Parametric Analysis On The Determination Of The Minimum Miscibility Pressure In Slim Tube Displacements
Authors: Flock D.L. , Nouar Akli 
Published: March 2010

The reservoir pressure required for the development of a dynamic vaporizing or condensing gas drive is often determined experimentally by what is referred to as the "slim-tube" displacement test.

Early work has shown that, provided the displacement is stable, the determination of the minimum miscibility pressure is the only function of the thermodynamic conditions (nature of fluids and temperature).

The purpose of this investigation is to verify whether the above condition holds/or conventional tests carried out in flat slim-tube coils. This has been done by examining the effect of tube length and injection rate on the displacement efficiency at various pressures covering the immiscible and multiple-contact miscible displacements.

It was found that gravity segregation has an adverse effect on the results, and in the early stage of the displacement, unstable flow occurs for the injection rates studied. These rates are of the same order of magnitude as those considered by other experimenters. Increasing the length of the model, tends to have an over-all stabilizing effect on the results.

For short slim-tubes, the usual indicators cannot be trusted to establish the MMP*. The use of longer slim-tubes yield the best estimate of the MMP.

Introduction

P. Deffrene et al.(1) have shown that the MMP is only dependent On the thermodynamic conditions (nature of the fluids and temperature), if proper care is taken to suppress any unstable phenomenon, namely, viscous fingering and gravity override. They used a vertical displacement, to take advantage of gravity segregation to suppress fingering, provided the rate used was smaller than the critical rate(2) defined as:

Equation (Available In Full Paper)

The criteria used for MMP estimation is to plot recovery versus operating pressure at gas breakthrough or after 1. to 1.2 pore volumes injection, and the pressure where the curve shows a clear break in slope, will correspond to the MMP. It is evident that the method will be influenced by the macroscopic sweep efficiency of the displacement making it imperative to eliminate the causes for poor sweep, which are: viscous fingering, and gravity override (horizontal systems).

However, as pointed out by Orr et al(3), there is no general consensus as to the experimental procedures or the criteria defining the MMP. They observe that the displacement lengths range from 1.5 m to 25.6 m, flow geometries vary from vertical to flat coils to spirals, and flow velocities vary over nearly two orders of magnitude. The MMP is generally determined on criteria based on recovery at some pore volume injected and/or, on the visual observation of the transition zone in a sight glass. These criteria will be discussed in more detail in a later section.

Our experiments were conducted on a flat slim-tube coil apparatus, because it is the most commonly used in the industry. The effect of tube length and injection rate on displacement efficiency, namely, recovery and shape of the effluent gas concentration profile, and consequently their effects on the criteria used for estimating the MMP were examined