Abstract

Solution gas libration is one of the key factors contributing to the success in solution gas drive in heavy oil reservoirs. Unconsolidated sands dilate with decreasing effective stress, and contract with increasing effective stress. This paper describes special experiments that were designed to investigate the effect of sand matrix deformation on the bubble nucleation and growth in heavy oil, and the oil production. The experiments involved flow of heavy oil with dissolved gas in reconstituted sand packs. Gas bubbles were allowed to develop in dilated sand packs by lowering the effective confining stresses in an undrained (closed) system. Solution gas in heavy oil was allowed to produce from the sand packs in constant depletion rates. Production rates under various conditions were compared. Based on the experimental results, a mechanistic model for solution gas drive in heavy oil was proposed to explain the linear relationship between the oil production and depletion pressure.

Introduction

The formation of dispersed gas bubbles (or foamy oil) in the heavy oil has been postulated to be an important factor contributing 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). The depletion tests results indicate that the recovery factors are higher in fast depletion tests of pressure decline rates of 2.1 to 3.5 kPa/min than those in slow depletion tests of pressure decline rates of 0.3 to 0.5 kPa/min. The total oil recovery under step pressure declines can be as high as up to 35 to 45% 4. In the constant pressure decline rates, the total oil depletion lies in a range of 17 to 29% 5. It appears that the viscosity or temperature plays a minor role in the production. These findings provide some valuable insight into the solution gas drive mechanism. However, the soution gas-oil flow properties, such as total compressibility and mobility, have not been quantified. How does the deformable reservoir sand matrix affect the solution gas drive behaviour? This paper attempts to answer some of these questions by conducting some specially designed depletion tests.

TESTING MATERIAL AND EQUIPMENTS

A schematic diagram of the pressure depletion test setup is illustrated in Figure 1. The test sand was packed inside a core holder of 41.2 cm in length and 3.8 cm in diameter. The core holder composed of a flexible Vitron sleeve and two steel end platens was subjected to a confining overburden pressure inside an insulated highpressure cylindrical chamber. A high-precision screw pump was connected to the high-pressure steel chamber to regulate the pumped water volume and the pressure.

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