Application of a Multiphase Coning Model to Optimize Completion and Production of Thin Oil Columns Lying Between Gas Cap and Water Zone
Authors: Cottin Rene H. , Ombret Rene L. 
Published: December 2007

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
A recently developed, high performance, numerical model was used to analyse three phase coning. Results, obtained in a series of simulations, have supplied general directions for optimizing completion and production of thin oil columns comprised between gas cap and water zone.
A correlation is presented for fast evaluation. Conclusions, valid only for reservoirs showing no major heterogeneity, depend mainly on relative permeability effects, gas cap expansion and water drive strength.
Under certain conditions, it appears that wells perforated under water oil contact and produced at high rate, as commonly practised on some fields, notably in East European countries, are practised on some fields, notably in East European countries, are giving the best results.

Introduction
Profitable production of oil is particularly questionable when accumulation is thin and comprised between gas cap and water zone.
Double coning phenomenon, which occurs in such cases, limits considerably oil production rate, so that one must sometimes renounce to start oil layer exploitation even if its volume in place is important.
This problem has, therefore, been studied for a long while both on fields and research laboratories. On fields, it is rather difficult to interpret results, because bottom hole conditions are never simple reservoir is more or less heterogeneous, cement jobs are uncertain, aquifer strength is unknown, etc. Moreover, it is not easy in practice, to vary completion conditions, like for instance to move several times the position of perforated interval on the same well. In laboratories, simulations on analogic models or petrophysical scale models, already difficult for two phase problems, become hazardous in presence of three phases, because all similitude conditions cannot presence of three phases, because all similitude conditions cannot then be respected together. On the other hand, because of the great number of parameters involved, theoretical hand calculations cannot be achieved without assuming drastic simplifications.
This is why, up to recent years, most of these works were too partial, approximate, or inconclusive to be reliable. partial, approximate, or inconclusive to be reliable. It appears now that modern two dimensional, three phase numerical models may help to clarify considerably this complicated subject.

BACKGROUND OF DOUBLE CONING THEORY
The first idea that occurs to prevent double coning, and the easiest for application, is to produce oil under double critical conditions, that is to say without breakthrough of gas and water into the well.
Several authors gave formulae to calculate the optimum position of perforated interval-corresponding to a maximum water free and gas free oil rate. All these formulae are of the same type. We shall compare further on their application with the results of our field experience and our numerical simulations. In all cases, oil rates obtained with this type of completion are very low, often below commercial limits.
To obtain more oil, one must accept to produce associated water or gas in spite of equipment expenses. Thus, for a long time, polyphasic high rate productions have been experimented and polyphasic high rate productions have been experimented and adopted on many fields, but generally without optimizing position of perforated interval.
In 1954, an A.P.I. committee, reviewing thin oil column completion practices, mentioned, with extreme caution, "reverse coning" field tests, involving perforation above gas oil contact or below water oil contact. Later, in 1965, J. Van LOOKEREN (ref. 9), studying this method thoroughly tried to prove, with theoretical arguments and laboratory experiments, that oil production can be improved considerably by perforating wells far down in water zone. Unfortunately, some of its basic assumptions were too simplistic to give absolute conviction.