Offshore oil drilling is a process dictated by the depth of oil and gas reservoirs buried underneath the ocean. By drilling into the seabed, crews can set concentrical tubes stacked upon one another into the sand or rock formations below. These tubes access and pump hydrocarbons to the surface, analogous to a water well dug into the ground and a pail used to bring the water to the surface. Offshore drilling, uses equipment to pass through more than 5,000 feet of water to reach the sea floor and additional depths of sand and mud to reach reservoirs of hydrocarbons. At this depth, several different types of tubes and valves are set. Specialized pressure systems pump the oil or natural gas up to the surface where it can be transported via ocean tanker to the mainland for subsequent refining.
Offshore drilling is very expensive, requires extensive planning and only occurs when oil prices are high enough to justify huge and necessary capital expenditures. Specialized drilling vessels costing $250,000 to $750,000 per day are required to make sure wells are drilled properly and safely the first time. Revisions, called workovers, cost about $6 million, reflecting the difficulty of getting drills, tubes, hydraulic and hydrostatic equipment into the sandy, muddy seafloor to reach reservoirs, often below 5,000 feet of water.
An offshore drilling well is composed of tubulars, tools, pumps and controls, engineered to produce hydrocarbons and sometimes inject other fluids. Different types of equipment are used depending on oceanography and type of hydrocarbon. Open holed tubes or cased hole tubes can be used, both of which are perforated and screened to interface with the reservoir, although the choice of which depends on the type of hydrocarbons involved. The type of tubes chosen is based on a number of factors including whether fluid injection is necessary to artificially induce the flow of hydrocarbons. Hydrostatic equipment is part of an offshore drilling well, monitoring pressure balances so that hydrocarbons flow to the well and pump to the surface properly. Safety equipment, like blowout preventers that seal the well, are also necessary, placed at the top of the well and called a "tree".
Drilling is done at sites with expected high productivity. Wells are designed to assure flow of hydrocarbons. An offshore drilling vessel or rig is established with heavy machinery that will drill bore holes into the seabed to the proper depth. Tubes will be inserted into the bored hole (the well) stacked on one another until the reservoir is reached. Open-cased ones will be placed at the reservoir level to allow hydrocarbons into the tubes. Pressure builds or is induced to force the hydrocarbons up through the well to the surface. At the wellhead, safety equipment, called a tree, is placed; the tree limbs are rams and rods that will seal the well under emergency conditions or additional piping is sometimes attached, directing and controlling the flow from the well.
Myriad combinations of equipment can be used. Site specifics, such as whether the well will be drilled through sand or rock, the depth of water and whether the reservoir contains natural gas or oil, influence each offshore well design. Different types of cement are implemented to stabilize the well against the reservoir structure (sand or rock). Cement mixed with nitrogen is very good at sealing wells that may encounter natural gas. Few standing platforms (those stabilized on stilts suitable for reservoirs in less than 200 feet of water) remain. Most offshore drilling occurs at much greater depths. Tubes with encased screened tubing stacked upon perforated tubing or combinations of such would be inserted into the well bore hole. The safety tree is designed to seal the well based upon evaluations of expected pressure flows sometimes originating near the Earth's mantle.
Offshore Drilling Well Design
Well design and equipment selection are all based upon site-specific circumstances that are expected to be encountered as determined by sophisticated geological surveys, engineering evaulations and analysis conducted years and months before the rig arrives at the reservoir location and simultaneously while drilling occurs. Unexpected encounters with different pressures, fluids or gases would require activating the safety tree until new more suitable equipment for the site can be manufactured, delivered and installed. Submerssibles continually monitor drilling to avoid workovers but cannot always identify equipment failures at 5,000 foot depths that can occur instantly and lead to catastrophes at the surface. Key site-specifc factors to consider are natural pressure flows, gas type to be encountered, oil grade to be encountered (e.g., light crude) and reservoir structure (sand or rock), and most importantly the amount of hydrocarbon expected to be recovered. The volume of hydrocarbon must be sufficient to cover the investment in offshore oil drilling.