Monitoring Primary-Depletion Reservoirs With Seismic Amplitudes and Time Shifts
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18219, "Monitoring Primary-Depletion Reservoirs With Seismic Amplitudes and Time Shifts," by A. Tura, T. Barker, P. Cattermole, C. Collins, J. Davis, P. Hatchell, K. Koster, P. Schutjens, SPE, and P. Wills, Shell Intl. E&P, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May.
In the high-porosity, poorly consolidated turbidites of the deepwater Gulf of Mexico (GOM), production-induced compaction can be the drive mechanism when aquifer support is weak and before pressure support by secondary-recovery water injection begins. Time-lapse (4D) seismic-monitoring time shifts occur in areas of depletion and in the overburden, and they indicate compartmentalization in the reservoir. Compartmentalization information can help place new production and injection wells better, as well as new sidetracks for optimized field development.

Introduction
Travel-time changes are more sensitive to reservoir compaction caused by depletion than to fluid-saturation changes. As a result, travel-time changes are simpler to interpret compared to amplitude changes. In primary-depletion reservoirs, the 4D amplitude and travel-time changes are not large. Small-to-moderate amplitude changes manifest at the oil/water contact (OWC) or in large pressure-depletion areas. The travel-time changes can be on the order of one to several time samples of the typical seismic-sampling interval of 4 milliseconds. In the GOM, the overburden can contain large faults, salt bodies, steep dips, near-surface channels and bodies, diffractors, and overpressured regions. As a result, 4D monitoring of these reservoirs requires highly repeatable seismic data with very accurate positioning of sources and receivers between surveys.
In the GOM, it is often difficult to obtain high repeatability with 3D streamer surveys because of strong currents. A novel two-boat, three-pass acquisition method to overcome acquisition-repeatability issues was investigated and used by the Shell Mars/Europa 4D study team over the Mars and Europa fields. It was possible to match amplitude changes computed from synthetic seismic (by use of history-matched reservoir-simulation models) as well as travel-time changes computed from geomechanical models of these fields.

Mars and Europa Fields
Field data from Mars and Europa fields demonstrate the concepts detailed in the full-length paper. Mars is a large field in approximately 4,000-ft water depths with approximately 500 million bbl of recoverable oil remaining from 18 vertically stacked producing reservoirs. Most production is from four main reservoirs, with up to 7,000 psi pressure depletion and with the deepest reservoir being the largest. Waterflooding began in 2004 to recover remaining reserves. Time-lapse surveys of this field are important to monitor the waterflood and detect compartmentalization.
Europa is a subsea-completed field and is connected to the Mars platform. It has two producing reservoirs, with up to 4,000 psi pressure depletion, and has been undergoing primary depletion since January 2000. Well performance indicated that both producing reservoirs are compartmentalized by small-scale faults, and production problems have been encountered including well failures and fluid-flow impairment. Time-lapse seismic was used in this field to detect compartmentalization and movement of the OWC.