BP Trinidad and Tobago (bpTT) has been developing highrate gas fields in Trinidad & Tobago since 1999, and has six high rate gas fields currently on production, with several more in planning stages. All of the wells require sand control and this has resulted in five sandface completion types (Open Hole Gravel Pack, Cased Hole Frac Pack, Cased Hole Gravel Pack, Stand Alone Screen and Orientated Perforating). Based on the experience and field performance, open-hole gravel packing has become the preferred option. The techniques used in completing these high rate gas wells as open-hole gravel packs have included both water-packs and shunt-packs. The experiences gained from these operations have now become part of BP's open-hole gravel pack best practices.
The paper details the completion evolution in BP's offshore Trinidad and Tobago high rate gas fields and the relative performance of these completion types from sand control and well productivity standpoints.
Characteristics of bp Trinidad & Tobago's High-Rate Gas Fields Currently, there are six high rate gas fields that are being operated by bpTT in Trinidad & Tobago: Amherstia, Flamboyant, Immortelle, Kapok, Mahogany, and Cannonball. There are three other fields in the planning phase. The six producing gas fields contain multiple stacked and faultsegmented reservoirs with recoverable reserves ranging from 80 bcf to 1.5 tcf.1 Detailed discussions of the reservoir characteristics for most of these fields can be found in several earlier publications.2,3
The reservoirs in these fields are generally characterized as fine sand, with D50 varying from 50 to 125 microns, uniformity coefficient (D40/D90) of 4.5 to 15, sorting coefficient (D10/D95) of 23 to 95, and fines content (D < 44 microns) of 14 to 47%. From a sand size distribution standpoint, based on available guidelines in the literature, gravel packing and frac packing are the preferred sand control techniques.4,5
As of mid 2006, approximately 50 high-rate gas wells were completed in these six fields, with 10 cased and perforated (no sand control), 2 standalone screens, 8 casedhole frac-packs, 9 cased-hole gravel packs and 23 open-hole gravel packs. These completions are reviewed below, with some examples for each in detail. Additional relevant information to BP Trinidad & Tobago field developments maybe found in the literature.6–14
This technique was applied in deeper reservoirs with relatively high strength, UCS in the range of 1,600 to 2,000 psi. A total of ten C&P completions were performed in high rate gas wells, four in Amherstia, four in Flamboyant and two in Immortelle fields.
C&P-1 Well: This well was completed in June 2002, targeting 23U/L & 24U-sands in Amherstia, at a measured depth of 12,290–13,566-ft (9,834–10,292-ft TVD), with 9–5/8-inch casing set at 14,350-ft. The reservoir section was drilled with a water-based fluid. Sand prediction models from the operator and the service company teams had independently confirmed that this well was a good candidate for a cased and perforated completion with oriented perforating. Tubing conveyed perforating (TCP) assembly consisted of a gross length of 1,301-ft, with a net perforation length of 460-ft, 4-spf and 180o phased 4–1/2-in guns and a packer. The guns were run with weighted spacers and swivels to orient the guns to perforate on the high and low sides of the casing. The upper completion string consisted of a production seal assembly (stung inside the packer), 7-in tubing string, downhole pressure gauge, and a subsurface safety valve. Four intervals varying from 60 to 240-ft were perforated with ∼800 psi underbalance, with a total perforated interval length of 460-ft across the 1,276-ft interval with 76o inclination. The initial reservoir pressure was 4,260-psi, with a bottomhole static temperature of 177oF.