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Q913 re1 w5 lec 17

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  • 1. Reservoir Engineering 1 Course (1st Ed.)
  • 2. 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
  • 3. 1. 2. 3. 4. 5. Oil Recovery Waterflooding Considerations Primary Reservoir Driving Mechanisms Secondary Recovery Project Flooding Patterns
  • 4. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 5
  • 5. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 6
  • 6. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 7
  • 7. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 8
  • 8. 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 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 10
  • 9. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 11
  • 10. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 12
  • 11. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 13
  • 12. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 14
  • 13. Waterflooding: Lithology and Rock Properties Reservoir 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 15
  • 14. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 16
  • 15. Waterflooding: 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 (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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 17
  • 16. Waterflooding: 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 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 20
  • 17. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 21
  • 18. Oil Recovery Range for Driving Mechanisms 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. Driving Mechanism Rock and liquid expansion Solution gap Gas cap Water drive Gravity drainage Combination drive 2013 H. AlamiNia Oil Recovery Range, % 3–7 5–30 20–40 35–75 <80 30–60 Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 22
  • 19. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 23
  • 20. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 24
  • 21. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 25
  • 22. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 26
  • 23. Volumetric Undersaturated Oil Reservoirs Volumetric 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 27
  • 24. 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 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 29
  • 25. Initializing a Secondary Recovery Project Cole (1969) lists the following factors as being important when determining the reservoir pressure (or time) to initiate a secondary recovery project: 1- 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. The mobility of the oil will increase with decreasing oil viscosity, which in turns improves the sweeping efficiency. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 30
  • 26. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 31
  • 27. 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 wells A 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 32
  • 28. Initializing a Secondary Recovery Project (Cont.) 5- Effect of delaying investment on the time value of money A delayed investment in injection facilities is desirable from this standpoint. 6- Overall life of the reservoir Because operating expenses are an important part of total costs, the fluid injection process should be started as early as possible. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 33
  • 29. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 34
  • 30. Requirement for Fluid Injection Projects The 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 35
  • 31. 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. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 37
  • 32. 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 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 38
  • 33. 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 patterns Peripheral injection patterns Regular injection patterns Crestal and basal injection patterns 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 39
  • 34. Regular Injection Patterns The patterns termed inverted have only one injection well per pattern. 2013 H. AlamiNia Reservoir Engineering 1 Course: W5L17 Principles of Waterflooding 40
  • 35. 1. Ahmed, T. (2006). Reservoir engineering handbook (Gulf Professional Publishing). Ch14
  • 36. 1. 2. 3. 4. Recovery Efficiency Displacement Efficiency Frontal Displacement Fractional Flow Equation