Explaining the steps in the deep sea drilling process.
The Basics of Deep Sea Drilling
If you have not viewed our interactive illustration of deep sea drilling, please check that out at www.deepseafacts.com
Below are more detailed explanations for each step of the deep sea drilling process.
Permission to Explore
After examining existing seismic testing data, companies must bid in New Zealand’s block offer process to gain permits to explore. To win a permit an explorer must be deemed competent by the Government. More information on the Block Offer process can be found here.
How Oil Companies Decide Where to Drill Offshore Oil and Gas Wells
When it comes to deciding where to drill an offshore oil or gas well, oil companies look at a variety of information and then decide where the best spot might be to begin drilling.
First, an oil company will hire a seismic survey vessel to scan underneath the sea floor to look at the various rock and sediment formations in the ground.
Scientists on the survey boat send sound waves from equipment towed several miles behind the boat. The sound waves reflect off the various layers of ground underneath the sea bed and echo back to the survey boat.
High tech computers on board the survey boat are able to turn these reflected sound waves into an image showing the various features of the earth directly below.
Types of Rigs Used
Once more seismic data is captured; a work programme in place, companies must find an drilling rig that is capable of drilling the well. As the search for oil and gas has expanded into deeper water, drilling rigs have become larger and more sophisticated, and the people operating them are more highly skilled.
Most major oil companies do not own their own drilling rigs. Instead, they outsource the job of actually drilling the oil well to a drilling contractor who has the people, equipment, and expertise to drill oil wells in the safest, most efficient, and environmentally friendly way possible.
There are several different types of drilling rigs. But the two major types involved in deep-water offshore drilling are:
Semi-submersibles are what most people think of when they hear the term ‘oil’ rig.
So why is this type of drilling rig called a semi-submersible? Once these units are over the proposed well drilling location, they flood their huge ballast tanks with seawater and partially submerge below the surface of the water. This provides a very stable platform to drill from.
What you typically do not see is that most semi-submersibles are built with two huge pontoons on the bottom. These pontoons are what provides the buoyancy to keep the unit floating. Since the majority of a semi-submersibles buoyancy is located well beneath the surface of the water, semi-submersibles are very stable.
Semi-submersibles can maintain their position over a proposed oil or gas well two separate ways.
The traditional method of keeping a semi-submersible on location is through the use of anchors. Up two twelve anchors are run out away from the unit and set on the ocean floor. The tension in the anchor chain or cable is increased by use of a large winch and once the drilling rig is positioned over the well, only a few minor adjustments need to be made to keep the unit on location.
Semi-submersibles were typically limited to drilling in water depths less than 600 metres, but today’s advanced semi-submersibles can be anchored in water depths over 1500 metres.
The second method of keeping a semi-submersible on location is through what’s called a dynamic positioning system or “DP” system.
With a DP system, the semi-submersible uses position information from Global Positioning Systems (GPS) and beacons on the ocean floor to constantly monitor its position. The DP computer uses thrusters to hold the rig over the well.
A modern dynamically positioned drilling rig stays within 2 metres of a wellhead more than 2kms below the surface of the water.
Over the last 15 years, modern drillships have been built to meet the growing demand for highly capable ultra-deepwater drilling rigs.
Built on traditional ship bodies, these drilling rigs are massive in size and can stretch nearly 3 football fields in length.
Drillships have a much larger storage capacity and better global mobility than a semi- submersible rig.
Today’s generation of deepwater drillships use the dynamic positioning system for maintaining their position.
Drillships are capable of working for extended periods without the need for constant resupply.
Another benefit of a drillship is their speed and manoeuvrability. If a major storm approaches, drillships can secure their operation and sail out of harms way.
The First Step: “Spudding”
The first step in actually drilling a well is called “spudding”. When drilling wells offshore, this involves forcing up to 100m and up to 36″ diameter metal tubing called “conductor” into the ground. This 36″ diameter casing will form the backbone of the well and provide the support we’ll need for the remaining phases in our well construction plan.
The 36″ casing is lowered to the seafloor by “drill pipe”. Each section of drill pipe is anywhere from 30-45 feet long and about 6″ in diameter. The drill pipe is connected end to end and gradually lowered down into the well and back up to the surface as needed.
To help speed up the time connecting hundreds of sections of drill pipe together every time you want to lower a drill bit, casing, or other piece of equipment into the well, the drill pipe is stored in the derrick three or four sections at a time.
Each section of drill pipe is called a “joint“, and when two or more “joints” connected and “racked back” in the derrick they are then called a “stand“.
Joint: One section of drill pipe.
Stand: Two or more sections of drill pipe “racked back” in the derrick.
Racking Back: The process of storing one or more ”stands” of drill pipe in the derrick.
As the casing gets deeper and deeper into the ground, some rigs use large “hammers” to push the casing to the desired depth.
When the 36″ casing is set to the correct depth, we unlatch the drill pipe from it and pull the drill pipe back to the surface so that we can get the next section of casing and proceed to step 2 of our well drilling program.
Second Step: Drilling a Hole for Second String of Casing
The next step in our well drilling plan is to lower a drill bit down inside the 36″ casing we just set into the seafloor. Once the drill bit enters inside the 36″ casing we’ll drill 300m to 1000m into the ground.
The drill bit is connected to the drill pipe which runs all the way back to the surface of the water to the drill ship.
As the bit is rotated in the well bore (the hole that is cut into the ground), high pressure drilling fluids called “mud” is pumped down the centre of the drill pipe and out through nozzles in the drill bit.
As the drill bit cuts away at the rock formations, the drilling fluid then carries the chipped rock pieces out of the hole to prevent them from building up on the bottom of the well.
Drilling fluid, or “mud” as it is referred to on a drilling rig, has several important functions besides clearing out rock bits from around the bit.
First, it keeps the bit cool as it turns through layer after layer of hard rock formations. Second, it keeps the bit and the drill pipe lubricated to help keep it from getting stuck in the hole. Perhaps most importantly, the “mud” helps prevent the well from caving in or “taking a kick”. Mud is the primary barrier for keeping hydrocarbons in the well, the weight of the mud is adjusted to ensure that it is heavy enough to top hydrocarbons coming up the well from hydrocarbon bearing zones.
After you’ve drilled deep enough to “run” your 22″ casing, you bring the drill bit and all the drill pipe back to the surface and then begin lowering the 22″ casing back down to the seafloor. Using an underwater Remotely Operated Vehicle (ROV), we are able to line the 22″ casing up so that it will pass through the 36″ casing and into the freshly drilled hole.
This second section of casing we are installing (the 22″ casing) is usually between 300m to 1000m in length.
The Third Step: Cementing the First Two Casing Sections in Place
Once the 22″ casing is set inside the 36″ casing, the two different sized casings are cemented in place. This is accomplished by pumping cement down the drill pipe and out through a special nozzle on the end of the pipe.
This is a critical step in the operation. The cement must be mixed precisely; the nozzle is lowered to the correct position inside the casing. Once the cement has been pumped down the drill pipe and back up around the sides of the casing (filling in the space between the casing and the drilled well hole it takes anywhere from 4-12 hours for it to harden or “set”. The cement helps “glue” the casing to the rock and provides a vertical barrier on the outside of the casing.
The Fourth Step: Connecting the Blowout Preventer (BOP) and Marine Riser On Top of the Well Head
Formations in the ground are under pressure.
To control these formation pressures, a large underwater control valve called a Blowout Preventer (BOP) is placed on top of the wellhead. This is why it’s very important to have run the first two sections of casing and properly cemented them in place.
The BOP sits directly on top of the wellhead on the ocean floor. Mud is the primary barrier to keep hydrocarbons from coming to the surface but in the event of a “kick”, giant valves inside the BOP can seal off the well keeping any oil, gas, and any excessive pressure contained within the well.
Ensuring the BOPs are properly maintained and tested is one of the highest priorities of both the drilling contractor and the oil company involved in drilling the well.
The BOP is rigidly connected to the drilling rig by way of marine riser. Drill pipe can be lowered down through the marine riser, through the BOP, into the wellhead, and then down into the well to drill deeper into the ground. As the drilling fluid or mud is pumped through the drill pipe and out through the drill bit, it circulates all the way around up through the marine riser back to the oil rig.
As mentioned earlier drilling fluid helps clear the rock bits or “cuttings” that are constantly being chipped away as the drill bit drills deeper into the ground. Marine riser allows these “cuttings” to be brought back up to the oil rig to be collected and disposed of.
Once the mud returns to the drilling rig, the cuttings are filtered out and the mud is reused.
The Fifth Step: Drilling the Remaining Sections of the Well
The remaining sections of the well are drilled the same way as the 22″ casing was drilled in the earlier step only now the cuttings and mud are circulated back up to the drillship to be processed.
The drill crews will drill deeper into the ground so that the next section of casing can be run and cemented into place. There is no set limit on how long each section of casing will be, this decision is made by the drilling engineers that have closely analysed the well data and data from nearby or “offset” wells.
As the drill bit continues to make its way towards the target the drilling crew closely monitors the mud being circulated up the well for traces of hydrocarbons and other attributes as well as the pressure of the formation.
Determining if a Well has a Producible Amount of Oil or Gas
Once the geologists are happy with the depth of the well, a series of test called “logs” will be conducted to determine how much oil and gas (if any) is present in the formation. The process is called logging because the information is “logged” into a database as it is collected.
This is accomplished by sending high tech measurement devices into the hole that can detect various features of the formation.
The potential production rate of the well might also be tested by bringing hydrocarbons to the surface to flow test the well.
If the explorer has been fortunate to make a viable commercial discovery, the company will design the most efficient and effective way extract the resource.
At the end of drilling the well, the well is “completed” by completion engineers.
Once all the oil and gas has been extracted, or if no oil and gas is found, the well is plugged with cement.
• The excess casing and other equipment is removed and salvaged as appropriate;
• Cement plugs are placed at key in the borehole to prevent migration of fluids between the different formations.
• The surface is reclaimed.