An Investigation of the Effect of Oil Composition on Heavy Oil Solution-Gas Drive

Abstract

Abstract Solution-gas drive has been implemented successfully for some heavy-oil reservoirs in Canada and Venezuela since the 1980's. Nevertheless, the mechanisms of heavy oil-solution gas drive remain unclear. The objective of this study is to probe the phenomenon experimentally. To this end, a series of depletion experiments were conducted that employ heavy crude oils from two different fields in Venezuela and two viscous mineral oils. The experiments were conducted at reservoir temperatures for crude oils and room temperature for mineral oils with similar viscosity. The sandpack is made up of Ottawa sands with a porosity of about 20-30% and permeability of roughly 2.7-5 Darcy. The sandpack is placed in a specially designed aluminum coreholder that allows visualization of gas phase evolution during depletion using X-ray computed tomography (CT). In addition, a visualization cell with a window size of 5 x 40 x 0.05 mm (width × length × thickness) was installed at the outlet of the sandpack to monitor the flowing-gas-bubble behavior versus pressure. Bubble behavior observed at the outlet corroborates CT measurements of in-situ gas saturation versus pressure. Importantly, these results help us to understand the gas flow pattern inside the sandpack. Results show that solution gas-drive is effective even at reservoir temperatures as high as 80°C. Oil recovery ranges from 12-30% OOIP; the higher the depletion rate, the greater the recovery rate. Secondly, both depletion rate and oil composition affect the size of mobile bubbles observed. At a high depletion rate (0.035 PV/hr), a foam-like flow of relatively small pore-sized bubbles dominates the gas and oil production of both crude oils. Conversely, at a low depletion rate (0.0030 PV/hr), foam-like flow is not observed in the less viscous crude oil; however foam-like flow behavior is still found for the more viscous crude oil during the early and middle periods of gas and oil production. No foam-like flow is observed for the mineral oils. Third, intermittent gas-bubble flow causes the produced gas-oil ratio (GOR) to fluctuate over time. Fourth, CT images show that the gas saturation distribution along the sandpack is not uniform at our test conditions; high gas saturation at the outlet is observed for virtually all oils. Away from the outlet, gas saturation is also heterogeneous and apparently related to heterogeneities within the pack. As the pattern of produced gas switches from dispersed bubbles to free gas flow, the distribution of gas saturation becomes even more heterogeneous. This indicates that a combination of pore restrictions and gravity forces significantly affects free gas flow.