Mensa Project: An Overview
Authors: McLaughlin D.C. 
Published: October 2008

This paper presents an overview of the Shell Offshore Inc. (SOI) Mensa Project, a very remote subsea gas development in 5300 feet of water in the Gulf of Mexico (ref. OTC Papers 8577, 8578, 8579, 8580, 8581, 8626, 8627, 8628, 8629, 8630, and 8631 for more technical details). Planning and execution of the Project required a challenging balance throughout between the desire to utilize proven solutions from SOI's Popeye Project(l), the requirements to extend and develop new technology, continuing emphasis on cost and cycle time reductions, and working within a newly created Subsea Systems Alliance. Mensa was successfully installed and the first well started up in July 1997. It represented application of an innovative, cost-effective field development system, which established dramatic new offshore standards for water depthand offset distance.

Introduction

In 1987, SOI discovered Mensa, a gas field in the Mississippi Canyon Area of the Gulf. A second (appraisal) well was drilled in 1988, continuing a large gas discovery. However, since the find was in water much deeper titan any offshore development at that time, its development feasibility was uncertain. Joint subsurface and development feasibility studies were initiated in 1989, including consideration of a Tension Leg Well Jacket concept and subsea options. By early 1991, it was apparent that a satellite subsea development was the only viable economic solution, but the technology was not ready. Deepwater diverless subsea technology had been demonstrated by this time, however the applications had been m less than 3000 feet of water with relatively short offsets, were lower pressure (5000 psi systems) and had utilized guidelines for guiding/positioning drilling and subsea equipment to tile seafloor. Mensa was much higher pressure, with very long offset, and well beyond the practical water depth limit for using guidelines. The basic strategy adopted then was to develop SOI's Popeye prospect such that it could serve as a technological stepping stone to Mensa. Mensa was put in the long range operating plan to follow Popeye. In 1993, SOI completed a trade with leaseholders of the two blocks north of the Mensa exploration wells, and a four-block unit was formed. SOI is the Operator and has a 100% ownership in the unit. As interest increased in accelerating all of SOI's deepwater prospects, additional preliminary design trade-off and operability studies were conducted for Mensa, including consideration of a Spar buoy to provide local chemical injection and controls. A totally subsea system wasselected by late 1994, and a development plan with conceptual design developed by spring of 1995. Although accelerated by two years from tile original long range plan, it was felt that the technology development for Mensa could be achieved within acceptable risks. The Mensa Project Team was formed and the project approved in late May 1995. First production was achieved in July 1997, several months ahead of the base case schedule.

System Description

The Mensa reserves were deposited in a single thick turbidite reservoir within Mississippi Canyon Blocks 686, 687, 730, and 731. The development plan includes three subsea wells directionally drilled from the spread well "cluster" area on Block 687 to bottomhole locations on Blocks 686, 687, and 730.