Abstract

Production of oil from reservoirs overlain by a gas cap causes the gas-oil interface to advance towards the producer and curve downwards. With conventional wells, the rate of production that is required for the operation to be economic is usually too high to avoid direct bypassing of gas to the producer. However, with horizontal wells, the critical production rate is higher and may allow economic production without gas bypass. A stable interface can be maintained closer to the well giving much higher recoveries for comparable rates.

Gravity stabilized, vertical gas displacement can also be utilized in reservoirs underlain by an active aquifer. In such operations, if the producer is kept at a potential equivalent to that of the aquifer, then free water production can also be avoided. The pressure is maintained by the injection of the gas so that the oil is pushed out rather than sucked out.

Equations relating the critical flow rate to maintain stability of the interface and the recovery are developed in the paper and compared to experimental data from vertical Hele-Shaw models. The study examines the effect of production rate, well spacing, initial height of the reservoir and nature of fluid on the recovery for downward displacement of oil by gas to horizontal wells located at the base of the reservoir. Experiments have been carried out at conditions both above and below the critical rate. It is found that at very high rates the interface becomes unstable almost instantaneously and oil is displaced by a dominant finger of gas. At lower rates the interface is stable for most of the displacement and larger recoveries are obtained. More closely spaced well and higher initial height of the reservoir give much better rates and recoveries. The agreement between the predictions and experimental data is good.

Introduction

There are many oil reservoirs in the world which have either an active water layer below the oil zone or a gas cap above it or both. Production of oil from such reservoirs causes the oil-water interface, or the gas-oil interface, to move towards the production well. The interface becomes curved and, with conventional wells, assumes a shape resembling that of a cone; this phenomenon is known as coning, either water coning or gas coning (Figure 1) or hath. Coning, because it allows bypass of free gas or water, creates serious inefficiencies: loss of production, lower oil cull poor sweep of the reservoir and lesser recovery of the oil in places. Mathematically, gas coning and water coning are analogous. Although this paper addresses gas coning, the ideas presented here are applicable to either case.

There exists a maximum rate, called the critical rate, for production of oil without bypass of free gas. This critical rate is usually too low to be economic. Horizontal wells offer an attractive alternative in some cases where conventional wells have poor performance due to coning problems. A horizontal well usually has a more extended contact with the reservoir than a vertical well.

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