me4105 nus offshore oil and gas technology lecture 10

8/11/2019 ME4105 NUS Offshore Oil and Gas Technology Lecture 10

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Offshore Oil & Gas Technology


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Gas injectionin secondary recovery is currently and

usually only applied to reservoirs which have a gas capwhere gas drive can use the effects of gravity (the

density difference between the gas and the oil.

Injection gas may come from the produced oil after

separation. But this gas is a marketable product and gasinjection may be costly in terms of deferred revenue, as

well as the required equipment costs. On the other hand,

a powerful incentive for gas injection is if they is gas

stranded from market due to absence of an export route,

or the available pipeline already has full capacity, andwhen flaring is not permitted. Gas injection is also an

effective way of disposing of produced gas while

conserving it for future recovery. This is particularly so

for gas injection offshore or in other remote locations.

Improve Oil Recovery


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Immiscible gas injection uses gases that will not mixwith the oil this includes natural gas, flue gas, andnitrogen. The natural gas produced with the oil can be

reinjected into the well to maintain formation pressure.Immiscible gas injected into the well behaves in a mannerto that of a gas-cap drive. Gas injection requires the use ofcompressors to raise the pressure of the gas so that it willenter the formation.

Miscible gas injectionin Enhanced Oil Recovery usesgas which are miscible with the oil produced includespropane, methane under high pressure, methane enrichedwith liquid hydrocarbons, nitrogen under high pressure andcarbon dioxide used alone or followed by water. All of

these are effective in displacing most of the trappedreservoir oil. However certain economic consideration andpractical field application problems may restrict the use ofsome.

The injection gas should be inexpensive, do not corrodemetal equipment in the well and should not mix with naturalgas in the reservoir to form an explosive combination.

Gas Injection


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Miscible Gas flooding The principle of miscible displacementis to reduce or

eliminate the interfacial tension forces between the

displacing and displaced fluids, so that the residual oilsaturation in the swept zone can be reduced.

In the 1970s, carbon dioxide began to be used frequentlyas an injection gas. During a carbon dioxide flood, CO2is usually brought to the in by pipeline from CO2wells or

trucked in as a liquid. It is also available as a byproduct ofpower, chemical and fertilizer plants and coal gasification.When CO2is injected into the reservoir, it is miscible withoil, making the oil more fluid. The CO2gas then pushesthe fluid oil through the reservoir toward producing wells. Itcan often recover about 35% of the remaining oil.

Because of the very low viscosity of the CO2, it tends tofinger and break through to producing well leavingunswept areas in the reservoir. To prevent this, alternating

volume of water and gas can into the reservoir in a Water-alternating-gas(WAG) process.


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Chemical Flood A chemical floodis a process in which different fluids are

injected into the depleted reservoir in separate batches

(slugs). The fluids, each serving a different purpose,move as separate fronts from the injection wells, throughthe reservoir rock toward the producing wells.

In a micellar-polymerflood, a slug of reservoir water is firstinjected to condition the reservoir as it moves ahead of other slugs

of injected chemicals. Next, a slug of surfactant solution is injection into the reservoir

called a surfactant flood. It acts as a detergent, reducing thesurface tension of the oil and washing the oil out of the reservoirpore spaces. The oil forms droplets suspended in the water called

a microemulsion. The next slug is water thickened by polymers.Pressure on the polymer water from the injected well drives thesurfactant and oil microemulsion front ahead through the reservoirrock toward producing well.

A chemical flood can be used only for sandstone reservoirsbecause carbonates absorb the surfactants. It can recovers about

40% of the remaining oil but is an expensive process.


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Chemical flood

Source: N. Hyne, Nontechnical Guide to Petroleum geology, Exploration, Drilling & Production


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Thermal Recovery Thermal recoverytechnique utilize heat to make heavy

oil (


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Cyclic steam injection



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Thermal Recovery A steamfloodor steam driveuses both injection and

production wells. The superheated steam is pumpeddown the injection wells into the a heavy oil reservoir.

The steam heats the heavy oil to greatly reduce its

viscosity. As the steam gives up its heat, it condenses

into hot water that drives the oil toward the producing

wells. The pattern of injection and producing wells in a

steam flood is similar to that of a waterflood but are very

closely spaced. The recovery will vary between 25 to

65% of the oil in place.

Fireflooding , or in situ combustion, generates heat ina reservoir by injecting air into the well and starting a fire

in the formation close to the injection well. The fire and

the air flow move simultaneously toward the production

wells.


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Fireflood



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The most common fireflood is forward combustioninwhich the fire and injected air originate at the injected well.

The oil flows toward the producing wells. In dry

combustion, only air is injected. In wet combustionor

combination of forward combustion and waterflooding

(COFCAW), water and air are injected either together oralternately. The generated steam from water helps drive

the oil.

The recovery from a fireflood can be 30 to 40% of the oil in

place. Corrosion of equipment is a problem because of thehigh temperatures and corrosive gases that are generated.

Time-lapse seismic methods can be used to trace the

movement of the subsurface fire front.

Thermal Recovery


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Floating Production Systems FPS (Floating Production Systems) such as semi-

submersibles and tension leg platforms do not in generalhave oil storage capacity and hence require FSU(Floating Storage Units) if employed in remoteroffshore areas where export of products by pipeline isnot economical.

Spars and FPSO(Floating Production Storage andOffloading) vessels posses storage capacity whichmakes them more attractive for production in remotelocation where no export pipeline is available. Thiseliminates the cost of a separate storage facility or anexport pipeline. Oil can be offloaded directly to a shuttletanker for onward transportation to the refinery.

FPSOs are also idea for the exploitation of deepwatermarginal field as well as small field with short production

lives.


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Offshore production


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Deepwater production (water deep up to 600m) in Foinaven using FPSO


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NanHai ShengLi FPSO


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Schiehallion FPSO World largest new-built vessel of its type

capable of 950,000 barrel of oil


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Texacos Captain FPSO

FFloating Production Technology, A technology supplement to Harts Euroil, 1996


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Petrojarl Foinaven FPSO offloading to a FSU vessel

E l d d i


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Exploded view

Floating Production Technology, A technology supplement to Harts Euroil, 1996

E l d d i


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Exploded view



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Turret of an FPSO


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Surface handling of well fluid Oil and gas are not usually merchantable as they come

from the wellhead. Typically, a well stream is a high-velocity, turbulent, constantly expanding mixture ofhydrocarbon liquid and gases mixed with water andwater vapour, solids such as sand and shale sediments,and sometimes contaminants such as carbon dioxide

and hydrogen sulfide. Several step are necessary to getoil and gas ready to transport and its next stop.

The well stream is first passes through a series ofseparating and treating device to remove the sedimentsand water, to separate the liquids from the gases, and totreat the emulsions for further removal of water, solids,and undesirable contaminants. The oil is then stabilised,stored, and tested for purity. The gas is tested forhydrocarbon contents and impurities, and gas pressureis adjusted to pipeline or other transport specifications.


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Surface facili ties

Oil and gas processing


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Oil and gas processing

Source: Gerding, Fundamentals of Petroleum, 3rdEd.


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Typical process flow diagram

me4105 nus offshore oil and gas technology lecture 10

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