Partial upgrading of heavy oil reserves
Partial upgrading using solvent deasphalting addresses multiple issues associated with the recovery of stranded heavy oils
Mitra Motaghi, Priyanka Saxena and Rahul Ravi
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As the world’s population aspires to a better quality of life, global energy consumption and the demand for transportation fuels can only be projected to increase for the foreseeable future. This increase in energy demand can, in turn, only be met by economically mobilising all available energy resources. As the demand for transportation fuels outpaces the supply of traditional crudes, increased production from non-traditional hydrocarbon deposits, such as Canadian tar sands, Venezuelan heavy oils and other South American stranded crudes, becomes economically attractive.
Today, heavy oils, extra-heavy oils and bitumen represent only a small portion of worldwide oil production, and viable solutions to monetise these stranded resources could likely double current proven oil reserves. The strategic importance of monetising these crudes in the face of the overall gap in global supply and demand presents interesting challenges for many block owners, while opening up unique opportunities for technology providers to offer niche upgrading solutions.
These heavy and extra-heavy oils are regarded as unconventional because of the difficulties associated with production, dewatering, transportation and processing of these valuable resources. The methods of production and the degree of upgrading are highly dependent on local infrastructure and the availability of natural gas and power. This, in turn, means that remote fields and offshore heavy oil fields are not well suited to installing traditional, capital-intensive upgrading processes that involve hydrogen addition or carbon rejection. In such cases, the economic solution may gravitate towards the use of minimal upgrading at remote fields, sufficient to enable the transport of these materials for processing in more conventional upgrading or refinery assets.
High viscosity and density make the production of heavy crude oil a difficult and energy-intensive task. Traditional recovery methods utilise steam flood, cyclic steam stimulation, steam-assisted gravity drainage (SAGD) or solvent injection. While all of these are now proven and commercially practised, the costs associated with the generation of steam and power remain an issue that negatively affects the cost of oil production. For example, in the case of SAGD, a 3–5 steam-to-oil ratio is needed to extract a barrel of bitumen from tar sands. In many facilities, this can only be accomplished by burning imported natural gas (as most of these heavy deposits have little or no associated gas), whole crude or crude-derived light distillate products.
Due to the relatively small difference in density, dewatering of these extra-heavy crudes and bitumens at the production site often involves the use of lengthy residence times, large storage capacity, addition of expensive chemicals or diluent-assisted dewatering. The associated capital investment and the cost of utilities and chemicals for the conventional dewatering process can add up to a significant 25–50¢/bbl oil.
High viscosities mean that transporting these heavy crudes remains the single largest inhibitor to the effective monetisation of these stranded assets. The only sustainable economic means of transportation is through pipelines, which requires that the material be upgraded to a transport viscosity specification. This often translates to about 250–300 cSt at pumping temperature conditions.
By necessity rather than by choice, the dilution of heavy oil with a less viscous hydrocarbon such as condensate, natural gasoline or naphtha is widely practised. While this may appear to be simple on the surface, it is clearly an expensive proposition. Figure 1 illustrates the amount of diluent needed to lower the viscosity of heavy crude deposits to make them economically transportable. To meet transport viscosity specifications, many block owners use a diluent fraction that could be as high as 30% of the transported heavy oil. In many areas, the availability of diluent itself poses a significant problem. While recycling may mitigate the issues linked to availability, it still involves substantial operating costs associated with the distillation of the solvent from the crude mix. Recycling also involves a large capital investment incurred by the dual pipeline arrangement required for the supply of diluent, the additional capacity required for moving the diluted crude, as well as the diluent recovery facilities.
The type and volume of diluent requires careful scrutiny, as incompatibility with the crude can lead to asphaltene precipitation in the pipeline, which may cause fouling-related operating problems.
Some owners use heated pipelines in order to enhance the oil’s flow properties. Implementation of heated pipelines involves complex design criteria to address expansion of the pipelines, pumping/heating stations, heat losses and so on, as well as greater corrosion rates arising from elevated temperatures. However, the biggest issue is the low availability of the asset, which seems to be the primary reason why many owners seek to abandon this approach in search of more reliable options. Loss of electricity, heating medium and pipeline flow can cause heavy crude oil to solidify, requiring expensive and time-consuming processes to clear, which may result in an extended loss of service.
Another method for reducing the viscosity of heavy oil, and thus enabling it to be transported by pipeline, is the use of thermal treatment (with or without a catalyst) in the field in moderately scaled reactors. During thermal treatment, asphaltenes will begin to precipitate and subsequently form deposits, which, if not controlled, will cause instability in the resulting oil. Moreover, in upgrading the viscosity of crude oil by cracking, the olefins and diolefins produced will tend to polymerise, making the crude unstable and unsuitable for transportation through pipelines. Therefore, most refiners are reluctant to process cracked stocks in their crude diet, which only drives down the marketability and value of the crude.
Heavy crudes can cause a threefold increase in the amount of resid coming to the refinery. Beyond the obvious strain imposed on the capacities of vacuum distillation units and delayed cokers (or other bottom-of-the-barrel processing units), they cause a severe imbalance in the refinery, often leading to under-utilisation of light end processing assets.
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