Mobility Control In Dynamic Gravity Segregation Flow Systems
Authors: Nzekwu B.L. , Bennion D.W. 
Published: March 2010

Abstract
Experiments were carried out in a vertical sand pack to study the effect of gravity segregation on liquid displacement by a miscible liquid, a gas and a combination of the two. The advantages of using a foaming agent during simultaneous gas-liquid injection were also investigated.
A gamma-ray attenuation technique was adapted to track the shapes of the moving displacement front and monitor in-situ saturation profiles.
The results show that high gas mobility and low density lead to early gas breakthrough and poor sweep efficiency. Even in miscible liquid-liquid displacements a density difference of 0.079 g/cm3 leads to gravity override or under-ride by the displacing liquid. During simultaneous gas-liquid injection the presence of gas increases the severity of the over-ride/under-ride effect and reduces the displacement efficiency of the miscible liquid. In-situ generation of foam is an effective method of controlling the mobility of the gas and improving the conformance of the displacement front.

Introduction
The purpose of this study was to investigate the characteristics of gravity segregating systems and the use of foaming agents to control the mobility of air and water injected simultaneously into a liquid-saturated porous medium.
One of the major factors responsible for poor conformance and low recovery efficiency of several displacement processes(1) is gravity segregation caused by a density difference between displacing and displaced fluids. For example gas-driven processes such as CO2-flooding, alternating water-gas injection, steam injection and in-situ combustion are affected by varying degrees of segregated flow behavior.
Several laboratory experiments and numerical studies of fluid flow in porous media have confirmed the importance of a density difference in the occurrence of segregation. Slower rates of segregation were observed in brine-liquid systems than in brine-air systems(2). With miscible liquids the length of the mixed zone was influenced by gravity segregation, and a greater reduction in displacement efficiency was observed when the mixed zone was narrow compared to the height of the experimental model(3). The presence of a broad transition zone tended to nullify the influence of a difference in density(4,5). Gravity segregation is generally thought to be time-dependent and more prevalent in thick heavy oil reservoirs(6). However, some field projects have also shown that segregation can occur instantaneously at the wellbore(7,8), and in static gas-liquid systems only a short time is required for an initially uniformly distributed gas phase to reach a fully segregated state(9). For unit mobility ratios, two miscible fluids will segregate in dynamic flow by the same amount they segregate in a static system(10). Studies in models simulating both linear and five-spot patterns show that volumetric sweep efficiency at breakthrough can be as low as 20% of those with no gravity segregation(11). This reduction in performance is more pronounced with an increase in vertical or horizontal permeability, an increase in density difference, a decrease in production or injection rates, and an increase in mobility ratios(6).
Gravity override has been observed in nearly all steam-drive Projects(12). For reasons of convenience, theoretical studies often assume vertical combustion fronts, even though gravity effects are usually evident(12).