Alteration In Heavy Oil Characteristics During Thermal Recovery
Authors: Venkatesan V.N. , Shu W.R. 
Published: March 2010

Abstract
Significant visbreaking, or a permanent reduction in viscosity of heavy oils, can occur during in-situ thermal recovery processes such as steamflooding and combustion. This paper reports a study on the mechanism of in-situ visbreaking. Isothermal visbreaking studies were conducted af 572 °F (300 °C) for up to 480 hours using heavy crudes from San Ardo, Celtic, and Cold Lake. The viscosity of the oil, as well as asphaltene content, distillation, elemental analysis, etc., were monitored as a function of time. The experimental results showed a permanent reduction in oil viscosity by a factor as high as ten, without significant formation coke, after 120 hours of soaking. The observed initial rate of viscosity reduction increases with increasing hydrogen content of the oil. Heavy oil is viewed as a colloidal system containing asphaltenes dispersed in a solvent phase commonly known as maltenes. Under this framework, visbreaking is interpreted as being caused by one of these three factors: a decrease in maltene viscosity, a reduction in asphaltene content, or a decrease in a third parameter called "reduced specific viscosity ", which characterizes the "solvent power " of the maltenes with respect to the asphaltenes. The decrease in maltene viscosity and the increase in its solvent power dominate the visbreaking mechanism. The effect of reduction in asphaltene content on visbreaking is relatively minor.

Introduction
High viscosity is a major constraint in the production and processing of heavy oils and oil sand bitumens. Thermal recovery processes employing steam or combustion are effective in temporarily lowering the oil viscosity and enhancing its production. Field evidence indicates that a permanent reduction in viscosity or "visbreaking " can accompany these processes which can result in partial upgrading of the heavy oil(1). In-situ visbreaking is characterized as a mild thermal cracking with the retention of products in the liquid phase. The extent of vis breaking depends upon the temperature, heating time, and the oil characteristics. Visbreaking as a recovery mechanism during steam recovery processes has been studied by Shu and Hartman(1) using numerical simulations. Their analysis showed that heavy oil visbreaking could have a significant effect on oil production depending upon the mode of steam process and the reservoir characteristics.
The present work reports a study on the alteration in the oil characteristics during visbreaking for heavy crudes from San Ardo (California), Celtic (Saskatchewan), and Cold Lake (Alberta). Isothermal visbreaking experiments were conducted at 300 °C for up to 480 hours. Such information will be useful for numerical simulation studies, for evaluation of field data, for developing improved processes, and for designing surface upgrading facilities.

Background
Extensive information is available on the surface visbreaking of heavy oils and petroleum residue in the refining literature. In such studies, thermal cracking of the feed oil is carried out at a high temperature (near 450 °C) for short contact time (usually in seconds and minutes). On the contrary, during in-situ visbreaking, the reservoir oil is exposed to lower temperatures (200 – 325 °C) for longer periods of time (usually days and months). Few laboratory studies simulating the in-situ visbreaking conditions have been reported.