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  • 1. Reservoir Engineering 1 Course (2nd Ed.)

2. 1. Reservoir Fluid Behaviors 2. Petroleum Reservoirs A. Oil B. Gas3. Gas Behavior 4. Gas Properties: A. B. C. D.Z Factor Isothermal gas compressibility (Cg) Gas formation volume factor (Bg) Gas Viscosity 3. Multiphase Behavior Naturally occurring hydrocarbon systems found in petroleum reservoirs are mixtures of organic compounds that exhibit multiphase behavior over wide ranges of pressures and temperatures. These hydrocarbon accumulations may occur in the gaseous state, the liquid state, the solid state, or in various combinations of gas, liquid, and solid.2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties4 4. Petroleum Engineers Task These differences in phase behavior, coupled with the physical properties of reservoir rock that determine the relative ease with which gas and liquid are transmitted or retained, result in many diverse types of hydrocarbon reservoirs with complex behaviors. Frequently, petroleum engineers have the task to study the behavior and characteristics of a petroleum reservoir and to determine the course of future development and production that would maximize the profit. 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties5 5. Classification of Reservoirs and Reservoir Fluids Petroleum reservoirs are broadly classified as oil or gas reservoirs. These broad classifications are further subdivided depending on: The composition of the reservoir hydrocarbon mixture Initial reservoir pressure and temperature Pressure and temperature of the surface productionThe conditions under which these phases exist are a matter of considerable practical importance. The experimental or the mathematical determinations of these conditions are conveniently expressed in different types of diagrams commonly called phase diagrams. One such diagram is called the pressuretemperature diagram. 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties6 6. Typical P-T Diagram for a Multicomponent System2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties7 7. Pressure-Temperature Diagram Although a different hydrocarbon system would have a different phase diagram, the general configuration is similar. These multicomponent pressure-temperature diagrams are essentially used to: Classify reservoirs Classify the naturally occurring hydrocarbon systems Describe the phase behavior of the reservoir fluidTo fully understand the significance of the pressuretemperature diagrams, it is necessary to identify and define the following key points on these diagrams: Cricondentherm (Tct), Cricondenbar (pcb), Critical point, Phase envelope (two-phase region), Quality lines, Bubblepoint curve, Dew-point curve 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties8 8. Oil Reservoirs Depending upon initial reservoir pressure pi, oil reservoirs can be subclassified into the following categories: Undersaturated oil reservoir. If the initial reservoir pressure pi, is greater than the bubble-point pressure Pb of the reservoir fluid Saturated oil reservoir. When pi is equal to the bubblepoint pressure of the reservoir fluid Gas-cap reservoir or two-phase reservoir. If pi is below the bubble point pressure of the reservoir fluid The appropriate quality line gives the ratio of the gas-cap volume to reservoir oil volume. 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties10 9. Crude Oils Crude oils cover a wide range in physical properties and chemical compositions, and it is often important to be able to group them into broad categories of related oils. In general, crude oils are commonly classified into the following types: Ordinary black oil Low-shrinkage crude oil High-shrinkage (volatile) crude oil Near-critical crude oilThe above classifications are essentially based upon the properties exhibited by the crude oil, including physical properties, composition, gas-oil ratio, appearance, and pressure-temperature phase diagrams. 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties11 10. Ordinary Black OilA typical p-T diagram for an ordinary black Liquid-shrinkage curve for black oil oil Reservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties 2013H. AlamiNia12 11. Low-Shrinkage OilA typical phase diagram for a low-shrinkage Oil-shrinkage curve for low-shrinkage oil oil Reservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties 2013H. AlamiNia 13 12. Volatile Crude OilA typical p-T diagram for a volatile crude oil 2013H. AlamiNiaA typical liquid-shrinkage curve for a volatile Reservoir Engineering 1 Course: Petroleum Reservoirscrude oil Gas Properties 14 13. Near-Critical Crude OilA schematic phase diagram for the nearA typical liquid-shrinkage curve for the nearcritical Properties 2013H. AlamiNia critical crude oil Reservoir Engineering 1 Course: Petroleum Reservoirs Gas crude oil 15 14. Liquid Shrinkage for Crude Oil Systems2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties16 15. Gas Reservoirs In general, if the reservoir temperature is above the critical temperature of the hydrocarbon system, the reservoir is classified as a natural gas reservoir. On the basis of their phase diagrams and the prevailing reservoir conditions, natural gases can be classified into four categories: Retrograde gas-condensate Near-critical gas-condensate Wet gas Dry gas2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties18 16. Retrograde Gas-CondensateA typical phase diagram of a retrograde A typical liquid dropout curve (liquid system Reservoir Engineering 1 Course: Petroleum Reservoirs Gas Propertiesa condensate19 shrinkage volume curve for 2013H. AlamiNia system) 17. Near-Critical Gas-CondensateA typical phase diagram for a near-critical Liquid-shrinkage curve for a near-critical gasgas condensate system 2013H. AlamiNia condensate reservoir Reservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties 20 18. Wet GasPhase diagram for a wet gas2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties21 19. Dry GasPhase diagram for a dry gas2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties22 20. Compositions of Various Reservoir Fluid Types2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties23 21. Reservoir Fluid Properties To understand and predict the volumetric behavior of oil and gas reservoirs as a function of pressure, knowledge of the physical properties of reservoir fluids must be gained. These fluid properties are usually determined by laboratory experiments performed on samples of actual reservoir fluids. In the absence of experimentally measured properties, it is necessary for the petroleum engineer to determine the properties from empirically derived correlations. 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties26 22. Natural Gas Constituents A gas is defined as a homogeneous fluid of low viscosity and density that has no definite volume but expands to completely fill the vessel in which it is placed. Generally, the natural gas is a mixture of hydrocarbon and nonhydrocarbon gases. The hydrocarbon gases that are normally found in a natural gas are methanes, ethanes, propanes, butanes, pentanes, and small amounts of hexanes and heavier. The nonhydrocarbon gases (i.e., impurities) include carbon dioxide, hydrogen sulfide, and nitrogen. 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties27 23. Properties of Natural Gases Knowledge of PVT relationships and other physical and chemical properties of gases is essential for solving problems in natural gas reservoir engineering. These properties include: Apparent molecular weight, Ma Specific gravity, g Compressibility factor, z Density, g Specific volume, v Isothermal gas compressibility coefficient, cg Gas formation volume factor, Bg Gas expansion factor, Eg Viscosity, gThe above gas properties may be obtained from direct laboratory measurements or by prediction from generalized mathematical expressions. 2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties28 24. Behavior of Ideal Gases The gas density at any P and T:Specific VolumeApparent Molecular WeightSpecific GravityStandard Volume2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties29 25. Ideal Gases vs. Real Gases In dealing with gases at a very low pressure, the ideal gas relationship is a convenient and generally satisfactory tool. At higher pressures, the use of the ideal gas equation-of-state may lead to errors as great as 500%, as compared to errors of 23% at atmospheric pressure.2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties30 26. Behavior of Real Gases Basically, the magnitude of deviations of real gases from the conditions of the ideal gas law increases with increasing pressure and temperature and varies widely with the composition of the gas. The reason for this is that the perfect gas law was derived under the assumption that the volume of molecules is insignificant and that no molecular attraction or repulsion exists between them. Numerous equations-of-state have been developed in the attempt to correlate the pressure-volume-temperature variables for real gases with experimental data.2013H. AlamiNiaReservoir Engineering 1 Course: Petroleum Reservoirs Gas Properties31 27. Gas Compressibility Factor Definition In order to express a more exact relationship between the variables p, V, and T, a correction factor called the gas compressibility factor, gas deviation factor, or simply the z-factor, must be introduced to account for the departure of gases from ideal