Mensa: Shell's Mississippi Canyon Block 731 Field - An Integrated Field Study

Pfeiffer, D.S., B. T. Mitchell, and G.Y. Yevi, Shell Offshore, Inc., New Orleans, LA 70116

Abstract

Shell Offshore Inc.'s Mississippi Canyon Block 731 Field (Mensa) is a large turbidite reservoir in 5300 ft water depth, containing in-place reserves of over 1.3 TCF of dry gas. The field is currently producing over 240 MMCFGD out of three wells, connecte d to a subsea manifold located five miles away. The gas and associated condensate (1.75 bbls/mmcf) are carried through a 63-mile flowline to Shell's West Delta 143 platform for processing. There are limited opportunities to intervene in such a system, and any additional work in the field (e.g., recompletions/workovers) is very costly. Therefore, an understanding of the reservoir geology is critical to optimize the development and minimize the need for future intervention work.

The vast majority of the gas reserves are in the late Miocene "I" sand, an amalgamated sheet sand deposited as a fan in a relatively unconfined basinal setting. Salt withdrawal on the south end of the basin has provided accommodation space, and the fan is proximal to a sediment entry point on the northwest edge of the field.

Based on 3D seismic and two well penetrations (the discovery and appraisal wells), the "I" reservoir was originally modeled as a very homogeneous sand connected to a large aquifer to the south that provided the field's drive mechanis m. However, when production from the first well commenced, initial pressure measurements did not support this model and reservoir simulations using these data suggested that the "I" sand was primarily a depletion-drive reservoir having little or no aquifer downdip. This had serious negative implications for the estimated recoverable reserves and the development strategy; the 63 mile-long flowline and glycol-based hydrate inhibition system required maintaining a reservoir pressure above 3200 PSI over the l ifetime of the field and the large aquifer was supposed to provide this support.

The Mensa subsurface team re-mapped the field using a high frequency seismic dataset that had not been previously available. The team re-evaluated both the geologic model for the "I" sand and other new but smaller gas reservoirs encoun tered in the last production well to be drilled in the field (MC-687 A-2). The results suggested that the original geologic model for the "I" sand was overly simplistic.

The reservoir is now viewed to be an amalgamated sheet sand complex, having at least two distinct sand lobes seen seismically and in well logs, one cutting down into the other, and with the possibility for there being at least a part ial permeability barrier between them. Some of the new reservoirs seen in the A-2 well appear to be juxtaposed with the "I" sand across a series of faults within the field, providing additional reserves that could be at least partially drained by the three producing wells. An erosional bypass channel bisects the "I" sand aquifer and may be limiting the amount of pressure support that the aquifer can provide. The aquifer itself may be composed of several sand lobes that may not fully communicate with each other although there is no current evidence seismically and there are no wet "I" sand penetrations. The presence of partial permeability barriers in the "I" sand (either from a post-depositional bypass channel or internal baffles) and potential access to more volumes in other reservoirs could all combine to give an early pressure history that would suggest a depletion drive. Over time, however, the team expects that these internal barriers will break down as the pressure differential across them increases (this has been Shell's experience elsewhere, e.g., Shell's South Timbalier 292 Field [Peccary]) and the reservoir will then behave more like an aquifer-driven system. Very recent pressure data appears to confirm this and no change in the developme nt is planned at this time.