Effect of Sand Matrix Deformation on Solution Gas Drive in Heavy Oil Reservoirs
Authors: Wong R.C.K. , Maini B. 
Published: March 2010

Abstract
Solution gas liberation 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 bubble nucleation and growth in heavy oil, and oil production. The experiments involved flow of heavy oil with dissolved gas in reconstituted sand packs. Gas bubbles were allowed to nucleate and grow in dilated sand packs by lowering the effective confining stresses in an undrained (closed) system. Then, oil was produced from the sand packs by reducing pressure. Production rates from these experiments were compared to those from constant pressure decline tests. 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 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 heavy oil maintains the released solution gas dispersed in the continuous oil phase, which is very different from conventional 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 test results indicate that recovery factors are higher in depletion tests with fast pressure decline rates of 2.1 to 3.5 kPa/min than those in depletion tests with slow 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 constant pressure decline rates, total oil depletion lies in the range of 17 to 29%(5). It appears that oil viscosity or temperature plays a minor role in production. These findings provide valuable insight into the solution gas drive mechanism. However, the solution gas-oil flow properties, such as total compressibility and mobility, have not been quantified. How does the deformable reservoir sand matrix affect 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 set-up is illustrated in Figure 1. The test sand was packed inside a core holder 41.2 cm in length and 3.8 cm in diameter. The core holder, composed of a flexible Viton sleeve and two steel end caps, was subjected to a confining overburden pressure inside an insulated high-pressure cylindrical steel chamber. A high-precision screw pump was connected to the high-pressure chamber to regulate the pumped water volume and pressure. A vessel containing live oil or water was connected to the inlet end of the core holder so that live oil or water could be injected into the sand core using a displacement pump.