me4105 nus offshore oil and gas technology lecture 9

8/11/2019 ME4105 NUS Offshore Oil and Gas Technology Lecture 9

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Offshore Oil & Gas Technology


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Artificial Lift When a well is first complete, the fluid is expected to flow

to the surface by natural reservoir energy for some time.At some time during their economic life, however, most

oilwells will require some form of artificial lift to help raise

the fluid to the surface and obtain the maximum recovery

of oil for maximum profit to the producer. The most

common methods of artificial lifts are those that use gasand those that use pumps.

Types of pumps used are:

Sucker rod pump(beam pump, rod pump, etc)

Surface hydraulic pump,

Electrical submersible pump(ESP),

Downhole hydraulic jet pump

Progressive cavity pump (PCP)


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The selection and design of artificial lift systems require a

prediction of the operating conditions across the entirewell life. The following are considered during the selection

process:

Flowing wellhead pressure or gathering system

backpressure Well productivity index and well production requirement

Water cut development, and producing gas-liquid ratio

Mixture viscosity which will affect frictional losses in the

system Reservoir pressure depletion

Casing and tubing size limitations

Environment impacts of surface system and energy

input.

Artificial Lift


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The downhole environment and fluid characteristics alsoplay an important role in the selection of the lift system.

The following factors will influence the performance and

overall reliability of the lift system, depending on the lift

method selection: Crude density and bubble point pressure

Crude viscosity and emulsification tendency,

Solution gas-oil ratio

Fluid corrosivity

Scaling tendency

Produced solids content and abrasive nature

Artificial Lift


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Gas Lift Gas liftis a method of injecting high-pressure gas into

the casing, passing it into the tubing (through an orificeor a mechanical valve), which then aerates the fluid in

the tubing, lightens the fluid column and raise the fluid by

the expansion of the gas.

There are two modes of operation: continuous gas

injection into the tubing at the predetermined depth and

intermittent injection at high instantaneous rates for

short period of time.

The system comprises:

Base packer and tubing string

Side-pocket mandrels spaced in the tubing with

retrievable pressure controlled gas valves

Gas compression and distribution system.


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Gas lift continuous injection

Source: Gerding, Fundamentals of Petroleum, 3rdEd.


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Gas lift intermittent cycle


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Gas lift system


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Gas liftis commonly used when there is a large supply of

gas (produced natural gas or an inert gas) is economicallyavailable. The natural gas is usually treated to remove theheavier components.

Apart from the central compressor facility (comprisingmainly the compressor station, gas-oil separator and glycol

dehydrator), all gas lift equipment are large downhole.Offshore oil wells that need gas lift are usually completedwith gas lift valves and mandrels at different depths.

Gas lift is an inexpensive technique when many wells canbe serviced by one central compressor facility.

Gas lift is a flexible method of artificial lift, but the systemsare sensitive to flowing backpressure, fluids viscosity, andwell productivity index. This is an optimum amount of gasinjected to achieve lift and there are also limits to the

systems ability to lighten the fluid column.

Gas Lift


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Sucker Rod Pump Sucker rod pumpsis the most common method of

artificial lift. A pumping unit at the surface lifts a rodrunning downhole connected to a pump. Each stroke liftsa volume of fluid. The system consists of: Subsurface pump which includes one standing valve and one

traveling valve (ball and seat type non-return valves)

Sucker rod string connected to the pump downhole and to thepumping unit at surface

Production tubing and stuffing box at wellhead providingflowpath, and seal around the sucker rods to prevent leakage

Pumping unit that converts motor power through a gearbox toprovide lift

Motor and sheave providing power and defining pumping speed(strokes per minute).

It is sensitive to gas at intake. Most common problemsinclude parted rods, tubing leaks, worn pump and gas

lock.


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Sucker rod pump



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Sucker rod pump



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How a sucker rod pump works

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


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Electric submersible pump(ESP) uses a downhole

multistage centrifugal pump driven by a downholeelectric motor, and are particularly suited to high volume,

high water cut lift applications. The system consists of:

A multistage centrifugal pump where the stage count

defines the head generated, and the outer diameter

defines capacity

A seal chamber which prevents wellbore fluids from

migrating along the system shaft into the motor

An electrical cable transmitting current to the motor,

and wellhead electrical penetrator

A surface transformer, motor starter/controller,

junction box, and power distribution system

Production tubing and a surface production choke

Electric Submersible Pump


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Electr ic submersible pump



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Hydraulic Pumps Subsurface hydraulic pumpor Hydraulic

submersible pump(HSP) systems use a power fluid

pumped to drive a downhole pump. The power fluidpowers a downhole multistage turbine which in turn drive

a downhole multistage centrifugal pump.

Downhole jet pumpsuse a power fluid to a drive jet

pump to entrain the well fluids and pump it to thesurface. A jet pump works by accelerating the power

fluid to high velocity through a nozzle thereby reducing

the jets static pressure. The reduction in the static

pressure then causes the well fluid to be entrained intothe mixing chamber where mixing takes place. Pressure

recovery then follows in the diffuser and the combined

fluids then rise to the surface. Jet pump has the

advantage of no moving parts and can handle liquid-gas

mixtures.

H d li i t


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Hydraulic pumping system



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Reserve-flowdownhole jet pump


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Progressive Cavity Pump Progressive cavity pumps(PCP) are novel pumps. A

progressive cavity is created by a single helical rotor,

which turns eccentrically inside a double-threaded helical

elastomeric stator and moves the fluid on. The number

of seal lines determines the pressure boost capabilities,

and the amount of slip in the pump. Fluids viscosity and

the amount of compression loaded into the fit also affectthe system efficiency.

These systems are well suited to high-viscosity fluid, and

can handle moderate amount of sand production.

PCPs can be configured with two different drivemechanisms. The earlier and more used method is a

surface right angle drive motor coupled to the downhole

pump by sucker rod. The other is to couple the PCP to a

downhole motor as in the case of the ESPs.


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Progressive cavity pump


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Improved Oil Recovery Primary recoveryis recovery of oil through primary

productionby the original reservoir drive energy. Itdepends on the type of reservoir drive, oil viscosity, andreservoir permeability but averages 30 to 35% of the oilin place and can be as low as 5%.

Improved oil recovery(IOR) is any activity thatincreases the recovery above that of primary recoveryand includes drilling extra wells, which can intersectreservoir areas that have otherwise been missed.

Advance drilling techniques, e.g. extended reach and

horizontal well drilling technology since 1990, and multi-lateral and multi-branch drilling technology creatingmultiphase drainholes from a single well since 1995,have increased reservoir exposure and improve wellperformance.

R i R M h i


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Reservoir Recovery Mechanism

Source: Dawe, Modern Petroleum Technology, Vol 1, Upstream


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Improved oil recovery can be obtained by supplyingenergy to the reservoir. This can be done by the

following means:Secondary recovery(nowadays initiated at or near

the beginning of production), which involves addingexternal energy but without any fundamental changes

to the physical properties of the fluids. This energy isadded either by water or gas injection.

Enhanced oil recovery(EOR), also known asTertiary recovery, which involves adding externalenergy and creating fundamental changes to the

physiochemical properties of the system, e.g. addingchemicals or heat to the reservoir to effect changes influid density, fluid viscosity, the interfacial forces, or tochange the reservoir wettability which affects thedistribution (position) of the oil, gas and water within

the pores.

Improved Oil Recovery


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Improve Oil Recovery Secondary recoveryactually consists of replacing the

natural reservoir drive or enhancing it with an artificial, or

induced, drive. Generally the use of injected water ornatural gas into the production reservoir is the mostcommon method.

Waterfloodingis the least expensive and most widelyused improved recovery method, but it is not considered

an advanced recovery method because it is used typicallyin secondary recovery. In the method, water is injectedinto the formation to move oil to the production wells.

Although water for injection may be supplied from waterwells drilled specifically for this purpose, the water that isproduced with the oil may also be used.

Regardless of whether the water is injected into formationfor disposal, pressure maintenance, or waterflood, thefluid must meet certain requirements. The injection watermust be clear, stable, deoxygenated and similar to thewater in the formation where it is being injected. Hence,

waterflood can be expensive.


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Waterflooding

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


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Although secondary recovery may be attractive in theory,

not all reservoirs are suitable for waterflooding, for

example, if they have discontinuities such as sealingfaults or high-permeability thief zones which cannot be

controlled. Selecting where to inject the water can be

problematic.

Some waterfloods may take up to two years of injectionbefore any increase in production occurs.

After a field has been in operation for some years, the

wells will produce an increasingly large quantity of water

(or gas), up to perhaps 99% water-cut. The field isabandoned when it becomes uneconomic to lift and

process the produced fluids. At this stage, huge volumes

of water have to be processed, e.g. over a 24-fold

increase in water volume in going from 80% to 99%

water-cut for a constant oilproduction rate.

Improve Oil Recovery


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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 gas

injection 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, and

nitrogen. The natural gas produced with the oil can bereinjected into the well to maintain formation pressure.Immiscible gas injected into the well behaves in amanner to that of a gas-cap drive. Gas injection requiresthe use of compressors to raise the pressure of the gas

so that it will enter the formation. Miscible gas injectionin Enhanced Oil Recovery

uses gas which are miscible with the oil produced includes propane, methane under high pressure,methane enriched with liquid hydrocarbons, nitrogenunder high pressure and carbon dioxide used alone orfollowed by water. The principle of miscible displacementis to reduce or eliminate the interfacial tension forcesbetween the displacing and displace fluids, so that theresidual oil saturation in the swept zone can be reducedto near zero

Gas Injection

me4105 nus offshore oil and gas technology lecture 9

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