Gas Recycling

in Gas

Condensate

Reservoirs What?

Why?

How?

11/25/2014

1

Submitted by

Ahmed Farag Rizk.

Mohamed Ata Farahat.

Ali Yahya Gergis.

Semak Zaghlol.

Mokhtar Ahmed Husieen.

Mohamed Fathy Salem.

Tarek Ali.

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Agenda Five Reservoir Fluids.

Retrograde Gas.

Reservoir Regions.

Condensate Problem.

Method of Implementation.

Process Efficiency.

Advantages and Disadvantages.

Economics.

Case Study.

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Five Reservoir Fluids Reservoir

Fluids

black oil Volatile oil Gas

Condensate Wet Gas Dry Gas

Black Oil Volatile

Oil

Retrogra

de Gas

Wet Gas Dry Gas

Initial

Producin

g GLR

(scf/STB)

<1,750 1,750-

3,200

>3,200 >15,000 100,000

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Five Reservoir Fluids

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Retrograde Phenomena

The formation of liquid hydrocarbons in a gas

reservoir as the pressure in the reservoir

decreases below dewpoint pressure during

production.

It is called retrograde because some of the

gas condenses into a liquid under isothermal

conditions instead of expanding or vaporizing

when pressure is decreased

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Retrograde gas‐condensate

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Retrograde gas‐condensate

Gas Condensate reservoir

Near Critical Lean gas

condensate Rich gas

condensate

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Near‐critical Gas‐condensate

Reservoir

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Lean Gas Condensate & Rich

Gas Condensate

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Liquid Dropout %

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Reservoir Regions

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Reservoir Regions

Region I - Near Wellbore

Close to the wellbore with high condensate saturation.

Both gas and condensate are flowing simultaneously.

Region I exists only when bottomhole flowing pressure is less than P*(pressure at which condensate saturation is equal to the critical saturation).

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Reservoir Regions

Region 2‐condensate build up Where condensate is dropping out of the gas phase.

Exists when the reservoir pressure declines below the dew point pressure.

The liquid drop out begins as the dew point is

approached. However, it is not mobile since the condensate saturation is less than Sc.

Therefore, in this region only gas phase is mobile whereas condensate is immobile.

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Reservoir Regions

Region III – Single Gas Phase

Region in the reservoir which contains only the

original reservoir gas.

This is the farthest region in the reservoir .

reservoir pressure is greater than dew point pressure.

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Gas Condensate Reservoir

Problems

Condensate Blockage.

Gas permeability reduction.

Loss of Condensate.

Liquid loading problems

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Possible Solutions

Reduce pressure drawdown.

Maintain pressure above dewpoint by gas cyclic or injection.

Hydraulic fracture.

Horizontal wells.

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Why Gas Re-injection Solution Disadvantage

Reducing D.D. Deal with the well not the

reservoir.

Gas cyclic Short term benefit

Hyd. Frac. Increase permeability till

condensate accumulation

happen.

Horizontal wells High cost

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Why Gas Re-injection?

Maintain reservoir pressure.

Long-term benefit.

The injected gas will be produced later.

Condensate will not be lost inside the reservoir.

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Gas Recycling Target: Maximum recovery of the valuable condensate

Definition: The process of keeping the reservoir pressure above the

dew point pressure to minimize or eliminate the formation of condensate at the reservoir conditions .

Statistics: The condensate recovery factor by depletion ranges

between 20‐40%.

This recovery factor can be increased with cycling to between 60‐75%.

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Gas Recycling Data required:

(1) Geologic data. (2) Rock and fluid properties. (3) Reservoir pressure history. (4) Condensate, gas, and water production

data, from the date of discovery. (5) Proposed future production rates. (6) Gas- and/or water-injection data, past and

future. (7) Productivity, injectivity & Backpressure test

data on wells.

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Implementation Method

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Injection Pattern Developed field: In which gas recycling starts after

long period of natural depletion

Undeveloped field: By model study well arrangements are then selected.

Injection fluid with the maximum possible contact with the crude oil system.

Well pattern selection depend mainly on the comparison of which is economically practical and which is theoretically efficient.

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Injection Pattern

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Injected Gas Type:

HC.

N2.

CO2.

Source:

Closed system.

Open system.

Semi-closed system.

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Handling of Production

Main Equipment

Separator , Compressor & fractionation equipment.

Desulphurization:

Reagents used: Sodium carbonate solution (regeneration

by air current).Sodium phenolate (regeneration by heating),Amines (regeneration by heating).

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Handling of Production Dehydration:

Various desiccants are used, both solids (silica gel, activated aluminum, calcium sulphate, anhydrite, fluorite, etc.) and liquids (glycols). There is practically no economic method for the removal of oxygen from gas.

Filtration:

Injection gas must be free from solid or liquid particles. Scrubbers and filters are thus installed in the system so as to remove all particles larger than a few microns.

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Reservoir cycling efficiency

𝐸𝑅 = 𝐸𝐴𝐸𝑉𝐸𝐷

EA Area enclosed by injected gas divided

by total reservoir area.

EV Pore space invaded by injected gas

divided by total thickness.

ED Volume of wet HC swept out of

individual pores divided by same pores at

the beginning of cycling.

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Factors Affecting Efficiency

Mobility ratio:

The viscosity of lighter dry gas is less than that

of wet gas.

𝑀 =𝑘𝑑𝑔 ∗ 𝜇𝑟𝑔

𝑘𝑟𝑔 ∗ 𝜇𝑑𝑔

M should be ≤ 1

Gravity difference:

Gravity difference may accelerate the early

breakthrough of dry gas

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Factors Affecting Efficiency

Formation volume

factor:

The FVF of the dry gas

is greater than for the

wet.

There is a volume

difference.

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Factors Affecting Efficiency

Vertical Permeability:

Case 1

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Factors Affecting Efficiency

Vertical Permeability:

Case 2

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Factors Affecting Efficiency

Type of injected gas: Effect of different injection gases on condensate saturation

reduction during the production period.

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Advantages Available techniques that used for that method.

Availability of different types of gases.

Maintain pressure of reservoir.

Increase amount of condensate recovery.

Increase gas permeability.

Deviated or crooked holes can be injected by gas.

Easy maintenance for surface equipment.

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Disadvantages large amount of gas is required.

Huge compressor is required to inject gas.

Complex process to isolate gas and its impurities.

Condensate liquid builds up near wellbore causing a reduction in gas permeability and gas productivity.

Impurities with gas ( H2S & Co2 ) can cause corrosion.

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Economics

Profit = revenue – cost

Present day value (PDV)

Expectations of condensate and gas

prices.

Condensate demand.

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Economics

cost

capital

compressors

Additional pipelines

New wells

Additional separation units

operating

gas Power

consumption

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Economics

Revenue

Normal recovery

Additional gas

Additional condensate

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Economics

Additional recovery determination must

be so accurate.

Well pattern selection depend mainly on

the comparison of which is economically

practical and which is theoretically

efficient.

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Economics Suggested procedure in developing a cycling

project:

1. Determine the reserve and the expected recovery.

2. Determine the expected additional recovery.

3. Draw up a development plan.

4. Determine the required surface facilities.

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Economics 5. Economic analysis: a) Taxes.

b) Markets.

c) Economics of size and design of total physical plants:

i. Costs of total physical plants per unit of capacity.

ii. Costs of operations per unit of capacity.

iii. Costs of taxes.

iv. Revenue from markets. v. Estimates of intangible risk factors.

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Case Study The North Sea Gas-Condensate System The gas has dew point pressure of 6,750 psi at 280 F

and contains 73.19 mol% methane and 8.21 mol% C7+.

The maximum liquid dropout of 21 .6% occurs at 3100 psi.

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Case Study several injection gases including Methane, Nitrogen,

Carbon Dioxide, and various combinations of theses gases were injected at 3100 psi.

The liquid dropout at this pressure is 26%.

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Case Study

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Case Study

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Case Study

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Case Study

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Case Study Conclusions The study shows that the gas injection process is a

viable option for reducing the liquid blockage in the near wellbore region.

Results of the study indicate that all the injection gases used in this investigation can actually increase the liquid blockage when they are injected with insufficient volume.

The gas injection process is particularly effective when initiated before the maximum liquid dropout is reached.

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Reference Understanding gas condensate reservoirs.

Petsoc-77-01-06,recovery of retrograde condensed liquids by revaporization during dry gas injection.

Spe-1813-pa,equilibrium revaporization of retrograde gas condensate by dry gas injection.

Spe-68170-ms,investigation of revaporization of retrograde condensate.

Api-41-221,practical economics of cycling.

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