What's new in production: Lighten up

LEONARD KALFAYAN, CONTRIBUTING EDITOR 

“Lighten up” is not something anyone likes to hear; it just doesn’t sound good. However, in our industry, what would really sound good is if heavy oil could “lighten up,” or be upgraded in situ (in the formation) to light oil. Upgrading heavy oil to light oil in situ has been a long-standing desire in the industry, and many efforts have been made to this end. 

Industry estimates vary regarding the percentages of heavy and extra-heavy oil reserves and light oil reserves. They depend on how reserves are defined (proven, producible, etc.) and the cutoff API gravity of heavy oil and light oil, which is typically 20 API to 22 API. In any case, more extreme estimates suggest that heavy and extra-heavy oil (e.g., bitumen) represent over 70% of global oil reserves when broadly defined, and about 70% of heavy oil and over 80% of bitumen resources are in the Western Hemisphere (Venezuela, Canada). The Eastern Hemisphere has much more conventional light oil. However, it can be agreed that there is a lot of heavy oil. 

Heavy oil is more difficult and more costly to produce than lighter oils, as they cause more downhole problems—such as asphaltene deposition—they are more costly to refine or they just cannot be economically produced. Nevertheless, the depletion of light crude oil reserves is largely the reason that has driven the petroleum industry to focus on exploiting heavy crude oils, despite the significant challenges in recovery and processing. 

When I was a young engineer with the Union Oil Company of California, I was working on cyclic steam stimulation of heavy oil wells in California, in a field that produced 15 API to 18 API gravity crude. I remember very well when a senior technical consultant to the president of the research division came unexpectedly into my office. He asked me to come up with a method to increase the temperature of the steam we were injecting in that field to 900°F, so that the oil in place could be thermally cracked to higher gravity light oil. He was disappointed when I told him that was not possible. I was then told that I needed to think out of the box. Ultimately, he did accept my explanation that we could only generate and inject steam plus steam liquid at about 500°F and 700 psi for wells in that field. I won’t go into it here. However, he piqued my interest in heavy oil production, including how one might upgrade heavy oil in situ.  

I enjoyed working on steam stimulation methods for some years, including chemical enhancement of the steam and liquid phases and combining oil viscosity reduction with formation stimulation. There were exciting successes, long since forgotten, only to be found in expired patents and technical literature that no one would think of referencing. In any case, methods like successful cyclic steam injection (huff and puff) or steam flooding—as well as solvent treatment methods and others—do not truly upgrade heavy oil in situ. There may be some degree of upgrading of the oil contacted, but these methods primarily reduce viscosity of the oil only, to enhance lifting of the crude. 

Can nanoparticles help? I realize there may be some weariness about reading yet another article about nanoparticles (NPs). However, the idea of applying such tiny materials for in situ upgrading of heavy oil does have merit, and it should not be dismissed, especially in combination with thermal (steam) stimulation. NPs can be effective in heavy oil upgrading because of their very high surface area-to-volume ratio, which provides abundant active sites for chemical reactions. Also, NPs have tunable surface chemistry, enabling customization for specific interactions. NPs also have the ability to form stable dispersions in both oil and water, ensuring that they reach and interact with the oil effectively and as desired in the reservoir. 

NPs can induce catalytic cracking and hydrogenation of heavy crude oils. NPs such as nickel (Ni), iron (Fe), cobalt (Co), copper (Cu) metals or their oxides act as catalysts to break bonds in complex heavy oil components, such as asphaltenes and resins. Cleaving of carbon-sulfur (C–S), carbon-carbon (C–C) and carbon-carbon double (C=C) bonds are examples. The result of these cracking reactions is conversion of heavy oil components to lighter hydrocarbon components along with the generation of gases, including hydrogen, which is important in that hydrogen saturates unstable olefins and diolefins that can otherwise cause oil instability or emulsions during production and transport. 

Combining steam stimulation and NP technology. In a steam-based recovery processes (typically 350°F to 650°F), NPs lower the activation energy barriers for key thermal reactions. This effect accelerates the breakdown of heavy oil molecules, substantially reducing oil viscosity, and it improves oil quality (increases API gravity). NPs can also increase the efficiency of steam injection by enhancing chemical upgrading, along with the physical heating of the crude oil. Not all of the oil in place needs to be upgraded; the oil that is upgraded, with its lower viscosity and higher API gravity, serves to dilute heavy oil that has not been upgraded or upgraded to the same degree, thereby improving its mobility as well. 

Certain NPs also have surfaces that strongly adsorb asphaltenes, which can prevent their deposition—a common issue in wells producing heavy crude oils. NPs can also favorably alter the wettability of rock surfaces, shifting the formation pore surfaces from oil-wet to more water-wet. Oil is displaced and produced more readily if the formation is in a water-wet state. 

Of course, it’s not so simple. Not surprisingly, performance is highly reservoir specific. Sandstones and carbonates behave differently, and reservoir temperature, pressure, mineralogy and heterogeneity are all important factors. Logically, though, NPs work best or most synergistically in combination with steam and solvents, as pretreatments to injection or in conjunction with injection, but they do not function quite as well in standalone applications.  

There is much more that can be said about NPs, their heavy oil upgrading applications and their potential. Interested readers are encouraged to explore the literature further, including pilot applications by Ecopetrol in Colombia, NanosTech (Canada) and research and pilot applications by the University of Calgary, among others. Methods for enhancing production of heavy crude oils must be successfully developed, given the necessity to tap into and cost-effectively produce more of this massive amount of such challenging, heavy hydrocarbon reserves.