Asphaltene precipitation and deposition in the near wellbore region: a modeling approach

In this work, a new single-well model to simulate asphaltene precipitation, deposition, and plugging of oil wells during primary production is developed. A four-component (asphaltene-oil-gas-water), four-phase (solid-oil-water-gas) limited compositional formulation is described to account for asphaltene flow with crude oil and its precipitation, deposition, and plugging near the well, with the resulting effect on well productivity. The model accounts for the asphaltene precipitation phenomena directly through the use of laboratory values of asphaltene solubility versus pressure data. Four to six primary variables are solved simultaneously in the model. These include the pressure, three of the four phase saturations, the gas saturation (bubble point) pressure, and the asphaltene saturation pressure. The model is implemented in cylindrical (r-z) coordinates to match the flow direction around the well. This model is applied to a typical well configuration using a recombined asphaltic UAE crude oil sample. The asphaltene solubility versus pressure relationship is first evaluated experimentally using the gravimetric method. Then, the asphaltene precipitation and deposition around a typical production well is then simulated. Results of the model demonstrate how the asphaltene precipitation zone develops around production wells as the pressure at the well declines, thus precipitating asphaltenes as the pressure drops below the upper asphaltene precipitation pressure. Also, they show the growth of deposited asphaltene near the well with time, as the plugging problem takes a few hundred days to develop after the start of production. Application of this model will help operators to better forecast the onset of asphaltene precipitation as it relates to flow rate history of the well and the pace of plugging induced by asphaltenes, and thus better plan remedial measures. (C) 2004 Elsevier B.V. All rights reserved.