Abstract

The displacement of oil by bottom water to a horizontal well in a verticallyconfined reservoir was studied experimentally in a Hele-Shaw cell. Thestability of the water-oil interface was investigated at different flow ratesand viscosity ratios_ A stable interface with higher recoveries was obtainedfor lower flow rates and for the cases where water and oil had the sameviscosities. The interface became unstable for higher flow rates and/orviscosity ratios. Fingering flow, which results ill bypassing of oil, reducesultimate oil recovery. For a given viscosity ratio, the higher flow ratesusually result in a lower oil recovery at breakthrough. However, in some caseswhere multiple fingers fanned, the oil recovery for higher flow rates was foundto be higher than that for lower rates. Gil recovery at breakthrough wascorrelated with a dimensionless flow rate.

A novel numerical method was developed in this study to simulate [he pistontype of displacement found in Hele-Shaw cells. The advancement of the water-oilinterface and the corresponding oil recovery, as well as the pressurebehaviour, were predicted. A single finger was identified for different flowrates when the viscosity ratio was adverse. The simulation results were ingeneral agreement with corresponding experimental data..

Introduction

Reservoirs underlain by bottom water zones are very common and constitute asignificant resource. The effective exploitation of these resources wouldprovide an offset to the oil production decline from lop quality reservoirs.Production from such reservoirs usually involves the completion of the wellnear the top of the reservoir so as to reduce the production of freewater.

If the interfacial effect is ignored, then there are two kinds of force exertedon the interface during production:

  • the viscous force that results from theflowing of fluids in the reservoir and

  • the net gravitational force thatresults from the difference in density of two phases.

For upwards displacementof oil by water, the gravitational force tends to stabl1iu: the water-oilinterface if water has the higher density than oil When the difference betweenvertical pressure gradients across the interface becomes greater than thedifference of gravity pressure gradients, the water-oil interface becomesunstable and viscous fingering occurs. Otherwise, the water-oil interface isstable. Figure 1 shows schematically the shape of a stable water cone for avertical well am.! a corresponding crest for a horizontal well. For verticallyconfined reservoirs, the cumulative oil production for isolated wells at thebreakthrough depends on the volume of water cone or crest underneath theproduction well (Butler, 1992). Cumulative oil production by bottom water drivecan be improved by horizontal wells in place of vertical wells because of theextended length of water crest (see Figure 1).

When the viscosity of water is less than that of oil, which is the usual casein actual reservoirs, the upwards displacement of oil by bottom water may causethe interface lo become unstable at high production rate because of the adversemobility ratio between water and oil phases.

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