Experimental Investigation of Asphaltene Deposition in Capillary Flow

This paper reports a set of experiments carried out to examine asphaltene deposition in a glass microcapillary pipet, as a proxy to asphaltene deposition in reservoir pores. A new capillary-flow experiment was designed to ensure capillary-dominated flow, low inertial forces, negligible dispersion, and insignificant gravity effects, and the end-effect was limited to 0.1% of capillary length. This was achieved by maintaining the flow rates in the range 5 <= Q(T) <= 60 mu L/min. The asphaltene precipitation was induced by bringing into contact a heavy oil sample, diluted with toluene, with a number of different precipitants (n-pentane, n-heptane, and n-octane). The deposition of asphaltene was monitored by imaging the capillary tube and by measuring the pressure drop across it. A new, simple model has been developed to relate the pressure drop to the change in the thickness of the deposited layer and subsequently to the change in permeability. The model indicates that the thickness varies as one over the fourth power of the pressure drop, while the change in permeability is proportional to the square root of the pressure drop. A series of experiments has been carried out to examine the effects of different flow rates, precipitant concentration, and the nature of the precipitant on asphaltene deposition. The deposit growth was generally monotonic and reasonably uniform, indicating the lack of erosion and entrainment. The results indicate the deposition rate increases with decreasing carbon number of the precipitant, while it is not unduly influenced by changes in a flow rate. No noticeable deposition was observed when the precipitant/solvent ratio was 1.22 despite the ratio exceeding the minimum threshold value of 1.08, obtained from gravimetry.