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| HOME > CHIKYU >
CHIKYU SCIENCE AND TECHNOLOGY >
Drill the Earth |
| D/V CHIKYU is a first riser equipped science drill ship to reach the deeper
part of the Earth. What is the difference from the previous scientific drilling? Why does D/V CHIKYU can
drill such a deep part? We approach to the mystery of the drilling ability of D/V CHIKYU. |
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D/V CHIKYU has a riser drilling system, which has been
used in the oil and gas industry. For scientific research applications it is the first time to have such
a system. Riser pipe guides the drill pipe for reentering to the well, and downhole measurement tools
and devices for lowering and setting in the hole. “Mud” is the artificial drilling fluid, which is chemically
and physically composed. Mud is circulated from the drilling vessel by using high pressure pumps onboard
down through inside the drill pipes, and returns up the annulus between the drill pipe and the borehole,
the casing and riser pipe to the vessel. There are multiple benefits of the riser drilling with having
the mud circulations while drilling and of the borehole itself. This is the major key when drilling to
depths thousand of meters below the sea floor. Further Blow Out Preventer (BOP) makes safe drilling even
where there is unexpected high pressure flow of gas, oil or other formation fluids from the well.
By use of the riser,
- Circulate drilling fluid, mud,
- Re-enter drilling pipe easily,
- Lower large diameter of measuring/monitoring devices, not limited
to the inner diameter of the drilling pipe.
By circulation of the mud
- Increase mud water weight, to counter-balance the formation pressure,
- reinforce wall of the borehole by mud ingredients (mud cake),
- exploit viscosity of mud to displace drilling cuttings (scraped formation particles) smoothly
- sample
drilling cuttings (scraped formation particles) on board to evaluate formation layers.
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| Riser-less Drilling System |
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The existing scientific drilling system uses only drill pipes
to perform the drilling, and then circulated seawater to push out the cuttings.
The advantage of this method is that many boreholes can be drilled in a short period of time. The disadvantages
are corruption of the borehole wall due to seawater penetration and hence a decreasing core collection
rate, much the same way that a hole dug on a beach collapses when inundated with water.
- Many boreholes can be drilled in a short period of time.
- Difficult to drill through potential oil and gas bearing formations, because the risk of environmental damage
by oil and gas spill is high.
- Difficult to drill unconsolidated layers such as submarine fan, because the hole may collapse.
- Difficult to drill deeper than 2,000m below ocean floor, because of the hole cleaning and stability problems.
- Because of the hole cleaning and stability problems, it is difficult to exploit the hole for subsequent
long-term observation.
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| This system is generally used to core medium to hard formations. It is most effective
for coring consolidated formations without fractures. |
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A cylindrical sample taken from geological layer (core), which consists of sediments and rocks, can provide us with many messages about the earth. D/V CHIKYU is equipped with several types of core barrel systems to apply different geological conditions to allow efficient and uninterrupted core collection. The core sampling system is composed with three parts: the core bit, the inner barrel, and the outer barrel. There is an opening at the center of the bit that can cut off the core to the same diameter as the Internal diameter of the inner barrel, and the core is pushed into the inner barrel as the bit's penetrating into the formation. The wireline coring system allows us to recover the core without pulling the drill string/pipe out from a few thousand meters below the sea floor to save the operating time.
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| This system is generally used for unconsolidated soft sediments. In general, The RCB, because of the rotation of the core bit, will disturb the soft sediment samples. On the other hand, the HPCS has a sharp knife-like cutting shoe with hydraulic pressure to allow it penetrating to soft sediment. This system makes the soft core sample be undisturbed. |
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| This system is effective to use for fractured hard rock and for soft and hard alternating layers. Hydraulic pressure is used to drive/rotate the downhole motor equipped on the top of the inner barrel. This mechanism allows us to control the rotation rate and the applied weight on the core bit independently of the main drilling system to obtain high quality cores. |
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| This system is used for sampling the core with maintain in-situ pressure. The core sample will be retrieved in the pressure tight chamber. The chamber is also possible to keep gas and fluids in the cored formation, and to measure the pressure and temperature through a port on the core barrel, when it is pulled out from the hole to the vessel. |
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