Abstract

Comparisons of simulations and laboratory experiments in a crossbedded sandpack model, 30 cm × 10 cm × 3 cm in size, with moderate permeability contrasts, is reported. A nuclear tracer imaging technique, applied to monitor the two dimensional local fluid saturation development, showed that a low permeable bed dominated fluid flow behaviour. A full field commercial simulator was found applicable to model the flow.

Introduction

Improved understanding of fluid flow in heterogeneous reservoirs and a more detailed reservoir characterisation of heterogeneous reservoirs become increasingly more important. This is due to the growing ability to make use of more detailed reservoir descriptions and a related increased interest in the oil industry to apply IOR techniques in heterogeneous reservoirs. Evaluation and application of different IOR techniques require an improved understanding of the impacts from various kinds of heterogeneities on local fluid saturation development.

This paper focuses on cross bedded reservoirs. In heterogeneous reservoirs with layered structures enhanced oil recovery can generally be obtained by placing foam or polymers in high permeable regions of the reservoir to avoid bypass of low permeable regions and thus increase the sweep efficiency. To accurately place the blocking agents in the high permeable regions a good description and understanding of the fluid flow behaviour in heterogeneous systems are required. Also other enhanced oil recovery techniques frequently discussed in recent literature, e.g. WAG stimulation, can seriously be distorted from both capillary and permeability heterogeneities in the reservoir, if not properly accounted for.

Bedded systems are geological ubiquitous in all fluvial and colean deposits. The beds exist on all scales ranging from millimetre to tens of metres. Coarse materials are deposited when the wind or water have high transport energy, which corresponds to high load capability. As the current changes to lower velocity the load capability will decrease and finer materials deposit. Alternating high and low loads result in alternating sequences of coarse and fine material deposited, thus creating a bedded structure with permeability and capillary heterogeneities.

Most earlier studies reported in the literature have focused on fluid flow either parallel or transverse to bedding planes as the extreme cases of bedded heterogeneities. The aims of these studies have been to determine impacts from capillary, viscous and gravity forces in heterogeneous systems with regard to flow dynamics, oil production rates and remaining oil saturation.

Water injected parallel to the bedding planes results in different frontal velocities in the high permeable and low permeable layers, assuming dominance of viscous forces. The different waterfront velocities results in a separation of the waterfronts, with an advanced waterfront in the high permeable bed and a lagged waterfront in the low permeable bed. The difference in fluid saturation between the layers leads to two kinds of crossflow:

  • capillary crossflow due to capillary pressure continuity across the surface dividing the high and low permeable beds;

  • viscous crossflow due to different pressure drops in the two adjacent beds, thus creating a pressure difference between the layers, which induces viscous crossflow, described by Darcy's law.

Pressure continuity across the surface dividing the layers induces continuous capillary pressure and a resulting discontinuous saturation distribution is determined by the capillary pressure functions in the two layers.

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