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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