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. Introduction During the vapour extraction (VAPEX) process, solvent contacts the heavy oil at the so-called transition zone, which is formed between the solvent vapour chamber and the untouched heavy oil zone. If the solvent concentration in the heavy oil becomes high enough before the solvent-diluted heavy oil reaches the production well, asphaltene precipitation occurs in the reservoir and the heavy oil is in-situ deasphalted 1,2. Thus the quality of such deasphalted heavy oil is much better than that of the original heavy crude oil. On the other hand, the precipitated asphaltenes may be deposited onto the sand grains so that the heavy oil reservoir permeability is significantly reduced. It is well known that the heavy oil production rate strongly depends on the viscosity of the solvent-diluted heavy oil and the reservoir permeability 3,4. Therefore, it is important to evaluate the beneficial and detrimental effects of in-situ deasphalting on the enhanced heavy oil recovery. Asphaltene precipitation in the VAPEX process has long been observed and studied in the laboratory tests. It was speculated by Butler and Mokrys 5 that asphaltene precipitation might occur if a light hydrocarbon solvent is used to extract heavy oil at a high operating pressure. The influence of asphaltene precipitation and possible deposition on the heavy oil production rate in the VAPEX process has been studied. The major concern is whether the precipitated asphaltenes may flow along with the produced heavy oil or cause partial reservoir plugging that results in low heavy oil production rate.