Total E&P Angola operates the deep offshore ROSA field on Block 17. On the fifth development well in the field. Two zones in the Oligocene sandy layers are transversed at a well deviation of 48° and required the implementation of the stacked frac-pack technique. The perforated length of zones was approximately 118 ft and 213 ft.

Multiple fracture initiation and early screen out were expected, requiring consideration for an enhanced frac–packing method. In order to facilitate placement reliability, a concentric annular packing system was chosen to limit slurry bridging during placement in combination with an aggressive pumping design (25 bbl/min for the lower zone and 32 bbl/min for the upper zone). This paper will discuss the system used for successful frac packing and how it provided alternate paths for the slurry flow when a bridging occurred while keeping the same high flow rate.

Data collected on the two zones during treatment have shown that early annular bridging had occurred during slurry placement, but the system had successfully created a high-velocity flow path with minimal pressure drop to bypass the bridge. Once the bridge was bypassed, the full flow area was again used, minimizing turbulence and potential for damaging screens.

The paper will describe:

  • The target zones

  • The concentric annular packing system design

  • Integration of the concentric system in the dual stacked frac packs architecture

  • Hole pattern design and velocity calculations in shroud orifices

  • The service tool design for stack frac-packs

  • Pumping treatment analysis

  • Observations gathered during the job

  • How technique and job-treatment design versus use of a regular frac-pack system or other alternate-path systems was justified.

Although alternate path technique has been used in frac-packing more than 30 times globally, the stack frac-pack performed on this case history well (WELL-5) holds the record for being the longest treated length, steepest deviation, and highest pumping rate ever attempted thanks to the concentric annular packing system originally designed for the horizontal gravel pack completions.

High completion efficiency was achieved. Productivity data are provided (skin, PLT, etc).

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