Mensa Field: Deepwater Gulf of Mexico Case Study

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 159741, ’Mensa Field, Deepwater Gulf of Mexico - Case Study,’ by Muhammad Razi and Peter Bilinski, Shell Exploration & Production, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 8-10 October. The paper has not been peer reviewed. The Mensa field is a deepwater Gulf of Mexico (GOM) geopressured gas field, producing from Upper Miocene sand with faulted four-way closure. During the early life, pressures fell sharply, with the field performing as a dominantly depletion-drive reservoir. The trend indicated isolated compartments. After a few years of production, pressures leveled off significantly. This midlife trend seemed to be in contact with originally isolated gas compartments and an aquifer. The late-life pressure trend indicates aquifer influx that effectively blocked the main producing wells from a significant gas area and pressure support. Introduction The Mensa field is a deepwater GOM subsea development in Mississippi Can-yon (MC) Blocks 686, 687, 730, and 731 (Fig. 1). The field was discovered in 1987 with Well MC731-1, which penetrated 110 ft of net gas pay in the I sand. Appraisal Well MC730-1, drilled in 1988, penetrated 168 ft of net gas pay in the same sand. On the basis of seismic interpretation and well information, the I sand was modeled as a thick high-net-/gross-pay-ratio (NTG) sand with 29 to 33% porosity, 250- to 1,000-md permeability, and a gas-in-place volume of up to 1.5 Tcf (Fig. 2). North/south-striking faults divide the main I sand into eastern and western fault blocks. The fault tips out to the south such that the fault blocks are in good pressure communication (confirmed later by depleted pressures recorded in Well A2). A basinwide aquifer was modeled in the south, with the original gas/water contact (GWC) set at 15,750 ft subsea. The I sand was assumed to produce by moderate-to-strong aquifer support. The Mensa field faced many challenges. Deep water (approximately 5,300 ft) resulted in high subsea-well and system costs. Nearest facility was approximately 68 miles away (West Delta Block 143), involving high development cost to connect. Low ultimate recoveries because of the almost flat structure (approximately 5° dip), large aquifer, and high-permeability sand (wells water out immediately after water breakthrough). Poor economics because of the low condensate yield and low gas prices at that time (approximately USD 1.5/Mcf). High subsea-intervention cost and long time required (e.g., wireline job, acid treatment, or workover/sidetrack). High rock compressibility (30 to 60 microsip), which resulted in collapsed casing and sand-control issues.