miscible drive & carbon dioxide flooding

Outline

Introduction Miscible Drive

• Miscible Slug Flooding• Basic Methods of Miscible Drive• Improved Miscible Drive Methods

Carbon Dioxide Flooding• CO2 Immiscible Flooding• CO2 Miscible Flooding• CO2 Demand, Sources, Transportation

Developed

Types of Reserves

Proved PossibleProbable

Undeveloped

Quantity of crude oil, condensate, natural gas anticipated to be commercially recoverable:

1. from known accumulation

2. under existing economic condition

3. under current government regulation

ReserveIntroduction

1. Lowering Mobility Ratio (M):

As M decreases, the volumetric sweep efficiency will increase and oil recovery will increase.

2. Increasing Capillary Number (Nc):

As Nc increases, the residual oil saturation will decrease and oil recovery will increase

Approaches of EOR Processes

o

w

o

w

k

kM

/

/

cos

ForceCapillary

Force ViscousNc

Introduction

Miscible Drive

Miscible Oil Displacement: It is the displacement of oil by fluid with which it

mixes in all proportions without the presence of an interface, all mixtures remain as a single phase.

Miscible Agents:

1. Propane, LPG mixtures, and Alcohols.

2. Miscible CO2 drive.

3. Natural gas, and High pressure gas (N2).

4. Surfactant slug.

Miscible Drive

The practical interest of miscible displacement became apparent when it was discovered that:

1. To attain miscibility it is sufficient to inject a slug of solvent of limited volume displaced by a much cheaper follow up fluid. (absolute miscibility)

2. Under certain conditions of pressure, temperature and phase composition various fluids may become miscible with reservoir oil. (thermodynamic miscibility)

Miscible Slug Flooding

A certain volume of solvent is placed in contact with the oil with which it is miscible, and is then followed up with a fluid C which is immiscible with the oil O but miscible with the solvent S

C S O

Typical displacement systems used are:1. Oil, LPG, gas2. Oil, alcohol, water


The theory of miscible displacement shows that between two miscible fluids in motion a mixing zone is formed, the size of which is proportional to the square root of time.The size of the bank of the pure solvent continually

decreases as the sweep progresses. The volume of the solvent slug injected should be

such that the bank of pure solvent is not exhausted before the miscible mixture breaks through at the producer, otherwise C and O, which are not miscible, would come into contact, (Miscibility Rupture).


The major difficulty in planning a miscible displacement by solvent slug lies in the selection of an adequate slug volume, neither too small, to avoid the risk of a miscibility rupture, nor too large, to protect the economics of the project.

Thermodynamic Miscibility

During the injection of gas into an oil reservoir, as long as the original fluid are not completely different in composition, there will be a gradual exchange of components between the two fluids and their composition will become more alike.

Eventually part of the gas phase and part of the oil phase will no longer be separated by an interface and will thus become miscible.

The phase exchange are governed by the equilibrium constant Ki for each component.

Yi: molecular fraction of component i in the vapor phase.

Xi: molecular fraction of component i in the liquid phase.


i

ii x

yK


Ki = f (P,T,PK)

PK: Convergence pressure

For a given PK, Ki tends to a value of 1 with decreasing temperature and with increasing pressure.

Thus, it is apparent that low temperature and high pressure are favorable conditions for the successful implementation of a miscible displacement project.

The Ternary Diagram

Thermodynamic miscibility can be more readily described if we represent complex mixtures of HC by a combination of three arbitrary components made up of groups of HC with similar thermodynamic properties:

1. The light components, methane C1 and possibly N2

2. The intermediate components, C2-C6, the intermediate HC play a major role in thermodynamic equilibrium

3. The heavy components, C7+

The Ternary Diagram

Having chosen the three components, we can draw an equilibrium triangle of which each apex represents one of the components.

The Ternary Diagram

At any given pressure and temperature, the point (M) may represent, according to its location inside the triangle, either a single phase or a diphase fluid.

For a given combination of P & T, the curves bounding the diphase region; the bubble point curve and the dew point curve.

Critical point (C) at which the mixture at the critical pressure and temperature.

The Ternary Diagram

For any given saturated liquid A, there is a corresponding saturated vapor B with which it is in equilibrium, the line AB is known as tie line.

The Ternary Diagram

It can be seen that high pressure and low temperature are very favorable conditions for miscible displacement, since they reduce the size of the diphase region

Basic Methods of Miscible DriveThe main standard methods of miscible drive are:

1. High pressure gas injection.

2. Enriched gas injection.

3. LPG slug injection.

4. Alcohol slug injection.

High Pressure Gas InjectionTwo types are commonly used in high pressure gas

injection:

1. Natural (HC) gas injection.

2. Inert gas injection.

High Pressure Natural Gas Injection

High Pressure Gas Injection

A. Phase Conditions in The Reservoir:

The oil must be rich in intermediate components.

The experience of various operators indicates that a miscible bank is created after the injected gas has traveled a dozen meters from the injection well. The quantity of unrecoverable oil under these condition will clearly be negligible.

High pressure gas injection is also known as “high pressure gas drive” and “vaporizing gas drive”.



B. Miscibility Pressure: On the ternary diagram drawn at reservoir T,

miscibility can only be achieved between gas and oil at a pressure equal or greater than the “miscibility pressure”, at which the tangent at the critical point passes through (O).



C. Application of High Pressure Natural Gas Injection:

a) High reservoir pressure (deep formation)

(3000-4500 psi)

b) Oil reach in intermediates (gravity ≥ 35 API)



High Pressure Inert Gas Injection

Once miscibility has been achieved, most of the gas injected is only needed to push forward the miscible front and fill up the porous medium.

It is possible to inject at first a limited volume of natural gas (around 5% of the pore volume) sufficient to ensure miscibility with the reservoir oil, and then replace the injection of expensive natural gas with that of a cheaper gas.

A suitable gas, approximately 12% CO2 & 88% N2, may be obtained by the combustion of relatively small volumes of separator gas.

CH4 + 2O2 + 8N2 → CO2 + 2H2O + 8N2


Enriched Gas Injection

A. Description of The Process:

In this case the formation of a miscible bank is achieved by way of the intermediate components in the natural gas.

The process is also known as “condensing gas drive”.


As the composition of the oil changes from O to ot the residual oil behind the front swells due to the absorption of light and intermediate components from the gas.

At a certain stage the oil saturation will have increased sufficiently that the oil becomes mobile and a bank of oil of composition ot will be formed.

At the end of this process there is no residual oil, in contrast to high pressure gas drive in which the resulting heavy oil op is unrecoverable.


B. Operating Conditions:

In case of enriched gas injection the operating parameters are pressure, and the composition of the injected gas (can be made richer by the addition of butane and propane or even LPG)

LPG Slug Injection

In this method, the miscible bank is formed at the outset by the injection of LPG of composition L, followed by the injection of dry gas G.

The LPG is fully miscible with the reservoir oil in place (O).

Alcohol Slug Injection

Most miscible displacement process, such as those we have already discussed, suffer from the disadvantages:

1. High reservoir pressures are required.

2. The areal sweep efficiency is relatively poor because of the large mobility contrasts between gas, solvent and oil.

3. Natural gas and LPG are not always available in sufficient quantity in the oil field.


These constraints have led to the search for methods of miscible displacement in which water is the driving fluid.

An obvious possibility is the use of alcohols as a slug between the oil and the water, since they are miscible with both liquids.


At first isopropyl alcohol was studied, this has the disadvantage of being expensive and absorbing water very rapidly, thus reducing its efficiency.

Other studies have shown that part of the isopropyl alcohol can be replaced, at the leading and trailing edges of the slug by methyl alcohol.

The methyl alcohol rapidly absorbs water, leaving the isopropyl alcohol at the center of the slug practically water free and thus retaining its oil displacement efficiency.


Finally, if normal butyl alcohol is used in front of and methyl alcohol behind the isopropyl alcohol, the total slug volume required is reduced to 10% pore volume.

Even though this type of miscible displacement has not yet found commercial application due to the high cost of various alcohols studied.

Improved Miscible Drive MethodsIt has been shown that the injection of natural gas

under conditions leading to miscible displacement suffers from the following disadvantages:

1.Poor vertical sweep efficiency in heterogeneous reservoir.

2.Poor areal sweep efficiency.

To improve matters:

1.Pre-injection of water.

2.Chasing the miscible slugs with water.

Pre-Injection of Water

The injection of solvent in stratified reservoir normally results in the most permeable layers receiving many times the solvent volume required to achieve miscible displacement throughout the field, before the least permeable layers have even received the minimum volume required.

During the pre-injection of water the most permeable zones take more water than the least permeable zones, so the injectivity to solvent in the most permeable zones suffers a greater reduction than that in the zones of lower permeability, the result is a more even distribution of the solvent subsequently injected.

Improved Miscible Drive Methods

Miscible Slugs Driven by WaterIn miscible displacement by gas the gas-oil mobility

ratio is often very unfavorable and thus the sweep efficiency is poor.

The mobility ratio may be reduced by injecting water with the gas.

Improved Miscible Drive Methods

Carbon Dioxide Flooding

• It may be miscible or immiscible drive.• Properties of CO2:

1. CO2 is colorless, odorless, inert, and noncombustible gas.

2. It has molecular weight of 44.01.

3. The phase behavior of pure CO2 is shown in the opposite figure on a P-T diagram.


• Properties of CO2:

4. CO2 density varies with pressure and temperature as does its viscosity and compressibility factor.

5. The CO2 is more soluble in oil than water (2 to 10 times more).

6. In solution with water CO2 increase water viscosity and forms carbonic acid, which has a beneficial effect on shaley rocks (reduction in pH stabilizes) and on calcareous rocks (dissolving effect).


Factors Making CO2 an EOR Agent:

1. Reduction in crude oil viscosity and increase in water viscosity.

2. Swelling of crude oil and reduction in oil density.

3. Acid effect on carbonate and shaley rocks.

4. Miscibility effects.

Reduction in Crude Oil Viscosity and Increase in Water Viscosity:

• Oil viscosity is reduced significantly when CO2 is dissolved in crude.

• This reduction in crude oil viscosity and an accompanying small increase in water viscosity reduces the water-oil mobility ratio.

Factors Making CO2 an EOR Agent


Swelling of Crude Oil:• As a result of CO2 dissolved in the crude, the oil’s volume will increase from 10 to 20% or more.

• Oil swelling increases the recovery factor; since for a given residual oil saturation, the mass of the oil remaining in the reservoir and expressed in standard conditions is lower than if the abandoned oil was CO2 free.

Factors Making CO2 an EOR Agent


Factors Making CO2 an EOR AgentAcid Effect on Carbonate and Shaley Rocks:

• Carbon dioxide in solution with water forms carbonic acid, which in turn, dissolves the calcium and magnesium carbonates.

• This action increases the permeability of the carbonate rock, improving the well injectivity and, in general, the fluid flow through the reservoir.

• CO2 has a stabilizing effect on shaley rocks, reducing the pH and preventing the shale from swelling.


Factors Making CO2 an EOR AgentMiscibility Effects:

• Carbon dioxide is not first-contact miscibility with reservoir oil.

• Carbon dioxide may develop miscibility through multiple contacts under specific conditions of P & T and with specific oil compositions.


CO2 Immiscible Flooding

• Immiscible CO2-oil displacement is best suited to medium and heavy oils, since the oil viscosity reduction is greater and more significant.

• The CO2 flooding process involves alternating injections of CO2 and water until a certain amount of CO2 has been injected, then water is injected continuously.

• The water-alternating-gas process is characterized by an improved mobility ratio and additional recovery over that of water flooding without CO2.


CO2 Immiscible Flooding

• In addition, the swelling effect of crude oil with CO2 increases the oil formation volume factor so that residual oil behind the water flood is smaller in volume at surface conditions.

• Also, oil swelling within the pore spaces displaces water out of the pores, resulting in a decrease in the wetting phase saturation.


CO2 Miscible Flooding

Multiple-Contact Miscibility:• CO2, at appropriate pressure, vaporizes or extracts heavier HC (C5 through C30) from the oil and concentrate them at the displacement front where miscibility is achieved.

• Dynamic miscibility with CO2 is possible through a vaporizing gas drive mechanism for reservoir fluid compositions lying to the right of the limiting tie line on a pseudo ternary diagram.



Multiple-Contact Miscibility:• The difference between the vaporizing gas drive mechanism with CO2 and with natural gas (methane) is that dynamic miscibility with CO2 does not require the presence of intermediate-molecular-weight HC in the reservoir fluid.



Miscibility Pressure:• The minimum miscibility pressure (MMP), above which dynamic miscible displacement with CO2 is possible, can be determined from displacement techniques and miscibility experiments:

1. Gravity-stable experiment2. Slim tube experiment3. Visual cell observation4. Correlations



1. Gravity-Stable Experiment:Use a vertical sand packed and oil

saturated test column in which CO2 injected at the top displaces the oil vertically downward at a rate slow enough to maintain a gravity-stable flow.The experiment is run at different

increasing pressure, and the ultimate oil recovery obtained are plotted versus the respective pressures on the same graph.



2. Slim Tube Experiment:

are performed in a 40-ft-long & ¼-in-diameter coiled stainless steel tube sand packed and oil saturated at given (P) and (T).



3. Visual Cell Observation:

can describe the gradual color change of the single phase effluents as the transition zone changes from displaced oil to injected CO2.

4. Correlations:

Correlations for estimating miscibility pressure have been made since reservoir temperature, oil composition, and characteristics are factors affecting this pressure.


Design Considerations

Before any calculations are made, one must consider whether displacement can be miscible or immiscible and whether flow is vertical or horizontal.

If the crude oil gravity is medium or light and the reservoir is deep or of medium depth, then miscible displacement should be considered.

When the reservoir is of medium or shallow depth with medium or heavy gravity oil, the miscibility pressure, if it exists, cannot be reached without surpassing the formation parting pressure.


Design Considerations

In this case, displacement has to be immiscible but will include the beneficial effects of viscosity reduction and oil swelling.

The reservoir displacement developed in non dip reservoirs with not so thick pay zone is controlled by the mobility ratio of CO2 to oil.

Vertical displacement is controlled by the effect of gravity.


Flood Design and Performance Predictions

CO2 flood design and performance predictions differ from reservoir to reservoir and for different operation strategies.Reservoir Pressurization:

is an initial phase of the CO2 miscible flood in which the reservoir pressure, reduced by primary depletion or some other recovery mechanism, has to be increased so that miscible displacement can occur.The reservoir pressure is usually increased by

injecting water.


Flood Design and Performance Predictions

CO2 Requirements:

are determined in many ways depending on the reservoir geometry and displacement direction and on the miscibility conditions and injection strategy.

CO2 Injection Pressure:

When CO2 is injected, care must be taken to ensure that the injection pressure are always below parting pressure.


CO2 Demand, Sources, Transportation

CO2 Demand:Carbon dioxide miscible displacement is one of the

most promising EOR methods.

CO2 Sources:A reliable source of supply for CO2 is very important

because the gas must be available on a continuous basis in large volumes for long period of time.The CO2 gas used must have a purity of 90% or more.

• Effects of presence of other gases (N2, H2S, water vapor) with CO2



CO2 Sources:1. Naturally occurring high-pressure gas reservoir

with high purity CO2.

2. Natural gas field containing CO2 which is removed in gas processing plant.

3. CO2 produced as by-product.

The sources which produce CO2 as an off gas are coal, oil, gas fired power plants, cement &ammonia plants, refineries, and ethanol &ethylene oxide plants.



Transportation of CO2: The method of transportation of CO2 from its source

to the field depends on whether the CO2 is liquid or gas.CO2 is liquefied at its source and transported to the

project sites by trucks, cars, or in storage tank located on barges.The CO2 necessary for large long-term projects is

transported most economically through a pipeline as vapor at P between 1400-2000 psi (which are above critical pressure) so that two phase flow does not occur.



Operational Problems:1. Corrosion.2. Asphaltene deposition.

Preliminary Screening Criteria:1. Reservoir suitability (there is many res. Factors which to be considered to decide applying this technique or not).

2. Determine CO2 source (volume requirement, and supply method).

3. Assess injection/production facilities (modification or new, metallurgy, process economics).

miscible drive & carbon dioxide flooding

Documents

displacement of oil

miscible fluids

miscible mixture

gas oil

miscible agents

miscible drivemiscibility

oil recovery

slug of solvent