Packerless, Single-Selective, Gravel- Packed Completions Designed to Improve Recovery at Cognac Field
Authors: Omar M. Rashid 
Published: December 2007

Packerless, Single-Selective, Gravel- Packerless, Single-Selective, Gravel- Packed Completions Designed to Packed Completions Designed to Improve Recovery at Cognac Field

Summary
At Cognac field, most reserves are contained in complex deltaic sands. A substantial thickness of low-permeability, shaly sands are overlain by high-permeability, clean sands. Because of the extreme variation in productivity, flow from the lower section was observed to be nonexistent or poor in common completions over the entire sand. The "packerless," single-selective, gravel-packed completion scheme was designed and installed initially to produce only the lower sand sections. Subsequent production produce only the lower sand sections. Subsequent production log evaluation indicated that the desired production profile was not achieved. Therefore, several modifications profile was not achieved. Therefore, several modifications were adopted on wells completed during the final phase, including independent backsurging of perforations and circulating gravel packs. A production log confirmed the effectiveness of the modifications. This modified completion technique has resulted in an increase in recovery from the bottom, lower-permeability sand sections and in high production rates with a minimal increase in expense. production rates with a minimal increase in expense.

Introduction
The Mississippi Canyon Block 194 field (Cognac) is situated on four tracts located in the Mississippi Canyon area, 12 miles [19.3 km] southeast of the mouth of the Mississippi River (Fig. 1). The platform is a 24-pile, 62-well-slot, fixed structure, set in a water depth of 1,025 ft [312.4 m] Shell Offshore Inc. operates the field in a joint venture with Petro-Lewis Corp., Sonat Exploration Co., Getty Oil Co., Amoco Canyon Co., Eugene Shoal Oil Co., Phillips Oil Co., Koch Industries, Inc., Odeco Oil and Gas Co., Ocean Oil and Gas Co., and Murphy Oil USA, Inc. Development drilling began in Oct. 1978 and was completed in Aug. 1981, after 61 wells had been drilled. The final phase of initial well completions was finished in Feb. 1983. The Cognac field production peaked during Sept. 1983 with average rates of about peaked during Sept. 1983 with average rates of about 85,000 B/D [13 514 m3/d] oil and 120 × 10 6 cu ft/D [3.4 × 106 3/d] gas. More than 85 % of the oil reserves are contained in five major reservoirs that range in depth from 7,200 to 9,500 ft [2195 to 2896 m] subsea and dip from 6 to 10 deg. Average pay thicknesses vary from 20 to 85 ft [6 to 26 m] true pay thicknesses vary from 20 to 85 ft [6 to 26 m] true vertical. Porosity ranges from 25 to 33% and permeability from 100 to more than 2,000 md. permeability from 100 to more than 2,000 md. Fig. 2 is a composite log that illustrates the complex deltaic depositional pattern of these lower-Pleistocene/ upper-Pliocene sediments. The massive, clean, high-resistivity sand at the top of these intervals is associated with high-energy delta deposition. This grades down toward the base of the intervals with lower-energy deposition, leaving a fine-grained, shaly sand interbedded with thin, high-permeability, laminated sand. Fig. 3 is a typical log section of a completed interval that depicts the decreasing sand quality from top to bottom. Permeabilities in this section range from more than 2,000 to less than 200 md, top to bottom. During the first completion phase (1979) at Cognac, entire intervals from top to bottom were perforated overbalanced with electric-line casing guns, backsurged, and "squeeze" gravel packed. A spinner survey in 1979 in one well with 85 ft [26 m] of perforations indicated that 100% of the flow was from 16 ft [4.9 m] at the top of the sand with no flow from the lower 69 ft [21 m] of perforations (Fig. 4), a result predicted for most of the perforations (Fig. 4), a result predicted for most of the completions. There was concern that considerable reserves could be left behind in the lower sand members, particularly in a water-injection program. Establishing inflow particularly in a water-injection program. Establishing inflow from these lower-permeability, bottom sand members is the subject of this paper.
Packerless Single-Selective Scheme Packerless Single-Selective Scheme To evaluate the productivity of only the lower sand members, a special gravel-packed selective scheme was designed (see Fig. 5). Because of the limited separation between the upper and lower parts of the sand, a conventional single-selective completion--i.e., individual, viscous gravel packs across the upper and lower sand members separated by an isolation packer-was noteble. The 2 3/8-in. [6.0-cm] isolation tubing extending across the upper sand member inside the 4-in. [10. 2-cm] screen was assembled on the surface as part of the gravel-pack assembly. After production is established from the lower zone, the upper zone can be added and commingled with the lower zone at any time by wireline perforating the 2 3/8-in. [6.0-cm] isolation tubing; the lower-zone production can be shut off at any time with a wireline plug; and production can be shut off at any time with a wireline plug; and the upper zone can be produced exclusive of the lower zone, if the lower zone waters out. It was expected that production from the upper zone into the lower screen production from the upper zone into the lower screen would be isolated sufficiently by the gravel column and the associated pressure drop in the annulus between the upper and lower zones. The advantage of this completion scheme is that it allows for selective production with essentially the cost of a single completion.
JPT
P. 1819