The use of horizontal wells for oil reservoirs with 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 operations 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 the 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 rate 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 reser- voirs with underlying aquifers. With heavy oils, production at economic rates involves not only displacement by a rising water crest, but also, because of viscous instability, the fingering of water upward through the oil above the crest.
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