RU2740462C1 - Method of preventing formation of asphaltene, resinous paraffin deposits (arpd) in lift pipes during gas-lift operation of oil wells - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: invention relates to oil industry, in particular to methods of preventing deposit formation on oil-producing equipment of paraffin and other components of hydrocarbon material, for example, asphaltene-resin-paraffin compositions during its production and transportation. Proposed method comprises pumping working fluid into annular space of the well. Initial data are obtained on component compositions of formation fluid and associated petroleum gas, which is used as a working agent, then it is purified from hydrogen sulphide and carbon dioxide, then, purified associated petroleum gas is pumped without changing its hydrocarbon composition and temperature of oil saturation with paraffin is determined and its formation depth in the well taking into account the oil component composition change during pumping of associated petroleum gas, further, the ratio of light and heavy fractions of injected associated petroleum gas is changed and the temperature of oil saturation with paraffin and the depth of its formation for a given ratio of light and heavy fractions are determined. Then the factors are compared and the optimal version of the required amount of working agent consumption is selected at the lowest temperature of oil saturation with paraffin.

EFFECT: improving operating efficiency of gas lift wells complicated by asphaltene-resin-paraffin deposits, increasing overhaul period of their operation.

1 cl, 2 tbl, 4 dwg

Description

The invention relates to the oil industry, in particular to methods for preventing wax deposits in downhole equipment.

A known method of preventing the formation of asphalt-resin-paraffin deposits in oil and gas production wells (copyright certificate SU No. 124896, publ. 01.01.1959) by lowering the tubing into the well and using flow lines with a pre-applied protective coating. Smooth dielectric coatings, the material of which are plastics (vinyl plastic, polyethylene, etc.), are recommended for use.

The disadvantages of the invention are the high complexity in the manufacture of tubing (tubing) with a protective coating, fragility and weak adhesion (adhesion) of the coating to the metal, as well as a high likelihood of damage during transportation and running operations during current (overhaul) well workovers.

A known method for preventing the deposition of asphalt-resin-paraffin substances in the equipment of the well (patent RU No. 2029855, publ. 27.02.1995) by dividing the bottomhole product into oil and water components. The aqueous component is used as the hydrophilizing solution. Pumping starts with the water component. It creates a hydrophilic film on the surface of the equipment. Then the oil component is pumped out. The pumping of the water and oil components is alternated to restore the hydrophilic film.

The disadvantages of this method are the high complexity in carrying out the operation to hydrophilize the inner surface of the tubing and the short duration of the hydrophilic film on the surface of the equipment due to its constant washing away with well products.

There is a known method of combating wax deposits in lift pipes during gas-lift operation of a well (copyright certificate SU No. 1680954, publ. 01/30/1991), including supplying a working agent through a pipeline into a well and pumping condensate into the annulus in dosed portions with a continuous flow rate, which is characterized by the fact that, in order to increase its efficiency by preventing the formation of wax deposits while reducing costs for its implementation, an inhibitor of paraffinic deposits is additionally introduced into it before pumping condensate, and the condensate and an inhibitor of wax deposits are pumped into the pipeline to supply the working agent in the form of an aerosol ...

The disadvantages of this method are the need for additional laboratory studies for the selection and assessment of the effectiveness of wax inhibitors. In addition, it is proposed to use a mixture of hydrocarbon gas released from the formation fluid and air as the gas mixture injected into the well. On the one hand, the oxidative effect of oxygen on oil components leads to the formation of a stable oil-water emulsion in the tubing, for the destruction of which additional use of chemical and thermal methods is required; on the other hand, during separation it is also possible to form a fire and explosive mixture of hydrocarbon gas and air.

A known method of eliminating and preventing the formation of asphalt-resin-paraffin deposits in oil and gas production wells (patent RU No. 2248442, publ. 09/10/2003), in which, in order to prevent the formation of asphalt-resin-paraffin deposits in the well at the depth of sediment formation, a heating system consisting of a linear heating element is immersed in the well in the form of a metal conductor, supplying cores and a current switch between them in the head part. The closure is a local heater, with the help of which heating is carried out when the heating system is immersed in the well, which allows the plugs formed by deposits to pass. After immersion, predominantly associated heating is carried out by closing the current circuit formed by the metal conductor and the supply core, for this the closing element has a falling dependence of the resistance on the temperature rise.

The disadvantage of this method is the lack of temperature control during operation of the heating system and, as a consequence, possible overheating of the contactor and core of the linear heating element, its further electrical breakdown and failure of the heating system, i.e. decrease in the reliability of the heating system as a whole. Also, another disadvantage of the method is that the closure, by virtue of its design, has a diameter close to the inner diameter of the tubing, therefore, the heating element lowered into the tubing of the well will impede the flow of well fluid during production, since the cross section of the tubing is significantly reduced and, therefore, the inability to operate the heating systems for long-term operation in the well.

There is a known method of combating wax deposits in lift pipes during gas-lift operation of a well (copyright certificate SU No. 1219789, publ. 03.23.1986) taken as a prototype, including pumping condensate together with a working agent into the well according to the following technological process: condensate from the tank through a dosing pump with a flow rate determined by the formula is pumped into the annulus of a gas-lift well

C = 550 + 12 (k-7),

where C is the consumption of injected condensate per 1 ton of produced oil, g; k is the percentage of paraffin in the oil produced.

The disadvantage of this method is the need for continuous or periodic injection of condensate together with a working agent into the well; it does not provide complete removal of asphalt-resin-paraffin deposits over the entire interval of wax formation depths in the well due to the fact that the volume of injected condensate is determined without taking into account the determination of the depth of formation of paraffin deposits.

The technical result from the use of the invention is to increase the efficiency of the operation of gas-lift wells, complicated by the formation of ARPD in the lift pipes, reduce their forced downtime for cleaning work and increase the turnaround time by taking additional measures to prevent and reduce the intensity of the formation of these deposits.

The technical result is achieved by obtaining the initial data on the component compositions of the formation fluid and associated petroleum gas, which is used as a working agent, then it is purified from hydrogen sulfide and carbon dioxide, then the purified associated petroleum gas is injected without changing its hydrocarbon composition and the temperature T1 of oil saturation with paraffin and the depth of its formation in the well, taking into account the change in the component composition of oil during the injection of associated petroleum gas, then the ratio of light and heavy fractions of the injected associated petroleum gas is changed and the temperature T2 of oil saturation with paraffin and the depth of its formation for a given ratio of light and heavy fractions, then a comparison of the T2 and T1 indices is carried out, if T2> T1, the ratio of the light and heavy fractions of the injected working agent is changed again and the determination of the temperature T2 of oil saturation with paraffin and the depth of its formation, if T2 <T1, is taken. temperature as a new initial version, then the condition of the possibility of ensuring the required ratio of fractions based on the component composition of APG is checked, if the condition of possibility is confirmed, then this procedure is repeated for another variant of the ratio of light and heavy fractions, if not confirmed, then the optimal variant of the required amount of flow is selected working agent at the lowest temperature of oil saturation with paraffin.

The method is illustrated by the following figures:

fig. 1 - graph of changes in the temperature of oil saturation with paraffin;

fig. 2 is a graph for determining the depth of formation of wax;

fig. 3 - flow diagram of the method;

fig. 4 is a diagram of an algorithm for calculating a change in the composition during injection of associated petroleum gas.

The method is carried out in the following sequence (Fig. 3). For a candidate gas-lift well included in a complicated pool due to the formation of ARPD, initial data on the composition of the formation fluid and associated petroleum gas (APG) are obtained.

Associated petroleum gas, previously purified from hydrogen sulfide and carbon dioxide, is considered as a working agent for gas lift. The injected associated petroleum gas is selected with a certain composition, taking into account the composition and properties of the well product and its change when mixed with a working agent, in order to reduce the crystallization temperature of paraffin and the intensity of its formation.

The composition and amount of injected associated petroleum gas is selected as follows. Initially, an option is being considered for injecting purified associated petroleum gas without changing its hydrocarbon composition, which includes light fractions from CH ₄ to C ₄ H ₁₀ and heavier ones from C ₅ H ₁₂ to C ₁₁ H ₂₄ . Further, for this option, the temperature of oil saturation with paraffin and the depth of its formation in the well are determined, taking into account the change in the component composition of oil during the injection of associated petroleum gas.

Accounting for changes in the component composition is carried out in the following way:

Component composition of the gas-liquid mixture at pressure P ₁ and temperature T ₁

where z _i - total composition of i - component

x _i - component composition of the liquid phase i - component

- компонентный состав газовой фазы i - компонента с учетом закачиваемого газлифта

- component composition of the gas phase i - component, taking into account the injected gas lift

n _g - molar fraction of the gas phase

n _w - molar fraction of the liquid phase, n _w = 1-n _g

a is the ratio of the number of moles of injected gas to the number of moles of oil per unit time

K _i - equilibrium ratio i - component

K _i depends on the ratio of fugacity of the liquid and gas phases. In turn, fugacity is a function of the component composition.

The calculation of the component liquid mixture at a pressure P ₁ and temperature T ₁ received mole fraction n _r1 gas phase, liquid phase n _x1, n _x1 = 1-n _r1

The accepted component composition of the liquid phase x _{i was} obtained from the calculation at the first temperature (T ₁ ) as the initial total composition z _i when calculating for the conditions of the second temperature (T ₂ ) (z _i2 = x _i1 ), (index 1, 2, respectively, the first and second condition)

The number of moles of liquid at a temperature T = T _{2 is considered} equal to one, the composition of the oil is calculated according to the previous algorithm, the instantaneous mole fractions of the liquid and gas phases n _{x2 (m)} , n _{g2 (m)} are obtained, respectively. The actual molar fraction of the liquid phase is calculated by the formula:

Similarly, for conditions (P ₃ , T ₃ ) and (P _n , T _n ) the actual molar fraction of liquid is obtained:

and the molar fraction of the gas phase

where n _wn , n _gn molar fraction of liquid and gas phases at temperature and pressure (P _n , T _n ).

Based on the results of calculating the change in the composition of the gas-liquid mixture during the injection of associated petroleum gas, the change in the temperature of oil saturation with paraffin is determined by using well-known software products that allow studying the process of formation of paraffin deposits in the well, such as Multiflash, Flow Assurance OLGA, LedaFlow Software. The temperature of oil saturation with paraffin during the gas-lift method of well operation is a function that depends on the pressure, fluid temperature, oil composition, flow rate and composition of the injected associated petroleum gas. The pressure and temperature distribution curves of the flow were obtained from the results of the analysis of the PT profile in the PIPESIM program. The release of solid ARPD substances from oil begins when the flow temperature drops to the temperature of oil saturation with paraffin, therefore, the depth of the beginning of intensive formation of deposits (Fig. 2) corresponds to the intersection of the curves of the distribution of the flow temperature and the temperature of oil saturation with paraffin in the well (Fig. 1).

The resulting oil saturation temperature with paraffin will be the starting point for comparison with subsequent options.

The next step is to change the ratio of light and heavy fractions of the injected associated petroleum gas and carry out the calculation according to the above method. We get a new value of the oil saturation temperature with paraffin.

Having performed the procedure for various ratios of light and heavy fractions, we select the most optimal option based on the required amount of the working agent flow rate and the lowest temperature of oil saturation with paraffin.

The method is illustrated by the following examples.

For the conditions of a gas-lift well with a reservoir pressure of 19.2 MPa and a reservoir temperature of 135 ° C. Oil saturation pressure with gas - 13.5 MPa. The result of selection of the optimal composition of the injected working agent is presented. In this case, the injection pressure of associated petroleum gas is 10 MPa and its flow rate is 20,000 m ³ / day, the planned flow rate for liquid is provided (100 m ³ / day). Table 1 shows the composition of the original oil and injected associated petroleum gas.

Further, based on the initial data, the depth of formation of asphalt-resin-paraffin deposits in the gas-lift well is determined based on the calculation of the oil saturation temperature with paraffin, taking into account the change in the composition of the oil during the injection of associated petroleum gas.

If the data on the composition of the formation fluid are not separated into separate hydrocarbon components, then it is necessary to redistribute these components into separate pseudo-components using the Katz method:

Where

z _{C7 +} - molar fraction of C ₇₊ ;

n is the number of carbon atoms of the pseudo-component;

z _n is the mole fraction of the pseudo-component with the number of carbon atoms n;

n + is the last hydrocarbon component in the C ₇₊ group with n carbon atoms, such as 12+;

M _{C7 +} , γ _{C7 +} - measured molecular weight and specific gravity of C ₇₊ ;

M _n , γ _n - molecular weight and specific gravity of the pseudo-component with n carbon atoms.

The value of T _k , P _k , ω component C _{n + are} determined as follows:

where T _кi , Р _кi , - reference values of critical pressures and boiling points i - component, ω - acentric factor.

Then, the compositional composition is calculated at pressure P ₁ and temperature T ₁ at the gas injection depth using the following algorithm (Fig. 4).

x _i - component composition of the liquid phase i - component

y _i - component composition of the gas phase i - component

ƒ (n _ν ) - Gas phase characteristic function:

Function derivative:

(n _g ) _n - new value of the molar fraction of the gas phase:

- компонентный состав газовой фазы i - компонента с учетом закачиваемого газлифта.

- component composition of the gas phase i - component, taking into account the injected gas lift.

a is the ratio of the number of moles of injected gas to the number of moles of oil per unit time.

When injected into the wellbore a gas mixture at a specific flow R _g and planned production rate Q of the fluid _w gas flow through a gas lift valve is calculated according to the formula:

The number of moles of gas pumped through the gas-lift valve is calculated by the formula:

where: z _{gas lift} is the supercompressibility coefficient of the real injected gas, z≈1 (This parameter can be determined graphically from the known reduced pressure and temperature using the Brown-Katz graph);

n _{gas lift} is the number of moles of injected gas;

Р - gas pressure at the injection depth, Pa;

V is the volume of injected gas, m ³ ;

T is the gas temperature at the injection depth, K;

R is the gas constant.

The new composition of the gas phase is calculated by the formula:

The temperature and pressure values for calculating the amount of injected gas were obtained from the results of the analysis of the PT profile in the PIPESIM program.

Next, binary interaction coefficients are determined according to the following rules:

The interaction between the two hydrocarbon components increases with an increase in the relative difference in their molecular weights.

k _ij <k _{i (j + 1)}

and k _{(i + 1) j} <k _ij

Hydrocarbon components with the same molecular weight have a binary interaction coefficient equal to zero.

k _ii = 0

The binary matrix of interaction coefficients is symmetric

k _ij = k _ji

For the system contains components N ₂ , CO ₂ or CH ₄

Where:

i - refers to the main components N ₂ , CO ₂ , or CH ₄ , and j refers to other hydrocarbon components of the binary mixture, T _r is the reduced temperature T _r = T / T _k

For nitrogen - hydrocarbons

δ ₀ = 0.1751787 - 0.7043log (ω _j ) -0.862066 [log (ω _i) ] ²

δ ₁ = -0.584474 + 1.328log (ω _j ) +2.035767 [log (ω _i) ] ²

δ ₂ = 2.257079 + 7.869765log (ω _j ) +13.50466 [log (ω _i) ] ² +8.3864 [log (ω _i) ] ³

For methane - hydrocarbons

δ ₀ = -0.01664-0.37283log (ω _j ) +1.31757 [log (ω _i) ] ²

δ ₁ = 0.48147 + 3.35342log (ω _j ) -1.0783 [log (ω _i) ] ²

δ ₂ = -0.4114-3.5072 log (ω _j ) -1.0783 [log (ω _i) ] ²

For CO ₂ -hydrocarbons

δ ₀ = 0.4025636 + 0.1748927log (ω _j )

δ ₁ = -0.94812-0.6009864log (ω _j )

δ ₂ = 0.741843368 + 0.441775log (ω _j )

Adopting the procedure recommended by Petersen (1989) for calculating the binary interaction coefficients between components heavier than methane, for example, C ₂ , C ₃

where n is the number of carbon atoms of the C _n component.

for example

The rest kij will be determined:

for example

Then the coefficients of compressibility of the gas and liquid phases are determined.

Peng and Robinson proposed the following equation of state:

Where:

T _r = T / T _k

m = 0.3796 + 1.542266ω-0.2699ω ² if ω <0.49

m = 0.379642 + 1.48503ω-0.1644ω ² + 0.016667ω ³ if ω> 0.49

Rearranging Peng and Robinson's equation of state in the form of a compressibility factor gives

Z ³ + (B-1) Z ² + (A-3B ² -2B) Z- (AB-B ² -R ³ ) = 0

Where

For the gas phase taking into account

For the liquid phase:

Calculation of the parameters A and B for the gas and liquid phases, we obtain A _g , B _g and A _g , B _g

Apply A _g , B _g for the gas phase:

Z ³ + (B-1) Z ² + (A-3B ² -2B) Z- (AB-B ² -B ³ ) = 0

The solution to this cubic equation, the largest positive root gives the gas phase compressibility factor:

Z ^g - the largest positive root

Apply A _g , B _g for the liquid phase:

Z ³ + (R-1) Z ² + (A-3B ² -2B) Z- (AB-B ² -B ³ ) = 0

The solution to this cubic equation, the smallest positive root gives the coefficient of compressibility of the liquid phase

Z * - smallest positive root

Next, the equilibrium ratio "K _i " is determined

для каждого компонента в жидкой фазе, применяя уравнение:Using the calculated composition of the liquid phase x _i , the fugacity coefficient is determined

for each component in the liquid phase using the equation:

Where

определяют коэффициент фугитивности

для каждого компонента в газовой фазе, применяя уравнение:Taking into account the change in the composition of oil during the injection of associated petroleum gas, using the calculated new composition of the gas phase,

determine the coefficient of fugacity

for each component in the gas phase using the equation:

Where

The calculation of the component liquid mixture at a pressure P ₁ and temperature T ₁ received mole fraction n _r1 gas phase, liquid phase n _x1, n _x1 = 1-n _r1

The accepted component composition of the liquid phase x _{i was} obtained from the calculation at the first temperature (T ₁ ) as the initial total composition z _i when calculating for the conditions of the second temperature (T ₂ ) (z _i2 = x _i1 ). (index 1, 2, respectively, the first and second conditions)

The number of moles of liquid at a temperature T = T _{2 is considered} equal to one, the composition of the oil is calculated according to the previous algorithm, the instantaneous mole fractions of the liquid and gas phases n _{x2 (m)} , n _{g2 (m)} are obtained, respectively. The actual molar fraction of the liquid phase is calculated by the formula:

n _x2 = 1⋅n _x1 ⋅n _{x2 (m)}

Similarly, for conditions (P ₃ , T ₃ ) and (P _n , T _n ), the actual molar fraction of liquid is obtained:

and the molar fraction of the gas phase

where n _wn , n _gn molar fraction of liquid and gas phases at temperature and pressure (P _n , T _n ).

Based on the obtained results of calculating the change in the composition of the gas-liquid mixture during injection of associated petroleum gas according to the above algorithm, the change in the temperature of oil saturation with paraffin is determined by using known software products that allow to study the process of formation of paraffin deposits in the well, such as Multiflash, Flow Assurance OLGA, LedaFlow Software ... The temperature of oil saturation with paraffin during the gas-lift method of well operation is a function that depends on the pressure, fluid temperature, oil composition, flow rate and composition of the injected associated petroleum gas. The pressure and temperature distribution curves of the flow were obtained from the results of the analysis of the PT profile in the PIPESIM program. The release of solid ARPD substances from oil begins when the flow temperature drops to the temperature of oil saturation with paraffin, therefore, the depth of the beginning of intensive formation of deposits corresponds to the intersection of the curves of the distribution of the flow temperature and the temperature of oil saturation with paraffin in the well.

The result of the study shows that under optimal conditions, the temperature of oil saturation with paraffin decreases. (Fig. 1)

Thus, the proposed method improves the efficiency of wells with a high content of asphalt-resin-paraffin deposits by reducing the downtime of the well and increasing the turnaround time by reducing the intensity of the formation of deposits, reducing the temperature and depth of formation of ARPD.

Claims (1)

A method for preventing the formation of asphalt-resin-paraffin deposits (ARPD) in lift pipes during gas-lift operation of oil wells, including pumping a working agent into the annular space of a well, characterized in that initial data are obtained on the component compositions of formation fluid and associated petroleum gas (APG), which is used in as a working agent, then it is purified from hydrogen sulfide and carbon dioxide, then the purified associated petroleum gas is injected without changing its hydrocarbon composition and the temperature T _{1 of} oil saturation with paraffin and the depth of its formation in the well are determined, taking into account the change in the composition of oil during APG injection, then the ratio of light and heavy fractions of the injected associated petroleum gas is changed and the temperature T _{2 of} oil saturation with paraffin and the depth of its formation for a given ratio of light and heavy fractions are determined, then a comparison of the indicators T ₂ and T ₁ , if T ₂ > T ₁ , is made again the ratio of light and heavy fractions of the injected working agent is determined and the determination of the temperature T _{2 of} oil saturation with paraffin and the depth of its formation, if T ₂ <T ₁ , is repeated, the obtained temperature is taken as a new initial version, then the condition of the possibility of ensuring the required ratio of fractions based on the component composition is checked APG, if the condition of possibility is confirmed, then this procedure is repeated for another variant of the ratio of light and heavy fractions, if not confirmed, then the optimal variant of the required amount of flow rate of the working agent is selected at the lowest temperature of oil saturation with paraffin.

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