Mensa Project: Well Drilling and Completion
Authors: Burman S.S. , Norton S.J. 
Published: October 2008

Two subsea wells have been drilled and completed in Shell Offshore Inc. (SOI) Mensa Field (Mississippi Canyon Blocks 686, 730, and 731). A third well is currently being drilled. Drilling and completing wells in 5,300 feet of water poses unique design challenges from an equipment and operations standpoint. This paper discusses the drilling and completion designs, operations, procedures, and equipment that were necessary to accomplish the program.

Introduction

The Mensa field, located 140 miles southeast of New Orleans in the Gulf of Mexico, was discovered in 1988 and evaluated with two exploration wells. Development drilling commenced in September 1996 and production began in July of 1997. Mensa is a relatively dry, biogenic gas accumulation in a thick, Miocene turbidite sheet sand and channel-mouth deposition. The single pay sand is located at approximately 15,400' true vertical depth (TVD) with an initial bottom hole pressure of 10,040 psi and a bottom hole temperature of 176 OF. The true vertical sand thickness at the well locations varies from 70 to 140 feet with 30% porosity and an average permeability of 500 millidarcies. The three wells are arranged in a 600' equilateral triangle pattern on the seafloor. From the well locations, the wells flow through individual 6" flowlines to a manifold located 5 miles to the West. The production is then commingled and flows through a 12" flowline to the host platform located in West Delta Block 143 some 63 miles from the manifold.

Drilling

Producing high rate gas wells from a subsea system in 5,300' water depth (WD) requires construction of a stable, long term foundation and installation of protective and production casing strings capable of providing a bottom hole size large enough for the completion equipment. This section reviews drilling of the Mensa field by providing a review of the exploration wells, the site investigation, the development well design, and the results and conclusions of the development drilling effort.

History: Review of Discovery Wells.

In November, 1987 the first of two exploratory wells, OCS-G 7955 No. 1, was drilled in Mississippi Canyon Block 73 1 in 5,378' of water and discovered commercial hydrocarbons in an Upper Miocene turbidite sand package at a depth of 15,584' subsea. An appraisal well followed in August 1988 to further delineate the sand package. The wells were drilled using 30" jet pipe followed by 20" structural casing set into a chaotic sand package 1,850' below the mudline. Both wells had problems with shallow water flows (SWF) that continued until each well was permanently abandoned. After running the marine riser and blowout preventer (BOP), the wells were drilled to total depth of 16,000' TVD with a polymer/salt drilling fluid. Multiple strings were set below the 20" structural casing (13-3/8", 11-314", 9-518", and 7-518"). The major drilling problems were related to (1) shallow water flows wit11 kicks and losses in upper sections, (2) close tolerance between the pore pressures and fracture pressures, and (3) poor progress due to swelling shales.

Design Basis for Development Wells.

The Mensa design basis included the requirement to produce 150 MMSCFD from each of three wells.