Mechanisms of Solution Gas Drive in Heavy Oil Reservoirs

Abstract

Introduction Solution gas drive in heavy oil reservoirs (with viscosity in the range of 10 to 1,000 poise and API gravity in the range of eight to 15), which is often referred to as cold production, has a long production history in Canada. In a recent review paper, Dusseault et al.(1) presented the information that more than 5,000 wells in Canada have produced heavy oil by cold production. In addition to Canada, cold production from heavy oil reservoirs has been practiced in Venezuela(2), China(3), and Oman(4). Recoveries from some of the heavy oil reservoirs by cold production are estimated to be as high as 20﹪. The high recovery is often associated with a low pressure decline rate in the reservoir and the slow increase of GOR in the two-phase below the bubblepoint, as well as geomechanical effects. Recoveries in the range of 10 to 20﹪ below the bubblepoint pressure are believed unusual for very heavy oils. In this overview, we will briefly review the literature and then provide some insight into the relevant mechanisms for cold production. Literature Review An early paper by Smith(5) set the stage for the understanding of cold production from solution gas drive in heavy oil reservoirs. In his work, Smith provides an analysis of solution gas drive in heavy oil reservoirs from the examination of field data. He attributes the high efficiency of solution gas drive in viscous oils to:a significant reduction (around one order of magnitude) in the oil viscosity due to formation of small gas bubbles in the oil,simultaneous flow of continuous oil phase and discontinuous gas phase in the form of tiny bubbles,an increase in absolute permeability due to sand production, andhigh fluid compressibility due to high gas bubble density in the oil. In order to validate some of the postulations by Smith, the bulk of research has focused on the socalled pseudo-single phase model. In the pseudo-single phase model, a new terminology has evolved to distinguish solution gas drive in heavy oil reservoirs. The so-called foamy oil terminology was introduced in 1992 by Sarma and Maini(6); it was defined as a viscous (heavy) oil containing dispersed gas bubbles. Claridge and Prats(7) used the term " foamy heavy oil" and " foamy crude" mainly to imply drastic reduction in the oil viscosity due to dispersed as bubbles. The term " foamy oil" has had a wide acceptance because the oil samples at the wellhead produced from the reservoirs seem to be in the form of oil-continuous foam, with the appearance of chocolate mousse, and may contain a large volume of dispersed gas bubbles. Hu et al.(3) report that the density of the produced oil from a heavy oil pilot well from the Henan oil field in China is about 0.58 g/cm3, while the density after all the gas has left the oil (which may take several days) is 0.95 g/cm3.