Object Models with Vector Steering
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Object Models with Vector Steering1 Ragnar Hauge,2 Anne Randi Syversveen,2 and Alister MacDonald3 Object models are widely used to model the distribution of facies in a reservoir. Several computer programs exist for modelling fluvial channels or more general facies objects. This paper focuses on a marked point model with objects that are able to orient locally according to a vector field. In this way, objects with locally varying curvature are created. With this kind of objects it is possible to model complex depositional basins, that are not easily modelled with conventional methods. The new object type is called Backbone objects. The objects have a piecewise linear centerline and are able to follow the direction of a three-dimensional vector field locally in lateral and vertical direction. How well the objects follow the vector field is determined by three parameters. Use of different coordinate systems and mapping between the systems make it possible to generate Gaussian random fields that follow the shape and direction of the objects. The Gaussian fields can be used to model petrophysical variables, which is important for fluid flow modelling. KEY WORDS: facies modelling, vector field, Backbone objects, local orientation, turbidites.
INTRODUCTION Forecasting production of oil and gas is the primary objective of the evaluation of petroleum reservoirs. This involves modelling of fluid flow, which again requires modelling of petrophysical properties such as porosity and permeability. Petrophysical properties often differ from one facies to another, and should be modelled separately for each facies. To do this, a model for the distribution of facies in the reservoir is required. Therefore, a good facies model is of great importance for the petrophysical modelling and indirectly for the fluid flow modelling. Object models are widely used for facies modelling when the reservoir consists of objects of different facies within a background of some other facies. Modelling of fluvial channels is described by Deutsch and Wang (1996), Holden and others (1998), and Viseur, Shtuka, and Mallet (1998). More general models are 1Received
30 July 2003; accepted 11 April 2005; Published online: 20 May 2006. Computing Center, P.O. Box 114, N-0314 Oslo, Norway; e-mail: Anne.Randi. [email protected]. 3Roxar ASA, Kuala Lumpur, Malaysia. 2Norwegian
17 C 2006 International Association for Mathematical Geology 0882-8121/06/0100-0017/1
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Hauge, Syversveen, and MacDonald
introduced by Syversveen and Omre (1997) and Lia, Tjelmeland, and Kjellesvik (1996). The model introduced by Lia, Tjelmeland, and Kjellesvik (1996) has great flexibility in modelling trends in intensity, size, and shape of the objects. The model is also able to condition on complex well observations, but lacks flexibility in local orientation of the objects. It is possible to have trends for rotation and dip of objects, but the trend is a global control of the object, and cannot change direction locally according to the trend. An application of the general model on
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