Vietnam National University - Ho Chi Minh City University of Technology Faculty of Geology & Petroleum Engineering Department of Drilling - Production Engineering Course Reservoir Engineering Trần Nguyễn Thiện Tâm Email: trantam2512@hcmut.vn 12/11/2017 Reservoir Engineering 1 Chapter 3 Reservoir fluid properties and PVT analysis 12/11/2017 Reservoir Engineering 2 References Tarek Ahmed, Reservoir Engineering Handbook, 4th edition. Gulf Professional Publishing, 2010. 12/11/2017 Reservoir Engineering 3 Contents Reservoir fluid properties Classification of reservoir fluids 12/11/2017 Reservoir Engineering 4 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. 12/11/2017 Reservoir Engineering 5 Properties of natural gases 12/11/2017 Reservoir Engineering 6 Properties of natural gases 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., impurities) include carbon dioxide, hydrogen sulfide, and nitrogen. 12/11/2017 Reservoir Engineering 7 Behavior of ideal gases The kinetic theory of gases postulates that gases are composed of a very large number of particles called molecules. For an ideal gas, the volume of these molecules is insignificant compared with the total volume occupied by the gas. It is also assumed that these molecules have no attractive or repulsive forces between them, and that all collisions of molecules are perfectly elastic.
12/11/2017 Reservoir Engineering 8 Behavior of ideal gases Based on the above kinetic theory of gases, a mathematical equation called equation-of-state can be derived to express the relationship existing between pressure p, volume V, and temperature T for a given quantity of moles of gas n. This relationship for perfect gases is called the ideal gas law and is expressed mathematically by the following equation: pV = nRT where p = absolute pressure, psia V = volume, ft3 T = absolute temperature, °R n = number of moles of gas, lb-mole R = the universal gas constant, which, for the above units, has the value 10.730 psia ft3/lb-mole °R 12/11/2017 Reservoir Engineering 9 Behavior of ideal gases The number of pound-moles of gas, n, is defined as the weight of the gas m divided by the molecular weight M, or: m n M m pV RT M where m = weight of gas, lb M = molecular weight, lb/lb-mol Since the density is defined as the mass per unit volume of the substance, m pM g V RT where ρg = density of the gas, lb/ft3 12/11/2017 Reservoir Engineering 10 Apparent Molecular Weight M a yi M i i 1 where Ma = apparent molecular weight of a gas mixture Mi = molecular weight of the ith component in the mixture yi = mole fraction of component i in the mixture 12/11/2017 Reservoir Engineering 11 Standard Volume In many natural gas engineering calculations, it is convenient to measure the volume occupied by l lb-mole of gas at a reference pressure and temperature. These reference conditions are usually 14.7 psia and 60°F, and are commonly referred to as standard conditions. The standard volume is then defined as the volume of gas occupied by 1 lb-mol of gas at standard conditions.4 scf/lb-mol psc 14.7 where Vsc = standard volume, scf/lb-mol scf = standard cubic feet Tsc = standard temperature, °R (°R = °F + 460) psc = standard pressure, psia 12/11/2017 Reservoir Engineering 12 Density The density of an ideal gas mixture is calculated by simply replacing the molecular weight of the pure component with the apparent molecular weight of the gas mixture to give: pM a g RT 12/11/2017 Reservoir Engineering 13 Specific Volume The specific volume is defined as the volume occupied by a unit mass of the gas.
V RT 1 v m pM a g m with pV RT M where v = specific volume, ft3/lb ρg = gas density, lb/ft3 12/11/2017 Reservoir Engineering 14 Specific Gravity The specific gravity is defined as the ratio of the gas density to that of the air. Both densities are measured or expressed at the same pressure and temperature. Commonly, the standard pressure psc and standard temperature Tsc are used in defining the gas specific gravity: g g air psc M a RTsc Ma Ma g psc M air M air 28.96 RTsc 12/11/2017 Reservoir Engineering 15 Example A gas well is producing gas with a specific gravity of 0.65 at a rate of 1. The average reservoir pressure and temperature are 1,500 psi and 150°F.
Apparent molecular weight of the gas b. Gas density at reservoir conditions c. Flow rate in lb/day 12/11/2017 Reservoir Engineering 16 Example A gas well is producing a natural gas with the following composition: Component yi Mi CO2 0.11 Assuming an ideal gas behavior, calculate: a. Apparent molecular weight b.
Gas density at 2,000 psia and 150°F d. Specific volume at 2,000 psia and 150°F 12/11/2017 Reservoir Engineering 17 Behavior of 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 2–3% at atmospheric pressure. 12/11/2017 Reservoir Engineering 18 Behavior of real gases Numerous equations-of-state have been developed in the attempt to correlate the pressure-volume-temperature variables for real gases with experimental data.
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 into Equation 2-1 to account for the departure of gases from ideality. The equation has the following form: pV = znRT 12/11/2017 Reservoir Engineering 19 Behavior of real gases where the gas compressibility factor z is a dimensionless quantity and is defined as 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: Vactual V z Videal nRT / p Studies of the gas compressibility factors for natural gases of various compositions have shown that compressibility factors can be generalized with sufficient accuracies for most engineering purposes when they are expressed in terms of the following two dimensionless properties: • Pseudo-reduced pressure • Pseudo-reduced temperature 12/11/2017 Reservoir Engineering 20 Behavior of real gases These dimensionless terms are defined by the following expressions: p T p pr Tpr p pc Tpc where p = system pressure, psia ppr = pseudo-reduced pressure, dimensionless T = system temperature, °R Tpr = pseudo-reduced temperature, dimensionless ppc, Tpc = pseudo-critical pressure and temperature, respectively, and defined by the following relationships: p pc yi pci Tpc yiTci i i 12/11/2017 Reservoir Engineering 21 Behavior of real gases Based on the concept of pseudo-reduced properties, Standing and Katz (1942) presented a generalized gas compressibility factor chart as shown in Figure 2-1. The chart represents compressibility factors of sweet natural gas as a function of ppr and Tpr. This chart is generally reliable for natural gas with minor amount of nonhydrocarbons.
It is one of the most widely accepted correlations in the oil and gas industry. 12/11/2017 Reservoir Engineering 22 Example A gas reservoir has the following gas composition: the initial reservoir pressure and temperature are 3,000 psia and 180°F, respectively. Component yi Tci pci CO2 0.6 Calculate the gas compressibility factor under initial reservoir conditions. 12/11/2017 Reservoir Engineering 23 Behavior of real gases Equation pV = znRT can be written in terms of the apparent molecular weight Ma and the weight of the gas m: m pV z RT Ma Solving the above relationship for the gas specific volume and density, give: V zRT v m pM a 1 pM a g v zRT 12/11/2017 Reservoir Engineering 24 Example A gas reservoir has the following gas composition: the initial reservoir pressure and temperature are 3,000 psia and 180°F, respectively.
Component yi Mi Tci pci CO2 0.6 Calculate the density of the gas phase under initial reservoir conditions. Compare the results with that of ideal gas behavior. 12/11/2017 Reservoir Engineering 25 Behavior of real gases In cases where the composition of a natural gas is not available, the pseudo-critical properties, i., ppc and Tpc, can be predicted solely from the specific gravity of the gas. Brown et al.
(1948) presented a graphical method for a convenient approximation of the pseudo-critical pressure and pseudo-critical temperature of gases when only the specific gravity of the gas is available. The correlation is presented in Figure. Pseudo-critical properties of natural gases. 12/11/2017 Reservoir Engineering 26 Behavior of real gases Standing (1977) expressed this graphical correlation in the following mathematical forms: Case 1: Natural Gas Systems Tpc = 168 + 325γg − 12.5γg2 Case 2: Gas-Condensate Systems Tpc = 187 + 330γg − 71.1γg2 12/11/2017 Reservoir Engineering 27 Example A gas reservoir has the following gas composition: the initial reservoir pressure and temperature are 3,000 psia and 180°F, respectively.
Component yi Mi Tci pci CO2 0.6 Calculate the density of the gas phase under initial reservoir conditions by calculating the pseudo-critical properties. 12/11/2017 Reservoir Engineering 28 Effect of nonhydrocarbon components on the z-factor Natural gases frequently contain materials other than hydrocarbon components, such as nitrogen, carbon dioxide, and hydrogen sulfide. Hydrocarbon gases are classified as sweet or sour depending on the hydrogen sulfide content. Both sweet and sour gases may contain nitrogen, carbon dioxide, or both.
The common occurrence of small percentages of nitrogen and carbon dioxide is, in part, considered in the correlations previously cited. Concentrations of up to 5 percent of these nonhydrocarbon components will not seriously affect accuracy. Errors in compressibility factor calculations as large as 10 percent may occur in higher concentrations of nonhydrocarbon components in gas mixtures. 12/11/2017 Reservoir Engineering 29 Effect of nonhydrocarbon components on the z-factor Natural gases frequently contain materials other than hydrocarbon components, such as nitrogen, carbon dioxide, and hydrogen sulfide.
Hydrocarbon gases are classified as sweet or sour depending on the hydrogen sulfide content. Both sweet and sour gases may contain nitrogen, carbon dioxide, or both. The common occurrence of small percentages of nitrogen and carbon dioxide is, in part, considered in the correlations previously cited. Concentrations of up to 5 percent of these nonhydrocarbon components will not seriously affect accuracy.
Errors in compressibility factor calculations as large as 10 percent may occur in higher concentrations of nonhydrocarbon components in gas mixtures. 12/11/2017 Reservoir Engineering 30 Nonhydrocarbon Adjustment Methods There are two methods that were developed to adjust the pseudocritical properties of the gases to account for the presence of the nonhydrocarbon components. These two methods are the: • Wichert-Aziz correction method • Carr-Kobayashi-Burrows correction method 12/11/2017 Reservoir Engineering 31 The Wichert-Aziz Correction Method Natural gases that contain H2S and or CO2 frequently exhibit different compressibility-factor behavior than do sweet gases. Wichert and Aziz (1972) developed a simple, easy-to-use calculation procedure to account for these differences.
This method permits the use of the Standing-Katz chart, by using a pseudo-critical temperature adjustment factor, which is a function of the concentration of CO2 and H2S in the sour gas.