Gravity Effects in Miscible Displacement
Authors: Miller M.C. 
Published: December 2007

This paper was prepared for the 41st Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Dallas, Tex., Oct. 2–5, 1966. 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
Segregation of hydrocarbons and water can severely reduce the ultimate vertical sweepout efficiency of miscible displacement processes when a mobile water phase is present in all or part of the reservoir. Computer studies have shown that the decreased miscible sweep resulting from segregated flow can result in uneconomic operations in some fields in which economic operations were predicted for non-segregated flow.
Gravity override is enhanced as a result of segregation, and once the reservoir fluids segregate, the high relative permeability of the lighter fluids at the top of the reservoir increases the gravity override with the result that oil in the lower portion of the reservoir is not displaced miscibly.
The shape and movement of the ail bank formed during secondary and tertiary projects can be paramount to a successful field operation. Results from our computer model study show that an oil bank formed ahead of an overriding miscible fluid is forced down below the solvent tongue and moved relatively slowly toward the producing well. The oil bank is slowly recovered immiscibly at an extremely low rate by the water injected along with the gas. Thus at the economic limit of the field operation, a substantial amount of the oil displaced by the miscible fluid early in the reservoir life is left unswept at abandonment.
Most of the above information has been obtained from a two-phase flow, two-dimensional mathematical model, which includes the effect of the segregation of all hydrocarbon fluids in the presence of a denser, mobile, immiscible water phase present in watered-out systems on the sweepout behavior of the oil-bank front and miscible front. This behavior was previously unattainable from our physical [laboratory] flow models in which miscible fluids were used.

Introduction
The miscible slug process and the high pressure gas process are currently employed in our endeavor to maximize the oil recovery from reservoirs. Since residual oil saturations of 20 to 50 per cent after gas or water floods are common, the additional oil recovery can be considerable. In the high pressure gas process, multiple contact between the injected gas and the reservoir oil strips the light ends from the oil until a transition fluid is formed which is miscible with the reservoir oil. Although pressures in excess of 10,000 psi are usually required for methane to be completely miscible with crude oil, a miscible flood is achieved by the above mechanism in the 3,000- to 5,000-psi range.