Modeling Cold Production For Heavy Oil Reservoirs

Abstract The term "Cold Production" refers to the use of operating techniques and specialized pumping equipment to aggressively produce heavy oil reservoirs. This encourages the associated production of large quantities of the unconsolidated reservoir sand creating a modified wellbore geometry that could include "wormholes ∼, dilated zones, or possibly cavities. As well, produced oil in the form of an oil continuous foam resembling chocolate mousse, suggests a foamy solution gas drive occurs in-situ. This leads to anomalously high oil productivity and recovery because free gas stays entrained in the foam, thereby sustaining reservoir pressure. In a recent paper1, the mechanisms that lead to this increased productivity were outlined and the suitable reservoir types conducive to cold production techniques were identified. In this paper, these mechanistic concepts are extended to practical, intuitive modeling techniques that can be applied to existing "black oil"reservoir simulators by appropriate alterations to the input data. Importantly, these techniques have been found to match actual cold production behavior in applicable Western Canadian conventional heavy oil reservoirs. With a history matched model, these techniques can be used to extend the cold production scenario into the future, providing better estimates of ultimate recovery. As well, sensitivities to the process can be investigated, including exploring sensitivities to various reservoir and operating parameters (e.g., reservoir pressure, production rate strategies) and examining the impact of a preceding cold production primary depletion on subsequent secondary and tertiary recovery processes. Introduction In approximately the last ten years, many authors have written about the phenomena involved in producing heavy oil by solution-gas drive. Their work has been inspired by field observations of cold production in some of the heavy oil reservoirs in Canada and Venezuela where unexpectedly high oil rates and recoveries, as well as low gas-oil ratios have been attained. This work has included laboratory investigations of fluid and rock properties (including geomechanical studies of the so-called worm holing effects), conceptual postulation of mechanisms in the context of actual field behavior, as well as some attempt to mathematically capture and numerically model these mechanisms. Perhaps one of the first to set forth the mechanisms and possible mathematics was Smith2 at Husky, who also appears to be one of the first investigators to note that the anomalous production enhancement must arise from a combination of geomechanical and fluid effects (i.e., results cannot be "excused as high permeability channels resulting from sand production"). These two categories of mechanisms - geomechanical effects and fluid effects - are the subject of the mechanisms proposed in the literature for explaining cold production performance of heavy oil reservoirs. Geomechanical Effects Productivity of heavy oil wells experiencing cold production is typically much higher than would be expected - actual productivity exceeds radial Darcy flow predictions (using typical oil viscosities and air permeabilities) by factors of four to ten. The general observation made by numerous producers, that oil rates seem to correlate with sand production, has led many to infer that the production of sand improves inflow performance by increasing the effective permeability of the formation, via the creation of a system of wormholes.