Shell Oil Co. and partner BP Exploration Inc. will use a tension leg platform (TLP) in first phase development of giant Mars field about 130 miles off Louisiana in the Gulf of Mexico.
The 3,250 ft tall Mars TLP is to be installed in 1996 in 2,933 ft of water on Mississippi Canyon Block 807 by operator Shell. Upon installation, Mars TLP will set a water depth record in the gulf for a permanent drilling and production platform.
Mars oil and gas is to begin flowing in fourth quarter 1996, increasing steadily to a peak of about 100,000 b/d of oil and 110 MMcfd of gas in 2000. Shell's current production in the Gulf of Mexico amounts to 132,000 b/d of oil and 753 MMcfd of gas.
Shell estimates ultimate recovery in the $1.2 billion first phase Mars development at about 500 million bbl of oil equivalent (BOE). Although plans are not final, recovery during the next development phase is expected to exceed 200 million BOE.
Shell estimates about 85% of Mars reserves are oil and 15% gas. Recoverable reserves of 700 million BOE would rank Mars as the Gulf of Mexico's largest oil accumulation.
About 55% of first phase costs will go for fabricating and installating the TLP hull, deck, and production and processing equipment and laying Mars oil and gas pipelines. Drilling and completing first phase wells will account for the remaining 45%.
Shell and BP plan to predrill about 10 of the 26 planned Mars development wells. The George Richardson semisubmersible rig, owned by Sonat Offshore Drilling Inc., Houston, is to begin predrilling in fourth quarter 1993. The remaining wells are to be drilled by a contract rig mounted on the TLP.
Developing Mars in phases will enable Shell to minimize upfront costs and gain experience operating wells in the field before further development.
TLP DETAILS
When completely assembled on location, the Mars TLP is to be 3,250 ft tall from seafloor to crown block. Steel weight of the unit will be about 36,500 tons. TLP drilling and production topsides will have slots for 24 wells. Two subsea wells also could be tied back to the installation during second phase development.
The Mars TLP deck is to include five modules--drilling, processing, power, wellbay, and quarters--fabricated by McDermott Inc. at Morgan City, La. The deck is to be an open truss framing design with dimensions of 245 ft by 245 ft by 45 ft high and is to weigh about 7,200 tons.
Production and processing equipment installed on the TLP will include complete oil and gas separation, dehydration, and treatment units capable of handling 100,000 b/d of oil, 110 MMcfd of gas, and 25,000 b/d of produced water. The TLP accommodation module will include a control room, emergency response center, and housing for 94 persons.
Mars oil production will move through a 116 mile, 18 in. pipeline to an onshore terminal near Clovelly, La. Mars gas is to flow about 55 miles through a 14 in. pipeline to a pipeline interconnect in the South Pass area.
The 15,560 ton Mars TLP hull is to be comprised of four circular 661/2 ft by 162 ft steel columns connected at the bottoms by four 27 ft by 24 ft pontoons. Belleli SpA, Taranto, Italy, is to fabricate the hull.
The Mars TLP is to be anchored to piles on the seafloor by 12 tendons--three at each corner, each about 2,852 ft long with 28 in. diameter and 1.2 in. WT. Combined weight of the 12 tendons will be 6,150 tons. Each of the 12 piles is to be 84 in. in diameter, 375 ft long, and weigh 260 tons.
Mars TLP will be able to withstand simultaneous hurricane force waves of 71 ft and winds of 140 mph.
BP said a TLP was found to be the most economical of more than a dozen development schemes considered for the field.
DEEPWATER ACTIVITY
In addition to Mississippi Canyon 807, the Mars unit includes all or parts of five other blocks in the Mississippi Canyon area of the Outer Continental Shelf: 762, 763, 806, 850, and 851. The Minerals Management Service approved unitization of the Mars prospect in December 1992.
Shell acquired the leases encompassing Mars field at federal offshore lease sales in 1985 and 1988. Shell holds a 71.5% interest in Mars, BP 28.5%.
Shell and BP drilled the Mars discovery, well in 1989 on Mississippi Canyon 763, with the Discoverer Seven Seas drillship (OGJ, May 6, 1991, p.42). The companies since have drilled four more wells and six sidetracks and shot two 3D seismic surveys to delineate the prospect.
When Mars TLP is installed on Block 807 in 1996 it is expected to eclipse the water depth record scheduled to be set later this year by Shell's Auger TLP when it is installed in 2,860 ft of water on Garden Banks Block 426.
Mars and Auger will surpass Shell's Bullwinkle platform, set in 1988 in 1,350 ft of water on Green Canyon Block 65, as the world's tallest fixed platform. For 10 years before Bullwinkle was installed, Shell's Cognac platform held the water depth record for fixed platforms--1,025 ft of water on Mississippi Canyon Block 194.
Shell owns 32% of leased Gulf of Mexico acreage in water more than 1,500 ft deep. In 1987, the company set a water depth drilling record with a wildcat in 7,520 ft of water on the gulf's Mississippi Canyon Block 657.
BP holds interests in 314 federal leases in the Gulf of Mexico, 282 of them in deepwater areas.
GEOLOGIC SETTING
The Houston Geological Society (HGS) said Shell and BP have identified 14 significant Mars intervals at 10,000-19,000 ft.
Mars reservoirs are characterized by turbidite sands with average 30% porosity and permeability ranging to more than 2 darcies. Column heights exceeding 2,000 ft have been seen associated with six of the reservoir intervals.
HGS data place Mars field in a chalk floored sedimentary basin rimmed by salt canopies and diapirs likely formed when deeper salt was remobilized by depositional loading of lower Tertiary sediment.
Early depositional fill patterns suggest that laterally extensive turbidite sheet sands entered the basin through multiple points adjacent to isolated salt core highs. Traps for the deeper horizons appear to have resulted from stratigraphic onlap onto the basin margin or from truncation of the reservoirs by subsequently remobilized salt.
A May 1991 report by Petrie Parkman & Co. Inc. said the Mars discovery well found oil bearing zones at 14,500-18,100 ft subsea with net pay of 440 ft. Shell said only that Mars formations above 14,000 ft are Pliocene and deeper reservoirs Miocene.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
