Abstract

In situ oil sands are dense uncemented fine-grained sands that contain substantial amount of methane and carbon dioxide gases in pore water and heavy oil. Gas evolves when the pore pressure drops to the bubble point pressure (liquid-gas saturation pressure) due to a decrease in confining pressure or fluid production. The volume and pore pressure changes in live oil-filled sand specimens due to decrease in confining pressure under undrained condition were examined in laboratory. A mechanistic model based on kinetics of gas bubble growth due to solute diffusion in supersaturated oil liquid was formulated and presented to interpret the observed time-dependent nonthermodynamic equilibrium behaviour of pore pressure and volume changes. It was found that the bubble sizes could be estimated indirectly by matching the pore pressure response of the live-oil filled system.

Introduction

The formation of dispersed gas bubbles (or foamy oil) in the heavy oil has been postulated to be an important factorcontributing to the success in primary production of heavy oil reservoirs 1, 2. It has been hypothesized that the foamy nature of the heavy oil maintains the released solution gas dispersed in the continuous oil phase, which is very different from the convention oil behaviour. The flow behaviour, pressure responses and production rates of solution gas drive in heavy oils have been studied by several investigators using depletion tests (e.g., Sheng et al. 3; Wong et al. 4; Zhang et al. 5; Tang and Firoozabadi 6; Tsimpanogiannis and Yortos7). The depletion tests results consistently indicate that the recovery factors observed in the depletion tests of fast pressure decline rates are higher than those observed in tests of slow pressure decline rates. However, how the gas bubbles nucleate, grow, coalesce and flow is still in controversy, even though some experiments were equipped with visual aids. This paper proposes a novel technique to estimate the bubble size and density. This technique was developed from a mechanistic model based on kinetics of gas bubble growth due to solute diffusion in supersaturated oil liquid.

A Mechanistic Model for Gas Exsolution Under Undrained Unloading

Consider a sand specimen saturated with live oil that is encased inside an impermeable membrane (Fig. 1). The enclosed system is subjected to an external confining stress and internal pore pressure. The external stress is larger than the internal pore pressure resulting in a net (effective) confining stress. A stepwise drop in external confining stress is applied to the encased system in an instant (Fig. 1a). The internal pore pressure reacts to the step-wise drop in external confining stress (Fig. 1b). The instant drop in pore pressure depends on the total compressibility of the encased system including compressibilityof sand matrix, fluid, and gas phases. This instant drop in pore pressure disturbs the non-thermodynamic equilibrium existed in the gas solute concentration of the liquid, and leads to growth of the bubble due to solute diffusion. As the bubbles grow, the internal pore pressure increases under undrained condition because no gas or oil is allowed to escape from the enclosed system.

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