The problem of asphaltene deposition in oil production, specially in EOR (enhanced oil recovery) processes is serious. The deposited asphaltenes blocked the reservoir pores and greatly reduced the recovery rate. In order to solve this problem, it is necessary to investigate the mechanism of asphaltene deposition, to determine the onset point, and to find the factors affecting the depositional process. In this work, the viscosity and electrical conductance of oil-solvent mixtures were determined for detecting the onset point of asphaltene deposition for Shengli, Liaohe, ad Shanjiasi crude oils. It is found experimentally that the viscosity of the oil-solvent system decreases with the addition of heptane, but there is a knee point on the curve which corresponds to the onset point of asphaltene deposition. When toluene is used as a solvent, there is no such knee point. The experiments indicated also that with the addition of heptane, the electrical conductance of the system rises at first and then falls. On the curve's first derivative, a maximum appears and can be defined as the onset point of asphaltenes deposition; there is no such maximum in most cases when toluene is used as a solvent. The data obtained by these two methods are slightly different, but they correlate well. It is further found that the onset of asphaltene deposition in crude oil occurs earlier than in its residue, indicating that the colloidal system of crude oil is less stable than that of its residue. Furthermore, it seems that the sulfur content of the oil exhibits an important influence on the stability of the colloidal system and the onset point of the asphaltene deposition. The electrical conductance method is more sensitive and less time-consuming than the viscosity method for the determination of the onset point of asphaltene deposition.
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