Abstract

The application of three-dimensional (3-D) seismic methods to reservoir characterisation has gained wide acceptance in the last ten years. Almost seventy-five percent of additional reserves are now found in mature areas by redefining reservoir boundaries and internal heterogeneities using surface and crosswell seismic methods. However the implementation of, in particular, 3-D seismic methods can be very expensive and can rival the cost of drilling a dry hole.

A solution to the problem is to physically model a reservoir in the laboratory and to collect simulated seismic reflection data over the model. Then conventional, or unconventional, seismic processing techniques can be applied to the data prior to embarking on costly field seismic data collection. The physical modelling experiments enable an improved and corrected image of the subsurface to be developed, and a more effective drilling program to be designed.

We briefly review the 3-D seismic method and indicate its value in reservoir characterisation. Examples will be given of models developed for experiments in fault plane analysis and for imaging beneath high velocity near surface layers.

Introduction

The 3-D seismic method was first shown to be viable for subsurface imaging in the early 1970s (Ref. 1). This was the first successful application of physical modelling to solving 3-D seismic problems. The fact that the 3-D method has been rapidly accepted in the last 10 years has been documented (Ref. 2). The conclusion is that '3-D seismic surveys have become a cost-effective tool for mapping hydrocarbon reservoirs and can have a major impact on the volume of reserves estimated'.

The method needs the subsurface area of interest to be spatially sampled correctly with seismic traces. in marine operations this may be achieved with a ship towing many cables, such that each source and receiver combination represent a sample point. In land operations various positionings of sources and receivers may be used such that an area of the subsurface is 'covered' with seismic traces.

The examples in this paper show two very different applications of 3-D seismic to solving reservoir problems, both of which required the extensive application of physical models.

Fault Plane Analysis. When a seismic wave meets an acoustic contrast it may be reflected and/or refracted. If a wave reaches a fault with a relatively high impedance contrast across it, it will refract through the fault plane, resulting in a change in the travel path of the wave. When common midpoint gathers are produced, the wave refracting across the fault can cause changes in the trace gathers which can affect 'reflections' within the gather. When the data are stacked the result can give a different appearance to the image. Such a change could be the presence of an apparent fault at depth, which is in fact an artefact of the stacking process. When computing the reserves in a reservoir the assumption of the presence of such a fault could lead to incorrect reserve estimates. Consequently, it is very important to determine whether such an artefact does, or does not, exist.

Two dimensional (2-D) seismic data collected across a fault plane containing such an impedance contrast in the shallow section will produce a false image of a fault (Ref. 3). 3-D data collected along strike to the fault will not have raypaths passing through the fault plane. so these data will not create such an artefact. A 2-D section can be reconstituted from the 3-D data, along the plane of the original 2-D line, without the artefact indicating that the 'fault' does not exist.

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