Abstract

Field experiences have shows that the primary depletion behaviour in several heavy oil reservoirs worldwide is anomalous. It has been theorized that during primary production, the solution gas released from heavy oil does not disengage from the liquid immediately but remains dispersed in the form of micro-bubbles which tends to flow with the oil. Such gas-oil dispersions are termed as foamy oil. Claridge and Prats hypothesised that adherence of asphaltenes to these gas bubbles reduces the viscosity o foil. However, no experimental verification was reported to confirm this hypothesis.

Experiments were undertaken to test the above Low Viscosity Model and to obtain and improved understanding of foamy oil rheology. To assess the rheological properties of foamy oil, a series of experiments were conducted using a high pressure rotational viscometer. The effects of asphaltenes, dissolved gas content, temperature, and bubble sizes on apparent viscosity of foamy oil was systematically evaluated. Foamy oil viscosity was found to be independent of shear rate at high pressures. However, a mild non Newtonian behaviour was observed at low pressures. The viscosity of gas-oil dispersion was dependent on bubble size distribution with higher viscosities for smaller size bubbles. The hypothesis of reduced viscosity resulting from adsorption of asphaltenes on surfaces of gas bubbles could not be confirmed experimentally. The viscosity of foamy dispersion was found to be higher than or similar to the live oil viscosity. In addition, the rheological behaviour of dispersions prepared with de-asphalted oils was similar to that of the original crude oil dispersions. This would suggest that asphaltenes do not play a major role in the rheological properties of the dispersion.

Introduction

Reservoir oil viscosity is the most important parameter that influences the oil recovery process. The higher the oil viscosity, the lower is the ultimate oil recovery. Therefore, it is more difficult to recover heavy oil from the underground reservoirs than the conventional light oil. Reduction of crude oil viscosity using steam, in-situ combustion, and/or solvent forms the basis for most of the EOR processes for heavy oil.

Field experiences have shown that the prima depletion behaviour in several heavy oil reservoirs worldwide is anomalous. It has been theorized that during primary production, the solution gas released from heavy oil does not disengage from the liquid immediately but remains dispersed in the form of micro-bubbles which tends to flow with the oil. Such gas-oil dispersions are termed as foamy oil. Claridge and Prats (1995) hypothesized that adherence of asphaltenes to these gas bubbles reduces the viscosity of oil. However, no experimental ve;ification was reported to confirm this hypothesis.

In this study, experiments were undertaken to test the above Low Viscosity Model and to obtain an improved understanding of foamy oil rheology. To assess the rheological properties of foamy oil, a series of experiments were conducted using a high pressure rotational viscometer.

This content is only available via PDF.
You can access this article if you purchase or spend a download.