How are oil wells drilled?

The first oil well was drilled in 1849 in Azerbaijan (see Figure 1 - although some sources quote several years earlier in 1846 while other sources quote Colonel Edwin Drake's well in Pennsylvania in 1859). The Azerbaijan well was drilled by hand using a primitive cable device to a depth of 69 feet. Technology improvements in both equipment and technique over the decades now allow for drilling up to 49,000 feet in total length and up to 35,000 feet vertically below the earth's surface and offshore in water depths up to 11,200 feet (see Oilprice.com).

Modern drilling rigs drill through soil and rock with equipment that rotates a string of pipe with a diamond-tipped bit. While drilling, fluid called "mud" is pumped into the well bore to maintain pressure and prevent fluid or gas from prematurely entering the well. Mud can be either water-based, which can often be dispersed into the environment, or oil-based, which must be treated and disposed of appropriately. Additives, incuding polymers and clays, are added to achieve the "mud weight" (i.e., density) required to achieve the hydrostatic pressure needed to counter-balance the internal pressure of the formations being drilled through. Additional additives are included depending on specifics of the water, oil, and gas formations to prevent corrosion, foaming, and clogging. The five main purposes of drilling mud are:

• Transporting the drill cuttings, i.e., rock particles, up the wellbore for disposal.
  
  
• Controlling pressure by offsetting the hydrostatic pressure in the rock formations.
  
  
• Stabilizing rock formation with special additives to keep the rock pores open.
  
  
• Increasing buoyancy in the well bore to reduce the weight of the drill string as the well goes deepers.
  
  
• Adding lubrication to reduce the rock formation's friction, thereby reducing heat and helping to extend the life of the drill bit.

As the well penetrates deeper below the earth's surface, it will encounter reservoirs containing water, oil, and/or gas at various internal pressures. To ensure that the well can safely reach the required depth, the well must be carefully designed. Wellbore pressure should be greater than the formation pressure to prevent unwanted fluid migration into the wellbore but not so great that the drilling fluid is pushed into the formations. The deeper the well, the harder it is to maintain the required pressure balance. Additional pipes, called casing strings, are inserted into the wellbore and cemented in place at various depths to isolate higher reservoirs from the wellbore and enable drilling to extend deeper. Figure 2 shows a schematic of a wellbore design for a subsurface profile indicating multiple casing strings to handle the reservoir pressures.

Drilling is performed through a device called a blow-out preventer (BOP) which is supported on the outermost string of casing. The BOP is a safety device that prevents the uncontrolled release of oil or gas from a well during drilling operations. It was a failure in the subsea BOP during drilling that led to the massive Deepwater Horizon oil spill in the Gulf of Mexico in 2010 (see OWOE: What has the oil industry done since the BP Horizon disaster to prevent similar accidents?).

After the well is drilled, it is completed. This includes perforating the casing and any cement sheathing with a perforating gun that uses armor-piercing projectiles or shaped explosive charges to establish a flow path between the reservoir and the wellbore. In loose sand formations a gravel pack is typically installed, consisting of a steel screen that is packed with gravel that is sized to prevent the passage of formation sand into the wellbore and plugging the well. Relatively small (compared with the well casings) diameter tubing through which the oil or gas will be produced to the surface is then installed.

After the well is comleted, the BOP is replaced with the wellhead which regulates and monitors the flow of fluids from the the underground formations during production operations. It also prevents leaking of oil or natural gas out of the well and prevents blowouts from occuring due to an unexpected increase in formation pressure. The wellhead consists of three components:

• The casinghead consists of heavy fittings that provide a seal between the casing and the surface. The casinghead also serves to support the weight of any casing strings that are not cemented.
  
  
• The tubinghead, similar to the casinghead, provides a seal between the tubing, which is run inside the casing, and the surface. Like the casinghead, the tubinghead supports the entire length of the production tubing.
  
  
• The christmas tree (see Figure 3 - named because of its many branches that make it appear somewhat like a Christmas tree) is the piece of equipment that fits atop the casing and tubing heads, and contains tubes and valves that control the flow of hydrocarbons and other fluids out of the well.

See Video 1 for a summary of the drilling process and equipment used to drill an offshore subsea oil well in deep water.

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