The Eakring Oilfield Section 

• What is Oil?

  Oil is a generic term for fluids that are not miscible with water. The name comes from Latin oleum for olive oil.

   

  • Crude oil consists of a mixture of petroleum liquids and gases (together with associated impurities) pumped out of the ground through oil wells.

     

    • Petroleum (from Latin petrus–rock and oleum–oil) or mineral oil. It can be shortened to the prefix petro-, as in "petrodiesel". Petroleum is a thick, dark brown or greenish inflammable liquid, which, at certain points, exists in the upper strata of Earth's crust. It consists of a complex mixture of various hydrocarbons, largely of the methane series, but may vary much in appearance, composition, and properties.

     

    • In chemistry, a hydrocarbon is an organic compound consisting only of carbon and hydrogen. They all consist of carbon backbone and atoms of hydrogen attached to that backbone.

      The simplest hydrocarbon, methane is a gas with a chemical formula of CH₄.

      Lewis Structure:

         H
      
         |
      
       H-C-H  
      
         |
      
         H

    • Strictly speaking, petroleum consists entirely of aliphatic hydrocarbons, those composed of nothing but hydrogen and carbon.

      The four lightest hydrocarbons -- CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane) and C₄H₁₀ (butane) -- are all gases, boiling at -107°C, -67°C, -43°C, and -18°C, respectively (-161°, -88°, -46°, and -1° degrees F).

      The chains in the C_5-7 range are all light, easily vaporized, clear naphthas. They are used as solvents, dry cleaning fluids, and other quick-drying products. The chains from C₆H₁₄ through to C₁₂H₂₆ are blended together and used for Petrol (gasoline). Kerosene is made up of chains in the C₁₀ to C₁₅ range, followed by diesel fuel/heating oil (C₁₀ to C₂₀) and heavier fuel oils as the ones used in ship engines. These petroleum compounds are all liquid at room temperature.

      Lubricating oils and semi-solid greases (including Vaseline®) range from C₁₆ up to C₂₀.

      Chains above C₂₀ form solids, starting with paraffin wax, then tar and asphaltic bitumen.

       

      Boiling ranges of petroleum atmospheric pressure distillation fractions in degrees Celsius:

      petrol ether: 40 - 70 (used as solvent)

      light petrol: 60 - 100 (automobile fuel)

      heavy petrol: 100 - 150 (automobile fuel)

      light kerosene: 120 - 150 (household solvent and fuel)

      kerosene: 150 - 300 (jet engine fuel)

      gas oil: 250 - 350 (Diesel fuel/ heating)

      lubrication oil: > 300 (engine oil)

      remaining fractions: tar, asphalt, residual fuel

      

• Formation

  Millions of years ago much of the earth was covered by seas and swamps. As microscopic animals and plants in these seas died, their remains were deposited in the sea bed, they mixed with rock materials eroded from higher ground.

  Gradually, sediments many thousands of feet thick, built up and became compressed to form sedimentary rocks such as sandstone and limestone.

  Because of the lack of oxygen, the small organisms did not decay but were transformed by heat and high pressure into oil and gas. This biological material in rocks starts off largely as a waxy material known as kerogen.

  As more new rocks formed above, gravity, capillary action and water pressure forced the oil and gas out of this source rock and upwards through porous rocks.

  If further upward movement was eventually stopped by rock layer and if a side ways movement was prevented by a fold or fault in the earth's surface, then oil and gas are trapped in a reservoir. This is not an underground lake, as the name suggests: oil and gas are held in the pores between grains the same as water is held in a sponge.

   A few scientists, notably Thomas Gold, have suggested other, abiogenic, theories for the origins of crude oil.

   

  A Typical Anticline

   

• Exploration and Drilling - The Basics

   

  • Exploration

    The task of finding oil is assigned to geologists, whether employed directly by an oil company or under contract from a private firm. Their task is to find the right conditions for an oil trap -- the right source rock, reservoir rock and entrapment. Many years ago, geologists interpreted surface features, surface rock and soil types, and perhaps some small core samples obtained by shallow drilling. Modern oil geologists also examine surface rocks and terrain, with the additional help of satellite images. However, they also use a variety of other methods to find oil. They can use sensitive gravity meters to measure tiny changes in the Earth's gravitational field that could indicate flowing oil, as well as sensitive magnetometers to measure tiny changes in the Earth's magnetic field caused by flowing oil. They can detect the smell of hydrocarbons using sensitive electronic noses called sniffers. Finally, and most commonly, they use seismology, creating shock waves that pass through hidden rock layers and interpreting the waves that are reflected back to the surface.

     

    Photo courtesy Institute of Petroleum Searching for oil over water using seismology

    In seismic surveys, a shock wave is created by the following:

    • Compressed-air gun - shoots pulses of air into the water (for exploration over water)
    • Thumper truck - slams heavy plates into the ground (for exploration over land)
    • Explosives - drilled into the ground (for exploration over land) or thrown overboard (for exploration over water), and detonated
    The shock waves travel beneath the surface of the Earth and are reflected back by the various rock layers. The reflections travel at different speeds depending upon the type or density of rock layers through which they must pass. The reflections of the shock waves are detected by sensitive microphones or vibration detectors -- hydrophones over water, seismometers over land. The readings are interpreted by seismologists for signs of oil and gas traps.

    Although modern oil-exploration methods are better than previous ones, they still may have only a 10-percent success rate for finding new oil fields. Once a prospective oil strike is found, the location is marked by GPS coordinates on land or by marker buoys on the sea.

     

• Drilling

  Typical Drilling Rig

   

	SPUDDING IN
		During drilling a liquid mixture containing clays and other natural materials, called 'Mud' is pumped down the drill string forcing the rock cuttings up to the surface. These cuttings are analysed for indications of oil or gas.

The start of a well is known as 'Spudding in' the origins of this phrase are now lost but the phrase came from the early days of drilling in the USA.

A well is cut by a drill bit rotated on the end of lengths of pipe called a 'drill string'. The 'Mud'  forces the rock cuttings up to the surface and also helps to cool the drill bit - it also acts as a safety device against oil and gas pressure that may be encountered. Drill bits are usually made of hard tungsten but can also be diamond tipped. As drilling progresses deeper, 'Mud weight' is increased to counterbalance the pressure in the formations.

Only if extensive initial testing of the well indicates the presence of oil in commercial quantities is a system installed for the Production of oil.

• Production

Because of the low down-hole pressures encountered in the East Midlands fields, oil is pumped to the surface. This involves installing a small well cellar and concrete pumping base at the surface. A small 'rig' completes the well by running tubing down the borehole. The borehole casing and the cement sheath around it, is then perforated.