A 2D on 3D pilot for 4D feasibility: A success on a carbonate field
Authors: Hubans Christian , Zamboni Enrico , Brechet Emmanuelle , Leher Hubert , Giovannini Alain 
Published: March 2015

Abstract
It is nothing unusual now to see decisions taken to record time-lapse seismic surveys in unconsolidated sands or turbidite environments. The Gulfs of Mexico and Guinea are typical areas for which investment in seismic monitoring raises no questions. However, in other areas, such as carbonate fields, the question is less clear, because the physical behavior of carbonate rocks is difficult to describe accurately. Very few examples of 4D monitoring in carbonate environments can be found in the literature. Initially, this 4D acquisition set out to monitor the GWC tilt but on completion, it clearly showed that a 2D survey on a 3D reference delivers a very high-quality 4D pilot for a very reasonable cost. This article presents the seismic processing, the Q.C. and the 4D attribute computation between these 2D lines and the old 3D base. The final quality of dVp/Vp sections is surprisingly good with an S/N ratio of around 10, affording the opportunity to understand new phenomena related to the field production. Based on these results, the 4D full-field survey is far more likely to be successful.

Introduction
Over the last few decades, time-lapse seismic data have proved to be a useful means of monitoring production and fluid injection in reservoirs (e.g. Sandoe et al., 2009; Jack, 2008; Calvert, 2005). 4D is now commonly practiced in unconsolidated reservoirs where the production mechanisms are expected to play an important role. This information is proving highly valuable for current work.
The use of 4D is not such an obvious choice in other cases, either because the 4D responses are not expected to be strong, as for instance in carbonate reservoirs, or because the seismic design is very expensive (LoFS project). In these types of situations, managers often ask for proof of the added value and/or efficiency of the 4D approach in obtaining useful information as regards production of the field. The most common approach for making a decision is then to perform a feasibility study, a cost study and a VOI (Value Of Information) study. Using the combined results of these three studies, it is possible to determine what production phenomena can be detected with 4D seismic and what amplitude they have, the budget necessary to do so, and lastly the anticipated impact on field management and return on investment.
The alternative is to:
Shoot a reduced 4D seismic design and use it to check the 4D seismic response, the sensitivity of the experiment and the interest of this information for field management. It is a very practical way of demonstrating feasibility.
The approach has been used for onshore activity where repeatability is a killing factor for feasibility, mainly in desert areas where dune movement or ground coupling can handicap matters. It is less commonly used offshore, but we will show that a light seismic acquisition (a few 2D lines) can be extremely useful to prove the feasibility of a 4D seismic survey, to detect a wide variety of production impacts on the rock, and to establish confidence in the deliverables of a future full-field 3D monitoring acquisition.

Field description.
The case is a gas field in a carbonate environment. The main reservoir is a reef. Porosity is around 28%. A WGC (Water Gas Contact) is visible on the seismic.