Limitations on the Oil/Steam Ratio for Truly Viscous Crudes
Authors: Doscher T.M. , Ghassemi Farhad 
Published: December 2007

Scaled physical model studies have yielded results that suggest very viscous crude oils with a viscosity somewhat greater than that of San Joaquin Valley crudes cannot be economically recovered by an unassisted steamdrive.

Introduction

Steamdrive has been used successfully for almost two decades for the economic recovery of viscous crudes. One of the large targets for steamdrive has been implicitly assumed to be the large accumulations of very viscous crudes with a gravity less than 10API [1 g/cm 3].

Some of the early scaled physical model studies of the steamdrive indicated that the viscosity of the oil at steam temperature had a significant effect on the efficiency of the steamdrive. The experimental work indicated that the growth of a steam zone was not independent of the viscosity of the reservoir fluid, contrary to the implicit assumption made in the Marx-Langenheim analysis of the steamdrive. This raised questions about whether the steamdrive could be successful in recovering very viscous crudes and bitumens.

Some investigators point out that other factors, such as pattern size, optimal injection rates, etc., limit the application of the steamdrive. However, most of these factors are controllable and can be adjusted before the steamdrive is started.

Further, some investigators have concluded from laboratory and field results that the hot-water drive associated with steam injection made a significant contribution to recovering viscous crudes. There can be little question that the viscosity of the crude is a factor in water-drive efficiency. Even assuming that the viscosity of the crude will not affect its ultimate residual saturation to a water drive, the efficiency of a water drive in terms of the number of PV's required to reach the residual must be significantly affected by the oil viscosity. Therefore, the hot-water-drive components of steamdrive cannot be expected to contribute significantly in attempts to recover very viscous crude oils.

A series of scaled physical model studies was undertaken to elaborate on the effect of viscosity on steamdrive. Preliminary results have been reported earlier; but in view of the important issues raised by the findings of this work, the completed study is reported here.

Experimental Procedure

The use of scaled physical models and the procedures used for modeling steamdrive have been described already. To achieve the goal of this work, the models were saturated with prototype crudes that had a range of viscosities at 100F [38C] from 3 cp to more than 100,000 cp [0.003 to 100 Pa s]. The initial oil saturation was 85 to 92 % in a uniform, confined 6-acre [145 687-M2], five-spot pattern. The model simulated one quarter of a confined 6-acre [ 145 687-m 2], five-spot pattern with a uniform porosity of 35%. In most of the pattern with a uniform porosity of 35%. In most of the experiments the prototype reservoir thickness was 73.5 ft [22 m] and the absolute permeability in the range of 1 to 2 darcies. A few experiments also were conducted with a model that corresponded to a reservoir only 27 ft [8 m] thick.

In all experiments the steam initially coursed through the model at the top of the reservoir-i.e., it overrode the oil column either fortuitously or by initially establishing a gas saturation amounting to 1 to 3% of the reservoir volume. In actual oilfield operations, the steam may or may not enter initially at the top of the reservoir, depending on the reservoir lithology and the distribution of the saturating fluids. However, in a steamdrive the steam eventually does reach an upper barrier to its further flow, and the overall performance of the steamdrive will approach the performance of a reservoir in which the steam did enter initially at the top. Because initial entry in these experiments was always at the top, a measure of consistency in the various experiments was introduced.

To make a valid comparison between all the runs, the rate of steam injection was standardized at a value that had been determined earlier as optimal for prototype crude oils with a viscosity at 100F [38C] in the range of 1,000 to 5,000 cp [ 1 to 5 Pa s] (San Joaquin Valley crudes)-130 to 150 B/D [21 to 24 m3/d] of water (converted to steam) per acre. The viscosity of the crudes at the prototype steam temperatures varied from 0. 15 to 85 cp [0.00015 to 0.085 Pa s].

Finally, various runs of steamdrives were compared by use of the "stable" oil/steam ratio that developed following steam breakthrough (see Fig. 1).

JPT

P. 1123