HR: 15:30h
AN: NS34A-01 INVITED     [Abstracts]
TI: 3D and 4D Seismic Imaging in the Oilfield; the state of the art
AU: * Strudley, A
EM: alan.strudley@westerngeco.com
AF: WesternGeco, Schlumberger House, Gatwick, RH6 0NZ United Kingdom
AB: Seismic imaging in the oilfield context has seen enormous changes over the last 20 years driven by a combination of improved subsurface illumination (2D to 3D), increased computational power and improved physical understanding. Today Kirchhoff Pre-stack migration (in time or depth) is the norm with anisotropic parameterisation and finite difference methods being increasingly employed. In the production context Time-Lapse (4D) Seismic is of growing importance as a tool for monitoring reservoir changes to facilitate increased productivity and recovery. In this paper we present an overview of state of the art technology in 3D and 4D seismic and look at future trends. Pre-stack Kirchhoff migration in time or depth is the imaging tool of choice for the majority of contemporary 3D datasets. Recent developments in 3D pre-stack imaging have been focussed around finite difference solutions to the acoustic wave equation, the so-called Wave Equation Migration methods (WEM). Application of finite difference solutions to imaging is certainly not new, however 3D pre-stack migration using these schemes is a relatively recent development driven by the need for imaging complex geologic structures such as sub salt, and facilitated by increased computational resources. Finally there are a class of imaging methods referred to as beam migration. These methods may be based on either the wave equation or rays, but all operate on a localised (in space and direction) part of the wavefield. These methods offer a bridge between the computational efficiency of Kirchhoff schemes and the improved image quality of WEM methods. Just as 3D seismic has had a radical impact on the quality of the static model of the reservoir, 4D seismic is having a dramatic impact on the dynamic model. Repeat shooting of seismic surveys after a period of production (typically one to several years) reveals changes in pressure and saturation through changes in the seismic response. The growth in interest in 4D seismic has been driven by the need for improved recovery on declining fields, and for newer fields the requirement for optimised production. The vast majority of 4D activity is currently offshore based on the economic imperative in those environments. Much effort is being employed in the acquisition and processing domains to improve the resolution of the method, and to move from a qualitative to quantitative assessment of reservoir changes. Key developments in this field have been the improvements in data quality attained by new acquisition systems, growing maturity in processing of repeat datasets and the integration of seismic information into the reservoir management workflow. For the future there is much interest in the emplacement of semi-permanent seismic arrays to monitor reservoir changes on a very frequent basis (months rather than years). These arrays would also enable passive monitoring of reservoir changes between repeat seismic surveys. Despite the maturity and success of the 3D seismic method in the oilfield there are remaining challenges. Improvements to imaging schemes are required to address the need for ever more accurate subsurface images and the desire for improved reservoir performance is driving 4D technology forward. For the future we can expect even greater data densities than are currently acquired and developments in imaging based around increased computational resources.
DE: 0900 EXPLORATION GEOPHYSICS
DE: 0935 Seismic methods (3025)
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly