Abstract
Linear scaled physical model studies were designed and carried out to study the recovery of crude oil by the injection of carbon dioxide. Viscous and gravity forces and molecular diffusion were explicitly scaled to field conditions. Mechanical dispersion was not scaled, and is exaggerated in the models.
The basic prototype considered in this study is a homogeneous reservor with a permeability of 20 millidarcies and a distance between injector and producer of 462 feet and an equivalent thickness of 28 feet.
Because both the line drive, in contrast to a radial pattern, and the low permeability of the prototype reduce the relative influence of gravity segregation, and the exaggeration of mechanical dispersion in the model reduces the effects of viscous fingering and gravity override; the results of these studies yield optimistic results in comparison with those that may be expected under field operating conditions.
The results reveal that two factors dominate the recovery process: the high mobility of the carbon dioxide, and its solubility in the subject crude oil. The latter in turn appears to be a function of the density of the carbon dioxide.
The effects of pressure, temperature, oil saturation, and slug size on recovery were studied in some detail, and the effect of the composition of the oil to a limited extent. A degree of criticality is observed with respect to the size of the slug on its influence on recovery and the carbon dioxide required to recover a barrel of crude oil. By controlling the slug size, about half of the residual oil can be recovered at ratios of less than 10 MSCF/B whereas continuous injection will recover more of the residual oil but only at ratios in excess of 20 MSCF/B.
No banking of the crude oil is observed and carbon dioxide production occurs rapidly and concomitantly with oil. The complex phase behavior of carbon dioxide, the separation of the heavy fractions and solution of the light fractions in the dense carbon dioxide gas, occurs only after the carbon dioxide has reached a high saturation in the system, and therefore contributes little to the up-front (economic) recovery of residual oil.
