This paper addresses the techniques developed for design and installation of long interval, high angle gravel-packed completions. These techniques were used for single and multi-zone completions installed during eight rig-years of operations on Exxon's Lena guyed tower, located in the Mississippi Canyon 281 field. The paper also discusses the productivity and life of the different completion types.
A major part of the discussion will include the successful application of prepacking and water-packing techniques in the long, highly deviated completion intervals (up to 445 feet at angles approaching 70 degrees) typical at Lena, While some operators consider this technology antiquated, field experience at Lena demonstrates that water-packing has resulted in high performance completions in challenging completion environments.
INTRODUCTION
Exxon's Lena guyed tower is located in the Gulf of Mexico approximately 50 miles southeast of Grand Louisiana, in 1000 feet of water. The platform was installed in 1982 at Mississippi Canyon Block 280 for the development of Blocks 280, 281, and 360. Development drilling from 1983 to 1989 resulted in 93 completions in 56 wellbores.
The completion intervals are characterized by fine-grain, unconsolidated sands that are finely interbedded with shale. These Pliocene reservoirs, which have limited areal extent, dip at 20 to 40 degrees from a piercement salt dome. Radial faults, which dissect the reservoirs, add further complexity to the production strategy. Field characteristics and the resulting development strategies necessitated drilling aggressive S-turn wells to penetrate stacked targets and build-and-hold wells to reach objectives located up to 11,500 feet horizontally from the platform. Wellbore deviations averaged 45 degrees, with some approaching 80 degrees. Due to high incident angles at reservoir contacts and thick sand packages, gravel-packs were frequently installed across long completion intervals, some exceeding 400 feet. Lena well depths ranged from 7,400 to 15,300 feet measured depth and 7,000 to 12,000 feet true vertical depth.
Wells were typically designed with 7 inch casing for single completions, 7-5/8 inch casing for dual completions, and 9-5/8 inch production casing with a 7 inch liner for wells requiring protective casing due to high torque and drag or differential sticking conditions. Each well was completed with the drilling rig immediately following setting of production casing.
BASIS FOR GRAVEL PLACEMENT METHOD
From the beginning of the program, the completion designs were based on Exxon's previous operating experience and gravel-pack modeling performed in the mid-1970s1. Subsequent research2 conducted with a more elaborate model reinforced the design basis and permitted a more detailed investigation of gravel placement. This research was conducted using a 7 inch by 2-3/8 inch gravel-pack completion model that was 25 feet long and contained up to 12 perforations per foot. The model was fabricated from clear plastic so that visual observation of the gravel-packing process could be made. The assembly could be rotated on its stand, which allowed simulation of wellbore deviation up to 110 degrees from vertical.
Over 200 full-scale gravel-pack tests were performed with varying well deviations, pump rates, gravel concentrations and fluid viscosities.
