Density of gases

Gases, unlike liquids, are characterized by low density. The normal density of a gas is the mass of one liter at 0°C and a pressure of 1 kgf/cm2. The mass of one molecule of any gas is proportional to its density.

The gas density c varies proportionally to the pressure and is measured by the ratio of the gas mass m to the volume V it occupies:

For practical purposes, it is convenient to characterize different gases by their density relative to air under the same conditions of pressure and temperature. Since the molecules of different gases have different masses, their densities at the same pressure are proportional to their molar masses.

Density of gases and the ratio of their density to air density:

Basic gas laws

A characteristic feature of gases is that they do not have their own volume and shape, but take shape and occupy the volume of the container in which they are placed. Gases uniformly fill the volume of the vessel, trying to expand and occupy as much volume as possible. All gases are highly compressible. Molecules of real gases have volume and have forces of mutual attraction, although these quantities are very insignificant. In calculations according to real gases usually used gas laws for ideal gases. Ideal gases are conventional gases, the molecules of which have no volume and do not interact with each other due to the absence of attractive forces, and during collisions between them no other forces act except the forces elastic impact. These gases strictly follow the laws of Boyle - Mariotte, Gay-Lussac, etc.

The higher the temperature and lower the pressure, the closer the behavior of real gases corresponds to ideal gases. At low pressures, all gases can be considered ideal. At pressures of about 100 kg/cm2, deviations of real gases from the laws of ideal gases do not exceed 5%. Since the deviations of real gases from the laws derived for ideal gases are usually negligible, the laws for ideal gases can be freely used to solve many practical problems.

Boyle's Law - Mariotte

Measurements of gas volume under the influence of external pressure showed that there is a simple relationship between volume V and pressure P, expressed by the Boyle-Mariotte law: the pressure of a given mass (or amount) of gas at a constant temperature is inversely proportional to the volume of gas:

P1: P2 = V1: V2,

where P1 is the gas pressure at volume V1; P2 - gas pressure at volume V2.

It follows that:

P1 * V1 = P2* V2 or P * V= const (at t = const).

This postulate is formulated as follows: the product of the pressure of a given mass of gas and its volume is constant if the temperature does not change (i.e. during an isothermal process).

If, for example, we take 8 liters of gas under pressure P = 0.5 kgf/cm2 and change the pressure at a constant constant temperature, then the following data will be obtained: at 1 kgf/cm2 the gas will occupy a volume of 4 liters, at 2 kgf/cm2 - 2 liters , at 4 kgf/cm2 - 1l; at 8 kgf/cm2 - 0.5 l.

Thus, at a constant temperature, any increase in pressure leads to a decrease in the volume of gas, and a decrease in the volume of gas leads to an increase in pressure.

The relationship between gas volume and pressure at a constant temperature is widely used for various calculations in diving practice.

Gay-Lussac's and Charles' laws

Gay-Lussac's law expresses the dependence of the volume and pressure of a gas on temperature: at constant pressure, the volume of a given mass of gas is directly proportional to its absolute temperature:

where T1 and T2 are the temperature in Kelvin (K), which is equal to the temperature in °C + 273.15; those. 0°C? 273 K; 100 °C - -373 K, and 0 °C = -273.15 °C.

Consequently, any increase in temperature leads to an increase in volume, or, in other words, the change in the volume of a given mass of gas V is directly proportional to the change in temperature t of the gas at constant pressure (i.e., during an isobaric process). This position is expressed by the formula:

where V1 is the volume of gas at a given temperature; V0 is the initial volume of gas at 0°C; b - coefficient of volumetric expansion of gas.

When heating various gases at same number degrees, the relative increase in volume is the same for all gases. Coefficient b is a constant volume increment for all gases, equal to 1/273 or 0.00367 oC-1. This coefficient volumetric expansion gases shows by what fraction of the volume occupied at 0°C the volume of the gas increases if it is heated by 1°C at constant pressure.

The relationship between pressure and temperature follows the same pattern, namely: the change in pressure of a given mass of gas is directly proportional to the temperature at a constant volume (i.e. in an isochoric process: from Greek words“izos” - equal and “horema” - capacity), which is expressed by the formula:

Pt = P0 (1 + bt),

where Рt is the gas pressure at a given temperature; Р0 -- initial gas pressure at 0° C; b - coefficient of volumetric expansion of gas.

This dependence was established by J. Charles 25 years before the publication of J. L. Gay-Lussac and is often called Charles’s law. The dependence of volume on temperature at constant pressure was also first established by Charles.

As the temperature of a gas decreases, its pressure decreases, and at a temperature of -273.15 °C, the pressure of any gas is zero. This temperature is called absolute zero temperature. At the same time, the chaotic thermal movement molecules and the amount of thermal energy becomes zero. The given dependencies, expressing the laws of Charles and Gay-Lussac, make it possible to solve important practical problems in the preparation and planning of underwater dives, such as, for example, determining the air pressure in cylinders when the temperature changes, the corresponding change in air reserves and time spent at a given depth, etc. . P.

Ideal gas equation of state

If the relationship between volume, pressure and temperature is linked together and expressed in one equation, then the equation of state of an ideal gas is obtained, which combines the Boyle-Mariotte and Gay-Lussac laws. This equation was first derived by B.P. Clayperon by transforming the equations proposed by his predecessors. Clayperon's equation is that the product of the pressure of a gas of a given mass and its volume divided by the absolute temperature is a constant value that does not depend on the state in which the gas is located. One way to write this equation is:

In this case, the gas constant r will depend on the nature of the gas. If the gas mass is a mole (gram molecule), then the gas constant R is universal and does not depend on the nature of the gas. For a gas mass equal to 1 mole, the equation takes the following form:

The exact value of R is 8.314510 J mol -1 K-1

If we take not 1 mole, but any amount of gas having mass m, then the state of an ideal gas can be expressed by the Mendeleev-Claiperon equation, convenient for calculations, in the form in which it was first written down by D.I. Mendeleev in 1874:

where m is gas mass, g; M - molar mass.

The ideal gas equation of state can be used for calculations in diving practice.

Example. Determine the volume occupied by 2.3 kg of hydrogen at a temperature of + 10 °C and a pressure of 125 kgf/cm2

where 2300 is gas mass, g; 0.082 - gas constant; 283 - temperature T (273+10); 2 is the molar mass of hydrogen M. From the equation it follows that the pressure exerted by the gas on the walls of the vessel is equal to:

This pressure disappears either at m > 0 (when the gas almost disappears) or at V>? (when the gas expands without limit), or at T > 0 (when the gas molecules do not move).

Van der Waals equation

Even M. V. Lomonosov pointed out that the Boyle-Mariotte law cannot be true at very high pressures, when the distances between molecules are comparable to their own sizes. Subsequently, it was fully confirmed that deviations from the behavior of ideal gases will be significant at very high pressures and very low temperatures. In this case, the ideal gas equation will give incorrect results without taking into account the interaction forces between gas molecules and the volume they occupy. Therefore, in 1873, Jan Diederik van der Waals proposed making two corrections to this equation: for pressure and for volume.

Avogadro's law

Avogadro put forward a hypothesis according to which, under the same conditions of temperature and pressure, all ideal gases regardless of their chemical nature contain per unit volume equal number molecules. It follows that the mass of equal volumes of gas is proportional to their molecular mass.

Based on Avogadro's law, knowing the volumes of the gases under study, you can determine their mass and, conversely, by the mass of the gas you can determine its volume.

Laws of gas dynamics

Dalton's law. The pressure of a mixture of gases is equal to the sum of the partial (partial) pressures of the individual gases making up the mixture, i.e., those pressures that each gas would produce separately if it were taken at the same temperature in the volume of the mixture.

The partial pressure of the gas Pr is proportional to percentage With a given gas and absolute pressure Rab gas mixture and is determined by the formula:

Pr = Pa6с С/100,

where Pr - partial pressure gas in mixture, kg/cm2; C is the volumetric gas content in the mixture, %.

Illustrate this law can be done by comparing a mixture of gases in a closed volume with a set of weights of different weights placed on scales. Obviously, each of the weights will exert pressure on the scale regardless of the presence of other weights on it.

Effect of temperature and pressure on gas density Gases, unlike droplet liquids, are characterized by significant compressibility and high values coefficient thermal expansion. The dependence of gas density on pressure and temperature is established by the equation of state. The simplest properties are those of a gas that is so rarefied that the interaction between its molecules may not be taken into account. This is an ideal (perfect) gas for which the Mendeleev-Clapeyron equation is valid:

The influence of temperature and pressure on gas density p - absolute pressure; R - specific gas constant, different for different gases, but independent of temperature and pressure (for air R = 287 J / (kg K); T - absolute temperature. The behavior of real gases in conditions far from liquefaction differs only slightly from the behavior of perfect gases, and for them, within wide limits, one can use the equations of state of perfect gases.

Effect of temperature and pressure on gas density In technical calculations, gas density is usually given as normal physical conditions: T=20°C; p = 101325 Pa. For air under these conditions ρ=1.2 kg/m3. Air density under other conditions is determined by the formula:

Influence on gas density of temperature and pressure According to this formula for isothermal process(T = const): An adiabatic process is a process that occurs without external heat exchange. For an adiabatic process k=ср/сv is the adiabatic constant of the gas; cp - heat capacity of gas at constant pressure; cv - the same, at constant volume.

Effect of temperature and pressure on gas density An important characteristic that determines the dependence of the change in density with a change in pressure in a moving flow is the speed of sound propagation a. In a homogeneous medium, the speed of sound propagation is determined from the expression: For air a = 330 m/s; For carbon dioxide 261 m/s.

The influence of temperature and pressure on gas density Since the volume of a gas largely depends on temperature and pressure, the conclusions obtained from the study of droplet liquids can be extended to gases only if, within the limits of the phenomenon under consideration, changes in pressure and temperature are insignificant. 3 Significant pressure differences, causing a significant change in the density of gases, can arise when they move at high speeds. The relationship between the speed of movement and the speed of sound in it allows one to judge the need to take compressibility into account in each specific case.

Effect of temperature and pressure on gas density If a liquid or gas is moving, then to evaluate compressibility they use not the absolute value of the speed of sound, but the Mach number, equal to the ratio of the flow speed to the speed of sound. M = ν/a If the Mach number is significantly less than unity, then the droplet liquid or gas can be considered practically incompressible

Equilibrium of gas If the height of the gas column is low, its density can be considered the same along the height of the column: then the pressure created by this column is determined by the basic equation of hydrostatics. At high altitude column of air its density is various points is no longer the same, so the hydrostatic equation does not apply in this case.

Gas Equilibrium Considering differential equation pressure for the case of absolute rest and substituting the density value into it, we have In order to integrate this equation, it is necessary to know the law of change in air temperature along the height of the air column. It is not possible to express the change in temperature as a simple function of height or pressure, so the solution to the equation can only be approximate.

Gas equilibrium For individual layers of the atmosphere, it can be assumed with sufficient accuracy that the change in temperature depending on height (and for a mine - on depth) occurs according to a linear law: T = T 0 + αz, where T and T 0 are the absolute air temperature, respectively, at height (depth) z and on the surface of the earth α is a temperature gradient characterizing the change in air temperature with an increase in height (-α) or depth (+α) by 1 m, K/m.

Gas equilibrium The values ​​of the coefficient α are different in different areas along the height in the atmosphere or depth in the mine. In addition, they also depend on meteorological conditions, time of year, and other factors. When determining temperature within the troposphere (i.e. up to 11000 m), α = 0.0065 K/m is usually taken, for deep mines the average value of α is taken equal to 0.004÷ 0.006 K/m for dry trunks, for wet trunks - 0.01.

Gas equilibrium Substituting the temperature change formula into the differential pressure equation and integrating it, we obtain The equation is solved for H, replacing natural logarithms decimal, α - its value from the equation through temperature, R - the value for air equal to 287 J/ (kg K); and substitute g = 9.81 m/s2.

Gas equilibrium As a result of these actions we get barometric formulaН = 29, 3(Т-Т 0)(log p/p 0)/(log. T 0/T), as well as the formula for determining pressure where n is determined by the formula

STEADY MOVEMENT OF GASES IN PIPES The law of conservation of energy in mechanical form for an element of length dx of a round pipe with diameter d, provided that the change in geodetic height is small compared to the change in piezometric pressure, has the form Here, the specific energy losses due to friction are taken according to the Darcy-Weisbach formula For a polytropic process with a constant polytropic index n = const and under the assumption that λ= const after integration, the law of pressure distribution along the gas pipeline is obtained

STEADY MOVEMENT OF GASES IN PIPES For main gas pipelines, therefore, the formula for mass flow can be written

STEADY MOVEMENT OF GASES IN PIPES M ω At n = 1, the formulas are valid for steady isothermal gas flow. The hydraulic resistance coefficient λ for gas depending on the Reynolds number can be calculated using the formulas used for liquid flow.

When moving real hydrocarbon gases for an isothermal process, an equation of state is used where the compressibility coefficient z of natural hydrocarbon gases is determined from experimental curves or analytically - from approximate equations of state.

ω

PHYSICAL PROPERTIES OF GASES

1. Gas density – mass of 1 m 3 of gas at a temperature of 0 0 and a pressure of 0.1 MPa (760 mm Hg). The density of a gas depends on pressure and temperature. The density of gases varies within the range of 0.55 - 1 g/cm3.

Commonly used relative density by air (dimensionless quantity - the ratio of gas density to air density; at normal conditions air density 1, 293 kg/m 3).

2. Viscosity of gases – internal friction of gases that occurs during its movement. The viscosity of gases is very low 1 . 10 -5 Pa.s. Such a low viscosity of gases ensures their high mobility through cracks and pores.

3. Solubility of gases – one of the most important properties. The solubility of gases in oil or water at a pressure of no more than 5 MPa is subject to Henry's law, i.e. the amount of dissolved gas is directly proportional to pressure and solubility coefficient.

At higher pressures, gas solubility is determined by a number of indicators: temperature, chemical composition, mineralization groundwater etc. The solubility of hydrocarbon gases in oils is 10 times greater than in water. Wet gas is more soluble in oil than dry gas. Lighter oil dissolves more gas than heavier oil.

4. Critical temperature gas. For each gas there is a temperature above which it does not transform into a liquid state, no matter how high the pressure is, i.e. critical t(for CH 4 t cr = –82.1 0 C). Homologues of methane can be found in liquid state(for C 2 H 6 t cr = 32.2 0 C, C 3 H 8 t cr = 97.0 0 C).

5. Diffusion is the spontaneous movement of gases to molecular level in the direction of decreasing concentrations.

6. Volumetric coefficient of reservoir gas is the ratio of the volume of gas under reservoir conditions to the volume of the same gas under standard conditions

(T = 0 0 and P = 0.1 MPa).

V g = V g pl / V g st

The volume of gas in the reservoir is 100 times less than under standard conditions, because gas is supercompressible.

GAS CONDENSATES

Not only can gas dissolve in oil, but oil can also dissolve in gas. This happens under certain conditions, namely:

1) the volume of gas is greater than the volume of oil;

2) pressure 20-25 MPa;

3) temperature 90-95 0 C.

Under these conditions, liquid hydrocarbons begin to dissolve in the gas. Gradually the mixture completely turns into gas. This phenomenon is called retrograde evaporation. When one of the conditions changes, for example, when the reservoir pressure decreases during development, condensate in the form of liquid hydrocarbons begins to release from this mixture. Its composition: C 5, H 12 (pentane) and higher. This phenomenon is called retrograde condensation.

Gas condensate is the liquid part of gas condensate accumulations. Gas condensates are called light oils, since they do not contain asphalt-resinous substances. The density of gas condensate is 0.65-0.71 g/cm3. The density of gas condensates increases with depth, and it also changes (usually increases) during development.

There are raw condensate and stable condensate.

Crude is a liquid phase extracted to the surface in which gaseous components are dissolved. Crude condensate is obtained directly in field separators at separation pressures and temperatures.

Stable gas condensate is obtained from raw gas by degassing it; it consists of liquid hydrocarbons (pentane) and higher ones.

GAS HYDRATES

Most gases form crystalline hydrates with water - solids. These substances are called gas hydrates and are formed at low temperatures, high pressures and at shallow depths. In appearance they resemble loose ice or snow. Deposits of this type have been found in areas permafrost Western and Eastern Siberia and in the waters of the northern seas.

The problem of using gas hydrates has not yet been sufficiently developed. All issues of gas hydrate production come down to creating conditions in the formation under which gas hydrates would decompose into gas and water.

To do this you need:

1) decrease in pressure in the reservoir;

2) increase in temperature;

3) addition of special reagents.

Patterns and changes in the properties of oil and gas in reservoirs and fields

So as a result of physical and chemical changes in oils and gases that occur under the influence of water penetrating into deposits and changes in reservoir pressure and temperature. Therefore, for reasonable forecasts of changes in the properties of oil and gas during the development process, it is necessary to have clear ideas: a) about the patterns of changes in the properties of oil and gas by volume of the deposit before the start of development; b) about the processes of physical and chemical interaction of oils and gases with waters entering the productive formation (especially with injected waters of a different composition than formation water); c) about the directions of fluid movement in the productive formation as a result of well operation; d) changes in reservoir pressure and temperature during the period of reservoir development. Patterns of changes in the properties of oil and gas according to the volume of the deposit. Complete uniformity of the properties of oil and gas dissolved in it within one deposit is quite a rare event. For oil deposits, changes in properties are usually quite natural and manifest themselves primarily in an increase in density, including optical density, viscosity, content of asphalt-resinous substances, paraffin and sulfur as the depth of the formation increases, i.e. from the roof to the wings and from the top to the bottom in thick layers. The actual change in density within most deposits usually does not exceed 0.05-0.07 g/cm3. However, very often the gradient of density increase and its absolute values increase sharply close proximity to the water-oil contact. Often the density of oil above the insulating layer is almost constant. In deposits of the “open” type, confined to layers exposed to the day surface and sealed from the top with asphalt-kirk rocks, the density of oil decreases with increasing depth, reaches a minimum, and then increases as you approach the OWC. The described patterns are most typical for high deposits of deposits in folded regions. The main reason for their formation is the gravitational differentiation (stratification) of oils by density within the deposit, similar to the stratification of gas, oil and water within the reservoir. Significant changes in the properties of oils in the OWC zone and in upper parts open-type oil deposits are associated with oxidative processes.

For deposits in platform areas with a low oil-bearing level and an extensive OWC zone, gravitational stratification is much weaker and the main influence on changes in the properties of oils is exerted by oxidative processes in the zone underlain by bottom water.

Simultaneously with the increase in oil density, its viscosity, as a rule, the content of asphalt-resinous substances and paraffin increases, and the gas content and saturation pressure of dissolved gases decrease.

Despite the high diffusion activity of gases, variability in their composition within a single deposit is far from a rare phenomenon. It manifests itself most sharply in the content of acidic components - carbon dioxide CO 2 and especially hydrogen sulfide H 2 S. Zoning is usually observed in the distribution of hydrogen sulfide, expressed in a regular change in the concentrations of hydrogen sulfide over the area. There are usually no obvious regular changes in concentration along the height of the deposit.

Gas-condensate deposits without an oil rim with a low level of gas content and a low condensate-gas factor, as a rule, have a fairly stable gas composition, composition and yield of condensate. However, when the height of the gas-condensate deposit is more than 300 m, the processes of gravitational stratification begin to noticeably manifest themselves, leading to an increase in the condensate content down the dip of the formation, especially sharply for deposits with a high level of gas content and an oil rim. In this case, the condensate content in the lower areas of the deposit can be several times higher than in the roof of the deposit. In particular, examples are known when the condensate-gas factor in the wells of the near-water part of the deposit was 180 cm 3 /m 3, and near the gas-oil contact - 780 cm 3 / m 3, i.e., within one deposit, the condensate content varied by 4 times. Fluctuations of 1.5--2 times are common for many fields with high floors gas content when the condensate exits more than 100 cm 3 /m 3.

Physicochemical properties of oil and parameters characterizing it: density, viscosity, compressibility, volumetric coefficient. Their dependence on temperature and pressure

Physical properties reservoir oils are very different from the properties of surface degassed oils, which is determined by the influence of temperature, pressure and dissolved gas. Changes in the physical properties of formation oils associated with the thermodynamic conditions of their presence in the formations are taken into account when calculating oil and oil gas reserves, during the design, development and operation of oil fields.

Density degassed oil varies widely - from 600 to 1000 kg/m 3 or more and depends mainly on the hydrocarbon composition and content of asphalt-resin substances.

The density of oil in reservoir conditions depends on the amount of dissolved gas, temperature and pressure. With increasing pressure, the density increases slightly, and with an increase in the other two factors, it decreases. The influence of the latter factors is greater. The density of oils saturated with nitrogen or carbon dioxide increases slightly with increasing pressure.

The influence of the amount of dissolved gas and temperature is stronger. Therefore, the gas density is always less than the density of degassed oil on the surface. As pressure increases, the density of oil decreases significantly, which is due to the saturation of oil with gas. An increase in pressure above the saturation pressure of oil with gas contributes to a slight increase in oil density.

The density of formation waters, in addition to pressure, temperature and dissolved gas, is strongly influenced by their salinity. When the concentration of salts in formation water is 643 kg/m 3, its density reaches 1450 kg/m 3.

Volume coefficient. When a gas dissolves in a liquid, its volume increases. The ratio of the volume of liquid with gas dissolved in it under reservoir conditions to the volume of the same liquid on the surface after its degassing is called the volumetric coefficient

b=V PL / V POV

where VPL is the volume of oil in reservoir conditions; V POV - volume of the same oil at atmospheric pressure and t=20°C after degassing.

Since oil can dissolve very a large number of hydrocarbon gas(even 1000 or more m 3 in 1 m 3 of oil), depending on thermodynamic conditions, the volumetric coefficient of oil can reach 3.5 or more. Volumetric coefficients for formation water are 0.99-1.06.

The decrease in the volume of oil recovered compared to the volume of oil in the reservoir, expressed as a percentage, is called “shrinkage.”

u=(b-1) / b *100%

When the pressure decreases from the initial reservoir p 0 to the saturation pressure, the volumetric coefficient changes little, because oil with gas dissolved in it behaves in this region like an ordinary weakly compressible liquid, expanding slightly as the pressure decreases. As the pressure decreases, gas is gradually released from the oil and the volumetric ratio decreases. An increase in oil temperature worsens the solubility of gases, which leads to a decrease in the volumetric coefficient

Viscosity. One of the most important characteristics oil is viscosity. Oil viscosity is taken into account in almost all hydrodynamic calculations associated with lifting fluid through tubing, flushing wells, transporting well products through in-field pipes, and treating near-wellbore formation zones. various methods, as well as in calculations related to the movement of oil in the reservoir.

The viscosity of reservoir oil is very different from the viscosity of surface oil, since it contains dissolved gas and is under conditions of elevated pressure and temperature. With an increase in the amount of dissolved gas and temperature, the viscosity of oils decreases.

An increase in pressure below the saturation pressure leads to an increase in the gas factor and, as a consequence, to a decrease in viscosity. An increase in pressure above the saturation pressure for reservoir oil leads to an increase in viscosity

With promotion molecular weight Oil's viscosity increases. Also, the viscosity of oil is greatly influenced by the content of paraffins and asphalt-resin substances in it, usually in the direction of increasing it.

Oil compressibility. Oil has elasticity, i.e. the ability to change its volume under the influence of external pressure. The elasticity of a liquid is measured by the compressibility coefficient, which is defined as the ratio of the change in the volume of the liquid to its original volume when the pressure changes:

β P =ΔV/(VΔP) , where

ΔV – change in oil volume; V – initial volume of oil; ΔP – pressure change

The compressibility coefficient of reservoir oil depends on the composition, the content of dissolved gas in it, temperature and absolute pressure.

Degassed oils have a relatively low compressibility coefficient, of the order of (4-7) * 10 -10 1/Pa, and light oils containing a significant amount of dissolved gas - up to 140 * 10 -10 1/Pa. The higher the temperature, the higher the compressibility coefficient.

Density.

Density usually refers to the mass of a substance contained in a unit volume. Accordingly, the dimension of this quantity is kg/m3 or g/cm3.

ρ=m/V

The density of oil in reservoir conditions decreases due to the gas dissolved in it and due to an increase in temperature. However, when the pressure decreases below the saturation pressure, the dependence of the oil density is non-monotonic, and when the pressure increases above the saturation pressure, the oil is compressed and the density increases slightly.

Oil viscosity.

Viscosity characterizes the friction force ( internal resistance), arising between two adjacent layers inside a liquid or gas per unit surface when they move mutually.

Oil viscosity is determined experimentally using a special VVD-U viscometer. The principle of operation of a viscometer is based on measuring the time of fall of a metal ball in the liquid being tested.

The viscosity of oil is determined by the formula:

μ = t (ρ w – ρ f) k

t – time of ball fall, s

ρ w and ρ w - density of the ball and liquid, kg/m 3

k – viscometer constant

An increase in temperature causes a decrease in oil viscosity (Fig. 2.a). An increase in pressure below the saturation pressure leads to an increase in the gas factor and, as a consequence, to a decrease in viscosity. An increase in pressure above the saturation pressure for reservoir oil leads to an increase in viscosity (Fig. 2. b).

The minimum viscosity value occurs when the pressure in the formation becomes equal to the formation saturation pressure.

Oil compressibility

Oil has elasticity. The elastic properties of oil are assessed by the oil compressibility coefficient. Oil compressibility refers to the ability of a liquid to change its volume under the influence of pressure:

β n = (1)

β n – oil compressibility coefficient, MPa -1-

V n – initial volume of oil, m 3

∆V – measurement of oil volume under the influence of pressure measurement ∆Р

The compressibility coefficient characterizes the relative change in a unit volume of oil with a change in pressure per unit. It depends on the composition of the reservoir oil, temperature and absolute pressure. With increasing temperature, the compressibility coefficient increases.

Volume coefficient

The volumetric coefficient is understood as a value that shows how many times the volume of oil in reservoir conditions exceeds the volume of the same oil after gas is released on the surface.

in = V pl /V money

c – volumetric coefficient

Vpl andVdeg – volumes of reservoir and degassed oil, m 3

When the pressure decreases from the initial reservoir p 0 to the saturation pressure (segment ab), the volumetric coefficient changes little, because oil with gas dissolved in it behaves in this region like an ordinary weakly compressible liquid, expanding slightly as the pressure decreases.

As the pressure decreases, gas is gradually released from the oil and the volumetric ratio decreases. An increase in oil temperature worsens the solubility of gases, which leads to a decrease in the volumetric coefficient.