Monday, January 2, 2012

Heavy Crude and Tar-Sands

Densities and viscosities are a way to lead almost naturally into a discussion of hydrocarbons. When the Deepwater Horizon platform blew up in April, 2010, it released approximately 4,900,000 barrels (780,000 cubic meters) of crude into the Gulf of Mexico.  Some of that raw crude floated to the surface and was skimmed or burned - or ruined wetlands that protect the fragile Gulf coast from hurricane storm-surges. These hydrocarbons were the high-API-gravity volatiles that will come out of any newly-opened reservoir as part of the crude oil contained there. Some of the hydrocarbons drifted off into the Loop Current, and what was not metabolized by bacteria probably drifted out into the Atlantic Ocean and the Gulf Stream. These are the nearly-10 API gravity stuff found in most crude oil. But the low-gravity components simply blanketed vast stretches of the floor of the Gulf, suffocating all life forms beneath it. Because the temperatures are quite cold in the abyssal sea depths, bacteria will work on this stuff only very slowly - if at all. While the flow was on-going from the Macondo Well 3,000 meters below the sea surface, I thought about this problem. It's incredibly expensive to even drop a string of sampling bottles over the side to these depths. I developed an electrical geophysical method to map, track, and characterize this dangerous stuff - whose impact on the Gulf ecosystem is still not clearly understood.

Dear Sir. What are tar-sands? 
--Brandon R

I worked my way through undergraduate school by washing dishes during the winters, and by fighting forest fires during the summers. Fire-fighting is hard, dirty, and dangerous work, so after my Junior year at Berkeley I started working for an oil company near my home, where I could use my brain and education more. I initially worked in the southern San Joaquin Valley of California for Getty Oil company.

Oil had been discovered just north of Bakersfield early in the 20th Century, and the initial “gushers” were just that: when the drill-hole penetrated into a rock unit with sufficient porosity to host oil, the overlying rock pressure forced the stuff out in a violent fashion. It quite literally rained oil. The ground throughout most of Oildale, California, is an ugly and relatively uniform reddish-brown from the gushers the blew out during the early history of the field. After the gushers stopped, pumping began.

After pumping oil from the Kern Field for half a century, drill-cores showed that only about 15% of the oil had actually been extracted – the low-viscosity, easily-flowing stuff. The rest was what we call "tar-sands" or "heavy crude" - even longer-chained carbon molecules that entangle with each other to make a much higher viscosity hydrocarbon than the "light crude." Viscosity is just a measure of the "sludginess" of a semi-solid material. Maple syrup has a higher viscosity than water, so it pours slower.

Solid rock salt even has a significant viscosity - especially under pressure from overlying rock. This is why layers of salt in the deep sediments of the Gulf of Mexico tend to ooze up into diapirs or "salt domes". Oil geologists learned early on that salt stopped oil from migrating. It also folded and lifted up sediments above it to create "traps" where oil and gas could migrate upwards through porous sedimentary rocks until they were blocked by the salt (it's more complicated than that, of course). However, if you can find the salt domes with gravity or seismic geophysical surveys - salt domes are less dense than surrounding rock and thus a gravity survey above one will give a lower-gravity "bulls-eye" - then you only have to drill around the edges to get at the traps.

Back to viscosity. You can increase viscosity in salt by applying pressure and heat. You can also increase viscosity in a petroleum-based product by heating it - my grandma would use heat to liquefy wax, or to get molasses to flow out of a jar faster.

The Getty engineers thought long and hard about abandoning the old Kern Field - if 85% of the oil was still in it, this seemed like an incredible waste. It was hardly economic anymore to operate a pump for a day to get just a few barrels of oil out.  They finally developed huge steam-injection generators and conducted an interesting experiment. They used the remaining light crude from the Kern Field to heat water to 500 degrees C. The super-heated water was then injected into the old drill-pipe at ~500 psi (about 30 times normal atmospheric pressure, or about 3.4 mega-pascals, the SI metric unit for pressure) for 5 days, then the well was capped and allowed to "stew" for two days. Finally the drill-pipe was uncapped and the steam was allowed to vent for 5 more days.

Then the formerly nearly-solid oil literally poured itself out of the drill-pipe just from the pressure of the overlying rock. This was only a partial success story, however; the hot heavy crude could be poured into a bucket in liquid form; it was surprisingly light brown in color. After it cooled to room temperature, however, you could turn the bucket upside down and nothing would pour out - it had turned solid again.

You can imagine that dealing with this sort of heavy crude is more expensive, and you would be right. The oil from the Athabasca tar-sands in Canada requires a lot of effort and energy - mining expenses, local water, and heating - to extract it. The heavy crudes in Oildale, California, and in Venezuela can be extracted, but then must be mixed with light crude (in Venezuela this must be pumped down hundreds of kilometers from the Caribbean coast) into slurry that won't clog the return-pipe as it cools. heavy crudes must also be handled in a refinery in a far more complex manner.

I was a co-editor of a UNESCO book published years ago titled “The Future of Heavy Crudes and Tar-Sands.” We concluded that there was enough low-gravity hydrocarbons in Venezuela’s Llanos (plains) and Canada’s Athabasca tar-sands to power the industrial world for centuries at current rates of oil consumption – but only if the price of a barrel of oil was maintained high enough to pay for the extra costs of mining and lowering the viscosity so the oil could be refined into gasoline.

There are problems with exploiting the Athabascan tar-sands, however: there is an over-abundance of nickel and vanadium in the tar that can each be serious environmental pollutants. There is also the heavy need for local water to process the stuff - and this has seriously impacted local rivers in the area. Finally, extracting the oil is not done with drill-pipe, but by strip-mining the surface to access the tar-sands; this leaves huge scars on the arboreal landscape.

The engineering technology developed to exploit these resources sounds like a great example of human ingenuity - and it is. But there is one final, larger-scale down-side: if more and more hydrocarbons are consumed by a careless and ever-growing human population, the amount of CO2 and methane - powerful greenhouse gases - will drive our world's average temperatures ever high, ever faster.

Climate change has been going on for billions of years - see-sawing back and forth from a "snowball Earth" a billion years ago to a simmering Earth that several times saw forests in Antarctica. However, the anthropogenic (human-caused) contribution of burning hydrocarbons and destroying forests in the past two centuries has given the current climate a very, very sharp kick. From the perspective of a scientist who loves his heated home and his Honda car, this causes me very mixed feelings. Glaciers are in retreat worldwide, and vast icebergs the size of some states are breaking free of Antarctica and melting. Island nations in the Pacific and Indian oceans are already starting to disappear - literally - as ocean levels rise. The list of consequences are as horrific as they are diverse, and our very human desire for a luxurious energy-powered life, fueled by more and more hydrocarbons, lies at the bottom of it all.


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