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Exploration geophysics is the use of seismic, gravity, magnetic, electrical, electromagnetic, etc., methods in the search for oil, gas, minerals, water, etc.,

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1. Signal emitted by vibrator truck

2. Reflected wavesreceived by geophones

3. Data transmitted to laboratory truck

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1. Isochrones2. 3D seismic Maps

The geophysicist's seismic recordings are fed into powerful computers. The terrain is mapped by means of isochronic lines linking points on the ground at which the waves take exactly the same length of time to be reflected back to the surface. This method yields two and three-dimensional images of the underground strata, and the resulting seismic maps serve to determine whether certain strata are likely to contain hydrocarbons.

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1. Seismic vessel2. Hydrophones

In the oil man's jargon, exploration and production at sea is known as "offshore." Because it is not practicable to survey the terrain at sea, seismic methods are used systematically. And since ships can travel easily in all directions, seismic measurement is in fact easier at sea than on land.The geophysicist can thus obtain more data offshore than onshore and a more precise three-dimensional image, once the data have been processed.

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All these results are aggregated and studied. Geologists, geophysicists, petroleum architects, together with drilling, production and reservoir engineers all supply data to economists and financial planners. By juggling figures, parameters and probabilities, they seek to work out a possible strategy for developing the reservoir in the event of confirmation of the presence of hydrocarbons.

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1. Geophysicist2. Geologist

Each member of the exploration team has contributed to the performance of the mission. By collating and comparing their experience, know-how and findings, their ultimate conclusions are the result of a team effort. Those conclusions are stated briefly:No: the chances of a result are too slim; or...Yes: the "prospect", i.e. this highly promising reservoir, is worth taking a gamble. The team is prepared to "pay to see," making the decision to drill.

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FINAL DECISION. .

Geologists, geophysicists and reservoir engineers have concluded there is a "prospect" or possible producing zone. But to find out whether there really are hydrocarbons trapped in the rock, they are going to have to drill down to that zone.

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1. Usually set up directly over thethickest layer of hydrocarbons.

Some fields lie at depths equivalent to twelve times the height of the Eiffel Tower...the siting of the drill rig is determined based on the existing state of knowledge of underground conditions and the topography of the terrain. This is generally sited vertically above the thickest part of the stratum thought to contain hydrocarbons. The drilling team often operates under difficult conditions. This narrow-bore hole (with a diameter of 20-50 centimeters) is generally sunk to a depth of between 2,000 and 4,000 meters. In a few cases it may go beyond 6,000 meters, and one has even gone to a depth of 10 kilometers, or 30,000 feet.

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1. Hoist attachment2. Derrick (mast)3. Traveling block4. Hook5. Injection head6. Mud injection column7. Turntable driving the drilling pipes8. Winches9. Motors10. Mud pump11. Mud pit12. Drilling pipe13. Cement retaining the

casing14. Casing15. Drill string16. Drilling tool

The derrick, or "mast" in oil slang, is the visible part of the well. This is a metal tower several tens of meters tall, and its serves to lower the "drill-string" vertically into the ground. This drill string is in fact a set of drill pipes screwed end-to-end. In rotary drilling, this string transmits the rotating movement to the drilling tool (of drill-bit) and channels mud down to the well-bottom as the drilling progresses.

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1. Three-cone rock bit2. Diamond drill bit

The drill assembly consists of a derrick, drill-string, drive-shaft, and the drill-bit itself. The commonest kind of drill-bit consists of three cones made of extremely tough steel capable of eating into the rock face. When the rock is very hard, a diamond-tipped monobloc drill-bit is used

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1. Mud pit

2. Pump

3. Injection line

4. Injection head

5. Drilling pipes

6. Descending mud (in pipes)

7. Returning mud (in annular space)

8. Filter

9. Mud return for recycling

Specially-formulated mud, prepared under the supervision of the hoghead (oil man's slang for the mud engineer) is injected through the hollow drill-string in order to cool the drill-bit and consolidate the walls of the hole. The mud also helps prevent the oil, gas or water found in the strata crossed from gushing out at the surface. Finally, the mud cleans the well-bottom and carries the rock cuttings back along the pipes to the surface. The geologist analyzes these cuttings to understand the nature of the rocks traversed and detected signs of hydrocarbons

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Well Logging:

1. Well casing2. Cable retaining the

down hole probe3. Down hole Probe4. First probe sensor5. Second probe sensor6. Third probe sensor7. Measurements obtained

by the sensorsOnce a certain depth has been reached, the exploration crew conducts a series of measurements known as well-logging. An electronic probe is lowered into the well to measure the physical properties of the rocks traversed. These actual measurements either confirm or disprove the hypotheses formulated prior to drilling, and generally provide more accurate data. The sides of the well are then consolidated by means of steel tubes screwed together, and the casing is cemented to the terrain to keep the strata separate from each other.

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1. Coring tool

2. Core sample

3. Indicationsconcerning height of beds

4. Clues concerning typeof rock

The cuttings brought up the surface do not supply sufficient information for a thorough understanding of the rocks traversed: that's where core sampling comes in. The drill-bit is replaced by a hollow bit called a coring tool, which extracts a cylindrical sample of rock several meters long. A study of the resulting core sample yields information about the nature of the rock, its slope, structure, permeability, porosity, fluid content, fossils present, etc.

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1. In this example, one hole in five is positive

Drilling progresses very gradually, at a speed of a few meters per hour, slowing to just one meter an hour by the time one is down to 3,000 meters below the surface. Snags are encountered from time to time, and the entire drill-string has to be pulled out regularly for a change of drill-bit.An exploratory well takes from three to six months to drill. Four wells out of five, or even six out of seven in pioneer zones, fail to yield commercially viable quantities of oil or gas. Sometimes, though, the drill-bit strikes a hydrocarbon-impregnated rock, in which case the drilling crew conducts extensive well-logging to find out more.

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1. Economic data2. Choice of operating methods3. Geological data

The exploratory phase has been successful: a reservoir has been identified, with the prospect of producing profitably. Based on assumptions as to future oil or gas prices, the next step is to determine whether sales of products extracted from the reservoir will be sufficient to cover the high cost of studies, development, construction and funding, as well as production costs proper. The decision to bring a reservoir on-stream is a major one, as the investment outlay can run into several hundred million, indeed a billion, dollars

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