1Recovery 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. 2 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. 3 FACTORS TO CONSIDER IN WATERFLOODING • In determining the suitability of a candidate reservoir for water flooding, the following reservoir characteristics must be considered: 1. 2. 3. 4. 5. 6. 7. 4 Reservoir geometry Fluid properties Reservoir depth Lithology and rock properties Fluid saturations Reservoir uniformity and pay continuity Primary reservoir driving mechanisms will influence the location and number of platforms required. injection may be unnecessary. Reservoir Geometry • The areal geometry of the reservoir will influence the location of wells and.1. if offshore. 5 . • If a water-drive reservoir is classified as an active water drive. controls the sweep efficiency.2. 6 . in turn. 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. permits the injecting water to expand openings along fractures or to create fractures 7 . there is a critical pressure (approximately 1 psi/ft of depth) that. • In waterflood operations. Reservoir Depth • Reservoir depth has an important influence on both the technical and economic aspects of a secondary or tertiary recovery project. 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. if exceeded. • Maximum injection pressure will increase with depth.3. no exact data are available as to the extent to which this may occur.4.Net thickness • The clay minerals present in some sands may clog the pores by swelling and deflocculating when waterflooding is used.Porosity . • Tight (low-permeability) reservoirs or reservoirs with thin net thickness possess water-injection problems in terms of the desired water injection rate or pressure.Clay content . Lithology and Rock Properties • Reservoir lithology and rock properties that affect flood ability and success are: .Permeability . 8 . gives higher recovery efficiency. Fluid Saturations • In determining the suitability of a reservoir for waterflooding. 9 .5. in turn. • Note that higher oil saturation at the beginning of flood operations increases the oil mobility that. a high oil saturation that provides a sufficient supply of recoverable oil is the primary criterion for successful flooding operations. 10 .6. For example. Reservoir Uniformity and Pay Continuity • Substantial reservoir uniformity is one of the major physical criterions for successful waterflooding. if the formation contains a stratum of limited thickness with a very high permeability rapid channeling and bypassing will develop. 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.7. 11 . • The approximate oil recovery range is tabulated below for various driving mechanisms. therefore. • Note that these calculations are approximate and. Primary Reservoir Driving Mechanisms cont. oil recovery may fall outside these ranges. 12 .7. • 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 13 .Water-drive reservoirs • Water-drive reservoirs that are classified as strong waterdrive reservoirs are not usually considered to be good candidates for waterflooding because of the natural ongoing water influx. • Smaller gas-cap drives may be considered as waterflood prospects.Gas-cap reservoirs • Gas-cap reservoirs are not normally good waterflood prospects because the primary mechanism may be quite efficient without water injection. • If the vertical communication between the gas cap and the oil zone is considered poor due to low vertical permeability. 14 . gas injection may be considered in order to help maintain pressure. a waterflood may be appropriate in this case. but the existence of the gas cap will require greater care to prevent migration of displaced oil into the gas cap. In these cases. The typical range of water-drive recovery is approximately double that of solution gas drive. In effect.Solution gas-drive mechanisms • Solution gas-drive mechanisms generally are considered the best candidates for waterfloods. Because the primary recovery will usually be low. 15 . • Waterfloods in solution gas-drive reservoirs frequently will recover an additional amount of oil equal to primary recovery. we hope to create an artificial water-drive mechanism. the potential exists for substantial additional recovery by water injection. 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 16 . 17 . The mobility of the oil will increase with decreasing oil viscosity. which in turns improves the sweeping efficiency.Factors to determine the reservoir pressure (or time) to initiate a secondary recovery project Reservoir 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. ” This increases the effective size of any oil globules. 18 . this would dictate that the gas molecules enclose themselves in an oil “blanket. The amount of residual oil left in the reservoir would be reduced by the size of the gas bubble within the oil globule.EFFECT OF TRAPPED GAS ON WATERFLOOD RECOVERY • There are two different theories – First Theory (Cole (1969) ) – In this case. 19 . 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.EFFECT OF TRAPPED GAS ON WATERFLOOD RECOVERY • Second Theory • as water displaced the oil from the reservoir rock. EFFECT OF TRAPPED GAS ON RECOVERY 20 . Converting existing production wells into injectors. 2. 21 .drilling infill injection wells.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. 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 22 . perhaps only few production wells are converted into injectors in a nonuniform pattern. 23 .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. • Faulting and localized variations in porosity or permeability may also lead to irregular patterns. 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. 24 . Regular Injection Patterns 25 . 26 . the fluid is injected at the bottom of the structure.Crestal and Basal Injection Patterns In crestal injection. In basal injection. Many water-injection projects use basal injection patterns with additional benefits being gained from gravity segregation. 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. 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 – – – – – – 27 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 . • The vertical sweep efficiency is primarily a function of: – – – – Vertical heterogeneity Degree of gravity segregation Fluid mobilities Total volume injection .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: – – – – 28 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. Because an immiscible gas injection or waterflood will always leave behind some residual oil. ED. ED will always be less than 1.0.e.. EA. • All three efficiency factors (i. and EV) are variables that increase during the flood and reach maximum values at the economic limit of the injection project 29 .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. bbl/STB •Ŝo = average oil saturation in the flood pattern at a particular point during the flood 30 .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/day qw = water flow rate. For two immiscible fluids. water cut. or: • where – – – – 31 fw = fraction of water in the flowing stream. i.e.Fractional Flow Equation • The development of the fractional flow equation is attributed to Leverett (1941). oil and water. bbl/day qo = oil flow rate.. bbl/day . fw (or any immiscible displacing fluid). bbl/bbl qt = total flow rate. the fractional flow of water. is defined as the water flow rate divided by the total flow rate. fw = fraction of water (water cut). md qt = total flow rate. md kw = effective permeability of water. bbl/day o = oil viscosity. bbl/bbl ko = effective permeability of oil. ft2 32 . md = water–oil density differences. cp A = cross-sectional area. cp w = water viscosity. g/cm3 kw = effective permeability of water. 33 .Effect of Water and Oil Viscosities This 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. bbl/day Winj = cumulative water injected.Frontal Advance Equation iw = water injection rate. day (x)Sw = distance from the injection for any given saturation Sw. bbl t = time. ft 34 . Swc to Swf) where all points of saturation travel at the same velocity. Nonstabilized zone Saturation zone between Swf and (1 – Sor). where the velocity of any water saturation is variable.e.. 35 .Stabilized zone and nonstabilized zone The stabilized zone As that particular saturation interval (i. 36 . day • PV = total flood pattern pore volume.Break throw time • tBT = time to breakthrough. ft 37 . bbl • L = distance between the injector and producer. • the cumulative water injected at breakthrough 38 . 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 39 . • The vertical sweep efficiency is primarily a function of: – – – – 40 Vertical heterogeneity Degree of gravity segregation Fluid mobilities Total volume injection . EV.VERTICAL SWEEP EFFICIENCY • The vertical sweep efficiency. Is the fraction of the vertical section of the pay zone that is contacted by injected fluids. kg = effective permeability to oil. krw = relative permeability to oil. respectively ko. λw. respectively kro. and gas. water.Mobility • In general. λg = mobility of oil. water. water. respectively k = absolute permeability 41 . and gas. and gas. kw. the mobility of any fluid λ is defined as the ratio of the effective permeability of the fluid to the fluid viscosity where λo. Mobility ratio Substituting for λ: 42 . Flood Patterns 43 . • 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.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. the associated increase in the reservoir pressure might be sufficient to redissolve all of the trapped gas Sgt back in 44 solution. 45 .Stages of water flooding. 46 . 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 47 Stable and unstable displacement in gravity segregated displacement . Water Flooding عمرو واصر صابر ابراهيم عبد الحكيم عبدي حسه محمد حسه الجىايىى احمد عبد السميع محمد احمد محمد محمود فتحى محمد محمد على السيد رضوان .