4D Seismic Pilot Successfully Interprets Carbonate Reservoir
Authors: Carpenter Chris 
Published: February 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 193063, “A Successful 4D Seismic-Monitoring Pilot in a Middle East Carbonate Reservoir Context,” by F. Cailly, T. Al-Romani, C. Hubans, and A. Lafram, Total, and A. Al Kaabi, ADNOC, prepared for the 2018 SPE Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11–14 November. The paper has not been peer reviewed.
This paper describes interpretation results of a 4D seismic-monitoring program in a challenging Middle East carbonate reservoir. The program consists of a 4D pilot [oceanbottom cable (OBC)] over a giant field divided into two phases. The authors discuss the difficulties faced by both phases of the pilot, and prove that a reliable 4D signal can be extracted over a Middle East carbonate reservoir.
Introduction
The time-lapse, or 4D, seismic method consists of repeating seismic survey acquisitions over the production time of a field. Newer vintages are compared with previous ones, and the signal differences are interpreted to characterize effects of the production spatially. Though this technique is mature and has been widely used in clastic reservoir environments, it is still used rarely, in operational terms, to monitor carbonate fields.
An opportunity for a 4D proof-of-concept program in a Middle East marine carbonate environment was identified during a new OBC seismic acquisition in 2013. While the survey was intended for 3D imaging purposes, the operator decided to acquire a 4D pilot over a test area of approximately 25 km2 (Phase 1) while repeating as best as possible the 1994 acquisition design (baseline for this 4D study). The goal was, first, to process this test, and then, if a reliable 4D signal (above noise level) could be established, to propose interpretation in accordance with the field-production mechanisms.
Field-Production and 4D Monitoring Objectives
The principal oil-producing reservoir belongs to the Upper Jurassic and is a mixture of limestones, dolomite, and anhydrite. It contains the primary 4D monitoring target for this pilot. First oil was produced in the 1960s, followed by peripheral water injection in the 1970s and crestal gas injection in the 1990s. Thus, the 1994 4D baseline survey is an OBC seismic acquired after a rich and complex production history.
Four-dimensional monitoring is justified by the fact that many expensive wells are drilled every year and their positioning is not straightforward. In particular, the crucial fluid-movement prediction (gas and water) is challenging and, thus, is considered the main monitoring objective.
Phase 1 Acquisition Status Repeating baseline acquisition parameters during the monitoring survey is crucial. Any deviation from this principle is likely to generate a 4D noise that will be difficult to remove.
In this pilot, taking advantage of a seismic crew mobilization for a modern wide-azimuth OBC acquisition, the field operator decided to try to repeat, in a small patch, an older OBC survey as a 4D pilot. In fact, this legacy vintage, which has a very different narrow-azimuth design, only was repeated over three swaths. The geometrical repeatability was good, except in the obstruction areas.