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Marine Offshore Oil and Gas - Prospecting Under the Sea

written by: Willie Scott • edited by: Willie Scott • updated: 11/23/2011

Now we know how the oil and gas reservoir was formed, the next job is to locate it. Geophysical and geological surveyors combine their skills to determine if oil and gas are present through prospecting under the sea. Once a reservoir is confirmed, an exploration well is sunk to obtain samples.

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    The Exploration Phase

    Early prospecting for hydrocarbons was made easy by the oil or gas having leaked to the surface indicating a reservoir underneath, this being particularly prevalent in the Far East where the cap rock was thin. The next method involved land surveys looking for geographical signs of an anticlinal fault below the ground. When a survey indicated such a location an exploration well was sunk to confirm a reservoir.

    This was all very well for land prospecting but difficult to carry out at sea, what about us old seadogs. Well, the1920’s saw the first geophysical prospecting for oil and gas using seismics and we could use this method to search for offshore crude!

    This involved setting of charges on the surface and monitoring the return of the shockwaves as they bounced of the rock and sediment layers below. At a fixed distance from where the charges are set off a listening device known as geophones record the strength and the length of time elapsed by the return shock wave signal.

    The same principles are used today but are more sophisticated with the use of an arrangement of twenty-thirty compressed air guns creating the shock waves and a series of listening devices, the recorded data interpretation having computer enhancement. Along with the seismic survey several geological observations of the undersea structure, sedimentation, palaeontology and stratigraphy are made. These are carried out by core drilling and examination of the core samples.

    These geophysical and geological surveys combine to determine if hydrocarbons are present at the locations and, this can then be confirmed with the drilling of an exploration well and as the old offshore drillers saying goes - this is the only real way of discovering oil.

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    Drilling for Oil and Gas

    Once the presence of an oil and gas reservoir has been confirmed with the drilling of an exploration well, frantic activity follows as methods of extraction are prepared. These depend on further reservoir core tests which will reveal the extent of the field, grade and estimated amount of hydrocarbons present.

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    The Subsea Drilling Template

    After this a drilling template is laid on the seabed, positioned exactly where the geologist dictates by GPS and satellite. The template has a number of predetermined holes drilled in it depending on the number of wells to be drilled. It is grouted into the seabed and connected to the drilling rig using wire hawsers allowing for both accurate drilling and movement of the rig by the sea and wind.

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    The Blow out Preventers (BOP's)

    Next a very important safety measure is put in place – the Blowout Preventer known as the BOP. This is positioned over the well on the seabed and is operated by hydraulics; two types can be used, a rubber seal type which shuts off the supply of hydrocarbons - if there is no drill strings present it seals the hole. If a drill string is inside the pipe then it seals against this. The other type is a hydraulic ram which literally crushes the drill, pipe and anything else which may be down the hole when it is operated. In addition to preventing a blowout they also prevent the leaking of hydrocarbons from the well into the sea.

    From the BOP a marine riser is fitted. This large bore specialist pipe runs from the BOP to the drill rig platform and encloses the drill strings and bit. There are various riser protectors used against damage e.g. from ice-bergs and collisions. Additional BOP's can be fitted on the rig where the riser enters the deck.

    In the case of Deep water Horizon Oil Rig Disaster in 2010, the blowout preventer did not work. It could be operated by remote control from the control room but did not operate when activated. It was then tried manually using an ROV; still with no success. If the BOP had worked lives could have been saved and the major oil spill disaster averted.

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    The Drill Bit

    The drill bit can be made up of three rotating integrated heads made from titanium or similar very hard metal. It can also comprise of a single bit with industrial diamonds inserted on the leading edges of the head.

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    Drilling Mud

    As the drill enters the bedrock, the drillers pump mud made up from a mixture of barite (crushed barium ore), clay and water down the casing to the point of the drill bit. This very heavy viscous fluid prevents a blowback of the subsea pressures, lubricates and cools the bit acting as a conduit to remove the drill cuttings as it returns up the casing to the rig. Here mud and cuttings are then filtered and the mud recycled.

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    The Drill Strings

    The drill strings, which are about nine metres long, connect the bit to the rotating table on the floor under the derrick. As you can imagine it is very labour intensive and hazardous to join the drill strings together using couplings (known as tool joints). This is especially so in typical weather conditions like the North Sea and even more so when another string is to be added to the existing strand. This strand can be between four and nine Kilometres long if the well is not vertical making it very, very heavy.

    When another string is to be added, the drilling is stopped and the drill strand and bit gripped and held by a clamp connected and travelling block. This is vertically above the strand and operated by a system of pulleys and wire ropes on the crown block right at the top of the derrick. The new string is connected and then the strand is released and the rotating table starts driving the bit via the strings once again.

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    The Borehole Sequence

    The first surface hole is drilled with a 500mm diameter three faced bit to a depth of up to a thousand metres. Then the drill is removed and casings are inserted into the drilled well, this section of casings is known as conductor pipe. The second hole is then drilled with a smaller size of bit about 300mm and this takes the well down even further, and surface casings fitted. Eventually the final bit goes on the string and this drills the bottom hole about 200mm diameter and intermediate casings are fitted right down almost to the reservoir. The casings are grouted into the wall of the well and a packer is sent down as the different sizes of casings are fitted to expand them against the wall. The purpose of the casings is three fold; to add strength to the well, to ensure the hydrocarbons don’t leak into surrounding subsea rock on their way up and, to prevent ingress of dirt or water to the well.

    When the drill breaks into the hydrocarbons reservoir a final production casing is fitted right down into the reservoir. This last casing has one end blanked off and effectively isolates the well from the reservoir until it is perforated, allowing for metered discharge at the production phase and to prevent a gushing of gas and fluids up the risers.

    A simplified sketch of a typical drilling rig is shown below.

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Marine Offshore Oil and Gas

This series is about offshore oil and gas. It begins by with the formation, discovery, and drilling of the hydrocarbons. Next is the construction and installation of the platform, hydrocarbons processing, piping ashore and oil & gas refining. The last article is on decommissioning of the platform.
  1. The Formation of Hydrocarbons in Offshore Reservoirs
  2. Marine Offshore Oil and Gas - Prospecting Under the Sea