Effect of Open Fractures on Oil Recovery When Superimposed Upon Synthetic Models of Heterogeneous Fluvial Sandstone Reservoirs
Authors: England William , Townsend Chris 
Published: December 2007

Two distinct types of open fractures (an outcrop joint pattern and stochastically modelled fractures) have been added to realisations of a heterogeneous fluvial reservoir model in order to determine their effect on simulations of a waterflood and the change in effect of the sedimentary description parameters. Each of the model description parameters was given a high and low value. The value choice for the parameters in each of 24 model realisations was selected using experimental design. The reservoir simulation model comprised of a 288 × 288 m square with a quarter five spot well configuration (injector and producer at diagonally opposite corners). ECLIPSE simulations of a waterflood were run on each realisation and the results were analysed in order to determine the effect of the various description parameters.

Even though the reservoir facies in the model were highly permeable the addition of the open fractures had a statistically significant effect. The generally larger stochastic fractures had the largest effect, reducing recovery at water breakthough, whilst the joint pattern slightly increased production at water breakthrough and at 95% watercut. Of the stochastic fracture description parameters examined only fracture strike had an influence on recovery at water breakthrough and only the addition of the stochastic fractures had a slight influence on the sedimentary parameters. An additional study on the joint pattern showed that the permeability anisotropy direction caused by an in-situ stress field was highly influential on both recovery measures.

Any conclusions from this study should be treated with caution as they are probably case specific and the sizes of fracture apertures in the sub-surface are highly uncertain. However, it has been shown that small-scale heterogeneities can, in certain circumstances, have an influence on reservoir simulations. Only careful reservoir description will result in realistic reservoir simulation.

Introduction

The relative impacts of a number of sedimentary heterogeneities on oil recovery from a low net to gross North Sea type fluvial reservoir were investigated as part of a BP-Statoil Alliance joint R&D project (Ref. 1). Ref. 1 built-up a sedimentary model at a number of specific scales ranging from the modelling of fine scale laminations up to that of the distribution of channel belts within a flood plain. In the current study, open fractures have been added to the realisations of the sedimentary model in order to examine their additional effect on oil recovery. This was measured by running ECLIPSE simulations on the fractured realisations and statistically analysing the simulation results. From these analyses it was possible to determine the significance of a number of sedimentary and tectonic description parameters. This study aims to answer three main questions pertaining to the effect of open fractures in highly permeable sandstone facies:Does their presence have a major effect on recovery?Do they alter the influence of the sedimentary description parameters?Do their description parameters have a significant effect on recovery?

Unfractured Model

Sedimentary Model. Ref. 1 looked at the effects of sedimentary description parameters on the waterflooding of an heterogeneous fluvial reservoir. In the current study open fractures have been added to these realisations in order to determine their effect on recovery and to see if the influence of the various sedimentary description parameters is altered. The sedimentary model was generated at 4 different scales (Fig. 1). Data to build this realistic fluvial sedimentary model were taken from i) the lower part of the Raude Member of the Statfjord Formation (Late Triassic to Early Jurassic) from the Statfjord Field (North Sea) and ii) analogue outcrops. The four-level hierarchy of the geological models comprised (Fig. 1):

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