A. D. Keen
Gulf Coast News Editor
Exxon Co. U.S.A. has begun installing the largest multiwell satellite subsea template system in the Gulf of Mexico.
Exxon last week began installation of the 445 ton drilling and production template and 285 ton, 10 well manifold last week in about 1,500 ft of water on Mississippi Canyon Block 354 about 50 miles south of Grand Isle, La., to handle production from Zinc field.
DEVELOPMENT PLANS
According to the present development plan, Exxon will transport Zinc field gas to Alabaster platform in 468 ft of water on Mississippi Canyon Block 397.
Exxon expects to drill as many as six wells through the Zinc template, although more might be needed. The first three wells-to be drilled directionally in first quarter 1993 to sands at true vertical depths of about 7,000 ft-are expected to be producing by mid-1993. The next three wells will target deeper reservoirs.
Budgeted cost of building and installing Zinc template and drilling the first three wells is $150 million. Each of the first three wells is expected to produce about 15 MMcfd of gas.
Production from Zinc and Alabaster fields is expected to peak in 1995 at 115 MMcfd of gas and 2,500 b/d of oil and condensate. Field life is projected at 20 years.
Exxon owns 100% interest in Zinc field, which covers Blocks 354, 355, 398, and 399 in the Mississippi Canyon area of the central gulf. Exxon drilled the discovery well on Block 354 in 1977.
ZINC FACILITIES DESCRIBED
The Zinc subsea template-designed to promote use of proven industry components-sits on four 54 in. tapered piles set 225 ft into the seabed.
McDermott International's 497 ft DB50 derrick barge with a 3,500 ton revolving crane was to lower the 730 ton structure into position. Exxon officials expect some pile settling, so support for the template will be provided long term by well casing strings.
The 118 ft by 75 ft by 28 ft template has 10 well bays arranged five to a side, each with a hub connecting it to the subsea manifold assembly in the template's central corridor. Production from individual wells is to be commingled before transport to Alabaster through three 6 mile long subsea flowlines-an 8 in. and a 4 in. line for high pressure and an 8 in. line for low pressure. The 8 in. lines are looped to permit pigging, and the 4 in. line can be pigged into the high pressure 8 in. line.
Two umbilicals provide electrical power and control signals as well as fluid conduits for hydraulic functions and chemical injection.
Zinc's subsea trees provide nominal 3 in. production bores and nominal 2 in. annulus bores, the former equipped with primary and secondary underwater safety valves and the latter with master and crown valves. The trees are rated at 5,000 psi.
Tree control pods and subsea chokes are retrievable by remote offshore vehicle (ROV). Isolation valves on the subsea manifold can be ROV actuated.
PRODUCTION PLANS
Bottomhole reservoir pressure of Zinc's main producing horizon is 3,200 psi and bottomhole temperature 150 F.
Zinc well designs include single zone, cased hole gravel pack completions for sand control.
For redundancy, each production tubing string is equipped with two tubing retrievable surface controlled safety valves (SCSV) in series, plus a nipple for backup wireline-insertable SCSVs.
Deepwater mobile offshore drilling or workover units will be required for tree repair if a gravel pack fails, if both SCSVs fail to test, or if one fails to open hydraulically. SCSV redundancy is included to reduce the need for rig intervention.
Zinc gas is sweet and contains little carbon dioxide. Exxon will protect the tree bore and downstream flowlines by injecting corrosion inhibitor into the subsea tree. To prevent hydrate formation, Exxon will continuously inject methanol into the production system. In addition, flowline heaters have been installed at Alabaster platform's inlet.
Southern Natural Gas is to transport Zinc and Alabaster gas to market through a 20 in. gas pipeline.
For oil and condensate produced by the project, Exxon Pipeline Co. recently laid a 12.8 mile pipeline from Alabaster to Exxon's Mississippi Canyon Block 311 facility, which in turn is connected to Exxon Pipeline's Grand Isle gathering system. The Alabaster to Block 311 line is Exxon Pipeline's deepest water pipelay to date in the gulf.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
