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Reservoir Engineering 1 Course ( 1 st Ed.)

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1. Turbulent Flow in Gas Wells: LIT Approach (Case C)

2. Comparison of Different IPR Calculation Methods

3. Future IPR for Gas Wells

4. Horizontal Gas Well Performance

5. Primary Recovery Mechanisms

6. Basic Driving Mechanisms

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1. Oil Recovery

2. Waterflooding Considerations

3. Primary Reservoir Driving Mechanisms

4. Secondary Recovery Project

5. Flooding Patterns

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Oil Recovery Categories

The terms primary oil recovery, secondary oil recovery, and tertiary (enhanced) oil recovery are traditionally used to describe hydrocarbons recovered according to the method of production or the time at which they are obtained.

Figure illustrates the concept of the three oil recovery categories.

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Primary Oil Recovery

Primary oil recovery describes the production of hydrocarbons under the natural driving mechanisms present in the reservoir without supplementary help from injected fluids such as gas or water. In most cases, the natural driving mechanism is a

relatively inefficient process and results in a low overall oil recovery.

The lack of sufficient natural drive in most reservoirs has led to the practice of supplementing the natural reservoir energy by introducing some form of artificial drive, the most basic method being the injection of gas or water.

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Secondary Oil Recovery

Secondary oil recovery refers to the additional recovery that results from the conventional methods of water injection and immiscible gas injection. Usually, the selected secondary recovery process follows

the primary recovery but it can also be conducted concurrently with the primary recovery.

Waterflooding is perhaps the most common method of secondary recovery. However, before undertaking a secondary recovery project, it

should be clearly proven that the natural recovery processes are insufficient;

Otherwise, there is a risk that the substantial capital investment required for a secondary recovery project may be wasted.

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Tertiary (Enhanced) Oil Recovery

Tertiary (enhanced) oil recovery is that additional recovery over and above what could be recovered by primary and secondary recovery methods.Various methods of enhanced oil recovery (EOR) are

essentially designed to recover oil, commonly described as residual oil, left in the reservoir after both primary and secondary recovery methods have been exploited to their respective economic limits.

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Factors to Consider In Waterflooding

Thomas, Mahoney, and Winter (1989) pointed out that in determining the suitability of a candidate reservoir for waterflooding, the following reservoir characteristics must be considered:Reservoir geometry

Fluid properties

Reservoir depth

Lithology and rock properties

Fluid saturations

Reservoir uniformity and pay continuity

Primary reservoir driving mechanisms

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Waterflooding: Reservoir Geometry

The areal geometry of the reservoir will influence the location of wells and, if offshore, will influence the location and number of platforms required.

The reservoir’s geometry will essentially dictate the methods by which a reservoir can be produced through water-injection practices.

An analysis of reservoir geometry and past reservoir performance is often important when defining the presence and strength of a natural water drive and, thus, when defining the need to supplement the natural injection. If a water-drive reservoir is classified as an active water drive,

injection may be unnecessary.

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Waterflooding: Fluid Properties

The physical properties of the reservoir fluids have pronounced effects on the suitability of a given reservoir for further development by waterflooding. The viscosity of the crude oil is considered the most

important fluid property that affects the degree of success of a waterflooding project.The oil viscosity has the important effect of determining the

mobility ratio that, in turn, controls the sweep efficiency.

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Waterflooding: Reservoir Depth

Reservoir depth has an important influence on both the technical and economic aspects of a secondary or tertiary recovery project. Maximum injection pressure will increase with depth.

The costs of lifting oil from very deep wells will limit the maximum economic water–oil ratios that can be tolerated, thereby reducing the ultimate recovery factor and increasing the total project operating costs.

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Waterflooding: Reservoir Depth (Cont.)On the other hand, a shallow reservoir imposes a

restraint on the injection pressure that can be used, because this must be less than fracture pressure. In waterflood operations, there is a critical pressure

(approximately 1 psi/ft of depth) that, if exceeded, permits the injecting water to expand openings along fractures or to create fractures. This results in the channeling of the injected water or the

bypassing of large portions of the reservoir matrix.

Consequently, an operational pressure gradient of 0.75 psi/ft of depth normally is allowed to provide a sufficient margin of safety to prevent pressure parting.

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Waterflooding: Lithology and Rock PropertiesReservoir lithology and rock properties that affect

flood ability and success are:Porosity, Permeability, Clay content, Net thickness

In some complex reservoir systems, only a small portion of the total porosity, such as fracture porosity, will have sufficient permeability to be effective in water-injection operations.

The clay minerals present in some sands may clog the pores by swelling and deflocculating when waterflooding is used.

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Waterflooding: Fluid Saturations

In determining the suitability of a reservoir for waterflooding, a high oil saturation that provides a sufficient supply of recoverable oil is the primary criterion for successful flooding operations. Note that higher oil saturation at the beginning of flood

operations increases the oil mobility that, in turn, gives higher recovery efficiency.

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Waterflooding: Reservoir Uniformity and Pay ContinuitySubstantial reservoir uniformity is one of the major

physical criterions for successful waterflooding. For example, if the formation contains a stratum of

limited thickness with a very high permeability (i.e., thief zone), rapid channeling and bypassing will develop. Unless this zone can be located and shut off, the producing

water–oil ratios will soon become too high for the flooding operation to be considered profitable.

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Waterflooding: Primary Reservoir Driving MechanismsSix driving mechanisms basically provide the

natural energy necessary for oil recovery:Rock and liquid expansion

Solution gas drive

Gas cap drive

Water drive

Gravity drainage drive

Combination drive

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Primary Recovery

The recovery of oil by any of the above driving mechanisms is called primary recovery. The term refers to the production of hydrocarbons from

a reservoir without the use of any process (such as water injection) to supplement the natural energy of the reservoir.

The primary drive mechanism and anticipated ultimate oil recovery should be considered when reviewing possible waterflood prospects.

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Oil Recovery Range for Driving MechanismsThe approximate oil recovery range is tabulated

below for various driving mechanisms. Note that these calculations are approximate and, therefore, oil recovery may fall outside these ranges.

Driving Mechanism Oil Recovery Range, %

Rock and liquid expansion 3–7

Solution gap 5–30

Gas cap 20–40

Water drive 35–75

Gravity drainage <80

Combination drive 30–60

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Water-Drive Reservoirs

Water-drive reservoirs that are classified as strong water-drive reservoirs are not usually considered to be good candidates for waterflooding because of the natural ongoing water influx. However, in some instances a natural water drive could

be supplemented by water injection in order to:Support a higher withdrawal rate

Better distribute the water volume to different areas of the field to achieve more uniform areal coverage

Better balance voidage and influx volumes.

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Gas-Cap Reservoirs

Gas-cap reservoirs are not normally good waterflood prospects because the primary mechanism may be quite efficient without water injection. In these cases, gas injection may be considered in order

to help maintain pressure.

Smaller gas-cap drives may be considered as waterflood prospects, but the existence of the gas cap will require greater care to prevent migration of displaced oil into the gas cap. This migration would result in a loss of recoverable oil

due to the establishment of residual oil saturation in pore volume, which previously had none.

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Gas-Cap Reservoirs (Cont.)

The presence of a gas cap does not always mean that an effective gas-cap drive is functioning. If the vertical communication between the gas cap and

the oil zone is considered poor due to low vertical permeability, a waterflood may be appropriate in this case.Analysis of past performance, together with reservoir geology

studies, can provide insight as to the degree of effective communication.

Natural permeability barriers can often restrict the migration of fluids to the gas cap.

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Solution Gas-Drive Mechanisms

Solution gas-drive mechanisms generally are considered the best candidates for waterfloods. Because the primary recovery will usually be low, the

potential exists for substantial additional recovery by water injection.

In effect, we hope to create an artificial water-drive mechanism.

The typical range of water-drive recovery is approximately double that of solution gas drive. As a general guideline, waterfloods in solution gas-drive

reservoirs frequently will recover an additional amount of oil equal to primary recovery.

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Volumetric Undersaturated Oil ReservoirsVolumetric undersaturated oil reservoirs producing

above the bubble point pressure must depend on rock and liquid expansion as the main driving mechanism.In most cases, this mechanism will not recover more

than about 5% of the original oil in place.

These reservoirs will offer an opportunity for greatly increasing recoverable reserves if other conditions are favorable.

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Optimum Time to Waterflood

The most common procedure for determining the optimum time to start waterflooding is to calculate:Anticipated oil recovery

Fluid production rates

Monetary investment

Availability and quality of the water supply

Costs of water treatment and pumping equipment

Costs of maintenance and operation of the water installation facilities

Costs of drilling new injection wells or converting existing production wells into injectors

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Initializing a Secondary Recovery ProjectCole (1969) lists the following factors as being

important when determining the reservoir pressure (or time) to initiate a secondary recovery project:

1- Reservoir oil viscosityWater injection should be initiated when the reservoir

pressure reaches its bubble-point pressure since the oil viscosity reaches its minimum value at this pressure. The mobility of the oil will increase with decreasing oil viscosity,

which in turns improves the sweeping efficiency.

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Initializing a Secondary Recovery Project (Cont.)2- Free gas saturation

(1) In water injection projects. It is desirable to have initial gas saturation, possibly as much as 10%. This will occur at a pressure that is below the bubble point pressure.

(2) In gas injection projects. Zero gas saturation in the oil zone is desired. This occurs while reservoir pressure is at or above bubble-point pressure.

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Initializing a Secondary Recovery Project (Cont.)3- Cost of injection equipment

This is related to reservoir pressure, and at higher pressures, the cost of injection equipment increases. Therefore, a low reservoir pressure at initiation of injection is

desirable.

4- Productivity of producing wellsA high reservoir pressure is desirable to increase the

productivity of producing wells, which prolongs the flowing period of the wells, decreases lifting costs, and may shorten the overall life of the project.

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Initializing a Secondary Recovery Project (Cont.)5- Effect of delaying investment on the time value

of moneyA delayed investment in injection facilities is desirable

from this standpoint.

6- Overall life of the reservoirBecause operating expenses are an important part of

total costs, the fluid injection process should be started as early as possible.

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Initializing a Secondary Recovery Project (Cont.)Some of these six factors act in opposition to

others. Thus, the actual pressure at which a fluid injection

project should be initiated will require optimization of the various factors in order to develop the most favorable overall economics.

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Requirement for Fluid Injection ProjectsThe principal requirement for a successful fluid

injection project is that sufficient oil must remain in the reservoir after primary operations have ceased to render economic the secondary recovery operations. This high residual oil saturation after primary recovery is

essential not only because there must be a sufficient volume of oil left in the reservoir, but also because of relative permeability considerations. A high oil relative permeability, i.e., high oil saturation, means

more oil recovery with less production of the displacing fluid. On the other hand, low oil saturation means a low oil relative

permeability with more production of the displacing fluid at a given time.

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Flooding Patterns

One of the first steps in designing a waterflooding project is flood pattern selection. The objective is to select the proper pattern that will

provide the injection fluid with the maximum possible contact with the crude oil system.

This selection can be achieved by (1) Converting existing production wells into injectors or

(2) Drilling infill injection wells.

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Selection of Flooding Patterns

When making the selection, the following factors must be considered:Reservoir heterogeneity and directional permeability

Direction of formation fractures

Availability of the injection fluid (gas or water)

Desired and anticipated flood life

Maximum oil recovery

Well spacing, productivity, and injectivity

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Well Arrangements

In general, the selection of a suitable flooding pattern for the reservoir depends on the number and location of existing wells. In some cases, producing wells can be converted to

injection wells while in other cases it may be necessary or desirable to drill new injection wells.

Essentially four types of well arrangements are used in fluid injection projects:Irregular injection patternsPeripheral injection patternsRegular injection patternsCrestal and basal injection patterns

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Regular Injection Patterns

The patterns termed inverted have only one injection well per pattern.

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1. Ahmed, T. (2006). Reservoir engineering handbook (Gulf Professional Publishing). Ch14

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1. Recovery Efficiency

2. Displacement Efficiency

3. Frontal Displacement

4. Fractional Flow Equation

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