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Recovery mechanisms

• 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.

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Recovery mechanisms• Secondary oil recovery Refers to the additional recovery that results from the

conventional methods of water injection and immiscible gas injection.– Water flooding is perhaps the most common method of

secondary recovery.

• Tertiary (enhanced) oil recovery Various methods of enhanced oil recovery (EOR) are

essentially designed to recover 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

• In determining the suitability of a candidate reservoir for water flooding, the following reservoir characteristics must be considered:

1. Reservoir geometry2. Fluid properties3. Reservoir depth4. Lithology and rock properties5. Fluid saturations6. Reservoir uniformity and pay continuity7. Primary reservoir driving mechanisms

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1 .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.

• If a water-drive reservoir is classified as an active water drive, injection may be unnecessary.

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2. 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 oil viscosity has the important effect of determining the mobility ratio that, in turn, controls the sweep efficiency.

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3 .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.

• 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

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4 .Lithology and Rock Properties• Reservoir lithology and rock properties that affect flood

ability and success are: - Porosity - Permeability - Clay content - Net thickness

• The clay minerals present in some sands may clog the pores by swelling and deflocculating when waterflooding is used, no exact data are available as to the extent to which this may occur.

• Tight (low-permeability) reservoirs or reservoirs with thin net thickness possess water-injection problems in terms of the desired water injection rate or pressure.

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5 .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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6 .Reservoir Uniformity and Pay Continuity

• Substantial 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 rapid channeling and bypassing will develop.

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7.Primary Reservoir Driving Mechanisms• Six 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

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

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7 .Primary Reservoir Driving Mechanisms cont.

• The 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.

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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

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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.

• 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.

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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.

• Waterfloods in solution gas-drive reservoirs frequently will recover an additional amount of oil equal to primary recovery.

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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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Factors to determine the reservoir pressure (or time) to initiate a secondary recovery

projectReservoir oil viscosity Water 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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EFFECT OF TRAPPED GAS ON WATERFLOOD RECOVERY

• There are two different theories– First Theory (Cole (1969) )– In this case, this would dictate that the gas molecules enclose

themselves in an oil “blanket.” This increases the effective size of any oil globules. The amount of residual oil left in the reservoir would be reduced by the size of the gas bubble within the oil globule.

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EFFECT OF TRAPPED GAS ON WATERFLOOD RECOVERY

• Second Theory• as water displaced the

oil from the reservoir rock, the amount of residual oil left in the larger pore spaces would be reduced because of occupancy of a portion of this space by gas.

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EFFECT OF TRAPPED GAS ON RECOVERY

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SELECTION OF FLOODING PATTERNS

• 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.2.drilling infill injection wells.

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Types of well arrangements

• Essentially four types of well arrangements are used in fluid injection projects:– Irregular injection patterns– Peripheral injection patterns– Regular injection patterns– Crestal and basal injection patterns

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Irregular Injection Patterns

• Surface or subsurface topology and/or the use of slant-hole drilling techniques may result in production or injection wells that are not uniformly located.

• Some small reservoirs are developed for primary production with a limited number of wells and when the economics are marginal, perhaps only few production wells are converted into injectors in a nonuniform pattern.

• Faulting and localized variations in porosity or permeability may also lead to irregular patterns.

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Peripheral Injection Patterns

• The injection wells are located at the external boundary of the reservoir and the oil is displaced toward the interior of the reservoir.

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Regular Injection Patterns

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Crestal and Basal Injection Patterns

In crestal injection, as the name implies, the injection is through wells located at the top of the structure. Gas injection projects typically use a crestal injection pattern. In basal injection, the fluid is injected at the bottom of the structure. Many water-injection projects use basal injection patterns with additional benefits being gained from gravity segregation.

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OVERALL RECOVERY EFFICIENCY• The overall recovery factor (efficiency) RF of any secondary or tertiary

oil recovery method is the product of a combination of three individual efficiency factors as given by the following generalized expression:

• Where– RF = overall recovery factor– NS = initial oil in place at the start of the flood, STB– NP = cumulative oil produced, STB– ED = displacement efficiency– EA = areal sweep efficiency– EV = vertical sweep efficiency

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OVERALL RECOVERY EFFICIENCY

• The areal sweep efficiency EA Is the fractional area of the

pattern that is swept by the displacing fluid.

• The major factors determining areal sweep are:– Fluid mobilities– Pattern type– Areal heterogeneity– Total volume of fluid injected

• The vertical sweep efficiency EV

Is the fraction of the vertical section of the pay zone that is contacted by injected fluids.

• The vertical sweep efficiency is primarily a function of:– Vertical heterogeneity– Degree of gravity segregation– Fluid mobilities– Total volume injection

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OVERALL RECOVERY EFFICIENCY

• The displacement efficiency ED is the fraction of movable oil that has been displaced from the swept zone at any given time or pore volume injected. Because an immiscible gas injection or waterflood will always leave behind some residual oil, ED will always be less than 1.0.

• All three efficiency factors (i.e., ED, EA, and EV) are variables that increase during the flood and reach maximum values at the economic limit of the injection project

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The displacement efficiency ED

• The displacement efficiency is expressed as:

Where•Soi = initial oil saturation at start of flood•Boi = oil FVF at start of flood, bbl/STB•Ŝo = average oil saturation in the flood pattern at a particular point during the flood

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Fractional Flow Equation• The development of the fractional flow equation is attributed

to Leverett (1941). For two immiscible fluids, oil and water, the fractional flow of water, fw (or any immiscible displacing fluid), is defined as the water flow rate divided by the total flow rate, or:

• where – fw = fraction of water in the flowing stream, i.e., water cut, bbl/bbl– qt = total flow rate, bbl/day– qw = water flow rate, bbl/day– qo = oil flow rate, bbl/day

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fw = fraction of water (water cut), bbl/bblko = effective permeability of oil, mdkw = effective permeability of water, md

= water–oil density differences, g/cm3kw = effective permeability of water, mdqt = total flow rate, bbl/dayo = oil viscosity, cpw = water viscosity, cpA = cross-sectional area, ft2

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Effect of Water and Oil ViscositiesThis illustration reveals that regardless of the

system wettability, •Higher oil viscosity results in an increase in the fractional flow Curve.•Higher injected water viscosities will result in a decrease water flow rate with an overall reduction in fw.

Frontal Advance Equation

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iw = water injection rate, bbl/dayWinj = cumulative water injected, bblt = time, day

)x(Sw = distance from the injection for any given saturation Sw, ft

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Stabilized zone and nonstabilized zone

The stabilized zone As that particular saturation interval (i.e., Swc to Swf) where all points of saturation travel at the same velocity.

Nonstabilized zoneSaturation zone between Swf and (1 – Sor), where the velocity of any water saturation is variable.

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Break throw time

• tBT = time to breakthrough, day• PV = total flood pattern pore volume, bbl• L = distance between the injector and producer, ft

• the cumulative water injected at breakthrough

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AREAL SWEEP EFFICIENCY

• Is the fractional area of the pattern that is swept by the displacing fluid.

• The areal sweep efficiency depends basically on the following three main factors:– Mobility ratio M– Flood pattern– Cumulative water injected Winj

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VERTICAL SWEEP EFFICIENCY

• The vertical sweep efficiency, EV, Is the fraction of the vertical section of the pay zone

that is contacted by injected fluids.

• The vertical sweep efficiency is primarily a function of:– Vertical heterogeneity– Degree of gravity segregation– Fluid mobilities– Total volume injection

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Mobility

• In general, the mobility of any fluid λ is defined as the ratio of the effective permeability of the fluid to the fluid viscosity

where λo, λw, λg = mobility of oil, water, and gas, respectivelyko, kw, kg = effective permeability to oil, water, and gas, respectivelykro, krw = relative permeability to oil, water, and gas, respectivelyk = absolute permeability

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Mobility ratio

Substituting for λ:

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Flood Patterns

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Effect of Initial Gas Saturation

• When a solution-gas-drive reservoir is under consideration for waterflooding. It is necessary to inject a volume of water that approaches the volume of the pore space occupied by the free gas before the oil is produced. This volume of water is called the fill-up volume.

• So for effective water flooding we must achieve reservoir pressure 100-150 psi above bubble point pressure before beginning water flooding

• Because economic considerations dictate that waterflooding should occur at the highest possible injection rates, the associated increase in the reservoir pressure might be sufficient to redissolve all of the trapped gas Sgt back in solution.

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Stages of waterflooding.

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Water Fingering and Tonguing

• In a dipping reservoir, The condition for stable displacement is that the angle between the fluid interface and the direction of flow should remain constant throughout the displacement

Stable and unstable displacement in gravity segregated displacement

Water Flooding

صابر ناصر عمروعبده الحكيم عبد ابراهيمالجناينى حسن محمد حسن

احمد محمد السميع عبد احمدمحمد فتحى محمود محمد

رضوان السيد على محمد