DSC correction-based multiphase flow system hydrate inhibitor loss prediction method

Abstract

The invention relates to the technical field of natural gas hydrates, in particular to a DSC (differential scanning calorimetry) correction-based multiphase flow system hydrate inhibitor loss prediction method, which comprises the following steps of: firstly, acquiring water activity and vaporization enthalpy of a sample under target salinity and ratio through two DSC tests, and regression a thermodynamic model according to the water activity and vaporization enthalpy to obtain a liquid-phase activity coefficient; the saturated vapor pressure of the inhibitor is calculated through the vaporization enthalpy obtained through DSC inversion and an anchor point method, a component distribution coefficient is obtained through combination of Poynting pressure correction and a gas phase fugacity coefficient, and a Rachford-Rice flash evaporation equation is solved to obtain an equilibrium phasor and composition; and then a first-order total mass transfer model is introduced to correct the actual vaporization loss, and droplet entrainment correlation is superposed to obtain the total flow loss of the inhibitor under the multiphase flow system. According to the method, the total flow loss of the hydrate inhibitor can be accurately calculated under the working conditions of high pressure, low temperature and salt-containing multiphase flow, so that a theoretical support is provided for adjusting an injection strategy of the inhibitor in real time.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of natural gas hydrate, and particularly relates to a hydrate inhibitor loss prediction method based on DSC correction for a multiphase flow system. BACKGROUND

[0002] During deepwater oil and gas drilling and long-distance submarine transportation, the low-temperature and high-pressure environment near the submarine mud line meets the thermodynamic conditions for the formation of natural gas hydrate. The gas produced from the formation and water can easily form natural gas hydrate in this environment, which is deposited and attached to the pipe wall, causing the effective flow passage to decrease. If not properly handled, the pipe string can be easily blocked, causing great economic losses. The current natural gas hydrate prevention and control method commonly used in the field is to add hydrate inhibitors such as methanol (MeOH) and ethylene glycol (MEG) to the pipe string. After the inhibitors are dissolved in the water phase, they will be vaporized and lost to different degrees under the gas carrying of the multiphase flow system, causing the effective inhibitor concentration in the actual liquid phase to gradually decrease, and the situation of "excess upstream and insufficient downstream" occurs, which still increases the risk of natural gas hydrate formation. In order to ensure safety, the engineering must adopt the "excess injection" strategy to completely eliminate the formation of natural gas hydrate, which leads to a substantial increase in operating costs. According to statistics, 15-40% of the hydrate prevention and control costs of deepwater oil and gas fields are related to the excess injection and loss of inhibitors.

[0003] The loss mechanisms of the two commonly used hydrate inhibitors in the multiphase flow system are different. Methanol has strong volatility, and the vaporization loss is significant. The pipeline example of JMCampbell shows that the sum of methanol loss (gas + liquid hydrocarbon) can reach twice the required injection amount. The loss of ethylene glycol is mainly caused by the entrainment of the gas phase. The GoM practice of JPT points out that there is still considerable loss of ethylene glycol under the recovery system and the entrainment conditions of the circular flow. The existing research on fluid loss is mainly based on the measurement of the distribution ratio of alcohols in the gas / water / hydrocarbon phase under static conditions through multiphase flash experiments, and a flash calculation model is established which depends on the general property library and empirical coefficients, ignoring the influence of the activity coefficient of the liquid phase components on the inhibitor in water and the physical property parameters in the real production conditions, as well as the mass transfer limitation between the inhibitor and the gas phase and the entrainment effect of the gas phase under the multiphase flow conditions. This leads to the difficulty in real-time prediction of the flow loss of the hydrate inhibitor in the long-distance multiphase flow system, and the injection strategy of the inhibitor cannot be adjusted in real time according to the effective concentration of the inhibitor in the pipeline. SUMMARY

[0004] The present application aims at the above-mentioned defects existing in the prior art, and provides a hydrate inhibitor loss prediction method based on DSC correction, which is characterized by integrating differential scanning calorimeter (DSC) experiment and a multiphase flash-mass transfer-entrainment coupling model to accurately calculate the total flow loss of the hydrate inhibitor under the conditions of high pressure, low temperature and salt-containing multiphase flow, so as to provide theoretical support for real-time adjustment of the injection strategy of the hydrate inhibitor.

[0005] The hydrate inhibitor loss prediction method based on DSC correction provided by the present application has the technical scheme that comprises the following steps: I. Preparing a sample for DSC test according to the components of formation produced fluid, and obtaining the water activity and vaporization enthalpy of the sample by measuring the heat flow curve of the sample by using DSC; II. Calculating the inhibitor activity coefficient under the target formula by using the liquid phase model eNRTL to process the water activity of the sample; III. Obtaining the inhibitor saturated vapor pressure under the target formula by using the Clausius-Clapeyron straight line to process the vaporization enthalpy of the sample; IV. Calculating the Poynting correction index and the fugacity coefficient of each component; V. Calculating the gas / water distribution coefficient and obtaining the molar fraction of each component in the liquid phase and the gas phase after flash equilibrium by using the multiphase flash calculation based on the parameters obtained in steps II, III and IV; VI. Correcting the upper limit of the vaporization loss in step V by considering the mass transfer limitation in the gas-liquid mass transfer process to obtain the real vaporization loss of the inhibitor; VII. Calculating the loss rate of the inhibitor caused by the gas entrainment by considering the influence of high-speed gas phase entrainment on the loss of the inhibitor; VIII. Adding the losses from steps VI and VII to obtain the inhibitor loss of the multiphase flow system, and iteratively calculating the inhibitor loss rate along the whole pipe section.

[0006] Preferably, the sample preparation for DSC test in step I and the two types of DSC test processes are as follows: S1.1. Preparing a sample for DSC test according to the components of formation produced fluid and the inhibitor concentration after injection and sufficient mixing, and converting the liquid phase mass fraction into the molar fraction in the liquid phase and the gas phase x i 、 y i , which is used for balance equation calculation; S1.2. Scanning the temperature of the DSC test sample at the target temperature by using DSC to obtain the ice melting starting temperature T f , converting the water activity of the sample by using formula (1) α w ; (1), wherein, R = 8.314 J / (mol·K); T f T0 is the ice-melting temperature, K; α w a is the water activity, dimensionless; S1.3 The isothermal vaporization energy test is carried out on the sample by using the open DSC, and the heat flow curve is recorded, and the vaporization heat is obtained by integrating the baseline after deduction Q The mass loss before and after the DSC experiment is recorded by weighing and converted into the number of moles Δ n , and the vaporization enthalpy Δ H vap, in ; (2), wherein, Δ H vap, in is the vaporization enthalpy, J / mol; Q is the evaporation heat of the inhibitor, J; Δ n and Δ m are the number of moles and mass lost before and after evaporation, mol, g; M in is the molar mass of the inhibitor, g / mol; q corr is the heat flow, W; t is time, s.

[0007] Preferably, the detailed process of step two is as follows: The water activity a α w (3) is used to calculate the water activity coefficient a T , temperature, salinity) and the molar fraction of the sample formula x w are solved simultaneously to obtain the water activity coefficient a gamma w The binary interaction parameters are obtained by regression using the liquid phase model eNRTL (equations (4)-(6)) at the target salinity tau 12 with tau 21 ; and then the inhibitor activity coefficient a gamma in of the target formula is calculated by using equation (7) (3), (4), (5), (6), (7), wherein, x w is the mole fraction of water in the liquid phase, %; x in is the mole fraction of inhibitor in the liquid phase, %; gamma w DSC is the activity coefficient of water obtained from DSC experiment, dimensionless; gamma w model is the activity coefficient of water obtained from liquid phase model eNRTL, dimensionless; G 12 is the activity coefficient of inhibitor obtained from liquid phase model eNRTL, dimensionless; H 21 is the calculated intermediate parameter, dimensionless; k is the number of formulations; S is the objective function, dimensionless; gamma in is the activity coefficient of inhibitor of the target formulation, dimensionless; wherein, the non-random parameter ω is generally taken as 0.3, tau 12 the initial value is taken as 0.5, tau 21 the initial value is taken as 0.1, and substituted into formula (5) to calculate gamma w model the multiple of the activity coefficient of water obtained from DSC actual measurement in S1.2 is taken as the fitting target and the objective function is established, as formula (6); iterative calculation is performed until the calculated α w T of the model matches the actual measured value of DSC; the activity coefficient of inhibitor gamma w model is obtained by using formula (7) gamma in .

[0008] Preferably, the detailed process of step three is as follows: the Δ H vap, in given by DSC is used to calculate the Δ P sat ( T ) at the anchor point pressure P sat,in ( T ) by using Clausius-Clapeyron differential formula or piecewise integral formula; wherein the Clausius-Clapeyron differential formula is formula (8), and if the Δ H vap ( T ) is approximately constant Δ H vap ​, the linear equation (9) is obtained by integration, and the selected anchor point T , P sat ) to calculate the intercept of the linear equation C , then the target temperature T j and Δ H vap, in Substitute equation (9) to obtain the saturated vapor pressure of the inhibitor P sat,in ( T j ); (8), (9), wherein P sat ( T ) is the anchor point saturated vapor pressure, Pa; C is the linear intercept, determined by single-point anchoring; T is the temperature, K; P sat,in ( T ) is the saturated vapor pressure of the inhibitor, Pa.

[0009] Preferably, the detailed process of step four is as follows: Calculate the Poynting correction index using equation (10), which represents the integral correction of the liquid phase reference state from the saturated pressure P sat,in to the target P j pressure; (10), wherein П is the Poynting correction index, dimensionless; rho in is the inhibitor density, kg / m 3 ; P j is the pressure in the calculation section, Pa; Using the Peng-Robinson EOS equation, input the critical constants, eccentric factors and gas phase composition of each component to obtain the fugacity coefficient of each component phi i .

[0010] Preferably, the detailed process of step five is as follows: Gas / water partition coefficient calculation: the gas / water partition coefficient K i component i is the ratio of the molar fraction in the gas phase to that in the water phase at equilibrium, and for volatile components i, the composition of the gas phase is solved by equation (11) i Distribution coefficient in gas-liquid two-phase K i ; (11) where, K i Distribution coefficient in gas-liquid two-phase, dimensionless; gamma i Activity coefficient of different components, dimensionless; phi i Fugacity coefficient of components, dimensionless; P sat,i Saturation vapor pressure of different components, Pa; Rachford-Rice two-phase flash solution: Given the molar fraction of feed z i , the molar fraction of gas phase is solved by equation (12) using dichotomy β ; and the molar fraction of each component in the liquid phase and gas phase after flash equilibrium is calculated by equation (13) x i 、 y i ; (12), (13), where, β is the molar fraction of gas phase, z i is the molar fraction of different components in the total feed stream, x i is the molar fraction of component i in the liquid phase after flash equilibrium, y i is the molar fraction of component i in the gas phase after flash equilibrium.

[0011] The detailed process of step six is as follows: The upper limit of the thermodynamic loss of the inhibitor after the gas-liquid equilibrium in the calculation section is calculated by equation (14) (mass transfer is not limited), but the interface mass transfer is affected by the gas-liquid contact area, flow pattern and degree of turbulence, so only a part of the upper limit can be actually achieved, therefore a first-order overall mass transfer model (equation 15) is needed to convert the actual inhibitor vaporization loss; (14), (15), where, xi eq andxi vap is the maximum vaporization loss and the actual vaporization loss rate, dimensionless; k L is the liquid phase mass transfer coefficient, which is usually calculated by empirical correlations; a is the specific surface area, which is determined by the wellbore flow pattern, dimensionless; tau is the residence time, s, which can be calculated by the liquid phase volume / liquid phase flow rate.

[0012] The detailed process of step seven is as follows: The high-speed gas phase entrains liquid droplets and carries the inhibitor into the gas phase, and the entrainment rate of the liquid droplets varies with the flow pattern, Reynolds number and Weber number. First, the liquid droplet entrainment rate is calculated by formula (16) E Then, the loss rate of the inhibitor lost by the entrainment of the gas is calculated by formula (17); (16), (17), wherein, E is the liquid droplet entrainment rate, dimensionless; We, Fr and Re are the gas phase Weber number, Froude number and Reynolds number, respectively, dimensionless; rho L and rho G are the gas phase and liquid phase densities, respectively, kg / m 3 ; mu L and mu G are the gas phase and liquid phase viscosities, respectively, Pa·s; xi aero is the loss rate of the inhibitor caused by the entrainment, dimensionless; m in,aero is the mass of MeOH lost by entrainment, g / s; n F is the total feed molar number, mol / s; m g is the gas phase flow rate, g / s; M g is the gas phase molar mass, g / mol; w in is the inhibitor mass fraction, %.

[0013] The detailed process of step eight is as follows: Calculation section j The total loss rate of the inhibitor in the flow process due to evaporation loss and entrainment loss can be calculated by formula (18); the pipeline is cut into N sections, and the calculation is performed in order j =1→ N cumulatively, and it is assumedR in 1 =1, the fraction of the inhibitor that has not yet been lost at the inlet of the pipe, then the calculation section j The contribution to the total inlet loss is calculated by equation (19); the inhibitor fraction for the next calculation section is updated, see equation (20), and the cumulative solution can obtain the hydrate inhibitor loss rate in the whole pipe section, see equation (21); (18), (19), (20), (21), In the formula, xi total Loss_of_Total is the total loss rate of the inhibitor, %; Loss_of_Total is the total loss contribution of the calculation section j to the total inlet, dimensionless; R in and R out Loss_of_Total is the total loss rate of the inhibitor, %; Loss_of_Total is the total loss contribution of the calculation section j the inhibitor fraction at the inlet and the outlet, %; Ξ is the hydrate inhibitor loss rate in the whole pipe section, %.

[0014] Compared with the prior art, the beneficial effects of the present application are as follows: First, the present application is closer to the field conditions: The water activity and vaporization enthalpy of the real sample solution are directly measured by differential scanning calorimeter (DSC) experiment to obtain the empirical parameters required by the multiphase flash calculation model, avoiding the deviation caused by relying on general database, so as to realize accurate calculation of hydrate inhibitor vaporization loss under high pressure, salt-containing and non-ideal liquid conditions; at the same time, this method introduces the multiphase flow distribution in the real pipeline into the calculation, which can consider the additional loss caused by droplet entrainment, and is more in line with the actual operation characteristics of deepwater oil and gas pipelines; Second, the present application is widely applicable: The present application is not only applicable to loss evaluation of different types of inhibitors such as methanol and ethylene glycol, but also can cover various complex scenes such as subsea wellheads, long-distance subsea pipelines and ground separators; whether in undulating pipelines, variable-diameter pipelines or separation equipment, the present application can accurately calculate according to different working conditions and inhibitor characteristics, thereby providing a unified loss evaluation tool for the whole process of deepwater oil and gas development; Third, the present application can realize the calculation of the space-time distribution of hydrate inhibitor concentration: The inhibitor is usually injected into the well head or pipeline by single-point injection under field conditions, but due to the length of the flow channel of up to thousands of meters, the concentration distribution of the inhibitor along the way is often unknown, and targeted hydrate risk management cannot be achieved; the method can calculate the loss amount of the inhibitor at different positions and time conditions of the pipeline in real time by establishing a multiphase flashing-mass transfer-entrainment coupling model, so as to obtain the real-time concentration distribution of the inhibitor, thereby providing a scientific basis for the injection strategy, optimization control and risk prevention and control of the inhibitor. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 A flow loss prediction flowchart of hydrate inhibitors in a multiphase flow system based on DSC correction according to the present application; Figure 2 A heat flow curve measured by DSC in the process of measuring water activity by ice melting method; Figure 3 A heat flow curve measured by DSC in the process of measuring evaporation intensity. DETAILED DESCRIPTION

[0016] The preferred embodiments of the present application will be described below with reference to the accompanying drawings, and it should be understood that the preferred embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0017] Example 1, a hydrate inhibitor loss prediction method based on DSC correction for a multiphase flow system according to the present application, comprises the following steps: First, the water activity and vaporization enthalpy of the sample at the target salinity and ratio are obtained by two types of DSC tests, and the liquid phase activity coefficient is obtained by regressing the thermodynamic model; then the saturated vapor pressure of the inhibitor is calculated by the vaporization enthalpy inversed by DSC and the anchor point method, and the component distribution coefficient is obtained by combining the Poynting pressure correction and the gas phase fugacity coefficient, and the equilibrium phase and composition are obtained by solving the Rachford-Rice flashing equation; then a first-order overall mass transfer model is introduced to correct the actual vaporization loss, and the total loss of the inhibitor under the multiphase flow system is obtained by superimposing the droplet entrainment correlation.

[0018] To achieve the above object, the specific technical scheme adopted by the present application is as follows: S1 sample preparation and two types of DSC test S1.1 According to the components of the formation produced fluid and the inhibitor concentration after injection and sufficient mixing, the sample for DSC test is prepared, and the liquid phase mass fraction is converted into the molar fraction in the liquid phase and the gas phase x i 、 y i (for equilibrium equation calculation).

[0019] S1.2 The DSC test sample is scanned at the target temperature using DSC to obtain the ice melting initiation temperature. T f The water activity of the sample is converted using equation (1). α w .

[0020] (1), In the formula, R = 8.314 J / (mol·K); T f The melting point of the ice is the initial temperature, in K; α w Water activity is a dimensionless quantity. Note: The prepared samples need to cover a series of inhibitor and salt concentrations within the field area. Water activity at different temperatures / formulations should be measured using a closed DSC system via the ice-melt method. α w ( T The sample was cooled from +25°C to -40°C (5 K / min) to completely freeze (temperature, salinity); then the temperature was raised back to +25°C and scanned slowly at 0.1-0.5 K / min. The ice melt peak was recorded, and the initial temperature was extracted. T f .

[0021] S1.3 An isothermal vaporization energy test was conducted on the sample using an open-type DSC, and the heat flow curve was recorded. The heat of vaporization was obtained by integrating after baseline subtraction. Q The mass loss before and after the DSC experiment was recorded by weighing and converted into mole count Δ. n The vaporization enthalpy Δ is obtained using equation (2). H vap, in .

[0022] (2), In the formula, Δ H vap, in Here, enthalpy of vaporization is expressed in J / mol. Q The heat absorbed by the evaporation of the inhibitor is J; Δ n With Δ m The moles and mass lost before and after evaporation, in mol and g; M in The molar mass of the inhibitor is in g / mol. q corr t represents heat flux, W; t represents time, s.

[0023] Note: During the experiment, the sample was slowly heated from 20℃ to 80℃ (1-2℃ / min). The process of the inhibitor evaporating from liquid to gas requires the absorption of latent heat. To ensure clear resolution of the evaporation peak, the heat flux after baseline subtraction was calculated within the evaporation range.q corr Integral of heat of vaporization Q .

[0024] S2 Activity coefficient regression (γ) Water activity is calculated using equation (3) α w ( T , temperature, salinity) and the molar fraction of the sample formulation x w The activity coefficient of water is obtained by simultaneous solution gamma w The binary interaction parameters are obtained by regression using the liquid phase model eNRTL (equations (4)-(6)) at the target salinity tau 12 with tau 21 The inhibitor activity coefficient of the target formulation is then calculated using equation (7) gamma in .

[0025] (3), (4), (5), (6), (7), where, x w is the mole fraction of water in the liquid phase, %; x in is the mole fraction of inhibitor in the liquid phase, %; gamma w DSC is the activity coefficient of water obtained from DSC experiments, dimensionless; gamma w model is the activity coefficient of water obtained from the liquid phase model eNRTL, dimensionless; G 12 and G 21 are intermediate parameters calculated, dimensionless; k is the number of formulations; S is the objective function, dimensionless; gamma in is the inhibitor activity coefficient of the target formulation, dimensionless.

[0026] Note: Only one α w cannot be obtained directly gamma in , the binary interaction parameters tau12 With tau 21 Calculate. Non-random parameter ω generally takes 0.3, tau 12 Initial value is recommended to be 0.5, tau 21 Initial value is recommended to be 0.1, and is substituted into formula (5) to calculate gamma w model In S1.2, multiple α w ( T , temperature, salinity) measured by DSC are taken as fitting targets and an objective function (formula 6) is established, and iterative calculation is performed until the gamma w model match the measured values of DSC. The activity coefficient of the inhibitor can be obtained by formula (7) gamma in .

[0027] S3 DSC anchoring of saturated vapor pressure P sat (T) of Use Δ H vap, in and anchor point pressure P sat ( T ) (which can be a reliable reference point such as P sat or self-measured at 298.15 K) to obtain P sat,in ( T ) by Clausius-Clapeyron linear formula or piecewise integral formula. The Clausius-Clapeyron differential formula is formula (8), and if Δ H vap ( T ) is approximately constant Δ H vap in a small temperature range, the integral obtains the linear equation (9), and the intercept of the linear equation can be calculated by substituting the selected anchor point T , P sat into formula (9). C Then, the target temperature T j is substituted into formula (9) to obtain the saturated vapor pressure of the inhibitor H vap, in ( P sat,in ). T j ).

[0028] (8), (9), where, P sat ( T ) is the anchor point saturation vapor pressure, Pa; C is the linear intercept, determined by single point anchoring; T is the temperature, K; P sat,in ( T ) is the inhibitor saturation vapor pressure, Pa.

[0029] S4 Poynting correction index calculation The Poynting correction index is calculated using equation (10), which represents the integral correction of the liquid phase reference state from the saturation pressure P sat,in to the target pressure. P j

[0030] (10), where П is the Poynting correction index, dimensionless; rho in is the inhibitor density, kg / m 3 ; P j is the pressure in the calculation section, Pa.

[0031] S5 Gas phase fugacity coefficient φ (Equation of State EOS calculation) Using the Peng-Robinson EOS equation, input the critical constants, eccentric factors, and gas phase composition of each component to obtain the fugacity coefficient of each component phi i .

[0032] S6 Gas / water partition coefficient calculation The gas / water partition coefficient K i component i is the ratio of the mole fraction in the gas phase to that in the water phase at equilibrium, and for volatile components i , the partition coefficient of component i in the gas-liquid two-phase system is solved using equation (11) K i .

[0033] (11), where, K i is the partition coefficient in the gas-liquid two-phase system, dimensionless; gamma ​i dimensionless; phi i dimensionless; P sat,i Pa, saturation vapor pressure of different components.

[0034] S7 Rachford–Rice two-phase flash solution Given feed mole fraction z i , the gas phase mole fraction β is solved by using dichotomy to solve equation (12); then the mole fraction of each component in the liquid phase and the gas phase after flash equilibrium is calculated by formula (13) x i , y i .

[0035] (12), (13), where, β is the gas phase mole fraction, z i is the mole fraction of different components in the total feed stream, x i is the mole fraction of component i in the liquid phase after flash equilibrium, y i is the mole fraction of component i in the gas phase after flash equilibrium.

[0036] S8 Actual vaporization loss (mass transfer discount) The upper limit of the thermodynamic loss of the inhibitor after the gas-liquid equilibrium in the calculation section is calculated by formula (14) (mass transfer is not limited), but the interface mass transfer is affected by the gas-liquid contact area, flow pattern and turbulence degree, so only a part of the loss upper limit can be actually achieved, therefore a first-order overall mass transfer model (formula 15) is needed to convert the actual inhibitor vaporization loss.

[0037] (14), (15), where, xi eq and xi vap are the maximum vaporization loss and the actual vaporization loss rate, dimensionless; k L is the liquid phase mass transfer coefficient, which is usually calculated by empirical correlation; a is the specific surface area, which is determined by the wellbore flow pattern, dimensionless;tau The residence time, s, can be calculated using the liquid volume / liquid volume flow rate.

[0038] S9 Entrainment loss calculation xi aero (Inhibitor carried away by entrained droplets) The high-speed gas phase entrains droplets and carries the inhibitor into the gas phase. The entrainment rate of droplets varies with the flow pattern, Reynolds number and Weber number. First, the droplet entrainment rate is calculated using equation (16) E Then, the loss rate of the inhibitor lost by the entrainment of the gas is calculated using equation (17).

[0039] (16), (17), In the equations, E is the droplet entrainment rate, dimensionless; We, Fr and Re are the gas phase Weber number, Froude number and Reynolds number, respectively, dimensionless; rho L and rho G are the gas phase and liquid phase densities, respectively, kg / m 3 ; mu L and mu G are the gas phase and liquid phase viscosities, respectively, Pa·s; xi aero is the inhibitor loss rate caused by entrainment, dimensionless; m in,aero is the mass of MeOH lost by entrainment, g / s; n F is the total feed molar number, mol / s; m g is the gas phase flow rate, g / s; M g is the gas phase molar mass, g / mol; w in is the inhibitor mass fraction, %.

[0040] S10 Total inhibitor loss Calculation section j The total loss rate of the inhibitor in the flow process due to evaporation loss and entrainment loss can be calculated using equation (18). The pipeline is cut into N sections, and the calculation is performed in order j = 1→ N The total loss rate of the inhibitor in the flow process due to evaporation loss and entrainment loss can be calculated using equation (18). The pipeline is cut into R in 1 = 1 (the fraction of the inhibitor not yet lost at the pipeline inlet), then the calculation section jThe loss contribution to the total inlet is calculated by equation (19). The inhibitor fraction of the next calculation section is updated, see equation (20), and the cumulative solution can obtain the inhibitor loss rate in the whole pipe section, see equation (21).

[0041] (18), (19), (20), (21), wherein, xi total Loss_of_Total is the total loss rate of the inhibitor, %; Loss_of_Total_Inlet is the loss contribution to the total inlet of the calculation section, dimensionless. j R in and R out Loss_of_Total_Outlet is the inhibitor fraction at the outlet of the calculation section, %; Ξ is the inhibitor loss rate in the whole pipe section, %. j

[0042] Example 2, a DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system mentioned in the present application, specifically includes the following: The test temperature of this example is 10℃, the pressure is 30MPa, the pipe size is 0.1524m, the flow rate is 5m / s, the gas component is pure methane, the liquid phase contains NaCl, and the thermodynamic inhibitor is methanol MeOH, wherein the mass fraction of NaCl is 3wt%, and the mass fraction of methanol MeOH is 50wt%. In the total fluid component, the mole fraction of methane z CH4 =0.9 (gas-dominated, natural gas production environment), and the mole fraction of the liquid phase is 0.1. Under this condition, the gas density rho G =228.967 kg / m 3 , the viscosity mu G =3×10 -5 , the liquid phase density rho L =1013.542 kg / m 3 , the viscosity mu L =0.00128, and the flow loss amount of methanol MeOH under this condition is calculated.

[0043] Preparation of S1 sample and two types of DSC test S1.1 Convert the liquid phase mass fraction to the liquid phase mole fraction x i ​​, take 100 g liquid phase as a benchmark for convenient calculation: Methanol MeOH mass is 50 g, molar mass is 32.04 g / mol, so the number of moles is 1.5599 mol; similarly, the number of moles of NaCl is 0.05133 mol, and the number of moles of water is 2.609 mol. Therefore, the molar fraction of each component in the liquid phase is: x MeOH = 0.3695, x w = 0.6183, x NaCl = 0.0122. Since the total liquid phase accounts for 0.1, the molar fraction of each component in the total feed is: z MeOH = 0.03695, z w = 0.06183, z NaCl = 0.00122.

[0044] Prepare samples for DSC testing according to the components of the formation produced fluid. The samples prepared in this example cover a series of inhibitor and salt concentrations in the field interval, and the sample components are shown in Table 1.

[0045] Table 1

[0046] S1.2 Measure the ice melting onset temperature of the prepared different component samples by ice melting method using DSC T f : First, put 10-20 mg of sample into a sealed crucible, and reduce the sample from 25℃ to -40℃ at a rate of 5 K / min, so that the sample can freeze completely; then slowly scan at a rate of 0.1-0.5 K / min, and raise the sample to 25℃, to obtain the heat flow curve shown in FIG. 1; Figure 2 Figure 2 The baseline in FIG. 1 is obtained by using an empty crucible with the same temperature scanning program. The point where the heat flow curve deviates from the baseline in FIG. 1 is the Onset point, and the corresponding temperature T f is the temperature at which the sample begins to melt. Substituting it into equation (1) gives the water activity of this component α w T , temperature, salinity), which is listed in Table 2.

[0047] Table 2

[0048] S1.3 Put 5-15 mg of sample into an open crucible and weigh m sample,0 ​​The sample was slowly warmed from 20°C to 40°C (1-2 K / min) to evaporate the sample from liquid to gas, and the heat flow curve was recorded and the DSC baseline (empty crucible same temperature scan program) was subtracted to obtain the heat flow curve shown in FIG. 2. Immediately after the end of evaporation, the sample was cooled to room temperature and weighed to obtain the mass loss Δm Figure 3 m sample, end The mass loss Δm can be obtained. m The heat of vaporization ΔHv can be obtained by integrating the heat flow curve. Q The enthalpy of vaporization ΔHvap can be obtained by substituting the heat of vaporization ΔHv into equation (2). H vap, in In this example, the total heat of vaporization in the holding interval is 5.4261 J, and the mass loss due to evaporation is 4.967 mg, so the enthalpy of vaporization ΔHvap is 35 kJ / mol. Figure 3 H vap, in = 35 kJ / mol.

[0049] S2 Activity coefficient regression (γ) The components of each experimental point in Table 1 were converted into the mole fractions of water and MeOH in the liquid phase, which are listed in Table 3. The activity coefficients of water measured by DSC at different experimental points can be obtained using equation (3). gamma w DSC In the eNRTL binary formula of the liquid phase model, the non-random parameter ω is 0.3, and the binary interaction parameter tau 12 The initial value is 0.5, tau 21 The initial value is 0.1, which is substituted into equations (4) and (5) to calculate gamma w model The experimental measurements and the calculations of the liquid phase model are substituted into the objective function (6) to obtain the model calculation error. If the model calculation value does not match the DSC measured value, the binary interaction parameter is corrected and recalculated. The experimental gamma w The experimental measurements and the calculations of the liquid phase model are substituted into the objective function (6) to obtain the model calculation error. If the model calculation value does not match the DSC measured value, the binary interaction parameter is corrected and recalculated. The experimental gamma w DSC The binary interaction parameter is corrected until the model calculation gamma w model matches the DSC measured value, and the determined value of the binary interaction parameter tau 12 and tau 21 is obtained, which is substituted into equation (7) to obtain the activity coefficient of MeOH gamma in The related parameters calculated in this example are listed in Table 3. ​​

[0050] Table 3

[0051] S3 Saturation vapor pressure P sat DSC anchoring of (T) The saturation vapor pressure of MeOH at 298.15 K is 16900 Pa, which is used as the anchor point to calculate the intercept of the straight line equation in equation (9) C The target temperature T j and Δ H vap, in Substituting equation (9) gives the saturation vapor pressure of the inhibitor P sat,in ( T ). The intercept C = 23.886 in this example, and T j = 283.15 K, Δ H vap, in = 35 kJ / mol gives P sat, in = 8250 Pa.

[0052] S4 Poynting correction index calculation Substituting M in = 32.04 g / mol, rho in = 0.79 g / cm 3 , P j = 30 MPa, T j = 283.15 K, P sat, in = 8250 Pa into equation (10) gives the Poynting correction index П = 1.6859. This value represents that the Poynting factor of MeOH at 30 MPa, 283.15 K increases the gas / water partition coefficient by 68.59%. Similarly, the Poynting correction index of water is П = 1.2573.

[0053] S5 Gas phase fugacity coefficient φ (Equation of state, EOS calculation) Using the Peng-Robinson EOS equation, the critical temperature, critical pressure, eccentric factor, target temperature, target pressure, and gas phase mole fraction of each component are inputted, as shown in Table 4, to calculate the fugacity coefficient of each component phi iSince the model for this part is mature, the detailed calculation process is not described, and the calculation results of this case are also listed in Table 4.

[0054] Table 4

[0055] S6 Gas / water distribution coefficient calculation Substitute the parameters obtained from the above experiment and model calculation into formula (11) to obtain the gas / water distribution coefficient, where NaCl has no volatility and does not enter the gas phase, so it is assumed that K NaCl =0, for light gases such as CH4, the solubility in water at 10 ℃, 30 MPa is very low, as long as K CH4 far greater than 1, the result of solving S7 flash is not sensitive, and K CH4 =100 is used as an engineering approximation to save calculation amount. The parameters used for calculating MeOH are: gamma in =1.44, P sat, in =8250, П=1.6859, phi in =0.023, P j =3×10 7 , get K in =0.029; The parameters used for calculating H2O are: gamma w =1.1395, P sat, in =4240 (vapor pressure table), П=1.2573, phi w =0.044, P j =3×10 7 , get K w =0.0046.

[0056] S7 Rachford-Rice two-phase flash solution Substitute the mole fraction of CH4, MeOH, H2O in the total feed z CH4 =0.9, z in =0.03695, z w =0.06183 into formula (12), and use the dichotomy method to solve β=0.916, substituting into equation (13) yields the mole fractions of each component in the liquid phase after flash evaporation equilibrium. x CH4 =0.00986、 x MeOH =0.32156、 x w =0.66739, mole fraction of each component in the gas phase y CH4 =0.98637、 y MeOH =0.00933、 y w =0.00307.

[0057] S8 Actual vaporization loss (mass transfer discount) Using equation (14), the upper limit of the thermodynamic loss of the inhibitor within the calculation section is calculated to be 0.2303. This means that if "mass transfer is infinitely fast / contact is long enough", this section will lose up to 23.03% of the MeOH vaporization loss into the gas phase. Under normal oil and gas production conditions, the wellbore is turbulent, and the liquid phase mass transfer coefficient is... k L Take 10 -4 The specific surface area is taken as 3×10 3 The residence time is taken as 3s. Substituting the above value into the first-order overall mass transfer model (Equation 15), the actual inhibitor vaporization loss rate is calculated to be 13.67%.

[0058] S9 Entrainment Loss Calculation xi aero With a flow rate of 5 m / s and a pipe size of 0.1524 m, rho G =228.967 kg / m 3 , mu G =3×10 -5 , mu L =1013.542kg / m 3 , ​ L Substituting 0.00128 into equation (16), we can obtain the entrainment rate of the droplet as 4.4938%, and further substituting it into equation (17), we can obtain the MeOH loss rate caused by the entrainment of the droplet as 2.47%.

[0059] Total loss of S10 inhibitor Calculate the calculation segment using equation (18). j The total loss rate of the internal inhibitor during flow, due to evaporation and entrainment losses, is 16.14%. The pipeline is cut into... N Sections, in orderj = 1 → N cumulatively, set R in 1 = 1 (the fraction of inhibitor that has not been lost at the inlet of the pipe), then the calculation section j The contribution to the total inlet loss is calculated using equation (19). The fraction of inhibitor is updated for the next calculation section, see equation (20), and the cumulative solution gives the rate of inhibitor loss throughout the pipe section, see equation (21).

[0060] The above only describes some of the preferred embodiments of the present application, and any skilled person in the art can modify the above-described technical solutions or modify them into equivalent technical solutions. Therefore, the corresponding simple modifications or equivalent transformations according to the technical solutions of the present application are within the scope of protection claimed by the present application.

Claims

1. A method for predicting hydrate inhibitor loss in a multiphase flow system based on DSC correction, characterized by: Comprising the following steps: I. Preparing samples for DSC test according to the composition of formation produced fluid, measuring the heat flow curve of the sample by DSC to obtain the water activity and vaporization enthalpy of the sample; II. Using the liquid phase model eNRTL to process the water activity of the sample to calculate the activity coefficient of the inhibitor under the target formula; III. Using the Clausius-Clapeyron straight line to process the vaporization enthalpy of the sample to obtain the saturated vapor pressure of the inhibitor under the target formula; IV. Calculating the Poynting correction index and the fugacity coefficient of each component; V. Using multiphase flash calculation, based on the parameters obtained in steps II, III and IV, to solve the gas / water distribution coefficient and obtain the mole fraction of each component in the liquid and gas phases after flash equilibrium; VI. Considering the mass transfer limitation in the gas-liquid mass transfer process, correcting the upper limit of the vaporization loss in step V to obtain the real vaporization loss of the inhibitor; VII. Considering the effect of high-speed gas phase entrainment on the loss of the inhibitor, calculating the loss rate of the inhibitor caused by the gas entrainment effect; VIII. The inhibitor loss of the multiphase flow system comes from the superposition of the two processes of steps VI and VII, and the inhibitor loss rate along the pipe length is obtained by iterative calculation.

2. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The sample preparation for DSC test in step I and the two types of DSC test processes are as follows: S1.1 Prepare samples for DSC testing according to the composition of the formation produced fluid and the inhibitor concentration after injection and thorough mixing, convert the liquid mass fraction to mole fraction in the liquid and gas phases x i , y i , for the balance equation calculation; S1.2 Temperature scan of the DSC test sample at the target temperature using DSC to obtain the ice melt onset temperature T f ; and converting the sample water activity using equation (1) α w ; and converting the sample water activity using equation (1) （1）， wherein R = 8.314 J / (mol K); T f T0is the ice melt onset temperature, K; α w awis the water activity, dimensionless; S1.3 Isothermal evaporation energy test is carried out on the sample by using open DSC, and the heat flow curve is recorded, and the evaporation heat is obtained by integrating after baseline subtraction Q ; the mass loss before and after DSC experiment is recorded by weighing and converted into moles Δ n , and the evaporation enthalpy Δ H vap, in ; （2）， where Δ H vap, in is the vaporization enthalpy, J / mol; Q is the inhibitor evaporation heat, J; Δ n is the inhibitor evaporation heat, J; Δ m is the loss of moles and mass before and after evaporation, mol, g; M in is the inhibitor molar mass, g / mol; q corr is the heat flow, W; t is time, s.

3. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The detailed process of step II is as follows: The water activity is calculated using equation (3) α w ( T , temperature, salinity) and the molar fraction of the sample formulation x w The water activity coefficient is calculated simultaneously γ w The binary interaction parameters are regressed using the liquid phase model eNRTL (equations (4) - (6)) at the target salinity τ 12 With τ 21 The inhibitor activity coefficient of the target formulation is then calculated using equation (7) γ in ; （3）， （4）， （5）， （6）， （7）， wherein x w is the molar fraction of water in the liquid phase, %; x in is the molar fraction of inhibitor in the liquid phase, %; γ w DSC is the activity coefficient of water obtained from DSC experiments, dimensionless; γ w model a is the activity coefficient of water obtained from the liquid phase model eNRTL, dimensionless; G 12 G is the Gibbs free energy, J / mol 21 a is an intermediate parameter for the calculation, dimensionless; k N is the number of formulations; S J is the objective function, dimensionless; γ in Inhibitor activity coefficient, dimensionless, for the target formulation; where non-random parameter ω is generally taken as 0.3, τ 12 The initial value is taken as 0.5, τ 21 The initial value is taken as 0.1, and substituted into formula (5) to calculate γ w model The multiple of DSC actually measured in S1.2 are taken as the fitting target and the objective function is established, as formula (6); iterative calculation is carried out until the calculated α w T , temperature, salinity) as the fitting target and the objective function is established, as formula (6); iterative calculation is carried out until the calculated γ w model matches the actually measured value of DSC; the activity coefficient of the inhibitor can be obtained by formula (7) γ in .​ 4. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The detailed process of step III is as follows: The Δ H vap, in With the anchor point pressure P sat ( T ), the saturated vapor pressure of the inhibitor is obtained by Clausius-Clapeyron linear equation or piecewise integral equation P sat,in ( T ); wherein the Clausius-Clapeyron differential equation is formula (8), if Δ H vap ( T ) is approximately constant Δ H vap , the integral obtains the linear equation (9), substitutes the selected anchor point T , P sat to calculate the intercept C of the linear equation, then substitutes the target temperature T j and Δ H vap, in into formula (9) to obtain the saturated vapor pressure of the inhibitor P sat,in ( T j ). （8）， （9）， wherein P sat ( T ) is the saturation vapor pressure of the anchor point, Pa; C is the y-intercept, determined by single-point anchoring; T is the temperature, K; P sat,in ( T ) is the saturation vapor pressure of the inhibitor, Pa.

5. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The detailed process of step IV is as follows: The Poynting correction index, which represents the integral correction of the liquid phase reference state from the saturation pressure P sat,in to the target pressure of the system, is calculated using equation (10) P j . （10）， where П is the Poynting correction index, dimensionless; ρ in as the inhibitor density, kg / m 3 ; P j as the pressure in the calculation section, Pa; The Peng-Robinson EOS equation is used to calculate the fugacity coefficients of each component by inputting the critical constants, the eccentric factor and the gas composition of each component φ i .

6. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The detailed process of step V is as follows: Gas / water partition coefficient calculation: gas / water partition coefficient K i component i At equilibrium, the ratio of the mole fraction in the gas phase to that in the water phase, for a volatile component i , is solved for the component i Partition coefficient in the gas-liquid two-phase K i ; （11）； wherein K i Kg is the partition coefficient in gas-liquid two-phase, dimensionless; γ i activity coefficient of the different components, dimensionless; φ i for the component fugacity coefficient, dimensionless; P sat,i Saturation vapor pressure for different components, Pa; Rachford-Rice two-phase flash solution: Given feed mole fraction z i , the gas phase mole fraction is solved by dichotomy for equation (12) β ; and the mole fraction of each component in the liquid phase and gas phase after flash equilibrium is calculated by equation (13) x i , y i ; （12）， （13）， wherein β is the gas phase mole fraction, z i is the mole fraction of the different components in the total feed stream, x i is the composition of the flash equilibrium i in the liquid phase, y i is the composition of the flash equilibrium i in the gas phase.

7. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The detailed process of step VI is as follows: Using formula (14) to calculate the upper limit of the thermodynamic loss of the inhibitor after the gas-liquid equilibrium is fully reached in the calculation section (mass transfer is not limited), but the interface mass transfer is affected by the gas-liquid contact area, flow pattern and turbulence degree, and therefore can only reach a part of the loss upper limit, so a first-order overall mass transfer model (formula 15) is used to convert the real inhibitor vaporization loss; （14）， （15）， wherein ξ eq with ξ vap maximum vaporization loss and actual vaporization loss rate, dimensionless k L KLa is the liquid phase mass transfer coefficient, often calculated using empirical correlations; a S is the specific surface area, dimensionless, determined by the wellbore flow pattern; τ For the residence time, s, the liquid phase volume / liquid phase flow rate can be used.

8. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The detailed process of step VII is as follows: The high speed gas phase entrains liquid droplets and carries the inhibitor into the gas phase. The entrainment rate of the liquid droplets varies with the flow pattern, Reynolds number and Weber number. First, the entrainment rate of the liquid droplets is calculated by using equation (16) E Then, the loss rate of the inhibitor lost by the entrainment of the gas is calculated by using equation (17). （16）， （17）， wherein E is the droplet entrainment rate, dimensionless; We, Fr, Re are the gas phase Weber, Froude and Reynolds numbers, respectively, dimensionless; ρ L is the gas phase flow rate, g / s; ρ G is the gas phase molar mass, g / mol; 3 ; μ L is the gas phase flow rate, g / s; μ G is the gas phase and liquid phase viscosity, respectively, Pa-s; ξ aero is the inhibitor loss rate due to entrainment, dimensionless; m in,aero is the mass of MeOH lost to entrainment, g / s; n F is the total feed molar number, mol / s; m g is the gas phase flow rate, g / s; M g is the gas phase molar mass, g / mol; w in is the inhibitor mass fraction, %.

9. The DSC correction-based hydrate inhibitor loss prediction method for a multiphase flow system according to claim 1, characterized in that: The detailed process of step VIII is as follows: Calculation segment j The total loss rate of inhibitor due to evaporation loss and entrainment loss during flow can be calculated using equation (18); cut the pipeline into N segments, in order j =1→ N Cumulative, set R in 1 =1, the fraction of inhibitor that has not yet been lost at the inlet of the pipe, then this calculation segment j The contribution to the total inlet loss is calculated using equation (19); update the fraction of inhibitor for the next calculation segment, see equation (20), and the cumulative solution gives the loss rate of inhibitor throughout the pipe segment, see equation (21); （18）， （19）， （20）， （21）， where, ξ total Loss of total inhibitor, %; Loss of Total is the calculated section j Contribution to total inlet loss, dimensionless; R in With R out Loss of total inhibitor, %; Loss of Total is the calculated section j Fraction of inhibitor at inlet and outlet, %; Ξ is the loss of inhibitor rate over the entire tube section, %.

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