole oil, which indicates complete saturation of water in the gravel pack. Similarly, the points in the oil-filled section indicate an irreducible water saturation in the gravel pack. The few points between the clusters are from a transition zone, which is typical of a fluid interface in porous environments.

The formation oil volume was evaluated using the custom characterization shown in Track 4. The oil/water contact (OWC) is distinct and the saturation profile robust in comparison with the qualitative evaluation in Track 5.

Better decisions from better information

Calibration of the RSTPro tool for the gravel-pack and reservoir conditions produced a better definition of the borehole environment and more accurate saturation evaluation for decision making. The openhole interpretation was confirmed in a sidetrack well, which verified the position of the OWC and the minimal depletion.


Download: Evaluate Gravel-Pack Condition for Sidetrack Potential (0.23 MB PDF)

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Crossplot overlay of C/O ratios with characterization of the gravel-pack saturation.RSTPro log identifying gravel-pack quality and saturation.
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