High-Accuracy Oriented Perforating Extends Sand-Free Production Life of Andrew Field
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 93639, "High-Accuracy Oriented Perforating Extends the Sand-Free Production Life of Andrew Field," by A.J. Martin, SPE, Schlumberger; D. Robertson, BP; J. Wreford, SPE, Reservoir Management Ltd., and A. Lindsay, KCA Deutag, prepared for 2005 Offshore Europe, Aberdeen, 6–9 September.
BP began development of the Andrew field in 1996 with 13 producer wells and one gas-injection well. Oriented perforating was used in some wells to reduce the risk of sand production from the weaker sandstone intervals. At the time, the accuracy of oriented perforating was unknown. The full-length paper describes the evolution to high-accuracy orienting systems on three new Andrew wells and two workover wells.

Introduction
The Andrew field, a Paleocene sand-stone dome structure 8,000 to 8,400 ft true vertical depth subsea, is in the U.K. sector of the North Sea. Its average porosity is 20%, and permeabilities range from 150 to 1,000 md. The field has been on production since 1996 when four predrilled wells were completed. The development program called for an additional 10 horizontal producers and a vertical gas-injection well. The producers were perforated underbalanced with some perforation intervals oriented to minimize the risk of sand production. Two infill wells were drilled in 1998. Completion strategy uses horizontal wells with cemented liners. Studies showed the formation to be sensitive to kill fluids, so a no-kill perforating solution was chosen that uses a downhole isolation valve. Each well has perforated lengths greater than 3,200 ft. The field came off plateau production in 2000. In 2001, sand production that appeared to be related to pressure drop was detected in two wells. Perforation strategy was reviewed when a new well, Andrew A-15, was planned.

Original Perforations
The original horizontal wells were perforated underbalanced by circulating in base oil before the completion was run and an isolation ball valve provided well control. The isolation valve was installed just below the packer in the 9 5/8-in. casing, with the completion stinging into a sealbore at the top of the 5 1/2-in. liner. Once the completion and tree were installed and tested, guns were run with a hydraulic workover unit and positioned at depth. The guns were fired with the well approximately 300 psi underbalanced.
Wells A-02 to A-06 used a mixture of 3 3/8-in., 4-shots/ft (SPF) guns, with full 60°-phased guns in the stronger B-sandstone intervals and ± 25°-phased guns in the weaker A-sandstone intervals. The phased guns were oriented to shoot the high side of the hole, with perforations oriented 25° either side of the vertical. At the time, the assumption was made that gravity-assisted orientation would point the guns in the right direction with no consideration for alignment error.
The orientation technique used weights and ballistic swivels. The swivels were spaced approximately every 200 ft (10 guns) to allow sections to rotate independently. After additional sanding studies, the rest of the wells were perforated with full 60°-phased guns.