Behavior of Gases

Behavior of Gases

1. BEHAVIOR OF GASES Gas Properties

2. Definition of a gas A gas is a homogeneous fluid, generally of low density and low viscosity. Gas has no definite volume but assumed the volume of any vessel which it is placed . Specific laws that express the behavior of gases at various temperature and pressure are very important in petroleum technology. The gases are divided into ideal (perfect) gas and real ( non-ideal) gas. The properties of hydrocarbon gases are relatively simple since the parameters of pressure, Volume and temperature (PVT) can be related by a single equation. The basics for this equation an adaptation of a combination of the classical laws of Boyle, Charles and Avogadro. In the equation of state for an ideal gas, that is a gas in which the volume of the gas molecules is insignificant, attractive and repulsive forces between molecules are ignored, and maintain their energy when they collide with each other.

3. Behaviour of hydrocarbon gases PV = nRT the ideal gas law Where field units SI units P = absolute pressure psia bara V = volume cu.ft. m 3 n = number of moles of gas - - T = absolute temperature o Rankine o Rankine R = universal gas constant psia. Cu.ft. KJ/ kmol.K The above equation is valid at low pressure where the assumptions hold. However, at typical reservoir temperatures and pressures, the assumptions are no longer valid, and the behaviour of hydrocarbon reservoir gases deviate from the ideal gas law.

4. Behaviour of hydrocarbon gases In practice, it’s convenient to represent the behaviour of these “real” gases by introducing a correction factor known as the gas deviation factor, into the ideal gas law: PV = znRT the real gas law The z- factror must be determined empirically,(i.e. by experiment), but this has been done for many hydrocarbon gases, and correlation charts exist for the approximate determination of the z-factor at various conditions of pressure and temperature.

5. The Perfect Gas Laws Boyle’s Law: For a given weight of gas, at a given temperature, the volume varies inversely as the pressure. Charles’ Law ( Gay-Lussac’s Law): For a given weight of gas, at a given pressure, the volume varies directly as the absolute temperature. Absolute temperature: o R = o F + 460 o K = o C + 273 Avogadro’ s Law: Under the same conditions of temperature and pressure, equal volumes of all perfect gases contain the same number of molecules one lb-mole of any perfect gas occupies a volume of 379 standard cu ft (60 o F, 14.7 psia), and one g-mole occupies a volume of 22.4 standard liters (0 o C, 1 atm)

6. T 1 = constant (V 1 P 1 T 1 ) (V P 2 T 1 ) P 2 = constant (V 2 P 2 T 2 ) Step 1 Step 2 Step 1: Boyle’s Law Step 2: Charles’ Law Eliminating V in both equations: Thus, for a given weight of gas, P V T = constant Combination of Boyle’s Law and Charles’ Law

7. Combination of Boyle’s Law and Charles’ Law with Avogadro’s Law Combination of all four laws gives, R = a gas const ant, that has the same value for all gases If n moles of gas, and since n is the weight of gas divided by the molecular weight a l so known as the general gas law P V T n R atm liters o K grams/MW 0.0821 atm cc o K grams/MW 82.1 psia cu ft o R Ib/MW 10.73

8. Density of a Perfect Gas Gas Mixtures Since density is defined as the weight per unit volume, the general gas law can be used to calculate densities of gases at various temperatures and pressures. 1 . 2. 3. 4. (Volume %) = (Mole %)

9. Actual gases approach perfect gas behavior at high temperatures and low pressures. In a perfect gas the molecules themselves are assumed to be of negligible volume (compared to the volume of the gas) and to exert no attractive forces on one another. At high pressures and low temperatures this is not so since, under these conditions, the volume of the molecules themselves is no longer negligible and the molecules, being more closely packed, exert appreciable attractive forces on one another Van der Waals’ equation is used to describe non-perfect gas behaviors. Non-Perfect Gases where: a and b are constants whose values are different for each gas, a/v 2 accounts for the attractive forces between the molecules, and is added to the pressure because the actual pressure would need to be larger to produce the same volume than if no attraction existed, and The constant b represents the volume of the molecules themselves, and it is subtracted from V since the actual volume of space available to the gas is less than the overall volume of the gas. When V is large (at low pressure and high temperatures), it is obvious that Van der Waals’ equation reduces to the general gas law

10. where Z is known as the gas compressibility factor or gas z-factor . Gas z-factor is an empirical factor, determined experimentally, which makes the above equation true at a particular temperature and pressure. For a perfect gas, Z is equal to one. For an imperfect gas, Z is greater or less than one, depending on the pressure and temperature. The law of corresponding states expressed that all pure gases have the same z-factor at the same reduced temperature and pressure . Gas Compressibility Factor For an imperfect gas one can write the general gas law in the form where P r = reduced pressure, T r = reduced temperature, P c = critical pressure, and T c = critical temperature For an imperfect gas mixture

11. Gas Formation Volume Factor, B g B g is defined as volume in bbl which is being occupied by 1 SCF gas when the gas was brought to reservoir conditions or is defined as the volume of gas at reservoir conditions which can produce 1 SCF of the gas at surface standard conditions. B g Estimation Method V o , T o , P o , z o , n reservoir V 1 = 1 SCF T 1 = 60 o F P 1 = 14.7 psia Z 1 = 1.0 n surface

12. Combining equations:

13. Gas Formation Volume Factor [res bbl/SCF] or [ft 3 /SCF] Volume of an arbitrary amount of gas at reservoir T & P Volume of SAME amount at standard T & P

14. Gas Formation Volume Factor Gas Formation Volume Factor is the volume in barrels (cubic metres) that one standard cubic foot (standard cubic metre) of gas will occupy as free gas in the reservoir at the prevailing reservoir pressure and temperature.

15. Gas Formation Volume Factor Using equation of state and Z at standard conditions = 1.0

16. Gas Formation Volume Factor (McCain)

17. Reciprocal of B g often used to reduce risk of misplacing decimal point as B g is less than 0.01 E is referred to as ‘Expansion Factor’ Gas Formation Volume Factor

18. Gas Formation Volume Factor

19. Viscosity Of Gases When fluid flow in the reservoir is considered, it is necessary to estimate the viscosity of the fluid, since viscosity represents an internal resistance force to flow given a pressure drop across the fluid. Unlike liquids, when the temperature and pressure of a gas is increased the viscosity increases as the molecules move closer together and collide more frequently. Viscosity is measured in poise. If a force of one dyne, acting on one cm 2 , maintains a velocity of 1 cm/s over a distance of 1 cm, then the fluid viscosity is one poise.

20. Viscosity Of Gases But for practical purposes, the centipoise (cP) is commonly used. The typical range of gas viscosity in the reservoir is 0.01 – 0.05 cP. By comparison, a typical water viscosity is 0.5 – 1.0 cP. Lower viscosities imply higher velocity for a given pressure drop, meaning that gas in the reservoir moves fast relative to oil and water, and is said to have a high mobility.

21. Viscosity (cp) Pressure T increasing 100 o F 150 o F 200 o F 100 o F 150 o F 200 o F Gas Viscosity

22. Gas Density Density is the most commonly measured property of a gas and obtained experimentally by measuring the specific gravity of the gas (density of the gas relative to air = 1) As pressure increases, so does gas density, but the relationship is non-linear since the dimensionless gas compressibility (z-factor) also varies with pressure. The gas density ( ρ g ) can be calculated at any pressure and temperature using the real gas law: ρ g = MP/ zRT Where M is the molecular weight of the gas (Ib/mol or kg/ kmol )

23. Relationship between subsurface and surface gas volumes The most important use of the real gas law is to calculate the volume which a subsurface quantity of gas will occupy at surface conditions, since when gas sales contracts are negotiated and gas is subsequently sold, is referred to in volumes at standard conditions of temperature (T sc ) and pressure (P sc ). The relationship required is the gas expansion factor (E), and is defined for a given quantity (mass or number of moles) of gas as; E = volume of gas standard conditions scf vol. of gas at reservoir conditions rcf

24. Relationship between subsurface and surface gas volumes It can be shown using the real gas law, and the knowledge that at standard conditions z = 1.0, that for a reservoir pressure (P) and temperature (T): E = 1/z . T sc / T . P / P sc vol. / vol. The previous equation is only valid as long as there is no compositional change of the gas between the subsurface and the surface. The value of E is typically in the order of 200, in other words the gas expands by a factor of around 200 from subsurface to surface conditions. The actual value of course depends upon both the gas composition and the reservoir temperature and pressure. Standard conditions of temperature and pressure are commonly defined as 60oF (298K) and one atmosphere (14.7 psia or 101.3 KPa), but may vary from location to location, and between gas sales contracts.

25. Relationship between subsurface and surface gas volumes In gas reservoir engineering, the gas expansion factor, E, is commonly used. But in oil reservoir engineering it is often convenient to refer to the gas formation volume factor, B g , which is the reciprocal E, and is expressed in units of rb/scf (using field units) Hence Bg (rb/scf) = 1/ 5.615 E

26. Gas Formation Volume Factor, B g B g is plotted as a function of reservoir pressure and temperature at 0.60, 0.70, 0.80, and 0.90 gas gravities. Other gas gravities, B g may be obtained by interpolation. B g

27. Gas Solubility, R s The gas solubility (R s ) is defined as the number of cubic feet of gas measured at standard conditions which are in solution in one barrel of stock tank oil at reservoir temperature and pressure, or simply the volume of gas produced at the surface divided by the volume of oil in the reservoir where the gas was existed. Unit for R s is SCF/STB or cuft/bbl. Gas solubility in crude oil increases with the increase of pressure until it reaches the saturation pressure (P b ). At pressure above P b , R s is constant or unchanged. Gas solubility decreases as the temperature increases. At constant pressure and temperature, gas solubility in crude oil will decrease with the decrease of specific gravity of gas. At constant pressure and temperature, Gas solubility in crude oil will increase with the increase of o API gravity of the crude oil. Gas solubility is dependent on the type of gas liberation processes. Flash liberation process produces much bigger gas solubility as compared to differential liberation one. The gas solubility (R s ): main observations

28. Gas Solubility, R s : Important observations gas solubility for saturated crude oil gas solubility for under-saturated crude oil Gas liberation processes effects on gas solubility

29. Gas Solubility, R s : Estimation Methods gas solubility correlation as function of pressure and o API