The VAPEX (vapor extraction) is an oil recovery process, in which heavy oil or bitumen is mobilized by injection of a low molecular weight hydrocarbon solvent and is drained by gravity to a horizontal production well. It has attracted considerable attention because of its potential applicability to problematic reservoirs and the chance of in situ upgrading of the oil during the process.
Oil drainage rate under Vapex is controlled by the viscosity of solvent diluted oil and can be affected substantially by deasphalting. In situ de-asphalting can be advantageous since it reduces the oil viscosity and leads to production of upgraded oil. However, the precipitated asphaltenes can also plug the pores of the formation and cause severe damage to the permeability. The objective of the current work was to determine whether the beneficial effects of asphaltene precipitation will outweigh any formation damage. The effects of in situ precipitation and deposition of asphaltenes on the rate of oil drainage and the quality of the produced oil under different operating conditions were experimentally evaluated. The experiments were conducted in a physical model packed with realistic permeability sand and propane was used as the solvent. The quality of the produced oil samples was evaluated through the SARA technique and viscosity measurements.
To reduce the formation damage problem and resulting production restrictions from the deposition of precipitated asphaltenes, injection of a mixture of an asphaltene dissolving liquid solvent (toluene) and vaporized solvent (propane) was tested. Periodical cleaning of the production well with toluene injection was also evaluated. The performance of butane was compared with that of propane to see how the solvent affectsasphaltene deposition.
The experimental results show that the oil produced at higher injection pressures was substantially upgraded but the viscosity reduction by asphaltene precipitation did not lead to higher rates of production. The effect of viscosity reduction was negated by the accompanying damage to formation permeability. Injection of toluene with propane was successful in increasing the rate of production and the extent of upgrading was encouraging. The huff and puff injection of toluene into the production well, to remove damage from the near well zone, was not successful. It led to production of oil with higher asphaltene content and there was no improvement in the rate of oil drainage compared to lower pressure operation with minimal asphaltene precipitation
The heavy oil and bitumen reservoirs of Canada are one of the largest hydrocarbon resources in the world. The estimated original oil-in-place of the Canadian formations is more than 400 billion m3 which is almost twice of the total conventional oil reserves of the Middle East1. Being highly viscous and immobile in their original state, heavy oil and bitumen cannot be effectively recovered through primary and secondary recovery methods. To recover oil from the heavy oil and bitumen deposits, EOR (Enhanced Oil Recovery) methods which mostly act through viscosity reduction by means of heating or dilution, have been implemented. CSS (Cyclic Steam Stimulation), ISC (In-Situ Combustion), SAGD (Steam Assisted Gravity Drainage) and solvent injection based techniques are examples of these methods.
