An Improved Seabed Seismic 4D Data Collection Method for Reservoir Monitoring
Authors: Seymour R.H. , Barr F.J. 
Published: December 2007

A 4-D seismic campaign consists of two or more 3-D surveys acquired at time intervals of perhaps a year. Differences between the seismic results can allow fluid fronts and pressure domains to be mapped. Experiences with 4-D marine seismic have been very promising in European oilfields. Sometimes, however, interpretation has been made difficult by seismic noise and by the inevitable variability in repeated towed streamer surveying - the costs of using the alternative fixed seabed arrays generally have been prohibitive. Now, in water depths of up to 150 m, hydrophone and geophone sensors can be placed temporarily on the seabed for each 3-D survey. These yield improved resolution, signal to noise ratios and repeatability, while the high overheads of permanent seabed arrays are avoided. Highly productive ocean bottom cable 3-D seismic surveys have been collected worldwide, and during 1995 and 1996 the technique has been extended to cope with European sea conditions.

Introduction

Reservoir Monitoring. Whether the need is to improve the economics of a marginal field, or to extend the economic life of a declining field, reservoir monitoring is an essential oilfield management tool. Yet conventional means of monitoring, including downhole and production measurements, and their use to validate a simulator model providing projections of field performance, have serious limitations. For example, bypassed and undrained oil can be inferred only indirectly, and innumerable wells have in the past suffered from unexpectedly early water or gas breakthroughs. This is not necessarily anyone's fault as such, it is simply that conventional monitoring leaves a wide range of uncertainty resulting from the sparseness and imperfection of the acquired data, and from the necessity of working with a simplified model of the Earth in the simulator. More detailed characterisation of the reservoirs in three dimensions, and observation of fluid movements and pressure throughout the field would allow for more reliable models and hence more accurate predictions of the consequences of competing potential reservoir management strategies. In particular, well productivity and ultimate reserves could be optimised by more accurate placement of development and infill wells.

Seismic Assistance. Reservoir engineers now recognise that between and away from wells, approximations of reservoir structure and properties, and even of major saturation or pressure changes can sometimes be derived from the latest high resolution 3-D and 4-D seismic data. In technically and economically suitable cases, the exploitation of 3-D seismic attributes by reservoir engineers has become an accepted practice to assist reservoir characterisation. To achieve that level of acceptability, the technique needed to prove its value to asset managers both in theory and in practical application. Seismic assistance to reservoir monitoring ("time-lapse" or "4- D" seismic) has been in development for at least ten years. Now the pressure to reduce costs and to extend field life has spurred a widespread increase of interest in 4-D by operators, and this technology is currently undergoing a proving process similar to the example of 3-D seismic.

A 4-D seismic campaign consists of two or more successive 3-D surveys acquired at time intervals appropriate for a reservoir's fluid front velocities. Differences in the seismic response of the reservoir are caused by the fluid movements and by pressure changes (and in some cases by temperature changes, fluid property or compositional changes, or rock changes - swelling, compaction, fracturing etc. - caused by production or injection). These seismic differences can allow fluid fronts and pressure domains to be mapped between and beyond wells.

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