EAGE Workshop on 4D Seismic and Reservoir Monitoring: Bridge from Known to Unknown

Seismic monitoring or 4D has started onshore on a few cases more than 30 years ago but it has really developed over the last two decades, first in offshore fields where seismic image is generally of good quality, then onshore with more difficulties to achieve similar repeatability. Beyond the operational issues and the detectability limits, the impact of repeated seismic varies according to the complexity of the physical changes induced by the production in the reservoir and the overburden.

In this presentation, several case studies of offshore and onshore 4D seismic from CGG's experience are presented. The techniques used for monitoring, permanent or not, are adapted to the degree of complexity of the objectives. They provide valuable support to reservoir engineers for tracking production related changes in the reservoir in order to economically optimize oil recovery.

The elastic changes observed through the 4D seismic response need to be translated into physical changes, important for the production. The use of a Petro-Elastic Model (PEM) for 4D interpretation is critical to be able to understand, quantify or interpret the observed changes. The role of the PEM is to translate the changes in physical properties into the elastic domain during a forward modeling step and to constrain the possible range of variations in an inversion phase. Available production information can be used to impose realistic 4D rock physics constraints which reduce the inherent non-uniqueness of the inversion solution and produce results which are more accurate and consistent with the expected production effects. Calibration of the elastic 4D elastic responses to the various physical property changes are key to the process but often difficult due to lack of repeated well information.

The properties which change vary significantly from one case to another depending on the reservoir characteristics and the production schemes. In the simplest cases, it can be Pressure and Oil Saturation changes only, but the elastic changes can also be related to changes in temperature, presence of Gas out of solution or other injected fluids, compaction, salinity, … These changes go beyond “dynamic” properties in a flow model. Properties traditionally considered as “static” like thickness, porosity and permeability can also be affected significantly by production induced changes.

On complex cases, the unknown variables may outnumber the measured changes limited to seismic amplitude and TWT changes. In such cases, it is important to integrate other information than seismic like gravimetric, electro-magnetic or geo-mechanical measurements. For this integration, the PEM can be extended to model the electric response or to be stress sensitive in order to translate the resistivity or strain into the elastic domain. However, the mathematical equations linking the properties are not sufficient to solve the problem of integration of the various disciplines. There is a need to combine the information from the different numerical models used in the various fields. The elusive “Shared Earth Model” is not yet into existence and the Flow Model used for evaluating the economic objectives is not suitable for all disciplines. In particular, the Flow Model lacks overburden and parts of the sub-surface considered as non-reservoir. Therefore, it imposes fixed geometry and uses spatially varying scales that are adapted to the flow simulation. These limitations cause problems with other modeling disciplines and to accommodate associated uncertainties. We have developed tools to integrate the various measurements at various scales, vertical and horizontal, to accommodate and reconcile information from different disciplines in the elastic domain for the purpose of prediction of the seismic response but also to update flow model properties. We also work on the assimilation of the dynamic data (production and 4D seismic data) in the history matching process.

The various studies presented here were performed in fields having different maturity, permanent systems installed before production for steam assisted gravity drainage or deployed over offshore fields after decades of complex production. The cases presented cover also classical repeated acquisitions, both on land and offshore but also evolution from repeated surveys to a permanent system enabling seismic on demand and can be compared in terms of repeatability.

The complexity of the monitoring depends on the production objectives, whether to track the movements of injected steam or water, or to determine areas of by-passed production, or to identify the movement of the OWC (oil-water contact) across vintages in a thin oil column in order to plan an optimum continuation of the horizontal well trajectory. The monitoring difficulties depend on the complexity of the geology and the resolution of the seismic compared to the heterogeneities of the reservoir but also on the complexity of the production scheme with natural depletion, water or gas injection or both alternatively (WAG) or steam injection. These production schemes will cause different physical phenomena to occur affecting the seismic measurement. The response is the combination of pressure and saturation changes with non-negligible changes in temperature or even salinity, but also geo-mechanical responses with compaction or expansion leading to thickness and porosity changes.

The understanding of the different effects and the ability to predict their seismic effect through petro-elastic models are key to be able to separate them from the seismic measurements. In some complex cases, different combinations of production-induced changes can have a similar seismic response. Solutions are adapted to the complexity of the predicted seismic response and are illustrated through the different cases presented, from very simple cases where single seismic attribute changes like amplitude variations or time shifts are directly interpreted into saturation changes showing where injected water is going to very complex cases where the seismic 4D response alone cannot remove the ambiguity in the changes that occur within the reservoir. 4D feasibility, 4D seismic inversion and 4D petrophysical analysis linking the flow model with the seismic measurements are key to understand the problems and to help the monitoring and management of the field production.