DEEPWATER DRILLING & PRODUCTION Shell's commitment to deepwater taking it to record depths in US Gulf

Shell Offshore's deepwater developments.

Dev George
Managing Editor

Around the world, in almost every major offshore petroleum province, offshore operators are descending to ever deeper prospects as maturing fields and less productive nearshore prospects prompt the industry's search farther seaward to the continental slope and beyond. In many of the traditional regions - North Sea, Gulf of Mexico, West Africa - this movement has resulted in the discovery of a score of large, new fields that provide even greater impetus to deepwater exploration and production.

Shell Oil Company, through its US subsidiary Shell Offshore, is probing geological sections in ever greater depths in the Gulf of Mexico. The company currently holds some 3.5 million acres of Gulf of Mexico leases, 627 blocks, of which 33% lies in water depths of over 1,500 ft - far more than any other operator.

Since 1987, when the company drilled its first deepwater exploratory well in Mississippi Canyon Block 657, Shell has been a leader in the deepwater movement, vying with Petrobras, its Brazilian counterpart, in establishing record after record in the advance to greater and greater depths.

Shell's first deepwater field, Cognac, online in 1978 at a water depth of 1,025 ft, was surpassed in 1988 by the company's Bullwinkle on Green Canyon Block 65, with what was the world's tallest fixed platform standing in 1,350 ft of water. The Texaco TLP Jolliet took the record in 1989 at 1,760 ft water depth, until Shell's Auger TLP recouped it in 1993, when it was installed in 2,860 ft on Garden Banks Block 426.

Three subsea production systems also broke records during the past two years: Shell's Tahoe Field, with production at 1,500 ft water depth on Viosca Knoll 783; Rocky Field, at 1,755 ft on Green Canyon 110; and Popeye, at 2,100 ft on Green Canyon 116.

Next year, Mars

But these first ventures into the deep are but the initial phase in Shell's ambitious deepwater program. Next year, the company is planning the installation of an enormous tension leg platform on the Mars Field in Mississippi Canyon Block 807, at a Gulf of Mexico record water depth of 2,933 ft. Mars is beyond doubt the largest oil and gas discovery in the Gulf of Mexico in more than 20 years, with more than 700 million bbl of oil and gas equivalent in recoverable reserves. Shell holds 71.5% interest in the field, while its partner, BP Exploration, holds the remaining 28.5%. Shell is the operator.

Mars will cost about $1.2 billion to develop just in its first phase, which is aimed at recovery of 500 million bbl, but the technology applied will provide guidelines for further Shell development in even deeper water.

The discovery well was drilled in 1989 on Mississippi Canyon Block 763 by the Discoverer Seven Seas drillship. Four other wells and six sidetrack wells have been drilled and a 3D seismic survey was conducted since to evaluate the field. The prospect encompasses almost all of six Mississippi Canyon blocks.

Plans for the first phase call for installation of a tension leg platform (TLP) on Mississippi Canyon Block 807, about 130 miles southeast of New Orleans. Production should begin in late 1996 and is expected to reach a peak rate of about 100,000 b/d oil and 110 MMcf/d gas. Thereafter, Shell plans to give the field two or three years to pay for itself before launching phase two. The second phase will most likely involve installation of a second TLP and/or a complex of subsea wells tied back to the original TLP.

Ram-Powell

Following closely behind Mars is Shell's massive Ram-Powell TLP, which will make another giant step down the slope to a further record water depth of 3,218 ft. To be installed in mid-1997 on Viosca Knoll Block 956, about 125 miles east-southeast of New Orleans, Ram-Powell will be Shell's third and largest yet TLP.

Shell, the operator of the Ram-Powell Field, owns 38% interest in the project, while its two partners, Amoco and Exxon, each hold 31% interest. Overall development cost will be about $1 billion.

Production is expected to begin in late 1997, with estimated recovery set at 250 million bbl oil equivalent, with production of 60,000 b/d oil and 200 MMcf/d gas.

Plans call for three to four development wells to be predrilled yet this year, with further development wells to be done by a contract rig after the TLP's installation. An additional four subsea wells will also be drilled that are to be tied back to the TLP.

Mensa & Ursa

Perhaps the deepwater projects that stir the imagination of offshore explorationists the most, however, are Mensa and Ursa.

Mensa is an enormous gasfield discovered in 1987 that lies 140 miles southeast of New Orleans in Mississippi Canyon Block 687 at a depth of 5,400 ft. This depth will be more than 2,000 ft deeper than Petrobras's current world-record water depth for production (3,400 ft).

When Mensa begins production in 1997, it will yield approximately 300 MMcf/d gas, boosting Shell's present daily Gulf of Mexico production by more than 25%. Fully owned by Shell Oil Company, the field encompasses Mississippi Canyon Blocks 686, 687, 730, and 731, and is expected to produce at least 720 bllion cu ft/d gas.

Development of Mensa, set to cost about $290 million, will be carried out using a system of subsea wells tied back to a shallow water platform at West Delta 143, via a 68-mile flowline - the longest tieback in the world.

Ursa is Mensa's oil equivalent. It was discovered in 1991 with a wildcat drilled by Sonat's Discoverer Seven Seas drillship on Mississippi Canyon Block 854, but encompasses six blocks: Mississippi Canyon Blocks 808, 809, 810, 852, 853, and 854. Recoverable reserves are set at over 220 million bbl oil.

Two delineation wells and one sidetrack well have been drilled since, with another to be drilled later this year, but development of the field continues to be studied by Shell and its partners, BP Exploration (23%), Conoco (16%), and Exxon (16%). Shell holds the remaining 45% and is the operator of the field.