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Petroleum engineering general discussion FAQ

Petroleum Basics

How is oil formed, discovered and extracted? by LiamBlair
Posted: 28 Apr 04

There are two theories that exist on the formation of oil. The most commonly accepted one is the 'organic theory'. That is, over the past millions of years, organic material, such as plant growth etc decomposes over time, and through the cycles of the ocean, becomes buried deep underneath the ocean floor (and land to!). With the huge force of deep water, sand and rock, this organic material forms into a hydrocarbon.that is, carbon based material within water. Several types of hydrocarbon exist but is mainly oil and gas. (gas being a further by product of the decomposing organic material). Platonic plate movements on a global scale result in layers of rock of varying permeability, i.e. oil and gas can travel or not travel, through these layers depending on the type of rock present. Normally these several layers are 'pushed' or 'pulled' into each other through platonic movements and due to the varying density of the rock layers, usually form reserviours of different shapes and sizes. A point to remember here is oil and gas is lighter than water so will always try to float! even though it is in rock. Subsequently, different types of reserviours are formed. obviously the top layer of rock (the sea bed) is impearmable so oil and gas cannot escape. This layer is called the cap rock. Several types of reserviours exist and are catogorised according to the rock surrounding them. Finding them requires acoustic signals sent out by towing a buoy behind a boat and firing signals to identify in a 3D image what the sea bed looks like. sometimes oil companies manage to see whats beneath the sea bed also and into the sub-strata (the rock beneath). But generally the principle is the same. Acoustic sounding. When the sea bed formation is identified, geologists identify 'potential' targets to drill depending on whether it looks like oil/gas could have migrated through the rock and become trapped, ready for extraction. (recent methods involve using acoustic signals to identify the oil and gas itself rather than its rock formation). Then a drill rig is sent out to drill a sample, it drills, and the rock is further analysed to determine oil/water/gas/rock quantity,quality,depth.This can be done by taking samples at different depths and having geologists study it. (Take note, Shell UK recently underestimated their North Sea UK oil reserves and subsequently managers have been resigning in the wake of this). Extracting it requires 'completing' the well, i.e. filling it with concrete so oil/gas doesnt escape. then moving the drill rig away. Another oil rig comes along, re-drills the hole and starts to produce the hydrocarbons.When they re-drill the hole, they 'case' it as they drill deeper, this involves inserting large diameter pipes to prevent the walls collapsing. The oil/gas comes out of the ground by under their own pressure since they are all under pressure from the rock/sea water etc. (think bursting a water ballon slowly, all you do is make the hole). This is called overbalance drilling,where the pressure in the well (the drilled hole) is higher than the pressure of the reserviour. Under balance drilling is the opposite but presents many dangers including 'kickbacks' where oil and gas surge to the surface (subsequently experiencing a pressure drop and volume expansion) and can pose a threat to personnel health. The new oil rig on site normally has the equipment to process the oil and divide up the oil from gas, water and drill fluid. drilling fluid is used to (a)keep the drill bit cool (b)transport all the rock cuttings to the surface (c)as a back pressure against the rock formation, so the hole doesnt collapse in on itself (d)as a method to use wire-line logging, electrical currents sent down the well to provide production data such as pressure,temp etc down hole (e)in a way, helps to reduce the weight of the drill string by providing a level of buoyancy, drill strings i.e. connected pipes with a drill bit on the end, can reach several kilometres in length! Sometimes a 'new oil' rig is not needed in the sense that once the well has been drilled, companies can install whats called a 'subsea' manifold, which is basically a valve on the sea bed. and is linked to pipelines that either go back onshore or to the sea surface to a vessel or FPSO. Currently this is the way the market is moving, since deeper waters cannot sustain fixed leg platforms. plus new technology is making this allowable. Keywords to search for is:

Acoustic sounding+oil
Drilling fluid properties
Subsea manifolds
Bundled pipelines
definition of Operators, contractors and service providers
under balance drilling
over balance drilling
rock formations
platonic movements
types of oil reserviours/rock formations
shale-shakers
cap rock
drilling rig
production rig
FPSO
drill bit/cone
drill string
drill pipe

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