THE ORIGINS OF CRUDE OIL AND NATURAL GAS
Millions of years ago, many plants and animals died in the sea. The remains of those plants and animals fell to the bottom of the sea. At the same time, dust and sand also fell to the bottom of sea.
This dust and sand covered the remains of the plants and animals.
This process continued for many, many years.
The weight of the thick layers of dust and sand increased with time and squeezed the remains of the plants and animals.
The pressure changed the remains of the plants and animals to hydrocarbons (oil and gas).
The pressure also changed the dust and sand to rock.
Solids that fall to the bottom of a liquid are called sediments, so the rock that formed from the dust and sand is called "sedimentary rock".
Because sedimentary rock was formed under the sea, all the gaps and holes in the rock are filled with water.
These gaps and holes in the sedimentary rock are called "pores" so sedimentary rock is "porous" rock. Porosity means how many gaps and holes the rock contains.
There is sometimes a mixture of salt water and hydrocarbons in the pores of the sedimentary rock.
The hydrocarbons are lighter than water (oil floats on the top of water), so the hydrocarbons will try rise up through the water. If the pores in the rock are joined together, the rock is permeable and hydrocarbons can move through it.
The ability of a rock to let liquids move through, it is called "permeability".
A rock that is porous and permeable will let liquids through it.
Hydrocarbons in sedimentary rock move upwards through the rock until they are above the water. They can rise all the way to the surface where they can be lost into the air.
The hydrocarbons formed in the sedimentary rock can not rise all the way to the surface if there is something to stop them.
They are "trapped".
If the porous sedimentary rock is covered by a non porous rock, the hydrocarbons stop moving when they meet the non-porous rock.
This is because non porous rock is "impermeable", which means that fluids cannot pass through it. Because the impermeable rock is on the top of the trap, it is called a "cap rock".
|Anticline Oil Trap|
Anticline traps are formed by the movements of the earth's crust.
The other types of traps are fault traps and stratigraphic traps.
|Types of Traps|
The trap it self is called a "formation". The trap is formed by the shape of different rocks.
SEARCHING FOR FORMATIONS THAT MIGHT CONTAIN HYDROCARBONS
We cannot see under the ground. We cannot know if there are oil traps 4 to 5 kilometers below the surface of the earth.
We have to use scientific methods in our search for oil and gas formations.
The main method used is the "seismic survey".
In simple terms, vibrations are made at the surface of the earth. This done with a number of explosions or a heavy vibrator on a truck.
The vibrations travel down through the layers of rock under the ground. Some of the vibrations bounce off the layers of rock and others continue through the layers of rock. We are able to measure the speed and strength of the vibrations as they return to the surface.
Computers take all these measurements and draw a picture of the shape of the rock under the ground.
When we know the shape of the rocks, we can see if there are many formations that might be oil traps.
|Rotary Drilling Rig|
DRILLING FOR OIL AND GAS
The drilling rig equipments is powered by diesel electric. Thia means that very large diesel engines turn electric generators to produce direct electric current.
The direct current is used to power pumps, hoists, compressors and other equipment.
The mechanical equipment used on a rotary drilling rig is grouped into three areas:
1. The hoisting equipment - lift the drilling column in and out of the hole.
2. The rotating equipment - turns the drilling column, so that the drilling bit can make a hole.
3. The circulating equipment - pumps liquid down the hollow drilling column. It comes back up to the surface through the space between the drilling column and the walls oh the hole.
The main job of this liquid is to remove the small pieces of broken rock made by the drill bit. It also lubricates and cools the drill bit and drill pipe. It prevents blow out and supports the walls of the hole. A blow out is when the well flows out of control.
DOWN HOLE TABULAR
Different sizes of steel pipe are installed in the well as it is drilled deeper. The hole is wide at the surface 75 cm, and narrow when it reaches the depth were the oil is 24 cm or 18 cm.
1. Surface Casing
The earth has soft sandy and sticky layers near to the surface. There is also good fresh water near the surface. A large size hole is drilled trough these layers and large steel pipe is inserted.
This steel pipe is called "casing". Because it is at the surface of the well, it is called "surface casing".
This surface casing is cemented in place. It seals off the water zones and supports the walls of the hole.
2. Intermediate Casing
As the well is drilled deeper and deeper into the ground, problems can occur. These problems could make it difficult to continue to drill.
A second string of casing is then put into the well and cemented in place.
This casing string is called "intermediate casing" (intermediate means between two points).
3. Production Casing
The well is drilled down through the formation that contains the oil. Then the final casing string is put into the well and cemented in place.
This string of casing is called "production casing".
The production casing goes all the way to the surface.
If the production casing does not go all the way to the surface, which can happen due to high cost, it is called "a production liner".
4. Production Tubing
This the final string of steel pipe put into the well.
It is NOT cemented in place.
The bottom of the production tubing is held in place and sealed inside the production casing by a device called "packer".
The top of the production tubing is hung and sealed at the surface.
All gas and oil produced from the well flows up inside the production tubing.
When the production tubing gets old and worn it can be pulled out of the well and replaced.
That is why it is not cemented in place.
|Casing, Tubing and Packer in a Well|
There is no way for the oil or gas to enter the well through the sides of the bore (The bore is the hole made by the drill).
Holes have to be made through the production casing and through the cement into the oil bearing rocks.
This is done by using special explosive charges. The process is called "perforating" and the holes are called "perforations".
|Jet Perforation. Using Shape Charges|
The wellhead is the equipment at the top of the well. Is is used to keep control of the well. It forms a seal to prevent well fluids blowing ar leaking at the surface.
How strong the wellhead is depends on how high pressure in the well.
The wellhead is made up of casing and tubing heads, the christmas tree and pressure gauges.
1. Surface Casing Head
This is attached to the surface casing which is the first and largest diameter casing put into the well.
2. Intermediate Casing Head
This is attached to the casing that was run into the well to overcome problems when drilling. This casing covers formations that may cause blow outs or fall in and fill the hole.
This casing head is put on top of the intermediate casing head.
3. Production Casing Head
This is attached to the production casing that goes all the way to the bottom of the well. This casing head is put on top of the intermediate casing head.
4. Production Tubing Head
The tubing head is attached to the production tubing. Because th eproduction tubing is not cemented into the well, the tubing head also supports the weight of the production tubing.
5. Chistmas Tree
This is the collection of valves that goes on the top of the well head. The well is controlled at the surface and serviced by using these valves.
|Single Zone Wellhead and Christmas Tree|
|Wellhead and Christmas Tree|
Normally wells are drilled straight down vertically. Sometimes it is necessary or better to drill at an angel, especially offshore on a platform. Drilling at an angel is called "directional drilling". Some offshore reservoirs need many wells. With directional drilling more than one well can be drilled from offshore platforms and towers.
This is a newer technique which Petroleum Companies use a lot. Some of the oil producing formations are not very thick and the rocks have low permeability. It is difficult for the crude oil to flow into the well bore. By drilling horizontally through the oil bearing formation much more rock is in contact with the well bore. Then it is easier for the oil to flow into the well bore. Oil production is higher from the well and so there is more money for company.
CRUDE OIL RECOVERY
The word "recovery" refers to the movements of crude oil from an underground reservoir, up a well, to the surface of the earth.
Oil recovery depends on the principle that:
"A FLUID WILL MOVE FROM A PLACE WHERE PRESSURE IS HIGHER TO A PLACE WHERE PRESSURE IS LOWER"
In oil production, oil moves from the reservoir, where the pressure is relatively high, up the well to the surface of the earth, where the pressure is relatively low.
PRIMARY RECOVERY uses pressures that exist naturally in a reservoir to push oil up to the surface. The source of this natural pressure is called "DRIVE".
Some of the oil fields have a combination drive, with a gas cap above the oil and water below it. Both the gas cap and the water push the oil into the well and up to the surface.
Cap and |
RESERVOIR PRESSURE MAINTENANCE
Recall that a fluid will move from a place where pressure is higher to a place where pressure is lower. The bigger difference pressure more force is to move the liquid. One method used to overcome this problem is to inject water into the water zone underneath the oil.
To make oil flow more easily and quickly to the well bore, we can stimulate a well. To stimulate the well, new or bigger cgannels are cut in the rock.
1. Acid Stimulation
This is done by pumping acid down the well at high pressure. The acid dissolve the limestone rocks, making the cracks and channels in the rock larger.
|Cracks/Channels made larger by Acid|
2. Hydraulic Fracturing
Fluids containing sand are pumped down the well very fast and at very high pressure. The force pushing on the rock is very strong. The rock splits, opening up large crack. The sand enter the cracks in the rock. The sand grains keep the cracks open when the pumping is stopped. The cracks make it easier for the oil to flow the reservoir into the well bore.
MOVEMENT OF CRUDE OIL TO PROCESSING PLANTS
Oil that is produced from wells moves through a flowline trunkline system to the production degassing plants.
|Flow Line/Trunk Line System|
|Production Plant Inlet, Manifold|
OIL PRODUCTION DEGASSING PLANTS
Degassing means removing the gas from the crude oil. This is the first step of many steps that are needed to process the crude oil into the things that we want. The degassing of the crude oil takes place in separatord in the oil field.
|Crude Oil and Natural Gas Flow|
1. Degassing Separator
The degassing separator removes the gas that comes up with the crude oil. The separator is a large diameter cylinder closed at the ends. The oil and gas mixture enters this large cylinder. The speed or velocity of flow slows down and the pressure is reduced. The reduction in pressure causes the gas to escape (come out) from the crude oil. (It is just like when you remove the cap on a bottle of Pepsi. The gas bubbles appear from nowhere and float to the top because you have reduced the pressure). Because the flow has slowed down, the gas has enough time to escape from the oil. The gas then goes out of the top of the separator and the oil goes out of the bottom. It is simple. There is a lot of equipment inside and outside to help this happen. The principle is always the same:
- light things go up.
- heavy things go down.
|Horizontal Degassing Separator|
2. Gas Processing
The natural gas that was produced with the crude oil is called "associated gas". Once the gas is separated from the crude oil, it has any water vapour removed from it. The gas is then compressed by large compressors and sent through pipe lines to the gas processing plants. At the gas processing plants, the components that make up natural gas are separated by distillation. These processes will be covered in detail later in your training progamme.
REFINING OF CRUDE OIL
The basic job of a refinery is to change crude oil into useful products. The crude oil must pass through a series of processes. One after the other. These processes change the crude oil into the finished products used in the home and industry. The first and most important process is fractional distillation.
|Path of Crude Oil|
|Hydrocarbons in Light Crude Oil|
|Simplified Crude Fractionating Tower|