The production of petroleum oil is basically a very simple process – a hole is drilled down into the earth’s crust (on land or under the sea) until an oil-bearing rock formation is reached, whereupon the oil is forced up the drilled hole to the surface. The consequent “gusher” is a sight familiar to magazine readers or movie-goers.
The actual process is a good deal more complex than this, but the first stage of petroleum production can be regarded in this simple way, with the natural pressure of the subterranean zone being sufficient to force the oil to the surface, in what is known as the primary phase of production. Eventually the pressure falls as the oil is extracted, and the production rate starts to fall. At this point, the oil discharge is boosted by the use of downhole pumps, although such pumps (like the familiar “nodding donkey”) will usually have been fitted at, and may have been used from, the start of operation. The deeper the oilbearing formation, the more likely that some form of artificial lift will be needed.
When the flow of oil out of the rock formation into the bottom of the well is too low for its further discharge by pumping, then that is the end of the primary production phase. The amount of oil produced in this primary phase can be as little as 10% of the total content of the reservoir and as much as 30%. For most production wells, the oil is recovered as a mixture with water, and this “produced water” can be up to five times, or more, the volume of recovered oil. A major wellhead process then is the separation of the oil from its accompanying produced water.
As pumped production rates start to fall, the oil production process can then move into a secondary phase, in which a fluid is injected into the reservoir to repressurise it and force out more of the contained oil. This can be done by waterflooding or by gas injection. In waterflooding, clean filtered water is injected into the rock layer through specially drilled injection wells (or through disused production wells), to push residual oil along to the operating wells. An injection of gas into the gas cap above the formation achieves the same effect. Primary and secondary production together can recover a total of 15 to 40% of the original oil in the reservoir.
When production starts to fall at the end of the secondary phase, a tertiary phase can be embarked upon – also called enhanced oil recovery (EOR) – although this is still only used in a minority of cases. EOR can employ
specific gas injection, or thermal methods such as cyclic steam injection or steam flooding, or even, in the extreme, in-situ combustion. Among the specific gas injection processes is the use of carbon dioxide, which then offers a very valuable means of carbon sequestration. Microbial injection, although still only a method under development, shows considerable promise for the future. By the use of EOR, the total recovery from an oil-bearing rock formation can be increased to 30 to 60% or more.
This three-stage production process applies to liquid oil trapped below ground, which can be pumped to the surface. A significant proportion of the world’s total reserves of hydrocarbon fuels (some say as much again as the current total proved liquid oil reserves) exists as tar sands, in the form of a mixture of heavy oil and bitumen with sand. Huge deposits of this material lie in Canada, and production of “synthetic” crude oil from them is now a well-established process. If the deposits are at or close to the surface, then open-cast mining is used, followed by hot water processing and flotation to release the oil. For subterranean deposits, recovery is achieved by hot water injection.
Large liquid and gas flows of several kinds are thus involved in oil production, but before looking at the needs for filtration of these flows, brief mention must be made of the liquid suspensions used in the original drilling process, since oil cannot be produced from underground until a drilled well is available through which it can pass.
As the drill bit moves downwards, it is surrounded by a thick liquid suspension of clay-like materials – the drilling mud – whose purposes include:
• cooling and lubricating the drill bit,
• transporting the rock fragments to the surface,
• applying pressure to the walls of the bored hole to prevent collapse, and
• minimising fluid loss across permeable rock formations by forming a filter cake over the surfaces of these formations.
Although not specifically involved in oil recovery, drilling muds are a vital part of the overall oil production process, and have important applications for filtration – in their initial formulation, and in their recycling, to remove rock fragments. Drilling muds are a complex mixture, and therefore expensive in first cost, so it makes sound economic sense to recycle them as often as possible, which creates quite an exacting task for the mud recycle filter – to remove the maximum of the rock fragment content of the returned mud, while changing the basic composition as little as possible.
Recovery fluid flows
It will be apparent from the above descriptions that the oil recovery processes involve large flows of liquids and gases, most of which have essential needs for filtration and/or sedimentation to separate oil from water, and liquid or gas from solids. The most important liquid flow is, of course, the crude oil transported up the well bore for treatment at the surface (or on the oil production platform). This is carried by a flow of produced water, at least as large as the oil flow, and generally much larger. There will frequently, also, be a flow of associated natural gas from the oil reservoir, or there may be a gas flow that is being used to lift the gas from underground.
If the well is in secondary phase production, there will be a large water flow necessary for the waterflood injection, or possibly a gas flow to pressurise the reservoir. In tertiary phase, the various enhanced oil recovery processes will also have their operating fluid flows, especially for gas injection, with carbon dioxide flows increasingly important as part of the carbon separation and sequestration schemes required to combat global warming.
Although the production of oil from tar sands is, on the global scale, nowhere near that of pumped crude oil, its fluid flow rates will become large, as the reserves are developed. These will include, besides the produced oil, the residual water from the processing of open-cast sands, or the recovery of oil from underground.
Crude oil treatment
The recovery of crude oil from underground requires separation treatment in two main places: at the well bottom, and at the well head. In the very restricted space at the bottom of the producing well, solid/liquid filtration is necessary to prevent the passage up the well pipe of as much suspended solids as possible. This is done by the well screen, a zone of perforated material that is either built in to the end of the well pipe, or fitted as a sleeve over a very coarsely perforated part of the pipe. The well screen is a specialised form of filter and is, of course, used for water production as well as oil). It can be made from wire mesh, wire wound, perforated plate or porous metal fibre material. A good array of typical screen designs can be seen on the Weatherford/Johnson Screens web site (www.weatherford.com).
The design of the well screen will be tailored to the nature of the rock formation and the size of the solid particles to be retained, which is generally 50 μm or more. The prime objects of this stage of filtration are to prevent blockage in the well pipe, and to protect whatever pump is being used down-hole to carry the oil to the surface. Sandstone is the most common oil-bearing rock, so that sand particles are the most likely solid to need removing. Sand, which can vary in particle size from 100 to 400 μm, can be very abrasive, both to pumps and to the oil-carrying pipeline. The screen aperture is critical in this respect – too large in relation to the sand’s particle size, then the oil flow rate will be higher but too much sand will penetrate the screen, while if too close to the particles in size, then the oil will be very clean, but the flow rate will be low and the screen may quickly block. Careful choice of screen type and aperture size is thus necessary to avoid selecting the wrong screen. Perforated screens and woven wire meshes provide a more accurate and consistent aperture than does a mat of nonwoven metal fibres.
Once the oil reaches the surface, there is more working space for any required filtration, and the major separation requirement is to recover the crude oil from its mixture with the produced water. This is very often undertaken in liquid/liquid separators working by sedimentation, almost certainly in lamella separators for off-shore installations where not so much space is available. Production economies dictate that this separation should be as efficient as possible, since the separated water may be going to waste, carrying any unseparated oil with it.
In the liquid/liquid separator a further amount of suspended solids will also be separated, and this may be sufficient solids removal to enable the separated oil to be transported to its ultimate refinery destination without blockage of, or damage to, the transporting system. If not, then further filtration will be necessary at the well head, although the flow rates will be high, and the filters will have to be automatically (or easily manually) cleaned. Pressure leaf filters are frequently used for this purpose.
The water produced from an oil field may well be several times the quantity of the crude oil associated with it, and if it is to be disposed of in the surrounding environment (especially the sea) then it will have to be thoroughly cleansed of suspended oil and solids, in common with any other waste water discharge requirement. If, however, the oil well is in a secondary, waterflood operation, then it obviously makes sense for the necessary water to come from that produced by the well.
This water will be injected into an underground reservoir, and must be able to flow through the small passageways in the rock. This means that it will have to be filtered free of fine solids, possibly down to 2 μm at the point of injection (although there will not be the need to separate oil from it so thoroughly). Where there is the space so to do, this filtration can be achieved by deep bed (“sand”) filters, almost certainly using multi-media beds for most efficient operation. For large operations, a two-stage filtration may make sense, with a 10 μm limit for water flow through topside equipment, and further filtration down to 2 μm or less at the point of injection.
A very different water treatment exists in the production of oil from tar sands. Here the water is used, hot, to process the sands, to yield an effluent water high in sand content. There is no equivalent to water flooding, so there is need for it only when recycled to the main production process. Filtration will be needed although only to protect the operating equipment.
Natural gas produced in association with crude oil will not normally present a filtration problem – from solids at least, although it may need separation from oil or water droplets. However there is nowadays an increasing need for the injection of gases into underground strata, to improve oil production rates. This can be into the gas cap over the reservoir – when the need for filtration is low, or directly into the rock formation, both as an enhanced oil recovery process and as a sequestration method for carbon dioxide disposal.
The direct injection of gases will require that they be free from suspended solids, possibly down to the same size level as is the case for water injection, namely around 2 μm. This will be done in the same sort of filters as are used for engine intakes, using V-block minipleat filter panels, for example.
The oil production process is a good market for filtration and sedimentation equipment. Although some parts of it – the primary production – are relatively mature, others, in particular tertiary processing and recovery from tar sands, still have major growth ahead of them.
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Ken Sutherland has managed Northdoe Limited, his process engineering and marketing consultancy, for over 30 years. Northdoe is essentially concerned with filtration and related separation processes. He has written numerous articles for Filtration & Separation and for Filtration Industry Analyst, and also four books on separation processes, most recently an A to Z of Filtration and the fifth edition of the Filters & Filtration Handbook, both for Elsevier.