Abstract

The Use of horizontal wells for oil reservoirs which underlying water can be attractive because water production may be reduced.

In this paper, the displacement of oil by a rising water crest is considered. This is the main mechanism by which oil is produced where operation above Muskat's critical coning rate is required. The arrangement which has been studied involves a series of very long, equally-spaced, parallel, horizontal wells located just below a horizontal reservoir boundary. It is first assumed that the oil and water have the same properties and methods are given for calculating the progress of the interface.

Decreasing the well spacing improves che fractional recovery but reduces the production quantity per well. The method extends to infinite well spacing, i.e. to isolated wells.

The effect of oil and water densities and viscosities on the stability of the rising crestal interface are also considered. There is a critical production race beyond which fingering will occur. This critical rate for fingering is different from that for cresting or coning and it presents a serious limitation to the effectiveness of horizontal wells for the production of heavy oil from reservoirs with underlying aquifers.

With heavy oils, production at economic races involves not only displacement by a rising water crest, but also, because of viscous instability, the fingering of water upwards through the oil above the crest.

Introduction

This paper is concerned with the production of crude oil from reservoirs which are underlain by an active aquifer. Production at high rates from such reservoirs results in coning and premature high Water cuts. Conventional coning theory is largely concerned with the concept of the horizontal pressure gradients within the oil layer being reflected by a change in the vertical position of the oil-water interface. If the flow rate is low then the pressure gradient is insufficient to lift the water to the production well and water-free recovery is possible. There is a critical rate of production above which water is produced.

While this concept is applicable in the case of oils having fairly low viscosities within reservoirs of reasonably high permeability, it is frequently uneconomic to produce at rates below the critical ones. This is particularly true for heavy oils lying above water where the rate must be considerably above the critical one for the operation to be economic and where, as a result, water inevitably rises to the production well. This is usually completed at the top of the reservoir.

In these cases the mechanism becomes one of vertical displacement of the oil by the rising water cone and the quantity of dry oil which can be produced is related to the volume of the cone. More exactly, the dry production is equal to the volume of oil which has been displaced from the interior of the cone after making allowance for the initial oil saturation and the efficiency of displacement by the Buckley-Leverett mechanism.

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