OverviewE&P

7/28/2019 OverviewE&P

1/21

Petroleum Economics: TBP 1

Overview of Petroleum Exploration & Production

1. Origins of Oil and Gas

Oil and gas are derived almost entirely from decayed plants and bacter ia. Energy from the

sun, which fuelled the plant growth, has been recycled into useful energy in the form ofhydrocarbon compounds - hydrogen and carbon atoms linked together.

Of all the diverse life that has ever existed comparatively little has become, or will become oiland gas. Plant remains must first be trapped and preserved in sediments, then be burieddeeply and slowly 'cooked' to yield oil or gas. Rocks containing sufficient organic substancesto generate oil and gas in this way are known as source rocks.

Dead plants usually are dispersed and decay rapidly, but in areas such as swamps, lakes andpoorly oxygenated areas of the seafloor, vast amounts of plant material accumulate. Bacteriabreaking down this material may use up all available oxygen, producing a stagnantenvironment which is unfit for larger grazing and scavenging animals. The plant and bacteriaremains become buried and preserved in mud. In swamps the remains may form coals onburial.

Whether oil or gas is formed depends partly on the starting materials. Almost all oil formsfrom the buried remains ofminute aquat ic algaeand bacteria, but gas forms if theseremains are deeply buried. The stems and leaves of buried land plants are altered to coals.Generally these yield no oil, but again produce gas on deep burial.


2/21


On burial the carbohydrates and proteins of the plant remains are soon destroyed. Theremaining organic compounds form a material called kerogen. Aquatic plants and bacteriaform kerogen of different composition from woody land plants.

The processes of oil and gas formation resemble those of a kitchen where the rocks areslowly cooked. Temperatures within the Earth's crust increase with depth so that sediments,

and kerogen which they contain, warm up as they become buried under thick piles ofyounger sediments.

As a source rock, deposited under the sea or in a lake, becomes hotter (typically >100oC),

long chains of hydrogen and carbon atoms break from the kerogen, forming waxy andviscous heavy oil. At higher temperatures, shorter hydrocarbon chains break away to givemore valuable light oil and then, above about 160

oC, gas. The woody kerogen of coals yield

mainly methane gas, whose molecules contain only one carbon atom.

Once a source rock has started to generate oil or gas it is said to be mature. The mostimportant products generated are gas, oil, oil containing dissolved gas, and gascontaining dissolved oil which is called gas condensate. Condensate is the light oil which

is derived from gas condensates which are found at high underground temperatures andpressures.

In the North Sea, oil forms at 3-4.5km depth, gas at 5-6km. At greater depths any remainingkerogen has become carbonisedand no longer yields hydrocarbons. Burial to these depthsoccurs in areas where the Earth's crust is sagging.

MigrationMuch oil and gas moves away or migrates from the sourcerock. Migration is triggered both by natural compaction ofthe source rock and by the processes of oil and gasformation. Most sediments accumulate as a mixture ofmineral particles and water. As they become buried, somewater is squeezed out and once oil and gas are formed,these are also expelled. If the water cannot escape fastenough, as is often the case from muddy source rocks,pressure builds up. Also, as the oil and gas separate fromthe kerogen during generation, they take up more spaceand create higher pressure in the source rock. The oil andgas move through minute pores and cracks which mayhave formed in the source rock towards more permeablerocks above or below in which the pressure is lower.

Oil, gas and water migrate through permeable rocks inwhich the cracks and pore spaces between the rock

particles are interconnected and are large enough to permitfluid movement. Fluids cannot flow through rocks wherethese spaces are very small or are blocked by mineralgrowth; such rocks are impermeable. Oil and gas alsomigrate along some large fractures and faults which mayextend for great distances if, or when as a result ofmovement, these are permeable.


3/21


Oil and gas are less dense than the water whichfills the pore spaces in rocks so they tend tomigrate upwards once out of the source rock.Under the high pressures at depth gas may bedissolved in oil and vice versa so they maymigrate as single fluids. These fluids may

become dispersed as isolated blobs throughlarge volumes of rock, but larger amounts canbecome trapped in porous rocks. Havingmigrated to shallower depths than the sourcerocks and so to lesser pressures the single fluidsmay separate into oil and gas with the less densegas rising above the oil. If this separation doesnot occur below the surface it takes place whenthe fluid is brought to the surface. Water isalways present below and within the oil and gaslayers, but has been omitted from most of thediagrams for clarity.

Migration is a slow process, with oil and gastravelling between a few kilometres and tens of kilometres over millions of years. But in thecourse of many millions of years huge amounts have risen naturally to sea floors and landsurfaces around the world. Visible liquid oil seepages are comparatively rare, most oilbecomes viscous and tarry near the surface as a result of oxidation and bacterial action, buttraces of natural oil seepage can often be detected if sought.

Trapping Oil and GasOilfields and gasfields are areas wherehydrocarbons have become trapped in permeablereservoir rocks, such as porous sandstone orfractured limestone. Migration towards the surface

is stopped or slowed down by impermeable rockssuch as clays, cemented sandstones or salt whichact as seals. Oil and gas accumulate only whereseals occur above and around reservoir rocks soas to stop the upward migration of oil and gas andform traps, in which the seal is known as the caprock. The migrating hydrocarbons fill the highestpart of the reservoir, any excess oil and gasescaping at the spill point where the seal does notstop upward migration. Gas may bubble out of theoil and form a gas cap above it; at greater depthsand pressures gas remains dissolved in the oil.Since few seals are perfect, oil and gas escape

slowly from most traps. In many fields incoming oiland gas balance this loss, as in the Brent andEkofisk fields in the North Sea. Gas migrates andescapes from traps more readily than oil, but thesalt layers beneath the southern North Sea where much gas is trapped have proved a veryefficient seal because salt contains no pore spaces, and fractures reseal themselves.

Structural traps are formed where rocks are folded into suitable shapes (A) or reservoir andsealing rocks are juxtaposed across faults (B). Traps may also form when rocks are domedover rising salt masses (C). Stratigraphic traps originate where a suitable combination of rocktypes is deposited in a particular environment (D), for example, where a reservoir rock ofpermeable river sand is sealed by clays accumulated in the swamps which formed to coverthe river channel. In reality most traps are formed by more complex sequence of events and

cannot be classified so rigidly. For example (E), the reservoir rock was first folded and eroded,then sealed by an impermeable rock which was deposited later over the eroded structure.


4/21


Where a particular set of circumstances has combined to produce a group of oil orgasfields with similar trap structures or reservoir rock, this is termed a play.

In order to trap migrating oil and gas, structures must exist before hydrocarbongenerat ion occurs . In some parts of the North Sea trap structures existed 125 million yearsago, but were not filled with oil until 100 million years later. The rocks beneath the North Seaare sinking only a few millimetres in ten years, so generation only occurred after very longperiods of burial and 'cooking'. All oil and gas fields form by a chance combination of eventsthat produces the right sorts of rocks and structures, together with the right timing.

Forces That Shape the Earth's CrustThe white-hot, partially molten interior of the Earth is in

constant motion. This transmits itself to the more rigid outerlayer, the lithosphere, which is also constantly on the move.

Newlithosphereis createdalong mid-oceanridgeswheremoltenrock isinjected,cooling to

form newoceancrust, thetop layer of this young lithosphere. Thelithosphere moves away from the ridges inthe process of sea-floor spreading, and isdestroyed wherever it slides back into theEarth, along subduction zones. Since it isthicker and lighter than the oceanic crust,continental crust is not subducted and so ismostly much older than oceanic crust. Thegreat slabs of lithosphere between mid-ocean ridges and subduction zones are

called plates.The complex interactions ofoceanic and continental lithosphere,


5/21


powered by plate movements are called plate tectonics. In addition to the opening out ofocean basins, the main effects of plate tectonics are the growth and break-up of continents.

Continents grow by formation of new continental crustalong volcanic belts and by the addition of terranes, whichare pieces of continental material and ocean island arcs

formed elsewhere and rafted into older continents by sea-floor spreading. These collisions telescope the continentalcrust and produce mountain ranges. Conversely, wherethe spreading process locates itself under a continent, thecontinent may eventually split apart. A new ocean will formbetween the rifted parts which may then travel longdistances as parts of the moving plates. The rate of growthand horizontal movement of plates is anything from about2cm to 10cm per year, about the same as one's fingernails.The drifting of rifted continents may carry them throughseveral climatic zones, for example, from equatorial humidthrough tropical arid to temperate and arctic, over tens or hundreds of million years (F14).This is of great importance to the generation and trapping of oil and gas, as are the structural

disruptions brought about by plate tectonics.

Large areas of the continentalcrust are covered by layers ofsedimentary rock which arethickest in the middle of basins.Nearly all oil and gas is found insuch basins, which are formedover many millions of years bystretching of the crust combinedwith sagging. The North Sea is aclassic example. Most basinshave a two-tier structure; thelower tier is faulted into blockswhile the upper-tier is a simplesag . There are different theoriesto explain basin formation. The lithosphere may stretch uniformly like toffee, fracturing theupper brittle layers into tilted blocks, then sag as the underlying, partly-molten layer(asthenosphere) cools down. Alternatively the entire lithosphere may be detached along ahuge low-angle fault to which curved listric block-faults are linked. The reality may be a

combination or stretching atdepth with detachment high upin the lithosphere.

Compression of the upper

continental crust by platetectonic mechanisms results inbuckling and telescoping of rocklayers to form fold and thrustbelts. The telescoping is oftenrelated to a deep detachment,above which a stack of thrustsheets pile up. Large masses oflightweight granite givebuoyancy to the crust. The highsthat result may be marked byreduced deposition of sedimentsor actual emergence and

erosion. Beyond the thrust belt, rock strata may undergo compression. This tends to expel thecontents of basins upwards and outwards in a process termed inversion. The expulsion oftentakes place along the same listric faults that guided the basin's development. Basin inversion


6/21


is a very important mechanism in gas and oil field formation. It may create good structuraltraps for oil and gas, and may prevent "over-cooking" of the source rock. However, it mayalso permit the escape of hydrocarbons or cause erosion of source rocks or reservoir rocks.

2. Discovering the Underground Structure

Large-scale geological structures that might hold oil or gas reservoirs are invariably locatedbeneath non-productive rocks, and in addition this is often below the sea. Geophysicalmethods can penetrate them to produce a picture of the pattern of the hidden rocks.Relatively inexpensive gravity and geomagnetic surveys can identify potentially oil-bearingsedimentary basins, but costly seismic surveys are essential to discoveroi l and gasbear ing structures.

Sedimentary rocks are generally of low density and poorly magnetic, and are often underlainby strongly magnetic, dense basement rocks. By measuring 'anomalies' orvar iat ions fromthe regional average, a three-dimensional picture can be calculated. Modern gravitysurveys show a generalised picture of the sedimentary basins. Recently, high resolutionaero-magnetic surveys flown by specially equipped aircraft at 70 - 100m altitude show fault

traces and near surface volcanic rocks.

Shooting seismic surveys

More detailed information about the rock layers within such an area can be obtained by deepecho-sounding, orseismic reflection surveys. In offshore areas these surveys areundertaken by a ship towing both a submerged airorwater gun array, to produce shortbursts of sound energy, and a set ofstreamers of several kilometres length. Each streamercontains a dense array ofhydrophone groups that collect and pass to recorders echoes ofsound from reflecting layers. The depths of the reflecting layers are calculated from the timetaken for the sound to reach the hydrophones via the reflector; this is known as the two-waytravel time. The pulse of sound from the guns radiates out as a hemispherical wave front, aportion of which will be reflected back towards the hydrophones from rock interfaces. Thepath of the minute portion of the reflected wave-front intercepted by a hydrophone group iscalled a ray path. Hydrophone groups spaced along the streamer pick out ray paths that canbe related to specific points on the reflector surface. Graphs of the intensity of the recordedsound plotted against the two-way time are displayed as wiggle traces.

Seismic recording at sea always uses the common depth point (CDP) method. A sequenceof regularly spaced seismic shots is made as the survey vessel accurately navigates itscourse. Shots are usually timed to occur at distances equal to the separation of thehydrophone groups. In this way up to 120 recordings of the echoes from any one of 240reflecting points can be collected. Each represents sound, which has followed a slightlydifferent ray path, but has all been reflected from the same common depth point.


7/21


Data Processing

Processing recordings involves many stages of signal processing and computer summing.Firstly, wiggle traces from a single CDP are collected into groups. Displayed side by side insequence they form a CDP gather. Reflections from any one reflector form a hyperbolic curveon the gather because the sound takes longer to travel to the more distant hydrophones. Thiseffect is called normal move out (NMO). Correction is needed to bring the pulses to ahorizontal alignment, as if they all came from vertically below the sound sourc. The separatewiggle traces are added together, or stacked. Stacking causes true reflection pulses toenhance one another, and hopefully, random noise will cancel out. This process is repeatedfor all the CDPs on the survey line. The stacked and corrected wiggle traces are displayedside by side to give a seismic section. Most seismic sections used by the oil and gas industryare time-sections that have undergone a long sequence of data-processing steps designed toimprove the quality of the reflections and bring out subtle geological features. For particular

purposes, after the principal reflectors have been identified or 'picked', a time-section may beconverted to a depth-section. For this and also for NMO corrections before stacking, thevelocities of sound in the rock layers traversed by the section need to be known. Computeranalysis of traces during NMO corrections yields velocity values, but more accurate datacomes from special velocity surveys carried in wells in conjunction with sonic logging.

Dataprocessinglessens theimpact ofvariousundesirableeffects that

obscure thereflectedsignals; it alsocompensatesfor someintrinsicdeficiencies ofthe CDPmethod.Undesirableeffects includemultiples,where the

sound is reflected repeatedly within a rock formation and, because this takes time, registersas a deeper reflector; reflections between the water surface and the seabed are a similar


8/21


phenomenon known as ringing. Diffractions are hyperbolic reflections from the broken end ofa reflector; they mimic arched formations. Random noise, mainly unwanted reflections fromwithin rock layers, horizontally propagated and refracted sound, bubble pulsations from theairguns and other effects also need to be reduced. Stacking reduces multiples and randomnoise, but the main computer processing steps are deconvolution, muting and filtering, andmigration. Deconvolution ('decon') aims to counteract the blurring of reflected sound by

'recompressing' the sound to the clean 'spike' emitted from the source. The result is clearerreflections and the suppression of multiples. Muting cuts out parts of traces embodying majordefects such as non-reflected signals; filtering removes undesirable noise to enhance the bestreflections. Finally, migration corrects distortions caused by plotting inclined reflectors as ifthey were horizontal and vertically below the midpoint between shot and receiver; it alsocollapses diffractions. In this process, the seismic energy is relocated to its true subsurfacelocation, ready for interpretation.

Interpretation

Seismic sections provide 2-dimensional views ofunderground structure. By using special shootingtechniques such as spaced airgun arrays or towing thestreamer slantwise, or by shooting very closely spaced lines,it is possible to produce 3-dimensional (3D) seismicimages. These images comprise vertical sections andhorizontal sections ('time-slices').

Seismicstratigraphy is thestudy of thedepositionalinterrelationshipsof sedimentaryrock as deduced from an interpretation of seismic

data; it can be used in finding subtle sedimentary traps involving changes in porosity. 'Bright-spots', short lengths of a reflection that are conspicuously stronger than adjacent portionsmay indicate gas: the velocity of sound is sharply reduced in gas-bearing rock, producing astrongly reflective contrast. A gas-water or gas-oil interface may stand out as a noticeably flat

reflection amongst arched reflections.

The end-products of seismic surveys are interpretedsections showing geological structure down to finesedimentary details. Maps are used to describe thetopology of known rock units and 'isopach' maps areshowing the thickness of these units. For the maps,reflections are 'picked' and their depths at points alongparallel and intersecting survey lines plotted and

contoured.

Seismic sections that have been picked by hand aredigitised and the digital files entered into a gridding andcontouring program. Contour maps can be plotted or3D colour and shade enhanced images can begenerated to illustrate the subsurface structure. Some

rock layers produce wiggles with a distinctive character that can be followed right across asection; others may be identified by comparison with synthetic 'seismograms' made fromlogging and velocity surveys in existing wells in which the rock sequence is known.


9/21


The seismic maps are used to identify structures that would either repay more detailedseismic surveying or would warrant wildcat drilling. The interpreter studies the maps toidentify areas that are shallower and form a dome shape (an anticline) or a shallow areasurrounded by faults (a horst block) - within such structures it is possible that migrating oil orgas may have been trapped.

Initially 3D seismic surveys were used over the relatively small areas of the oil and gasfieldswhere a more detailed subsurface picture was needed to help improve the position ofproduction wells, and so enable the fields to be drained with maximum efficiency. In the early1990's, when exploration shifted to smaller and more subtle traps, 3D seismic surveysbecame more widely used for exploration work. The vast amount of data generated by even asmall 3D survey meant that computer workstations were an essential tool for interpreting thedata quickly. With a computer an interpreter can map a specific reflector by moving the cursoralong it on the screen or, when a reflector is strong and continuous, the computer can 'auto-pick' that horizon through the whole 3D data set. Digital files of reflector picks can betransferred directly from the interpreter's workstation to mapping software. Visualisationsoftware is an additional tool that allows the interpreter to view the whole 3D data set as acube and rotate or cut it at any angle, allowing a picture of the subsurface geometry to bequickly seen.

Latest developments

Recent increases in computing capacity have enabled the migration process to be appliedbefore stack, i.e. on the vast amounts of data collected in the acquisition phase. This pre-stack depth migration (PSDM) application is critical in areas with complex geologicalsubsurface structures, such as around/below salt domes and other high-velocity layers. Thishas led to the first reliable seismic images of sediments located below such complicatedoverburden structures.


10/21


Because of the greatly improved seismic resolution of 3D seismic imaging, there has been aneffort to reduce the cost of 3D data acquisition and shorten the time it takes to acquire andprocess the large volumes of data acquired. In the past it could take up to 24 months toprocess the recordings from a 3D survey. Acquisition time has been cut by specially designedsurvey vessels deploying up to ten multiple streamers at a time (F62), or by using multiplevessels. These techniques allow a swath of seismic data to be acquired in the same time it

previously took to record a single 2-dimensional line. Specially designed paravanes steer thecables away from each other. Their design reduces the drag of the streamer array, whichordinarily would be sufficient to stop even quite a powerful vessel. Modern streamers havemultiple global positioning system (GPS) sensors that constantly record the position of thestreamers relative to the vessel and the earth.


11/21


New techniques of data compression are being tried to allow the transmission of the rawseismic records from the acquisition vessel to the shore for immediate processing, in an effortto get the data to the interpreters faster.

Improved resolution and reduced acquisition/processing times have opened up the possibilityof shooting seismic at different time intervals over the same area of a producing field, in order

to detect changes. These changes with time will clarify how a field is behaving by revealingexactly where the fluids are or are not moving, or by revealing changes in pressure in differentparts of the field, thereby indicating how production might be improved. This is the so-called4D or time-lapse seismic, where time is essentially the "fourth dimension". Results in recentyears have been quite astonishing.

If seismic is to be acquired at regular intervals over the same field, then it can be economic topermanently install an array of hydrophones on cables buried just beneath the seafloor.Another recent development is that visualisation has been taken to a new level with theadvent ofVirtual Reality rooms, allowing 3D subsurface images to be displayed on largescreens and to be viewed from almost any angle. Different development options, such as theimpact of various drilling targets, can be simulated. Much of the benefit of this approachstems from the fact that communication and understanding are greatly enhanced when multi-discipline teams meet whilst "immersed" in such an environment.


12/21


3. Drilling

There are two basic types of drilling rigs - fixed platform rigs and mobile rigs. Fixedplat form r igsare installed on large offshore platforms and remain in place for many years.

Mobi le r igscomprise two types:jack-up rigs used inshallow water less than 100 metres deep and semi-submersible rigs used in deeper waters down to 1000metres or more. In very deep waters, drilling ships areused. Jack-up rigs have lattice legs which are lowered tothe seabed before the floating section carrying the derrickis raised above the sea surface. Semi-submersible rigsfloat at all times, but when in position for drilling areanchored and ballasted to float lower in the water withtheir pontoons below wave-level. Some have 'dynamic-positioning' propellers and can drill in very deep water.

The drilling derrick towers above the drill floor and is wheremost of the activity is concentrated. The derrick supports theweight of the drillstring which is screwed together from 9-metre lengths ofdrill pipe. Hoisting equipment in the derrickcan raise or lower the drillstring. At the bottom of the drillstringis a drill bit, which can vary in size and type. It is attached tothe drill collars, heavy pipe-sections that put weight on the bit.On semi-submersible rigs, a compensator keeps the drillstringstationary while the rig and derrick move as a result of wavemotion. The drill bit is rotated either by turning the wholedrillstring ("rotary drilling") or by using a downhole turbinewhich rotates as drilling fluid is pumped through it. In rotarydrilling, the rotary motion is imparted to the drillstring by the "top drive". This is an electro-

hydraulic motor suspended in the top of the derrick. It is attached to the top of the drillstringand imparts torque to it, causing it to rotate. To add a new section of drill pipe the drillstring isclamped in the drill floor with wedges (slips) and the top drive disconnected. The new joint is


13/21


screwed into the drillstring suspended in the drill floor, the top drive connected to the top ofthe new joint, and drilling restarted. The raising and lowering of the top drive and themaintenance of correct tension on the drillstring is controlled by the driller operating thedrawworks lever in a control cabin (called the "doghouse") on the drill floor.

Drilling fluid (also called "mud"), which is mainly water-

based, is pumped continuously down the drillstring whiledrilling. It lubricates the drilling tools, washes up rock cuttingsand most importantly, balances the pressure of fluids in therock formations below to prevent b lowouts.

In offshore drilling, the first step is to put down a wide-diameterconductor pipe into the seabed to guide the drilling and containthe drilling fluid. It is drilled into the seabed from semi-submersible rigs, but on production platforms a pile-driver maybe used. As drilling continues, completed sections of the wellare cased with steel pipe cemented into place. A blowout

preventer(BOP) is attached to the top of the casing. This is a stack of hydraulic rams whichcan close off the well instantly ifback pressure (a kick) develops from invading oil, gas orwater.

A typical problem faced while drilling is the drillstring sticking in difficult rock formations. Ahydraulic device known as ajar, mounted between the drill collars, can give the drillstring aseries of jolts. If that does not work, other techniques may be used, including spotting with oiland water. Special fishing tools can also retrieve stuck pipe and broken equipment (junk).

Drilling grinds up the rock into tea-leaf-sized cuttingswhich are brought to the surface by the drilling mud. Thedrilling mud is passed over a shale shaker which sievesout the cuttings. In exploration drilling, the cuttings aretaken for examination by a geologist known as a

mudlogger who is constantly on the lookout for oil and gas.Oil entrapped in the mud is detected by its fluorescence inUV light. Gas is extracted from the mud in a gas trap andsent under vacuum to a gas detector and analyser. Anincrease in the amount triggers an alarm to alert themudlogger and the drilling superintendent. If laboratorytests are needed on potential reservoir rock, a solid coreof rock can be drilled by a special hollow drilling bit. Eachshort length of core retrieved calls for the entire drillstringto be pulled out of the well and then reinserted, so coring is an expensive operation notundertaken lightly.


14/21


Getting the Most out of a WellVital information on the type of rock drilled and the fluids it contains often needs to beobtained either while actually drilling, or after drilling beforerunning casing. This is obtained by running electronic measuringdevices into the well - either while drilling (as part of the drillstring)or after drilling on "wireline". The various types of measurement

include: (1) electrical resistivity of fluids within the rock; (2) thespeed of sound through the rock; (3) reaction of the rock togamma ray bombardment; (4) production of gamma rays fromfluids within the rock due to neutron bombardment; and (5) naturalgamma radiation of the rocks. The data obtained give indicationsof rock type and porosity and the presence of oil or gas.

Other devices measure hole diameter, dip of strata and thedirection of the hole. Sidewall corers which punch or drill out smallcores of rock, geophones for well velocity surveys and seismicprofiling are also lowered into uncased wells. In deviated wellsapproaching the horizontal, flexible high-pressure steel coiledtubing may be used to carry wireline logging tools and for

performingwellboremaintenanceoperations. Ifoil or gas hasbeendetected in awell, a tool islowered on awireline tomeasure fluid pressures and collectsmall samples. If the flow rate of the wellneeds to be measured, a "well test" iscarried out. This involves running

production tubing with flow control valves and isolation packers into the well, then flowing thehydrocarbons to surface through the high pressure pipework containing pressure recordersand flowmeters.

4. Developing a DiscoveryWhen promising amounts of oil and gas

are found in an exploratory well, aprogramme of detailed field appraisalmay begin. The size of the field must beestablished, and the most efficientproduction method worked out in order toassess whether it will repay, with profit,the huge costs of offshore developmentand day-to-day operation. Appraisal maytake several years to complete and isitself very costly.

Appraisal draws together informationfrom all available techniques. Detailed seismic surveys build up an accurate 3-dimensionalimage of the discovery, and appraisal wells are drilled to confirm the size and structure ofthe field. Wireline logging in each new well yields data on porosity and fluid saturation and


15/21


the thickness of the hydrocarbon-bearing rocks, while production testing yields hydrocarbonsamples and information on reservoir productivity, temperatures and pressures. Oil, gas andreservoir rock samples are analysed in the laboratory. Most fields have both good and badfeatures which must be fully considered when deciding whether to develop.

Production may prove difficult and expensive if the reservoir rock is seriously disrupted by

faulting or contains extensive areas of poor permeability. Porosity and permeability may varydramatically where the reservoir rock consists of a variety of sediments, and may be muchreduced in areas where mineral growth has blocked the available pore spaces. Geologistscompare core samples from the deeply buried reservoir rock with present-day sediments toidentify the environment in which it accumulated. This environment is used to develop ageological model to help predict likely variations in the reservoir rock types and properties. If,for example, the best-quality reservoir rock is a dune sand or a beach sand, its likely extentand thickness can be estimated from the size and shape of a comparable modern dunecomplex or beach. The identification of microfossils that inhabited particular environments,such as shallow seas or brackish lagoons, helps confirm the model, as well as indicating theage of the reservoir rock. Geologists and reservoir engineers use the geological model toselect the best sites for production wells.

How Much Oil and Gas?

When deciding whether to develop a field, a company must estimate how much oil and gaswill be recovered and how easily they will be produced. Although the volume of oil and gas inplace can be estimated from the volume of the reservoir, its porosity, and the amount of oil orgas in the pore spaces, only a proportion of this amount will be recovered. This proportion isthe recovery factor, and is determined by various factors such as reservoir dimensions,pressure, the nature of the hydrocarbon, and the development plan.

Pressure is the driving force in oil and gas production. Reservoir drive is powered by thedifference in pressures within the reservoir and the well, which can be thought of as a columnof low surface pressure let into the highly pressured reservoir. If permeability is good and the

reservoir fluids flow easily, oil, gas and water will be driven by natural depletion into the welland up to the surface. Expansion of the gas cap and water drives oil towards the well bore.Gas and water occupy the space vacated by the oil. In reservoirs with insufficient natural driveenergy, water or gas is injected to maintain the reservoir pressure.


16/21


The proportion of oil that can be recovered from a reservoir is dependent on the ease withwhich oil in the pore spaces can be replaced by other fluids like water or gas. Tests onreservoir rock in the laboratory indicate the fraction of the original oil in place that can berecovered. Viscous oil is difficult to displace by less viscous fluids such as water or gas as thedisplacing fluids tend to channel their way towards the wells, leaving a lot of oil in thereservoir. The quoted recovery factorfor most North Sea fields is about 35 percent, but maybe as low as 9 percent where the oil is very viscous, or perhaps as high as 70 percent wherereservoir properties are exceptionally good and the oil of low viscosity. The recovery factor ingasfields is much higher, figures ofover 85 percent being quoted for most.

Each oil and gas reservoir is a unique system of rocks and fluids that must be understoodbefore production is planned. Petroleum engineers use all the available data to develop amathematical model of the reservoir. Computer simulations of different production techniquesare tried on this reservoir engineering model to predict reservoir behaviour during production,and select the most effective method of recovery. For example, if too few production wells aredrilled water may 'cusp' or channel towards the wells, leaving large areas of the reservoirupswept.

Factors, such as construction requirements, cost inflation and future oil prices must also beconsidered when deciding whether to develop an oil or gas field. When a company is satisfiedwith the plans for development and production, they must be approved by the Government,which monitors all aspects of offshore development.


17/21


Production PlatformsMost oil and gas production platforms in offshore rest on steel supports known as 'jackets', aterm derived from the Gulf of Mexico. A small number of platforms are fabricated fromconcrete. The steel jacket, fabricated from welded pipe, is pinned to the sea floor with steelpiles. Above it are prefabricated units or modules providing accommodation and housingvarious facilities including gas turbine generating sets. Towering above the modules are the

drilling rig derrick (two on some platforms), the flare stack in some designs (also frequentlycantilevered outwards) and service cranes. Horizontal surfaces are taken up by store areas,drilling pipe deck and the vital helicopter pad.

Concrete gravity platforms are so-called because their great weight holds them firmly on theseabed. They were first developed to provide storage capacity in oilfields where tankers wereused to transport oil, and to eliminate the need for piling in hard seabeds. The Brent Dplatform, which weighs more than 200 000 tonnes, was designed to store over a millionbarrels of oil. But steel platforms, in which there have been design advances, are nowfavoured over concrete ones.

Several platforms may have to be installed to exploit the larger fields, but where the capacityof an existing platform permits, subsea collecting systems linked to it by pipelines have beendeveloped using the most modern technology. They will be increasingly used as smaller fieldsare developed. For very deep waters, one solution was the Hutton Tension Leg Plat form:the buoyant platform, resembling a huge drilling rig, is tethered to the sea-bed by jointed legskept in tension by computer-controlled ballast adjustments.

Alternatively, a subsea collection system may be linked via a production riser to a Floating,Production, Storage and Offloading (FSPO) vessel; either a purpose built ship or aconverted tanker or semi-submersible rig. The oil is offloaded by a shuttle tanker.


18/21


Functions of a Production Platform

Oil platforms are an industrial town at sea, carrying the personnel and equipment needed forcontinuous hydrocarbon production. The most important functions are drilling, preparing wateror gas for injection into the reservoir, processing the oil and gas before sending it ashore, andcleaning the produced water for disposal into the sea. Power is generated on the platform to

drive production equipment and support life. All production systems are constantly monitoredfor leaks, since oil and gas are hazardous and extremely flammable.

The top of each production well sprouts a branching series of pipes, gauges and valves calledthe 'Christmas tree'. At this point, crude oil is a hot, frothy, corrosive, high-pressure fluidcontaining gas, water and sand. After separation, the crude oil is metered and pumped intothe pipeline, or stored until sent ashore by tanker. The gas separated from the oil may beused for fuel, or compressed and piped to shore or re-injected into the reservoir. Any gas thatcannot be used or piped ashore must be burnt in the platform'sflare. Very little gas is now flared. Processing systems for thegasfields of the southern North Sea are relatively simple. Anyliquids dissolved in the gas are removed, then the gas iscompressed, cooled, dehydrated and metered before being

piped to shore.

Production WellsTo develop offshore fields as economically as possible,numerous directional wells radiate out from a single platformto drain a large area of reservoir. For directional drilling specialweighted drill collars are used with a 'bent sub' to deflect the drillbit at a certain angle in the required direction. Wells whichdeviate at more than 65 degrees from the vertical and reach outhorizontally more than twice their vertical depth are known asextended reach wells. In order for the driller to guide thedeviated well to a specific target zone in the reservoir amon itor ing-whi le-dr i l l ing (MWD) 'd irect ional sub'is runabove the bit to relay information back to the surface on the bitlocation and inclination. This information can be transmitted to


19/21


the surface using a mud-pulse telemetry system or recorded in the directional sub andrecovered when the bit is changed.

As the angle of deviation from the vertical increases, the friction of the rotating drillstringbecomes excessive. Also, as drilling becomes slower the risk of sticking the drilling assemblyagainst swelling shales rapidly increases. Environmental restrictions limit the use of friction-

reducing oil-based muds in many areas, so that oil-contaminated cuttings from wells need tobe shipped back for onshore disposal. The alternate is water-based which needs additives toreduce its frictional effects, and to inhibit its chemical reactivity with the clays drilled.

Deviated wells which exceed 80 degrees from the verticalare known as horizontal wells and the horizontal section ofthe well is maintained in the reservoirs to give the highestproduction rate possible. Horizontal wells are used whenthe reservoir permeability is low, or the reservoir interval isvery thin or the oil and gas is being produced from verticalfractures in the rock. The flow from a horizontal well may beover 5 times the flow from a normal vertical well. The higherflow rates more than offset the higher cost of drilling a

horizontal well.

More than one horizontal section can be drilled in one well as a multilateral well (F96). Thistechnique is used to reduce drilling costs and to maximise the number of wells that can bedrilled from small platforms.

Getting Every Last Drop Out

Crude oil can contain acidic fluids including hydrogen sulphide and carbon dioxide whichcorrodes casing. If necessary high grade steel production tubing is inserted into the well tocollect oil and gas and protect the casing. Access to the reservoir is achieved either byperforating holes through the casing installed across the reservoir using small explosive

charges, or by running casing with pre-drilled holes or slots. Many sandstone reservoirs areliable to collapse and produce sand along with petroleum - in these wells "sand screens",


20/21


which filter out the sand particles downhole, are run. Flow from the well is controlled by valveson the "Chr istmas tree" at the wellhead.

For smaller fields, rather than beingdrilled from a large central platform, thewells are drilled from subsea clusters.

For these types of wells, the wellheadand Christmas tree is installed directly onthe seabed, with production from severalwells co-mingled at a subsea manifold.Subsea manifolds are often linked bypipelines and umbilical control lines backto a nearby platform, where engineerscan control and monitor the oil and gasproduction. Alternatively, the productioncan be piped to a Floating, Production,Storage and Offloading vessel (FPSO)for processing and export. Floatingproduction facilities are generally less expensive to install than fixed platforms but their

operating costs are higher. The disadvantage of floating production systems is the weathermay prevent the docking of the offloading shuttle tankers for several days during the winterperiod.

In oil reservoirs, to achieve as high a recovery factor as possible reservoir pressures must notbe allowed to fall too low as oil and associated gas are removed. It is desirable to maintainpressures above the point where dissolved gas in the oil comes out of solution to form freegas. Seawater is pumped into the water-soaked rocks beneath the oil zone in volumes equalto the sub-surface volume of the liquids produced. Water injection wells are usually locatedaround the periphery of an oilfield. Gas separated from oil on the platform may also becompressed and injected into the reservoir rocks to maintain pressure. Water and gasinjection can improve recovery of oil from less than 15 percent to more than 50 percent. Verydeep fields, such as Brae, with high pressures and temperatures may yield condensate, avaluable light oil which exists as dissolved in gas in the reservoir. Dry gas will be injected intothe reservoir to maintain pressure, thus avoiding condensate drop out, and to sweep the gascondensate to the wells. Downhole pumps have been used offshore when reservoir pressuresare insufficient to send the oil to the surface, as in the Beatrice Field. A more commontechnique is gas lift in which gas from the same nearby field is mixed with oil in the tubing tolessen the weight of the liquid column.

Flow from every oil and gas well is tested and monitored throughout the life of the well.Replacement of worn equipment such as tubing and valves helps prolong the life of the well.In less productive wells, well stimulation may be tried. High-pressure fluids are pumped down

the well to create deep fractures inthe reservoir rock through which oil

and gas can flow. These fractures areheld open by sand grains which areforced into the fracture with the fluid.Acid stimulation helps removeclogging mineral scale such ascalcium carbonate which may haveaccumulated during years ofproduction.

In extended reach and horizontalwells, coiled tubing is often used tocarry production equipment to thebottom of the well. Coiled tubing is

more flexible and much quicker to use than the conventional drillstring.


21/21

Getting Oil and Gas Ashore

Most offshore oil and all offshore gas are brought to shore by pipelines which operate in allweathers. Pipeline routes are planned to be as short as possible. Slopes that could put stresson unsupported pipe are avoided and seabed sediments are mapped to identify unstableareas and to see if it will be possible to bury the pipe. Pipeline construction begins onshore,

as lengths of pipe are waterproofed with bitumen and coated with steel-reinforced concrete.This coating weighs down the submarine pipeline even when it is filled with gas. The preparedpipe-lengths are welded together offshore on a laybarge . As the barge winches forward on itsanchor lines, the pipeline drops gently to the seabed, guided by a 'stinger'. The inside ofpipelines need to be cleaned regularly to remove wax deposits and water: to do this acollecting device known as a pig is forced through the pipe.

Where tankers transport oil from small or isolated fields,various oil storage systems may be used. These mayrange from cylindrical cells contained in some of themassive concrete structures, to seabed storage unitssuch as that employed at the Kittiwake field, or integralstorage such as that contained in the various Floating,Production, Storage and Offloading vessels. In essencethese FPSOs are floating storage tankers, as well asproduction and processing installations. FPSOs provide an important option for developingfields which may be remote from existing infrastructure or where the field recoverablereserves are uncertain, for example because of difficult geological conditions.

In onshore terminals, carefully landscaped to minimise their environmental impact, crude oiland gas undergo further processing. Any remaining water and gas are removed from oilwhich is stored at the terminal before transport to refineries. Gas is dried and then given itscharacteristic smell before entering the national grid. During transportation, great care istaken to avoid or deal effectively with spillage.

Documents

OverviewE&P