Low-Temperature Visbreaking Of Heavy Crude Oils
Authors: Chen N.Y. , Scott E.J.Y. 
Published: March 2010

The visbreaking of a heavy Celtic crude and a San Ardo core sample was monitored for a period as long as three and a half years over a temperature range of 204 °C to 280 °C to assess the feasibility of in-situ healing to produce a pumpable oil. Healing the oil samples at 204 °C effected an initial rapid decrease of their viscosity. But even after a long period of heating at this temperature, their viscosity reached some asymptotic values and remained far above that of a pumpable oil. Further visbreaking was observed only after the samples were heated to higher temperatures for extended times. For example, at 280 °C, the viscosity of the Celtic crude at 25 °C was reduced to below 250 cp in less than two years.

The experimental result is consistent with a simple reaction network which involves both parallel and sequential first-order irreversible reactions to form a medium-viscosity intermediate and a low viscosity final oil. These reactions have vastly different activation energies.

Preliminary data on a San Ardo core sample show mineral matters in the core sample has negligible catalytic effect on visbreaking at 204 °C.

Introduction

Heavy crudes are highly viscous oils. In some cases their viscosity exceeds 1 000 000 cp at reservoir temperatures. They cannot be produced by conventional means without having their viscosity reduced. At present, techniques such as solvent dilution or heating with high-temperature steam or wet combustion are being employed in the production of such oils.

Bowman and Carrigy(1) reviewed a number of in-situ recovery techniques and the experimental projects in Alberta, Canada on heavy oil production. Other novel in-situ recovery techniques have appeared recently in the patent literature. For example, Striegler and Howell(2) described a technique comprising drilling a continuous well bore through the oil reservoir between the injection well and the production well, and heating the oil by circulating a hot fluid through the well bore. Dauphine(3) and Fisher(4) proposed the use of electric induction heating technique for the in-situ process.

Thermal reduction of oil viscosity is a common practice in the refining of vacuum resides to heavy fuel oils. The commercial process is conducted to 400 °C to 450 °C and short residence times. If the thermal visbreaking were extrapolated to lower temperatures and longer residence times, one would expect that when the oil is heated to and maintained at about 200 °C for a period of a few years, its viscosity should be reduced sufficiently for production by conventional means.

However, results of on-site steam-soaking studies(5) with steam temperature in the order of 230 °C, showed little evidence of significant permanent viscosity reduction.

Shu and Venkatesan(6) studied the kinetics of thermal visbreaking of a Cold Lake (Alberta) bitumen at 260 °C to 325 °C with residence times of up to one month. They(7) also studied the chemical changes during thermal visbreaking for heavy crudes from San Ardo (California), Celtic (Saskatchewan) and Cold Lake at 300 °C for up to 20 days.