RIASSUNTO
Abstract
This paper describes a two-step frac/gravel pack completion procedure conducted from a jack-up rig offshore Gabon in 1993 and 1994. After an in-depth, sand production study, seven wells were completed using hydraulic fracturing and cased-hole gravel packs.
Because the Anguille formation spreads over a large gross interval, production screens of 50 to 100 m In length were used. single-stage internal gravel packs (IGPs) on such large intervals were considered impossible to achieve with reasonable skin values.
The frac-pack technique was an attractive way to achieve good proppant placement, considering that the formation being fractured would have a more homogeneous conductivity across the whole zone and a uniform proppant placement would be achieved in the perforations as well as in the annulus.
High-efficiency frac gel using cross-linked HPG and a low damage one using HEC were compared to determine the best fluid to be used for these completions. Bottom hole pressures during treatment operations, as well as post-treatment pressure build-ups from each well, were analyzed in order to make general recommendations on this hydraulic fracture design.
Introduction
Different sand control techniques have been used in the last 30 years in West Africa by Elf Aquitaine Production, but very often IGP was selected because the formations are usually very heterogeneous and shaly. This has been the case for 50% of the wells requiring sand production control. The average skin of wells completed with IGPs range usually between 10 and 15. chemical consolidation was used on 15% of these wells with good results on short and clean intervals. Open-Hole Gravel Pack (OHGP), when applied on clean and homogeneous intervals, resulted in an average skin near 5. On all these wells, the height of the producing interval was between 8 and 30 m. When the interval was larger, usually only one part of it was completed on each well. Productivity Index (PI) reduction was observed when implemented in shaly and silteous reservoirs.
The development project of Hylia offshore Gabon consisted of seven wells. The reservoir had a gross pay of 100 to 260 m consisting of very heterogeneous and shaly intervals. For economical reasons, it was necessary to produce the entire interval on each well to help minimize the pressure losses usually observed with a sand control completion. Chemical consolidation and OHGP was discarded. Only IGP was considered to be suitable to the reservoir. The frac-pack technique was selected because it maximizes the production over the entire interval.
SAND PRODUCTION PROBLEM
Reservoir description
In the Hylia field, oil-bearing reservoirs belong to the Anguille formation. This formation shows good petrophysical characteristics including porosity and permeability values ranging respectively from 20 to 27% and 100 to 300 mD. Average vertical depth of the objective is about 1500 m(SL) and water-oil contact was found in one exploratory well at 1555 m(SL). Gross pay is from 100 to 280 m. Net pay is from 75 to 150 m. Pore pressures are normal and very close to hydrostatic, with pore pressure gradients of 1.07 bar/10 m.
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