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Tishk International University Engineering Faculty Petroleum and Mining Department Petroleum Production Engineering I Lecture 3: Properties of Oil and Natural Gas 4th-Grade- Fall Semester 2022-2023 Instructor: Nabaz Ali nabaz.ali@tiu.edu.iq

Contents Introduction Properties of oil Properties of gas References 2022-2023 Nabaz Ali Reservoir Engineering I 2

Introduction Properties of crude oil and natural gas are fundamental for designing and analyzing oil and gas production systems in petroleum engineering. 2022-2023 Nabaz Ali Reservoir Engineering I 3

Oil Properties Oil properties include solution gas–oil ratio (GOR), density, formation volume factor, viscosity, and compressibility. The latter four properties are interrelated through solution GOR. 2022-2023 Nabaz Ali Reservoir Engineering I 4

Properties of Oil 2022-2023 Nabaz Ali Reservoir Engineering I 5

Solution Gas Oil Ratio - GOR The solution gas-oil ratio (GOR) is a general term for the amount of gas dissolved in the oil. Solution GOR in black oil systems typically range from 0 to approximately 2000 scf / bbl. Rs Vgas Voil where Rs solution gas oil ratio, scf/stb Vgas gas volume in standard condition, scf Vst oil volume in stock tank condition, stb 2022-2023 Nabaz Ali Reservoir Engineering I 6

Solution Gas Oil Ratio - GOR 2022-2023 Nabaz Ali Reservoir Engineering I 7

Density of Oil - API API gravity is a commonly used index of the density of a crude oil or refined products. API stands for the American Petroleum Institute, which is the industry organization that created this measure. o API 141 .5 go 131 .5 r o ,st go rw where oAPI API gravity of stock tank oil. go ro,st rw specific gravity of stock tank oil, 1 for fresh water density of stock tank oil, lbm/ft3 density of fresh water, 62.4 lbm/ft3 2022-2023 Nabaz Ali Reservoir Engineering I 8

Oil Formation Volume Factor - Bo Oil formation volume factor is defined as the volume of oil (and dissolved gas) at reservoir pressure and temperature required to produce one stock tank barrel of oil at the surface. The oil formation volume factor is expressed in units of reservoir volume over standard volume (usually rbbl/STB) Vres Bo Vst 2022-2023 Bo oil formation volume factor, rb/stb Vres oil volume in reservoir condition, rb Vst oil volume in stock tank condition, stb Nabaz Ali Reservoir Engineering I 9

Viscosity of Oil - µo Viscosity provides a measure of a fluid’s internal resistance to flow. Any calculation involving the movement of fluids requires a value of viscosity. This parameter is required for conditions ranging from surface gathering systems to the reservoir. Correlations for the calculation of viscosity can be expected to evaluate viscosity for temperatures ranging from 35 to 300 F. The principal factors affecting viscosity are: Oil composition Temperature Dissolved gas Pressure 2022-2023 Nabaz Ali Reservoir Engineering I 10

Viscosity of Oil The characterization factor is a measure of the aromaticity/paraffinicity of a crude oil or of a crude oil fraction. The Watson K factor will range from less than 10 for highly aromatic hydrocarbons and almost 15 for highly paraffinic hydrocarbons. The range for whole crudes is 10.5 to 12.9. 2022-2023 Nabaz Ali Reservoir Engineering I 11

Oil Compressibility - Co The compressibility of any substance is the change in volume per unit volume per unit change in pressure. The oil compressibility is a source of energy for fluid flow in a reservoir. In an undersaturated reservoir it is a dominant drive mechanism, but for a saturated reservoir it is over-shadowed by the much larger gas compressibility (cg) effects due to dissolved gas evolving from solution. The oil compressibility is a component in the calculation of total compressibility (ct), which is the value used in the determination of skin, dimensionless time, and all material balance considerations in the fluid flow calculations. Oil compressibility, when plotted versus pressure, shows a significant discontinuity at the bubble point pressure. Above this pressure (undersaturated condition), the oil is a single phase liquid (consisting of oil and dissolved gas). The compressibility of this liquid can be measured in the laboratory, and it is a weak function of pressure. At and below the bubble point pressure (saturated condition), gas comes out of solution causing a sharp increase in compressibility which causes the discontinuity shown in the plot. Once below the bubble point, oil compressibility becomes a much stronger function of pressure. 2022-2023 Nabaz Ali Reservoir Engineering I 12

Oil Compressibility - Co 2022-2023 Nabaz Ali Reservoir Engineering I 13

Oil Compressibility - Co Isothermal compressibility of undersaturated oil is defined as which reflects the change in volume with change in pressure under constant temperature conditions. Below the bubble point pressure, oil isothermal compressibility is defined from oil and gas properties to account for gas coming out of solution. The corresponding saturated oil compressibility is Above bubble point pressure, oil volume changes as a function of isothermal compressibility only. 2022-2023 Nabaz Ali Reservoir Engineering I 14

Properties of Natural Gas LNG: Liquefied Natural Gas LPG: Liquefied Petroleum Gas NGL: Natural Gas Liquids 2022-2023 Nabaz Ali Reservoir Engineering I 15

Ideal Gases vs Real Gases pV nRT 2022-2023 pV znRT Nabaz Ali Reservoir Engineering I 16

Specific Gravity of Gas - 𝜸𝒈 The specific gravity of a gas, γ, is the ratio of the density of the gas at standard pressure and temperature to the density of air at the same standard pressure and temperature. The standard temperature is usually 60 F, and the standard pressure is usually 14.696 psia. Nc MWa gg 28.97 MWa yi MWi i 1 Mwa: Apparent molecular weight Yi: mole percentage Mwi: molecular weight of component 2022-2023 Nabaz Ali Reservoir Engineering I 17

Density of Gas - 𝝆𝒈 Gas density can be calculated from gas law for real gas with good accuracy: m MWa p rg V zRT Density of A B C 2022-2023 Nabaz Ali Reservoir Engineering I 18

Gas Compressibility Factor - Z The compressibility factor of a natural gas is a measure of its deviation from ideal gas behavior. Its value is usually between 0.8 and 1.2, but it can be as low as 0.3 and as high as 2.0. It is used in the calculation of gas pseudo-pressures (y), and in converting gas volumes and rates from standard conditions to reservoir conditions (and vice-versa). It is sometimes called the super-compressibility factor, and is often confused with the term "compressibility" (which is the change in volume per unit volume per unit change in pressure). The gas compressibility factor directly affects the gas compressibility (cg). The compressibility factor is expressed as: Z Factor: Is the ratio of the actual volume of n-moles of gas at T and P to the ideal volume of the same number of moles at the same T and P The gas law for real gas : 2022-2023 Nabaz Ali Vactual z Videal gas pV nzRT Reservoir Engineering I 19

Gas Pseudocritical Pressure and Temperature If the gas deviation factor is not measured, it may be estimated from correlations. The correlations depend on the pseudoreduced temperature and pressure, which in turn depend on the pseudocritical temperature and pseudocritical pressure. The pseudocritical temperature and pseudocritical pressure normally can be defined most simply as the molal average critical temperature and pressure of the mixture components. Thus, Nc Nc p pc yi pci T pc yi Tci i 1 i 1 where Pci and Tci are critical pressure and critical temperature of component i, respectively. 2022-2023 Nabaz Ali Reservoir Engineering I 20

Pseudo-reduced pressure and temperature: p pr p p pc T T pr T pc Using Ppr and Tpr in the graph, Z factor can be found 2022-2023 Nabaz Ali Reservoir Engineering I 21

Formation Volume Factor of Gas - Bg Formation volume factor of gas is defined as the volume of gas at reservoir pressure and temperature required to produce one standard volume of gas at the surface. Gas formation volume factor has units of volume/volume and can be represented in a variety of units (e.g., rcf/scf, rbbl/scf, and rbbl/Mscf). 2022-2023 Nabaz Ali Reservoir Engineering I 22

Formation volume factor is defined as : where the unit of formation volume factor is ft3/scf: p sc T z V zT Bg 0.0283 Vsc p Tsc z sc p If expressed in rb/scf, it takes the form of: zT Bg 0.00504 p 2022-2023 Nabaz Ali Reservoir Engineering I 23

Gas Compressibility - Cg Isothermal gas compressibility is the change in volume per unit volume for a unit change in pressure. Gas usually is the most compressible medium in the reservoir; however, care should be taken so that it is not confused with the gas deviation factor, z, which is sometimes called the compressibility factor. Isothermal Gas compressibility is defined as: 1 V C g V p T Real gas equation of state: 𝑛𝑅𝑇𝑧 V2022-2023 𝑃 Nabaz Ali Reservoir Engineering I 24

Differentiating real gas’s equation of state with respect to pressure at constant temperature T gives: 1 z V z nRT 2 p p p p Substituting the above equation into isothermal gas compressibility’s equation yields: Gas compressibility for real gas 1 1 z Cg p z p 2022-2023 Nabaz Ali Reservoir Engineering I 25

References McCain, W.D. Jr. The Properties of Petroleum Fluids, PennWell, Tulsa (1990). Lee, A.L., Gonzalez, M.H., and Eakin, B.E. 1966. The Viscosity of Natural Gases. J Pet Technol 18 (8): 997–1000. SPE-1340-PA. http://dx.doi.org/10.2118/1340-PA Carr, N.L., Kobayashi, R., and Burrows, D.B. 1954. Viscosity of Hydrocarbon Gases Under Pressure. J Pet Technol 6 (10): 47-55. http://dx.doi.org/10.2118/297-G Gold, D.K., McCain Jr., W.D., and Jennings, J.W. 1989. An Improved Method for the Determination of the Reservoir-Gas Specific Gravity for Retrograde Gases (includes associated papers 20006 and 20010 ). J Pet Technol 41 (7): 747-752. SPE-17310-PA. http://dx.doi.org/10.2118/17310-PA Engineering Data Book. 1987. Tulsa: Gas Processors Suppliers Association. Katz, D.L. and Lee, R.L. 1990. Natural Gas Engineering—Production and Storage. New York City: McGraw-Hill. Ahmed, T. 1989. Hydrocarbon Phase Behavior. Houston: Gulf Publishing Co. Pedersen, K.S., Fredenslund, A., and Thomassen, P. 1989. Properties of Oils and Natural Gases. Houston: Gulf Publishing Co. Whitson, C.H. and Brule, M.R. 2000. Phase Behavior, Vol. 20. Richardson, Texas: Monograph Series, SPE. 2022-2023 Nabaz Ali Reservoir Engineering I 26

where R s solution gas oil ratio, scf/stb V gas gas volume in standard condition, scf V st oil volume in stock tank condition, stb oil gas s V V R Solution Gas Oil Ratio - GOR The solution gas-oil ratio (GOR) is a general term for the amount of gas dissolved in the oil.Solution GOR in black oil systems typically range from 0 to approximately 2000 scf / bbl. 2022-2023 Nabaz Ali Reservoir .

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