Asphaltene Precipitation and Its Effects on the Vapour Extraction (VAPEX) Heavy Oil Recovery Process

Abstract

Abstract Asphaltene precipitation is one of the most important physical phenomena during the solvent vapour extraction (VAPEX) heavy oil recovery process. After the asphaltene precipitation occurs, the produced heavy oil is in-situ deasphalted and thus has a lower viscosity and better quality. On the other hand, precipitated asphaltenes may plug some small pores of the reservoir formation and thus reduce its permeability. In this paper, a series of the VAPEX tests is conducted by using a rectangular visual sand-packed high-pressure physical model to study the detailed effects of solvent type, operating pressure, and sand-pack permeability on the asphaltene precipitation and subsequent deposition, which strongly affect heavy oil production and quality. It is found that when the operating pressure is close to the vapour pressure of pure propane or the dew-point pressure of a butane mixture, the occurrence and extent of asphaltene precipitation and deposition strongly depend on the sand-pack permeability. At a considerably high permeability of several hundred Darcies, asphaltene deposition occurs near the injector and solvent-diluted heavy oil drains quickly, which lead to a significant heavy oil viscosity reduction and a high oil production rate, respectively. When the sand-pack permeability is low and close to a typical heavy oil reservoir permeability, however, the residence time of the solvent-diluted heavy oil inside the physical model is long due to its low drainage velocity. A sufficiently high solvent concentration in the heavy oil causes severe asphaltene precipitation in this case. A large number of the precipitated asphaltenes are blocked and deposited at the pore throats so that the porous medium is plugged to some extent.