Measurements of dispersion coefficients and dispersivities of a gaseous substance displacing another one, at low pressures, have been the object of this study 1.
Two series of laboratory experiments, conducted at two different pressure ranges on unconsolidated porous media, were carried out at a Gas Plant Laboratory, located at 120 Km far from Neuqu?n, South of Argentina. Each series, at its turn, was made on two different types of porous media, a plastic hose and a metallic slim tube, each one having been filled twice, with natural sand grains of two different sizes the first one, and synthetic sand with two grain sizes, the second one.
Experiments and some results from the first series, at the lowest pressures, will be presented in this paper. These results are expected to provide a better understanding on the behaviour of gases at low pressures as a starting point to find a scaling factor, that may be used not only on displacement projects for reservoirs, but also in other applications in the gas industry.
The present works refers to some experiences carried out on dispersion, phenomenon by which molecules of two miscible fluids, when in contact, diffuse into one another, due to the presence of some forced flow, id est, in presence of velocity. Longitudinal dispersion occurs in the direction of flow, and transverse dispersion is the one perpendicular to the direction of flow. Both are important in miscible displacements.
The objective was to measure longitudinal dispersion coefficients on a gas natural displaced by means of an inert gas, nitrogen in this case, at a very low pressures, almost atmospheric ones. The work was done in several stages, the first of them being an intense searching and consulting of bibliography on the subject.
The steps that followed this searching were the construction of two unconsolidated porous media, performance of the experiments and interpretation of the obtained data, by means of two methods closely related.
Dispersion plays an important rol in miscible displacements, specially in those occurring in enhanced oil recovery. The importance of this study may be better understood if the enormous field of application in enhanced hydrocarbon recovery, specially in gas natural area, of increasing interest for the energetic sector, is considered. A big amount of work has been done on the field of miscible displacement, but almost always the actors have been liquid hydrocarbons, chemical solvents, etc, and not so much has been devoted to experiences with natural gas.
Enriched gas drive studies with its condensing and vaporizing processes have been mainly devoted to oil recovery. Nevertheless the possibility of recovering heavy gas fractions, trapped in the top of geological structures because of retrograde behaviour in the neighborhood of the mixture critical point, opens new fields of research on these subjects.
Besides the inherent difficulties when working with natural gas due to the risks involved and the fact of not having visual aid to inspect the tests inside the core, other problems must be taken into account when physical models to make experiments are intended to reproduce as close as possible reservoir conditions.
