Abstract

GU field high rate gas campaign was selected by management as a pace setter project to deliver NAG to the Nigerian LNG trains project as part of the overall strategy of implementing the revitalized performance improvement initiative in Nigeria. To deliver top quartile performance at lower unit technical cost without compromising on safety and sustainable community development, an innovative well design philosophy with game changer technology was proposed.

The approach utilized a number of innovative techniques; slim well design with 2-casing string philosophy; 1-skin strategy and casing flow design. The 9 5/8" string acted both as the surface and production casing with the shoe above shallowest hydrocarbon. The build up, landing and drain section was drilled in a single trip through several unstable and reactive shale sections hence the "all-in-one approach". The 7" production liner was combined with 5 ½" expandable sand screen lower completion in a single trip. The 5 ½ Expandable Sand Screen was compliantly expanded to the 8 ½" drain hole.

Unlike previous concepts where zonal isolation was provided with cement, zonal isolation was provided with swellable elastomers bonded and spaced out on the 7" production liner. GU-11ST well was successfully completed as a single string single gas producer with 7" 13 Cr completion tubing.

The well tested to a facility-limited rate of 52 MMscf/d with FTHP of 3423 psig and condensate rate of 3900 bbl/d psi. Extrapolated well test results showed that the well can produce at 120 MMscf/d at a draw down of ca. 18psi; with extra swing capacity to go up to 160 MMscf/d when required. Several well testing challenges were addressed during the cleanup and flow period.

The systems approach on well design in GU-11ST resulted in lower unit technical cost compared to other wells in the campaign. It has also proven to be a future enabler to implementation of the small location concept with light and less expensive rigs without compromising safety and well objectives.

Background

The GU field is a large, partially developed field situated in OML-28 in the seasonal fresh-water swamp area, some 110 Km west of Port Harcourt or 20 km North East of Yenagoa. The field was discovered with the drilling of GU-001 well in 1967 and is currently in the early appraisal/development stage. To date, eleven appraisal/development wells have been drilled. GU -02 and -11 were abandoned due to lack of completable interval.

The eleven wells, (5 verticals and 5 horizontals), drilled so far in the field encountered a total of 23 hydrocarbon bearing horizons within a depth range of 7300 ftss to 15000 ftss. Of these, 2 are oil bearing, 4 are oil and gas bearing while the remaining 17 contain non-associated gas.

SPDC's drive to increase and sustain production at reduced Unit Technical Cost favoured accelerated production achievable through the drilling of high-rate wells and delivery of a low cost initiative without comprising well integrity. As highlighted most of the well designs in the Niger Delta transverses depleted sands separated by highly stressed or faulted mud stones. In the GU field this has resulted in severe cost over run & in most cases a compromise in the well objective.

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