Subseismic Faults in Reservoir Description and Simulation, SPE 26500-MS

A program system based on a stochastic model of the fault pattern and properties has been implemented. The faults are modelled as fault planes and displacement of adjacent rock. Realizations of the fault characteristics in a reservoir can be generated.

The generated set of faults is applied to a reservoir description (permeability field). The fault planes may act like barriers to fluid flow. The displacement of rock can disconnect a high permeable zone and thereby reduce the overall flow properties. On the other hand, it can also connect high-permeable rock and improve flow.

Although the reservoir geometry and properties are changed by the faults, well observations may still be honoured by taking faults into account when creating the description.

Introduction

Faults have a large impact on fluid flow and are, hence important to include in a reservoir description. The largest faults, with displacement larger than approximately 15-30m, are usually recognizable on seismics. There are however many more of the sub-seismic faults which cannot be observed by even modern data acquisition methods.

This paper presents a stochastic model for sub-seismic faults. The model is based on phenomenological information as reported in references 1-5, among others. It incorporates what is thought to be the two most important, effects on fluid flow:

- Fault plane effect: Due to cataclasis and other mechanical and non-mechanical transformations, the rock properties change within a small region around the break surface. The fault surface may become a barrier to fluid flow. It may also become more conductive than the surrounding rock.

- Geometric effect: This is caused by the displacement of the rock units which may change the conductivity of reservoir intervals across the fault. For example, a high-permeable zone may be split into two non-connected parts. The displacement function however also connect two such zones and thereby improve the flow properties.

A simulation algorithm for generating fault realizations based on the, stochastic model has been defined. This algorithm is implemented in a computer program. The program also changes a given permeability field according to the faults simulated. Thereby the fault model can be combined with other models of the reservoir, e.g. that describing the facies architecture.

It is important that both seismic and well observations are honoured by the program. The large faults recognized on the seismic are treated separately and deterministically by the program.

Fault parametrization

Each sub-seismic fault is modelled as an elliptic fault plane located in the three dimensional domain and an associated fault operator moving points in space. This is based on work by Walsh and Watterson, and is illustrated in Fig. 1 where a fault is imposed onto a layered sequence.