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 the sea.
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.
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
The hydrocarbons are lighter than water (oil floats on the top of water) so the hydrocarbons
will try to rise up through the water. If the pores in the rock are joined together the rock is
permeable and the hydrocarbons can move through it.
The ability of a rock to let liquids move through it is called "permeability".
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.
If the porous sedimentary rock is covered by a non porous rock the hydrocarbons stop
moving when they meet the non‑porous rock.
|Anticline Oil Trap|
There are many different types of oil traps. The type depends on how the trap was formed.
Anticline traps are formed by the movement of the earth's crust.
|Types of Traps|
The sedimentary rock where the hydrocarbons have been trapped is called the "reservoir
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 rocks under
|Rotary Drilling Rig|
DRILLING FOR OIL AND GAS
The drilling rig equipment is powered by diesel electric. This means that very large diesel
engines turn electric generators to produce direct electric current.
The direct current is then used to power pumps, hoists, compressors and other equipment.
The mechanical equipment used on a rotary drilling rig is grouped into three areas.
· The hoisting equipment
· The rotating equipment
· The circulating equipment
1. THE HOISTING EQUIPMENT
The hoisting equipment lifts the drilling column in and out of the hole.
2. THE ROTATING EQUIPMENT
The rotating equipment turns the drilling column so that the drilling bit can make a hole.
3. THE CIRCULATING EQUIPMENT
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 of 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 outs and supports the
walls of the hole. A blow out is when the well flows out of control.
DOWN HOLE TUBULARS
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 through these layers and a 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.
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.
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 a packer.
The top of the production tubing is hung and sealed at the surface.
All gas and oil produced from the well flows up the inside of the production tubing.
When the production tubing gets old and worn it can be pulled out of the well and replaced.
|Casing, Tubing and Packer in a Well|
Recall that the production casing is cemented in the well through the formation that contains
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.
|Jet Perforation. Using Shape Charges|
The wellhead is the equipment at the top of the well. It is used to keep control of the well. It
forms a seal to prevent well fluids blowing or leaking at the surface.
How strong the well head is depends on how high the pressure is in the well.
The wellhead is made up of casing and tubing heads, the christmas tree and pressure
1. SURFACE CASING HEAD
This is attached to the surface casing which is the first and largest diameter casing put into
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 surface 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 the production tubing is not
cemented into the well the tubing head also supports the weight of the production tubing.
5. CHRISTMAS 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 angle, especially offshore on a platform.
Drilling at an angle 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 movement 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 a DRIVE.
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 the difference in pressure the more force there is to move the fluid.
As the oil flows out of a reservoir the pressure in that reservoir drops. There is less force
available to move the fluid.
Water is injected by pumping treated water down water wells to raise the level of the water
under the oil. Oil and water do not mix. The water lifts the oil which is queezed between the
water and the gas cap. This makes more pressure. More pressure, more flow of oil.
Companies also use gas injection to keep up the pressure. Gas produced with the oil is
removed from the crude oil, compressed, and injected down wells into the gas cap.
To make oil flow more easily and quickly to the well bore we can stimulate a well. To
stimulate the well, new or bigger channels are cut in the rock.
1. ACID STIMULATION
This is done by pumping acid down the well at high pressure. The acid dissolves the
limestone rocks making the cracks and channels in the rock larger.
|Cracks/Channels made larger by Acid|
Special chemicals are added to the acid to help it do its job better, and to protect the
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 cracks. The sand
enters 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 from the reservoir into the well bore.
MOVEMENT OF CRUDE OIL TO PROCESSING PLANTS
Oil that is produced from wells moves through a flow line, trunk line system to the production
|Flow Line/Trunk Line System|
Flow lines carry crude oil from individual wells. These flow lines can go direct to the plant or
they can go into a trunk line which goes to the plant.
|Production Plant Inlet, Manifold|
The flow from any well can be sent where we want it to go.
They are many manifolds like these in all of the plants.
OIL PRODUCTION DEGASSING PLANTS
Degassing means removing the gas from the crude oil.
This is the first step of the many steps that are needed to process the crude oil into the
things that we want.
|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 that simple.
There is a lot of equipment inside and outside the separator to help this happen.
The principle is always the same.
· light things go up heavy things go down
· heavy things go down
|Horizontal Degassing Separator|
There are many different types of separator used in the process plants.
You will learn about these later in the training program.
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
At the gas processing plants the components that make up natural gas are separated by
These processes will be covered in detail later in your training programme.
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|
The crude oil is heated to 385°C (725°F) so that it boils. The hot vapours and liquids are
then passed into a tall vertical steel cylinder called a fractionating tower.
The lightest vapours go to the top of the tower, the heaviest go to the bottom of the tower.
|Hydrocarbons in Light Crude Oil|
There are trays in the tower to collect the condensed vapour. There are pipes from the trays
so the liquids can flow out of the tower.
|Simplified Crude Fractionating Tower|
Remember that the products from the crude distillation are only the first stage of refinery
Reduced crude is further processed to get asphalt, fuel oil, diesel oil and kerosene.
Naphtha is separated into several fractions, which in turn are further processed.
Temperature and pressure are very important in the distillation process. Specific
temperatures must be maintained at each distillation point in the tower. This is so the
fractions will condense correctly.
You will learn how process temperature and pressure are controlled later in the program. You will also learn about refinery operations in much greater detail.
You can see from the following table how many different things come from crude oil.