Role of Asphaltene in Foamy-Oil Flow

Abstract

Abstract The role of foamy oil flow in cold production of heavy oil has attracted considerable attention in the literature. It has been suggested in several studies that there may be a link between the presence of high Asphaltenes content and the foamability of oil.However, a systematic examination of the impact of Asphaltenes on the performance of solution gas drive, in connection with foamy oil flow, has not been reported.This paper presents an experimental study that addresses this issue. The objective of this study was to examine whether or not the presence of Asphaltenes has a strong influence on the performance of foamy solution gas drive.To this end, parallel solution gas drive experiments were conducted with a heavy crude oil from Lloydminster area and a de-asphalted version of the same oil. To eliminate the influence of oil viscosity, the original crude oil was diluted with a 50–50 mixture of heptanes and toluene to reduce viscosity to the same level as that of the de-asphalted oil.The experiments were carried out in a visual sand pack that permited observation of bubble formation in the sand. The results show that the effect of asphaltene content varies with the depletion rate.At higher depletion rates, the oil recovery and production profile of crude oil with asphaltene is different from those without asphaltenes.The presence of asphaltenes, appears to facilitate bubble nucleation, decreases the critical super-saturation and helps in maintaining the dispersed gas flow by suppressing bubble coalescence.However, as the depletion rate declines, the incremental recovery due to asphaltenes diminishes.The two crude oil samples provided similar recovery and production profile when the depletion rate was the slowest one used in this work. Introduction Foamy oil flow is the name commonly used to describe a form of two-phase oil-gas flow in porous media in which the gas phase remains partially or completely dispersed in the oil (1).It is generally accepted that the better than expected production behavior of primary depletion wells in many Canadian and Venezuela heavy oil reservoirs can only be explained by postulating an unconventional flow mechanism that results in a drastic reduction of gas mobility and trapping of a disproportionately large volume of gas within the reservoir (2). Smith (3) proposed a heavy oil production mechanism in which the gas released from solution remained dispersed in the oil in the form of micro bubbles.Subsequently, Maini et al. (4) experimentally observed this dispersed gas phase and called it "Foamy Oil."Claridge et al.(5) proposed that the stability of dispersed micro-bubbles is related to the asphaltenes adsorption at the gas-oil interface, which protects bubbles against coalescence.In micro-model experiments, Bora et al.(10) observed that asphaltenes lower coalescence rates. Tang et al.(18) did not observe any differences in comparing crude and silicon oil with similar viscosity.This, however, was most likely due to the low viscosity of crude used in Tang's experiments. Various other authors also show that foaming and interfacial film stability increase with the asphaltene concentration.Recently, F. Bouget et al. (32) presented results showing significant shift in the crude properties at asphaltenes concentrations of around 10%.His experiments suggested a significant increase in foamability, bubble stability and viscosity of crude at asphaltenes concentration of 10% and over.Similar results were obtained by J-F.Argillier et al. (28).They reported significant increase in relative viscosity when the asphaltene contents were 10% or more. To the best of our knowledge, a systematic study carried out to evaluate the role of asphaltenes in foamy oil flow that eliminated other extraneous factors arising from the use of oils from different sources has not been reported. The work presented in this paper was intended to evaluate the role of asphaltenes in foamy oil flow using oil samples from a single reservoir source.For this, asphaltenes were removed from a part of the crude sample and the viscosity of the remaining oil sample was lowered by the dilution with 50–50 mixture of toluene and heptanes.Parallel flow tests at various depletion rates were carried out to compare the results of 50–50 diluted oil and the de-asphalted oil for oil rate, recovery, GOR and other parameters.