me4105 nus offshore oil and gas technology lecture 8

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

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Offshore Oil & Gas Technology


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Well & Reservoir Pressures Tubing pressure measured on the fluid in the tubing.

The pressure gauge at the top of the Christmas treemeasures the tubing pressure.

Casing pressure measured on the fluid in the tubing-casing annulus.

Bottomhole pressure measured at the bottom of thewell. The pressure is measured either as flowing, withthe well producing, or shut-in or static, after the well hasbeen shut-inand stabilisedfor a period of time such as24 hours.

Downdrawis the difference between shut-in and flowingpressure in a well.

The original pressure in a reservoir before anyproduction has occurred is called virgin, initialororiginal pressure. During production, reservoir pressure

decreases.


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The term wellor reservoir stimulationencompasses

several processes used to enlarge old channels or tocreate new pores the producing formation. Since oil

usually exists in the pores of sandstone or the cracks of

limestone formations, enlarging or creating new

channels causes the oil and gas to move more readily to

a well.

An early method of stimulating wells used nitroglycerine.

Using high explosives to improve a wells productive

capacity began in the late 1800s and continued until

acidizingand hydraulic fracturingwere developed inthe 1940s.

Although nuclear explosives were used in recent

experiments, the two most commonly used stimulation

methods are acidizing and hydraulic fracturing.

Well Stimulation


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Well Stimulation Well Stimulationare methods to increase well production.

These include acidizing, explosive fracturing, andhydraulic fracturing.

Acidizing A well can be acidizedor given an acid job

by pumping acid down the well to dissolve limestone,

dolomite, or any calcite cement between sediment grain.HCL (regular acid), HCL mixed with HF (mud acid) and

HF are acids that are commonly used. HCL is effective on

limestones and dolomites and HF is used for sandstones.

For formation with high temperatures, acetic and formic

acids are used. To prevent the acid from corroding thesteel casing and tubing in the well, an additive called an

inhibitoris used. A sequestering agentis an additive

used to prevent the formation of gels or precipitates of iron

that would clog the pores of the reservoir during a acid job.


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Acidizing other additives like antisludge agents,

prevents an acid from reacting with certain types ofcrude to an insoluble sludge that blocks channels, or

reduces permeability.

Fracture acidizing acid is pumped down the well

under high pressure to fracture and dissolve the

reservoir rock. Most limestone and dolomite formations

have low permeability. Acid injection into these low-

permeability formations, even at moderates, usually

results in a fracture type of acid treatment, meaning that

the pressure is high enough to cause the formation tocrack.

Interstitialor matrix acidizingis another type of

acidizing where acid is pumped down the well to enlarge

the natural pore of the reservoir and increase production.

Well Stimulation


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Acidizing enlarges existing channels and make new ones

Source: Gerding, Fundamentals of Petroleum, 3rdEd.


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Matrix acidizingconsists of treating at a rate and

pressure low enough to avoid fracturing the formation.

This technique is generally used when formation damageis present or when a water zone or gas cap is nearby and

fracturing might result in high water or excessive gas

production.

After the acid job, the spent acid, dissolved rock andsediments are pumped back out of the well during the

backflush. Acid job used to remedy skin damage on a

wellbore is called a wash job.

Explosive fracturingcommonly used from 1860s until thelate 1940s, was done with liquid nitroglycerin in a tin

cylinder called a torpedo. It is lowered down the well and

detonated. The explosion created a large cavity that was

then cleared out and the well was completed as an open

hole. This technique was both effective and dangerous.

Well Stimulation


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Hydraulic fracturingwas developed in 1948 and has

effectively replaced explosive fracturing. During the fracjobor hydraulic fracturing, a large amount of frac fluidunder high pressure is injected into the well to fracture thereservoir rock. This pressure actually splits the rocks.Frac jobsare done either in an open-hole or a cased wellwith perforations.

Hydraulic fracturing is used to accomplish four basic jobs:

Create penetrating reservoir fractures to improve theproductivity of a well,

Improve the ultimate recovery from a well by extendingthe flow channels farther into the formations,

Aid in improved recovery operations, and

Increase the rate of injection of brine and industrial

water material into disposal wells.

Well Stimulation


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Hydraulic fracturing

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


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A common frac fluidis a gel formed by water and

polymers, long, organic molecules that form a a thickliquid when mixed with water. Oil-based frac fluid andfoam-based frac fluids using bubbles of nitrogen, orcarbon dioxide can also be used to minimise formationdamage.

Hydraulic fracturing is done in three steps. A pad of frac fluid is injected into the well to initiate

fracturing the reservoir,

Next, a slurry of frac fluids with propping agentsor

proppants(small spheres, usually well-sorted quartzsand grains, ceramic spheres, or aluminum oxidepellets) are pumped down the well the extend thefracture and hold open the fracture after the pumpinghas stopped.

The well is then back flushed to remove the frac fluid.

Well Stimulation


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Frac jobsare described the amount of frac fluids and

proppants used. A typical frac job uses 43,000 gallons of

frac fluid and 68,000 lbs of sand. A massive frac jobusing more than 1,000,000 gallons of frac fluid and

3,000,000 lbs of sand.

Hydraulic fracturing is a very common well stimulation

technique that increases both the rate of production andultimate production.

It increases the production from 1 to 30 times the initial

rate with the high increased in tight reservoirs. Ultimate

production can be increased from 5 to 15%. A well can be fraced several times during its life. In some

instances, however, hydraulic fracturing can harm a well

by fracing into water. The hydraulic induced fractures

extend vertically into a water reservoir that floods the

well with water.

Well Stimulation


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Hydraulic fracturing an oil well



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Aerial photograph of a massive frac job



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Oil & Gas Production


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Oil & Gas Production After the well has been completed, oil & gas will be

produced from the reservoir to the surface. However,only a small percent of the oil wells completed flow freely

of their accord. Gas wells are produced by pressure

flowing through the formation. Some oil wells may flow

naturally due to a driving force during their early stages

of their productive life, but at some point before depleting

they will required an external energy source.

The first period in the producing life of a reservoir is

called primary recovery, or primary production.

During this stage, natural reservoir energy, either by

itself or in the combination with an artificial assist,

displaces the hydrocarbons from the pore of a formation

and drives it toward production wells and up to the

surface.


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Reservoir Drive Mechanism Reservoir drive mechanism, as the reservoir energies

are called, include dissolved-gas drive, gas-cap drive,

water drive, combination drive, and gravity drainage.

In a dissolved-gas drive(also known as solution-gas

drive), the lighter hydrocarbon components that exists

as a liquid in the reservoir before it is produced come out

in the form of gas as the reservoir is produced. Thedissolved gas coming out of the oil expands to force the

oil into the wellbore. In dissolved-gas drive reservoirs,

pressure decline rapidly and continuously, and wells

generally require pumping or some other artificial lifts atany early stage. Because of this characteristic, both

dissolved-gas and gas-cap drives are termed depletion

drives. The gas-oil ratio is low initially, then rises to a

maximum and drops. Recovery efficiency varies from as

little as 5% to as much as 30%.


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Reservoir Drive Mechanism

Source: Gerding, Fundamentals of Petroleum, 3rdEd.


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A gas-cap driveis a depletion drive in a reservoir thathas a gas cap. As pressure is reduced in a oil zone by

withdrawal, the gas cap expands and pushes oil out

ahead of it. Performance in this type of reservoir is

similar to dissolved-gas drive, but pressure may decline

more slowly because the gas cap provides a lot of drive

energy. Gas-oil ratio rise continuously in upstructure

wells with this drive, with little or no water production

except in those wells that penetrate the reservoir near its

edge. Because the whole system has more energy, sucha reservoir may have a long flowing life, depending on

the size of the gas cap. Oil recovery may be from 20 to

40 percent of the original oil in place.



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Water driveoccurs when there is enough energyavailable from water in the reservoir to move the

hydrocarbons out of the reservoir, into the wellbore, andup to the surface. The water in most water-bearingformations is under fluid pressure proportional to thedepth beneath the surface; in other words, the deeperthe water, the high the pressure.

Water is quite efficient at displacing oil from reservoirrock. As the oil is driven out of the reservoir, the watermoves in to replace it. The pressure remains high aslong as the volume of oil withdrawn is replaced by an

approximate equal volume of water. If the reservoirpressure remains high, the surface gas-oil ratio remainslow because little or no free gas is evolved in thereservoir. Because high reservoir pressure ismaintained, wells usually flow on their own until water

production becomes excessive and kills the well.



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Water production may start early and increase to an

appreciable amount as water encroaches into the oil and

into the producing wells. Expected oil recovery with

water drive is generally higher sometimes 50% or more

of the oil originally in place because of the greater

displacement efficiency of water over gas.

Water drive reservoirs can have bottom-water drive oredgewater drive. In a bottom-water drivereservoir, the

oil accumulation is totally underlain by water. A well

drilled anywhere through a reservoir penetrates oil first

and then water. In an edgewater drivereservoir, the oilaccumulation almost completely fills the reservoir. Water

occurs only on the edges of the reservoir, so only wells

drilled along the edges penetrates water. Well drilled

near the top of the structure penetrate oil only.



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A combination drivehas a gas cap above the oil andwater below it. Both the gas cap and the water drive the

oil into and up the wellbore to the surface. Another type

of combination drive as gas dissolved in the oil with the

water below it. Both the water and the gas coming out ofsolution drive the oil to the surface.

Gravity drainageis a less common type of drive. The

force of gravity is, of course, always at work in a

reservoir. Usually gravity cause oil to migrate upward bypulling the heavier water down beneath it. However, in

shallow, highly permeable, steeply dipping reservoirs

and in some deeper, nearly depleted reservoirs, the oil

may flow downhill to the wellbore.


me4105 nus offshore oil and gas technology lecture 8

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