This paper was prepared for the 48th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Las Vegas, Nev., Sept. 30-Oct. 3, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract

This paper reports on a study of the natural depletion performance of individual wells producing from massive-sand oil reservoirs. The study utilized a two-dimensional, two-phase semi-implicit numerical coning model to simulate the depletion behavior of example wells. The effects of production rate, selective completion interval, sand thickness and horizontal and vertical permeabilities were investigated. The study showed that for wells completed over the total productive thickness in sands having continuous vertical permeability, the natural depletion recovery permeability, the natural depletion recovery efficiency is reduced by producing at low rates. This supports the findings of Morse and Whiting who, based on results from a linear vertical simulation model, showed this to be caused by the vertical segregation of liberated solution gas to form a zone of high gas saturation at the sand top which provides a short circuit for gas to flow directly to the wellbore. They found that production at reduced rates permits more effective vertical segregation and adversely affects recovery efficiency. In contrast to their work, however, this study showed that if the completion interval is selectively placed in the basal one-half or less of the sand thickness, the production-rate effect reverses and recovery efficiency can be dramatically increased by producing at rates which minimize the coning of the vertically segregated gas. Moreover, it was shown that even for sands having a vertical permeability as low as 1 md., the recovery efficiency is essentially unaffected by producing at high initial rates, followed by successive reductions in rate upon the onset of increasing gas-oil ratios. That is, readjustments in the vertical gas saturation distribution following each rate reduction permit continued production at low gas-oil ratios, and the recovery efficiency will be essentially the same as that for continuous production at the low ultimate rate. This interdependence of recovery efficiency on production rate and completion interval was production rate and completion interval was investigated over wide ranges of sand thickness and horizontal and vertical permeabilities. The interdependence and permeabilities. The interdependence and magnitude of potential natural depletion recovery efficiency are shown to increase substantially for larger sand thicknesses and higher values of horizontal and vertical permeabilities. Although continuous vertical permeabilities. Although continuous vertical permeability is critical to the rate dependency permeability is critical to the rate dependency of the recovery process, its maximum influence is obtained for values as low as 10 md. The paper illustrates the formation of the vertical gas saturation distribution during the natural depletion process, and the effect of production rate on the shape of the gas-oil contact profile. profile.

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