Over the past 150 years, oil companies and prospectors have drilled more than two million wells around the world in hopes of striking oil. Many of the early wells turned out to be dry. We use sophisticated technologies and tried-and-true techniques to find oil and gas under the earth’s surface.
More recently, scientific techniques and new technologies have greatly improved the odds.
Before we put drill to soil or the seabed, we use typographical maps, aerial photography, sound waves, 3D projections and other tools to help us form an educated guess about the size, shape and consistency of the oil or natural gas that lies underneath.
Why are oil and natural gas so difficult to locate? The best way to answer this is to look at how oil and gas came into being in the first place.
How oil and gas form
If you’ve ridden in a car or bus, fossils got you where you needed to go. If you’ve used a gas stove, fossils cooked your food. The petroleum oil that becomes gasoline and many other useful products wouldn’t exist without tiny plants, algae and bacteria, which settled to the bottom of the sea as they died millions of years ago.
There’s no oxygen under the earth’s crust, so the organic matter in the sediment changed into a substance scientists call kerogen. And when the temperatures rose to 110˚ Celsius or higher the kerogen gradually changed into oil. Under hotter conditions it changed into natural gas. The process takes at least a million years.
Crude oil is a complex mixture of hydrocarbons. In other words, it is made up of hydrogen, carbon and traces of other substances. Its texture varies, but it is generally liquid. Natural gas is mainly made up of the chemical compound methane. It is gaseous, or lighter than air.
If the story ended there, oil and gas might never have become the global energy sources they are today. The deposits would be so scattered that we would have almost no chance of extracting them in usable amounts. Even after oil has formed in the rock, pressure continues to rise, squeezing the oil out or upwards through rocks that have more pores, or spaces, within them.
All oil moves like this. Some of it eventually reaches the surface and seeps out naturally into land or water, but most of it eventually comes up against a layer of rock that it can’t move through. This impermeable rock forms a seal or trap, and slowly, very slowly, the oil builds up. As it does, it forms a reservoir.
Reservoirs are rock formations that hold oil, natural gas or both within their pores, like a fossilised sponge. Reservoirs can be massive. Some may be as large as London, Hong Kong or New York.
If only finding them was as easy as mapping a city. Rocks also move over millions of years, as tectonic plates shift. Their formations can be extremely complex. This makes oil and gas reservoirs extremely difficult to find.
The science of searching
There may be no more unexplored frontiers on earth, but deep inside the earth there is plenty that we don’t yet know.
Oil companies have a range of technologies to help them locate oil and gas reservoirs deep beneath land and sea. But the search remains a complex business. Success is never certain.
Improving the odds
In the early days of oil exploration, oil companies and prospectors really had no idea what they were looking for. They focused their search on areas near seepages, where oil bubbled up naturally in pools. Then they sunk a drill and hoped for the best.
The rate of success has improved greatly since those early discoveries, from 10% or less to more like 50%. New technologies developed by BP and others may improve the odds even further.
Drilling is still the only sure way to find out whether there’s oil or gas down there. But drilling is expensive. Each project can cost tens of millions of dollars or more. So before we drill, we do as much planning as possible. And that can take years.
The geologist’s eye
We start with what we can see. Both geologists and geophysicists provide crucial insights at this stage in the exploration process. Geologists look at what rocks are made of and the formations they make in the earth. Geophysicists use physical characteristics, such as magnetic and gravitational properties, to guess the type and shape of subsurface rocks.
Aerial photography from aircraft and satellites can be revealing. The same tectonic shifts that formed mountains and other topographical features above the earth’s surface also shaped the rock formations down below. To the trained eye, these photographs can say a lot about what lies beneath the soil.
Aircraft can measure the gravitational pull over an area. Even small gravitational differences can reveal large clues about the density of underlying rocks.
But the most powerful tool available to us is the acoustic survey. Geophysicists use air guns to fire acoustic pulses down through the rock. The sound waves bounce back like echoes, revealing different layers and depths.
This data gives our experts the information they need to map reservoirs and identify whether they’re filled with oil, gas or merely water.
Needless to say, this sort of seismic study is a lot less damaging to the environment than random drilling. But there are still environmental risks to consider. Before we begin a seismic study, we speak with marine biologists where appropriate, and look thoroughly at any environmental issues. For example, work might be delayed so that it doesn’t interfere with animals’ feeding times, breeding periods or migrating seasons.
Visualizing success in four dimensions
Anyone who has seen a 3D film on a large-format screen can imagine what the next stage in our exploration process is like.
In a special room called a highly immersive visual environment, or HIVE, our geologists, geophysicists, computer scientists, drilling engineers and others come together to view the seismic data – in four dimensions. (The fourth dimension is time.)
On a large, curved screen three projectors display sophisticated renderings of the reservoir’s oil and gas deposits, the surrounding rock structures and the earth or sea above it. Viewed through 3D glasses, these projections help us to more accurately plan our next step: drilling into soil and rock to find out for certain whether these visualisations were correct.
Drilling for oil and gas
When all the experts have been consulted, the risks have been assessed, the environmental studies have been carried out and the data has been compiled into workable maps of the exploration site, it’s time to send in the drilling crew.
Before any drilling begins on land we may have to build access roads, construct a temporary power station or install wells for the water supply. In fragile habitats or very remote places helicopters or barges may be the only responsible way to get equipment and supplies into place.
Drilling for oil in the winds, currents and choppy waters of the open seas is even more challenging. The most difficult part is getting a drilling rig to stay in position despite the currents and waves.
Three types of rigs
In shallower water of up to 100 metres, we use what is called a jack-up unit. It starts out as a barge, which is towed into place. Legs extend to the sea floor and then the barge lifts out of the water, becoming a stable drilling platform.
In rougher seas or water up to 300 metres deep we use some of the water itself as a counterweight. A semi-submersible drilling rig is a platform attached to submerged pontoons. When the pontoons are flooded with water they lower into the ocean, meaning waves don’t affect the platform nearly as much.
In the most extreme deepwater situations we bring in the most heavyweight drilling option available, the drill ship. Drill ships are held in place by large anchors or by dynamic positioning systems, which use computer-controlled propellers to help the ship stay in place.
Studying the mud
Even at this stage, with crews and heavy machinery in place, there is a possibility that nothing will be found. Or the oil and gas discovered could be of such small quantities that extracting it would not be worthwhile.
As the diamond or tungsten drill bit goes into the hard rock, a substance called ‘mud’ is pumped down through the pipe. This mud isn’t really mud. It’s a fluid consisting of water, clay, additives and thickeners. It both cools the drill bit, which can get really hot, and flushes out the shards of cut rock from the reservoir.
As the mud comes back up through the outer part of the pipe, we get the first hard evidence showing whether we were right about the resources at the site. Geologists monitor the cuttings to check whether they’re coming out in the sequences they expected, while records of the mud and rock fragments are kept for further study later on.
Once the exploratory well has been drilled, various instruments are lowered into it so we can learn more about the reservoir. This way we measure the natural radioactivity and electrical resistance of the rocks, as well as the pressure and temperature of the fluids or gases. We are involved in exploration and production projects in more than 25 countries
Because crude oil and natural gas are hot and highly pressurized, we have to take great care to control pressure during the drilling process.
However, it’s not quite as dramatic a job as some old Hollywood movies suggest, with struck oil spewing violently out into the sky. As a precaution, modern wells are fitted with emergency valves to prevent this kind of blowout.
Everyone involved in a drilling project undergoes rigorous safety training. Risks are assessed at every step. Increasingly we plan exploration projects remotely, using data instead of site visits, which means fewer employees and contractors are exposed to potential dangers on the actual rig.