The Anatomy of a Blowout

To comprehend a catastrophic oil well blowout, we first need basic understanding of how petroleum collects in underground reservoirs and how exploration for those reservoirs works. For starters; oil, natural gas, and water collect in underground layers when their path to shallower layers is blocked by an impenetrable zone. Instead of collecting in “lakes” or “rivers” of oil, however, hydrocarbons accumulate in tiny pores within huge volumes of rock.

Being buried under miles of solid rock means that hydrocarbon reservoirs are under enormous pressure. The pressure increases, on average, by a factor of 0.433 psi/foot or 9.792 kPa/meter. This regular pressure gradient means that pressure at the bottom of a ten-thousand-foot well is more than 4300 pounds per square inch; compared to a pressure of about 30-35 psi for a car tire. Since liquids cannot be compressed, deeply-buried reservoir fluids seek any possible pressure relief.

Drilling a hole ten or fifteen inches in diameter from the surface to a deep reservoir provides just such relief. To keep oil and water from spurting out of a wellbore, drillers fill the hole with fluid of their own. Called “mud” or “drilling mud,” this fluid is carefully designed to carry out several different functions, one of which is to match the pressure in the underground layers and prevent the crude oil from rushing to the surface. Maintaining balance is relatively simple in areas of normal pressure, where pressure at depth can be predicted from the standard pressure gradient (above).

There are, however, subsurface layers in which the pressure is much higher than that predicted by the pressure gradient. Unexpected penetration of such an overpressured zone can result in a blowout, as can improper drilling practices or poor well design.

When a blowout occurs liquids in the reservoir stream into the wellbore, forcing tons of drilling mud and thousands of feet of steel pipe from the mouth of the well at the surface. The rising column of oil, water, and natural gas are under such vast pressure that they can reach supersonic speeds; more than 1100 feet per second. Crude oil and natural gas are both flammable, and are often ignited by the heat of friction in the moving column or by sparks as metal and chunks of rock smash against one another. In the early days of exploration, drill rigs often “burned to the ground” after a blowout; though such gushers were looked on favorably before scientists understood the environmental havoc wreaked by such a disaster.

A blowout is both an environmental and an economic disaster, for not only are large quantities of a valuable resource wasted, the infrastructure at the surface is destroyed. In the April, 2010, blowout in the Gulf of Mexico, the semi-submersible drillship Deepwater Horizon burned and sank at a cost of $600 million and eleven lives. Five thousand barrels of oil per day, valued at some $400,000, poured out of the breached drill pipe. Because of such costs, exploration companies take expensive measures to prevent blowouts.

The first line of defense against blowouts is the drilling mud, described above. Before drilling into potential overpressured zones, mud engineers “mud up” to increase the density of the fluid in the well. The second line of defense is casing, heavy-weight large-diameter pipe that is cemented in place to line the hole and isolate zones of different pressure. The final line of defense is a massive mechanical device called a blowout preventer or BOP.

Blowout preventers come in several designs depending on the manufacturer (leading makers include FMC, Hydrill, and Cameron). A BOP is placed at the ground surface or, for offshore work, at the seafloor; between the drill rig and the well head. BOPs are designed to trigger automatically upon detection of rapid uphole flow, or trigger remotely on command. Blowout preventers come in two types: the first is basically a giant rubber doughnut that can be activated to seal off the annulus – the space between the drill pipe and the casing. The second type consists of massive hydraulic rams that force hardened, edged surfaces inward to cut the drill string and seal the well with a thick metal wedge.

The worst-case scenario of a blowout is one in which reservoir fluids breach the cement holding the casing in place and reach the surface around the outside of the pipe – in this instance, even BOPs are of no use. There has been speculation that this is what happened at British Petroleum’s Macondo well off the Mississippi Delta (April, 2010).

If the BOPs fail and a blowout occurs, options for recovery of the well are few. One option is to collapse the wellhead with shaped charges (compare to John Wayne’s portrayal of Red Adair in the movie “Hellfighters”). A more likely scenario is to drill a relief well that intersects the blown well – a technological challenge, to be certain, but doable. The relief well is used to dump high density “kill fluids” – super-weight drilling mud – into the wellbore of the flowing well and, eventually, bring it under control. Drilling a relief well takes weeks or months, while the blowout continues to spew crude oil, and can cost tens of millions of dollars.

In spite of all the technology and wellfield expertise, blowouts still occur. The April 2010 is one of the largest ever, a list that includes the Pemex IXTOC I blowout, which poured 10,000 barrels of oil per day into the Gulf of Mexico in 1979-80; and the 1969 blowout of a Unocal well in the Santa Barbara Channel off southern California. The environmental damage caused by the Unocal blowout is responsible for California’s strict regulation of offshore drilling.


Glossary: http://www.glossary.oilfield.slb.com/search.cfm
more information: http://www.chron.com/disp/story.mpl/business/deepwaterhorizon/6973912.html

How Do Oil Companies Find Oil? Basic Petroleum Geology, Part III

The Hunt for Hydrocarbons

Last time, we learned that four things are necessary to create and capture oil. Those four are, an organic carbon-rich source that gets “cooked” deep underground; a reservoir rock layer with bazillions of tiny empty spaces to hold liquid petroleum and water; a shape to the underground rocks that will trap the fluids in a confined space; and non-reservoir (impermeable) rocks to seal the hydrocarbons inside the trap. Now that we know which puzzle pieces have to join to create an oil field, to find oil we have to look for places where all four pieces are present.

Scientists who work for oil companies will tell you, “There is no direct hydrocarbon indicator.” That’s a fancy way of saying that there is nothing we can see or measure that lets us just point to a spot and say, “Drill here!” and know that we’ll strike oil. Instead, we have to study blurry images of the rocks deep beneath the surface and use training and experience to interpret them. That’s how we hunt for places where source, reservoir, trap, and seal all come together in the right relationships.

The first people to use oil found puddles where it had leaked out on the ground’s surface, like at La Brea Tar Pits in California, USA. Oil found at the surface, however, is usually gummy and thick because it’s been exposed to air and water; so early use was often as salves and medicines (not a good idea, really, since petroleum is an organic poison). Not only that, but the amount available in these leaks, or "seeps," is small. Early entrepreneurs dug wells by hand near seeps looking for larger deposits and, when they were successful, also noticed that the oil was higher quality – it was lighter and thinner, and could be burned in a lamp, for instance. The first successful drilled oil well in North America, the Drake #1 in Pennsylvania, was located near a surface seep. The presence of a seep is the closest thing there is to a direct hydrocarbon indicator, but we still have to figure out which direction to go if we want to find the good stuff!

As we entered the age of oil, demand grew faster than wells drilled or dug near a few surface seeps could supply it. Short supply means higher prices, and this bigger payback for the work ushered in an era of surface mapping to hunt for oil. By studying the shapes and order of rocks exposed at the surface, early oil-company geologists (scientists who study the earth) could identify possible traps in the crumpled layers around basin edges. Drilling holes in geological structures – bent or broken sedimentary rocks - caused a boom in oil exploration in the early twentieth century. This method also introduced the oil-seekers to risk: even though a trap is visible, that doesn’t mean that the other three pieces of the puzzle are present. If there’s no reservoir rock, there can be no oil accumulation. If there’s no source, there is nothing to put in a reservoir. And if there is no seal, anything that does enter a trap simply leaks out. Early “wildcatters,” as oil-drillers were called, either learned how to identify which structures had the best chance of all four components being present, or they went broke drilling “dry holes.”

Where there’s money to be made, technology soon comes along to make it easier (or not as hard) to earn it. This has happened in the “oil patch” many times. The first big leap in exploration came soon after World War I began, when French scientists developed a way to record the order and thickness of rock layers encountered in a well, and well logging was born. Well log measurements were soon invented that helped scientists figure out which deeply buried rocks are sources, reservoirs, or seals. Almost a century later, well logs are still used to help geologists understand the rocks in the subsurface, though the sorts of information collected today are much more complex than the first logs.

Just before World War II, there was second forward leap in exploration. Research scientists devised a way to bounce sound waves off underground layers, and record the waves that return to the surface. By carefully measuring the time it takes for that sound to return, scientists can create a sort of “sound image” similar to the layering of the rocks underground. This method, called seismic exploration, created a new field for scientists who called themselves geophysicists. The new tool came along just in time: geologists doing surface mapping had found most of the fields visible on the ground. With seismic tools, the two groups of scientists – geologists and geophysicists – could work as a team to identify traps that aren’t visible from the surface.

Today, geologists and geophysicists work together exploring for oil fields using powerful computers and special software. Exploration geologists and geophysicists are like detectives: they spend their days following subtle clues, putting together complex puzzles for which there are never enough pieces – and when they’re done, the ultimate test of their puzzle-solving skill is an exploration well. As a famous geologist said more than fifty years ago, “All the easy oil has been found.” These days, the hunt for oil involves months and years of painstaking work by highly-trained professionals using state-of-the-art technology. Even with all that power brought to bear on the problem, only about one of every eight exploration wells drilled finds enough oil to pay the cost of the well.

And here you thought all they had to do was stick a pipe in the ground…


This is the third of a series of minilectures on the petroleum industry from the ground up

1) Where Does Oil Come From?
2) Where Do Oil Companies Find Oil?
3) How Do Oil Companies Find Oil? <== You are here.  Future installments include:
4) The Economics of Petroleum Exploration and Production
5) Refining
6) The Economics of Big Oil
7) The Future of Oil

Where Do Oil Companies Find Oil? Basic Petroleum Geology, Part II

The Essentials for an Oil Field: Source, Reservoir, Trap, and Seal

Last time, we learned how oil (petroleum) forms: it’s a slow process by which an oil source – a rock full of ancient plant and animal life - transforms deep underground, over hundreds of thousands or millions of years. The long time this takes explains why, on a human scale anyway, oil is a non-renewable resource. Geologists call this “cooking,” and even call the deeply-buried area a “kitchen” or a cooking pot. To extend that metaphor, after we cook dinner, it has to be brought to the dining table so we can eat. In the world of oil and gas, our dining table is what is called a reservoir.

Remember that one of the four things needed to turn organic matter into oil is pressure? Well, you can imagine that with thousands of feet of rock on top of it, our source layer has been subjected to a lot of pressure. At about the same temperature, pressure, and time that our oil finishes cooking; water trapped in the buried layers starts moving, pushing the newly-generated oil ahead of it. That happens because oil and water don’t mix, just like an oil and vinegar salad dressing never quite mixes. The water and oil move from the source bed into adjacent rock layers with tiny cracks, or tiny spaces between the grains. From there, all that oil and water has one purpose: to move to the surface, a movement called migration (did you know that petroleum migrates, just like birds and butterflies?). This step, like the cooking process, is very long and slow.

At this point, we need to talk about rocks for a minute. Oil-bearing rocks are almost always the kind of rocks called sedimentary, which form huge layers like some Texas-sized wedding cake. To our moving oil and water, each layer is one of two kinds of rock: it’s a reservoir or a seal. The difference is that a seal is impermeable to fluids like oil and water, meaning that it won’t allow them to move through it, forming a barrier. A reservoir is permeable, however, and fluids can move through it, either when migrating out of the kitchen or someday moving into a well. Our big slug of oil and water starts out in the deep parts of sedimentary basins, which are shaped much like what they’re called: gigantic shallow bowls. All those layers of rock are slightly tilted, following the curve of that bowl’s sides from rim to bottom. This allows our oil and water to keep moving toward the surface even when it can’t go straight up. Sometimes it makes it all the way to sunlight, forming an oil seep like the world-famous La Brea Tar Pits near Los Angeles in the USA.

Most of the time, however, our mix of oil and water runs into some sort of physical barrier. That barrier brings the migration to a stop. The permeable rock through which the oil and water have been migrating might run into a geological wall, in the form of a fault such as the San Andreas Fault (also near Los Angeles). This is quite literally a dead end, and those fluids get trapped because they can’t reverse course and go back downhill. The rock layer through which our fluids are migrating might end for other geological reasons, one of which is that the conditions for the layer’s creation or deposition did not exist everywhere. Again, the moving fluids become trapped because they can’t go backwards. A common occurrence is that all those sedimentary layers become crumpled or folded near the edges of our basin, forming sort of a three-dimensional roller coaster shape. The fluids migrate uphill into high spots, but then they can’t move down. All of these situations are common forms of traps – the third component needed for an oil field. If you think about it, though, the oil could just move sideways or across whatever is in the way – that is, it could move, unless there are layers of impermeable rocks or seals, surrounding it.

So to recap, to make petroleum you need a source of organic carbon that can be cooked in a huge geological pressure cooker. Once our dry bits of organic carbon are transformed by heat, pressure, and time into liquid petroleum; it (along with a lot of water) starts searching for a route to the surface. This is migration, when fluids move out of the source layer and into a layer that lets natural uphill movement happen. This rock layer, which is permeable to oil and water, is reservoir rock. If that reservoir rock ends or gets bent back downhill at some point, then the moving fluids are caught in a trap. They can only stay trapped, though, if there are non-reservoir rocks surrounding the trap that prevent them from moving straight up or going out the sides of the trap. These are the seals. Those are the four essential parts of an oil deposit: reservoir, source, trap and seal.

One more point to consider: sometimes people think of oilfields as underground lakes and rivers, but this is not the case. Oil is found in reservoir rocks, not in puddles and pools. A reservoir rock looks just as solid as any other rock, but under a microscope you’d see that it’s really made of tiny grains, and between those grains are even tinier empty spaces. You can make a model of a reservoir rock by dumping a handful of marbles into a water glass – there are lots of odd-shaped spaces left over because the marbles are spheres and don’t fit together like puzzle pieces. If you pour water into that marble-filled glass it can fill those void spaces, which geologists call pores. In good reservoirs, pores occupy 25% or more of the rock’s total volume; all filled with oil and water. That might not seem like much when you hold a single rock in your hand, but an oil field is a lot bigger than your hand. A good-sized oil field is miles across and the reservoir layers can be hundreds of feet thick. If you don’t think that’s a lot, let’s do the math: 25% of the volume of a cylinder one mile in diameter and 100 feet thick is still almost 180 million cubic feet! So zillions of spaces so tiny you can’t see them still add up to a whole lot of volume – and that’s where oil fields come from.

Now that we know how an oil reservoir is found, next time we’ll look at how oil companies find reservoirs.


This is the second of a series of minilectures on the petroleum industry from the ground up

1) Where does Oil Come From?
2) Where do Oil Companies Find Oil?  <== You are here. Future installments include:
3) How do Oil Companies Find Oil?
4) The Economics of Petroleum Exploration and Production
5) Refining
6) The Economics of Big Oil
7) The Future of Oil

Where Does Oil Come From? Basic Petroleum Geology, Part I

It's tough to wrap your head around the oil industry without a basic understanding of just what oil (petroleum) is and how it gets to consumers. When we see headlines like "ExxonMobil and Shell Post Record Quarterly Profits" on the same day we've paid almost $5.00 per gallon to fill our tanks, anger and disgust are natural reactions - as is suspicion that the companies making all that money are ripping us off; big time. The huge profits oil companies, large and small, make when oil prices rise into the stratosphere don't however, come (as some might believe) from manipulating prices at the pump. Those profits are, instead, the result of a series of happy accidents and calculated risks, just as are the profits from opening a body shop or buying a fast-food restaurant. The big difference between the oil companies and Wendy’s or Joe's Collision Service is that almost everyone knows where beef patties come from and how fenders get dented. Many people, on the other hand, haven't given much thought to how gasoline gets to the pump or even where it comes from in the first place. You want to know a little more? Let's begin at the beginning...
   
If you ask a third-grader, “Where does oil come from?” he'll probably say, "From dinosaurs!" Ask him again when he's a college senior, and he'll probably give the same answer. He's wrong: though they're among the biggest animals ever to walk the Earth, dinosaurs are way, way, way down the list of sources of oil. The biggest sources are at the other end of the size scale: microscopic plants and animals like algae; most of which were plankton floating in oceans and large lakes. When uncounted billions of those animals and plants died, their bodies settled to the bottom along with fine sediment, and the whole shebang ended up buried. That's a key to turning that dead organic matter into petroleum: burial. One of the reasons why dead dinosaurs - most land animals for that matter - didn't get turned into oil is that they were exposed to the air and oxidized, just like a hunk of iron left sitting behind the garage rusts away to nothing with enough time.

Now we have an underwater layer of sediment chock full of dead stuff - organic matter or complex carbohydrates (compounds of carbon, hydrogen, and oxygen). What's it gonna take to make petroleum out of that stuff? It takes four things:

First, it takes a high enough concentration of that organic matter: a "rich" layer can contain fifteen, twenty, or even fifty per cent organic carbon. As a general rule, concentrations of less than four to five per cent organic carbon are too lean to produce much oil. Just like it is with parents, richer is better.

Second, it takes pressure.

Third is heat: things don't even start until the temperature is somewhere between 120° and 190°F.

Fourth is time: lots of time.

The amounts of pressure, heat, and time necessary to "cook" organic carbon into the oil we humans crave are a complex system. Lower temperatures for long periods can generate as successfully as higher temperatures for shorter periods. Two of the three variables, though, require that the carbon-bearing layer be buried; and the deeper the better; though not too deep - if things get too hot, the oil "overcooks." Complicating matters is that different kinds of organic carbon generate different hydrocarbons at different pressure-temperature levels. An entire branch of geology (organic geochemistry) is devoted to the study of this process, which scientists call "thermal maturation."

This is the first step in the lifecycle of oil; creation of a petroleum source and its conversion from tiny bits of dry organic matter into the liquid we know as crude oil. Remember, there are four things, all of which must be present in the right amounts to generate oil: organic matter, heat, pressure, and time. And when we say time, we don't mean on the order of days, weeks, months, or even years. We're talking tens or hundreds of thousands of years; even millions of years. That last explains why fossil fuels such as petroleum are called "non-renewable" resources: the rate of replacement is so slow that, for all practical purposes, no new petroleum is being created.


Where Does Oil Come From? is the first in a series of posts on oil and the oil industry.

Stay tuned. Future installments will cover:

Where do Oil Companies Find Oil?
How do Oil Companies Find Oil?
The Economics of Petroleum Exploration and Production
Refining: It's not all Gasoline
The Economics of Big Oil
The Future of Oil