# Method of measuring liquid flow in gas-liquid mixtures

FIELD: oil and gas production.

SUBSTANCE: invention relates to gas-liquid systems coming from oil production wells. Mixture is separated into liquid and gas in separator. Liquid is periodically accumulated in separator container and then displaced with gas. During this operation, differential pressure for liquid reaching its lower and upper recorded levels and time required for filling recorded volumes are measured as well as absolute pressure and temperature of gas in container. Liquid flow value expressed in weight is calculated using special mathematical dependence. At oil field, liquid and gas enter separator from preliminary gas intake installation or from the first separation step.

EFFECT: increased accuracy of measurement due to avoided gas density registration and excluded necessity of using strictly cylindrically-shaped measuring container.

1 dwg

The invention relates to the field of measurement of liquid flow in gas-liquid mixtures coming from oil wells.

There is a method of measuring fluid flow in gas-liquid mixtures coming from oil wells [1. Gsanremo and other Automated measuring system for measuring the flow rate of oil wells. Scientific-technical journal “automation and implementation of telemechanics and communication in the oil industry, No. 1-2, 2001, S. 16-18]. The method involves separation of the mixture in the separator for liquid and gas, periodic accumulation of fluid in the tank of the separator and the displacement of gas by measuring the differential pressure when the liquid reaches the lower and upper fixed levels and time filling fixed volumes. The mass flow rate calculated using the known dependence in accordance with a hydrostatic method of measuring mass [2. GOST 29976-86 Oil and petroleum products. Methods of measurement of mass, Gosstandart of the USSR, 1986)], which provides a measure of the fluid mass in the open cylindrical tanks at atmospheric pressure.

The application of hydrostatic method unchanged leads to error due to not taking into account the density of the gas and which significantly affects the measurement of fluid flow, hydrostatic “weighing” is the environment of the compressed gas in the separator.

The second disadvantage of this method is that for determining the mass of fluid that fills the measuring capacity requires the constancy (or dimension), the square cross-section within the fill level [2], i.e. the shape of the tank should be close to the cylindrical within the permissible error in the measurement.

The technical task of the invention is to increase the measurement accuracy by eliminating the methodical error Δdue to the lack of accounting for the density of the gas, and eliminates the need to apply a measuring vessel, a cylindrical shape.

To solve this problem in the measurement of fluid flow in gas-liquid mixtures coming from oil wells, including the separation of a mixture of liquid and gas separator, the periodic accumulation of fluid in the tank of the separator and the displacement of gas by measuring the differential pressure when the liquid reaches the lower and upper fixed levels and time filling fixed amounts, additional measures absolute pressure and temperature of gas in the tank, and the mass flow rate is calculated from dependencies

where:

V_{1}and V_{2}- calibrated volume of the separator corresponding to the calibrated EIT is enum marks the height level H_{
1}and H_{2};

g - free fall acceleration;

ρ_{o}- gas density at standard conditions;

T_{1}T_{2}- the value of the absolute temperature of the gas inside the separator when the level of marks H_{1}and H_{2};

T_{o}=293K - is the absolute temperature at standard conditions;

P_{o}=101.3 kPa - a value of absolute pressure at standard conditions;

P_{a1}and R_{a2}- the measured value of the absolute pressure in the separator at the moments t_{1}and t_{2}fill the separator liquid to calibrated levels H_{1}and H_{2}respectively;

P(t_{1}), P(t_{2}) - measured values of hydrostatic (differential) pressure at the moments t_{1}and t_{2}respectively;

τ_{W}=t_{2}-t_{1}- measured the time of filling of the separator liquid from mark H_{1}to the level of H_{2}.

The invention is illustrated in the drawing, in which figure 1 is a diagram of the device for measuring the average mass flow of the liquid.

To implement the method can be used in the device of known construction [1]. The device includes a separator 1 with inlet pipes 2 and 3 and the discharge pipe 4 with a three-way valve 5. A suction pipe 4 through the valve 5 is connected to the channel 6 output fluid channel 7 gas outlet of the separator 1. Separ the torus equipped with sensors 8 and 9 of the upper and lower levels, sensor 10 differential pressure sensors 11 and 12 temperature and pressure in the separator. The separator 1 may be a separator second stage, to which liquid and gas are fed to the individual tubes.

Field liquid and gas installation pre-selection of gas (or the first degree of separation) through the inlet pipe 2, and 3 received in the separator 1, where a further separation of gas from liquid.

While the valve 5 is in an intermediate position in which the channel 6 output fluid (residence permit) and channel 7 gas outlet (VG) communicate with the discharge pipe 4, the uncontrolled part of the liquid goes into the outlet pipe 4, which excludes the possibility of measurement of liquid flow.

At the time of switching of the valve from its initial position, “B”, when channel 6 (residence permit) is open, and channel 7 (VG) is closed, in the position “a”, when channel 6 is closed, and the channel 7 is open, the process of measurement of fluid flow, when the gas is diverted into the reservoir through the channel 7 and the pipe 4.

When the liquid level marks H_{1}and H_{2}sensors 9 and 8 signals to the controller. After the signal from the sensor 8, the controller gives a command to the switching valve 5 from position “A” to position “B”, and the rate of growth begins to decline.

After switching the valve 5 is replaced fluid in the outlet pipe 4.

Gave the e command is issued to switch the valve from position “B” to position “A” and the cycle of measurement of liquid flow is repeated.

According to the way the average mass flow rate of G_{W}is calculated by the formula

where:

τ_{W}=t_{2}-t_{1}the time dimension G_{W};

V_{1}- defined in the calibration volume of the separator, limited by marking the height level H_{1}(H_{1}measured from the zero level ONU);

V_{2}- defined in the calibration volume of the separator between the marks of H_{1}and H_{2};

g - free fall acceleration;

P(t_{1}), P(t_{2}- the value differential (hydrostatic) pressure points achievement level marks H_{1}and H_{2}determined by the signal from the pressure transducer 10;

ρ_{o}- gas density at standard conditions;

T_{1}, T_{2}- the value of the absolute temperature of the gas inside the separator when the level of marks H_{1}and H_{2}determined by the sensor 11;

P_{a1}P_{a2}- the value of the absolute pressure in the separator in moments of achievement level values of H_{1}and H_{2}determined by the sensor 12;

t_{1}, t_{2}- moments of achievement level marks the height H_{1}and H_{2}determined by the sensors, level switches 9 and 8.

Formula (1) is derived based on the following considerations.

The pressure difference P(t) in the “ + ” and the negative” camera sensor 10 is expressed by the formula

where:

ρ_{W}and ρ_{g}the density of the liquid and gas;

N is a value below the height level at an arbitrary time t;

P_{+}and R_{-}- the value of the absolute pressure in the “plus” and “minus” camera sensor 10, expressed by the formula

where:

H_{A}- mark height of the sampling point “negative” pressure (assuming that the pulse tube To the “minus chamber” filled with gas with a density of ρ_{g});

P_{a}the absolute pressure in both chambers of the sensor.

Formula (2) obtained by subtracting the left and right parts of equations (3) and (4).

The density of liquid ρ_{W}is expressed by the formula obtained by the conversion formula (2):

Using formula (5) eliminates truncation error Δabove.

The process of filling of the separator liquid is described by the following formulas.

At time t_{1}volume V_{1}filled liquid mass m_{1}.

where ρ_{1}=ρ(t_{1}- the density of the fluid in the volume V_{1}at time t_{1}.

At time t_{2}the volume of the separator (V_{1}+V_{2}) filled with a fluid with mass m_{1}(t_{1})+m_{2}(t_{2}), where m_{2}
2- weight of fluid in the volume V_{2}at time t_{2}

where ρ_{2}- the average density of the fluid in the volume (V_{1}+V_{2}).

Weight gain liquid Δm on the interval t_{2}-t_{1}will be:

Mass ow rate (average value) is expressed by the formula

From the formula (7) mass m_{2}(t_{2}) is expressed by the formula

Substituting the expression m_{1}(t_{1}from formula (6) in formula (10), we obtain:

Substituting the expression m_{2}(t_{2}from formula (11) in formula (9), we obtain:

Using formulas (5) to obtain the formula for ρ_{1}and ρ_{2}-

where ρ_{r1}- gas density at time t_{1}and ρ_{r2}- gas density at time t_{2}.

Substituting the expression ρ_{1}and ρ_{2}from formulas (13) and (14) into formula (12), we obtain:

Express ρ_{r1}and ρ_{r2}through the gas density at standard conditions ρ_{o}(at temperature T_{o}=293K and pressure P_{o}=101.3 kPa).

To do this, use the equation comp is being a real gas [3. Accion and other Molecular physics, M., Nauka, 1976, S. 478].

where:

m is the mass of gas in the volume V at the absolute temperature T and absolute pressure P_{and};

R - universal gas constant;

Z is the compressibility factor, taking into account the difference between the properties of real gas from ideal.

From equation (16) gas density ρ in real terms is expressed by the formula

Gas density ρ_{o}in standard conditions, R_{o}T_{o}using formula (16) is expressed by the formula

Dividing each other left and right parts of equations (17) and (18), after identical transformations get the well-known formula

Using formula (19), we obtain formulas for ρ_{r1}and ρ_{r2}-

Substituting the expression ρ_{r1}and ρ_{r2}from formulas (20) and (21) into the formula (16), we obtain the desired formula (1). The proposed method does not require permanence ρ_{W}on the interval t_{2}-t_{1}that can be seen from formulas (6) and (7).

Method of measurement of liquid flow in gas-liquid mixtures coming from oil wells, including the separation of a mixture of liquid and gas separator, the PE jdicheskoe accumulation of fluid in the tank of the separator and the displacement of gas by measuring the differential pressure when the liquid reaches the lower and upper fixed levels and time filling fixed volume and calculating the mass flow of the fluid, characterized in that it further measure absolute pressure and temperature of gas in the tank, and the mass flow rate is calculated from dependencies

where V_{1}and V_{2}- calibrated volume of the separator corresponding to the calibrated values marks the height level H_{1}and H_{2};

g - free fall acceleration;

ρ_{0}- gas density at standard conditions;

T_{1}T_{2}- the value of the absolute temperature of the gas inside the separator when the level of marks H_{1}and H_{2};

T_{0}=293 K is the value of the absolute temperature at standard conditions;

P_{0}=101.3 kPa - a value of absolute pressure at standard conditions;

P_{a1}and R_{A2}- the measured value of the absolute pressure in the separator at the moments t_{1}and t_{2}fill the separator liquid to calibrated levels H_{1}and H_{2}respectively;

P(t_{1}), P(t_{2}) - measured values of hydrostatic (differential) pressure at the moments t_{1}and t_{2}respectively;

τ_{W}=t_{2}-t_{1}- measured the time of filling of the separator liquid from mark H_{1}to the level of H_{2}.

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