Bubble Geometry During Solution Gas Drive Process in Heavy Oils

Abstract Cold production of oil leads to the degassing of the light species and the formation of a bubbly phase, sometimes called the "foamy oil" effect. This bubbly phase is particularly observed with heavy oils, combining high viscosity and asphaltenes. Presence and behaviour of a foamy-oil effect appears to be critical to the cold production process. This process is not a well understood production mechanism because a wide range of different petrophysical parameters and experimental factors interact in a rather complex way. Over the past few years, a number of efforts have been made in many institutions, in order to understand and model the solution gas drive mechanism in primary heavy oil recovery. Conventional simulations succeed in matching actual field productions but are not reliable for prediction forecast purposes (large uncertainties on recovery factors). We have modelled, in a previous paper1, depletion experiments on laboratory-scale cores using a one-dimensional model at the Darcy scale, describing the multiphase flow of oil and gas. In this paper we analyse a mechanism of gas phase flow based on the geometry of the bubbles and its consequences on their motion. Actually some Pore Network Models (PNM) simulations show slender bubbles in the direction of the flow2. Here we present a theory to predict the aspect ratio of the bubbles and their velocities. The aspect ratio of the bubbles depends on the characteristics of the porous media in terms of pore size distribution and a threshold value of bubble that separate a statistically isotropic ganglia configuration from a slender ganglia configuration size is identified. Consequences on gas flow rate and critical gas saturation are deduced. Finally we introduce a mass balance theory that highlights a parameter, a(t), which is the ratio of the gas phase mobility over liquid phase mobility. This parameter plays a key role in understanding and modelling the multiphase flow. The slender bubbles model is implemented and the results are compared to the experimental data Introduction Many efforts have been made recently to understand and model the solution gas drive mechanism in primary heavy oil recovery. Present day models are unreliable, as even though conventional simulators succeed in matching field productions, they fail to capture the actual physics of the Solution Gas Drive (SGD) process in heavy oils. A "foamy-oil" effect related to the bubbly aspect of the recovered oil, enhanced oil recovery and higher critical gas saturation has been identified by some authors (9, 10) but its physical nature is not quite elucidated. When it comes to modeling the SGD we can distinguish two tendencies. The first one consists in replacing conventional PVT properties of the oil by those of a mixture "oil + dispersed gas bubble"(9, 11). The second one consists in describing the flow behavior through mobility and critical gas saturation keeping conventional PVT properties(12, 13, 14, 15). Our model comes within the last context. In several depletion experiments, the pressure decreases at a set rate over the duration of the experiment.