RU2637541C1 - Method for preventing hydrate formation in field systems of gas collection - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: according to method, inhibitor of hydrate formation is supplied to the loop. To determine the beginning of hydrate formation process, gas temperature is measured at wellhead, actual gas temperature at the loop outlet and ambient temperature are determined. Certain rated values of gas temperature are calculated by the determined model. The actual value of the temperature at the loop output is compared in real with the base value. Additionally, in real-time mode the pressure is measured at wellhead and at the loop output. The theoretical rated value of hydrate formation temperature is taken as the basic temperature value. If actual temperature at the loop output is reduced to values below the base value, the actual pressure value at the loop output is compared with the value obtained in previous measurement cycle, and actual pressure value at the wellhead is compared with the value obtained in previous measurement cycle. If this pressure at the wellhead is increased by certain value and the pressure at the loop output is reduced by certain value, the specific values of which are determined by the cognitive model for given loop, and these dynamics is maintained for a period of time also determined by the cognitive model, then the beginning of hydrate formation process is diagnosed. First, supply of inhibitor to the loop is increased. If pressure values at wellhead and the loop output do not go beyond specified cognitive model, then the theoretical rated value of hydrate formation temperature is corrected according to the cognitive model. The theoretical estimated value of hydrate formation temperature is determined by determined model defined by analytical expression.

EFFECT: optimized hydrate formation inhibitor consumption and improved operation reliability of gas collection field systems.

3 cl, 2 tbl, 3 dwg

Description

The invention relates to the field of natural gas production, and in particular to the field of preventing hydrate formation in field gas collection systems by supplying a hydrate inhibitor to them mainly in the Far North.

A known method of preventing hydrate formation by supplying an inhibitor, which is used as methanol, to well clusters through a separate methanol pipe. Methanol is supplied from the integrated gas preparation unit (UKPG) to the beginning of the loop at the wellhead [Makogon Yu.F. Gas hydrates, prevention of their formation and use. - M .: Nedra, 1985. - 232 p.].

The main disadvantage of this method is the unreasonably high consumption of methanol. For the typical thermobaric operating conditions of the plumes in the northern deposits, the theoretical methanol consumption can vary over a fairly wide range (from 0 to 300 g per 1000 m ^{3 of} gas). In practice, an additional margin of 20-25% is required for methanol consumption during loop inhibition in order to eliminate the risk of hydrates in the collector [Instructions for calculating methanol consumption standards for use in calculating maximum permissible or temporarily agreed methanol discharges for Gazprom facilities. Departmental guidance document WFD 39-1.13-010-2000. - Moscow, 2000]. In addition, in the presence of dilute aqueous solutions of methanol to prevent hydrates of insufficient concentration, the opposite effect of accelerated growth of crystalline hydrates occurs. Therefore, in most fields, when dosing methanol, it is assumed that economical consumption can cause a serious accident, the elimination of which will cost much more than the cost of methanol consumption with a “margin”.

There is also a method of preventing and destroying hydrates by supplying methanol to well boreholes through a methanol pipe, according to which the start of the hydrate formation process in the loop is determined by the gas temperature at the inlet of the gas treatment plant: when this temperature is reduced to a value at which hydrate formation can begin in the loop, it is fed methanol [Istomin V.A., Kwon V.T. Prevention and elimination of gas hydrates in gas production systems. - M .: IRC Gazprom LLC, 2004].

A significant disadvantage of this method is also a significant overspending of methanol on the gas treatment facility. This is due to the fact that the greatest difficulties in rationing the consumption of methanol are associated with the inhibition conditions of the "well - loop (collector) - inlet separator UKPG" system. This is primarily due to the fact that the modes of operation of wells, loops and reservoirs can vary significantly from one another. As a result, methanol consumption rates for them can also differ markedly. Among the factors contributing to this difference are the productivity of the wells, the length of the loops and their loading, which determines the temperature regime of their work; the amount of water carried out from the well and its mineralization; the amount of hydrocarbon fluid, etc. However, due to the lack of individual dosage systems and methanol pipelines to each well, as well as monitoring the distribution of methanol on individual outlets from the total methanol pipelines, methanol is supplied in the maximum required amount.

In addition, a decrease in the ambient temperature along the plume, the length of which can reach 10 km, a change in the direction and speed of the wind, the thickness of the snow cover, etc. reasons that do not require an increase in methanol consumption.

The closest in technical essence to the claimed invention is a method of controlling the process of preventing hydrate formation in infield loops of gas and gas condensate fields of the Far North [patent RU No. 2 329 371]. This method includes determining the beginning of the hydrate formation process in the plume by controlling the temperature of the gas entering the inlet gas treatment unit from the plume, and supplying the inhibitor to the well clusters through a separate pipeline, in this case, the actual temperature t _and gas at its outlet from the plume are measured, the gas temperature at the wellhead t _n and ambient temperature t ₀ , from the values of t _n and t ₀ calculate the calculated value of the gas temperature at the outlet of the loop t _p and compare the dynamics of its change in time with the dynamics of changes in actual temperatures s gas t _and and by the result of the comparison judge the beginning of the hydrate formation process and the need to supply a hydrate inhibitor to the loop. In this case, the calculated value of the gas temperature at the outlet of the loop t _p is determined from the relation

,

,

where Δt is the correction that takes into account the influence of factors (wind speed and direction, snow plume entering and loop insulation quality), which cannot be estimated within the deterministic model, at _k is the calculated value of the gas temperature at the loop exit, determined from the deterministic model

,

,

,Where

,

- длина газопровода, K_T - коэффициент теплопередачи в окружающую среду, D - диаметр газопровода, С_р - теплоемкость газа при постоянном давлении, ρ - плотность газа, Q - объемный расход газа в нормальных условиях.

is the length of the gas pipeline, K _T is the coefficient of heat transfer to the environment, D is the diameter of the gas pipeline, C _p is the heat capacity of the gas at constant pressure, ρ is the gas density, Q is the volumetric gas flow under normal conditions.

The disadvantage of this method is the lack of control of changes in pressure at the wellhead and at the inlet of the gas treatment facility, which reduces the reliability of determining the start of hydrate formation, since a decrease in temperature is not necessarily uniquely associated with the hydrate formation process. An error in determining the start of hydrate formation leads either to an unreasonable start of the supply of methanol, or to its excessive flow.

The objective of the proposed technical solution is to create a method for preventing hydrate formation in field gas collection systems, which allows optimizing the flow rate of a hydrate inhibitor.

The technical result is achieved by diagnosing the beginning of the hydrate formation process according to the dynamics of the change in time of two parameters of the gas flow at the wellhead and exit from the loop - temperature and pressure, while the theoretical calculated value of the hydration temperature calculated using the deterministic model and refined at the need for a cognitive model, and permissible pressure fluctuations, also determined by the cognitive model for a given loop.

The purpose of the invention is the optimization of the flow rate of the hydrate inhibitor by increasing the reliability of determining the beginning of this process.

со значением р₂, полученным в предыдущем измерительном цикле, и текущее значение давления на устье скважины

со значением р₁, полученным в предыдущем измерительном цикле, и, если это давление на устье возросло на некоторую величину Δ₁, а давление на выходе из шлейфа одновременно уменьшилось на некоторую величину Δ₂, конкретные значения которых определяют по когнитивной модели для данного шлейфа, и эта динамика сохраняется в течение времени τ, также определяемого по когнитивной модели, то диагностируют начало процесса гидратообразования и начинают (увеличивают) подачу ингибитора в шлейф, а если значения давления на устье скважины и выходе шлейфа не выходят за установленные когнитивной моделью пределы изменения, корректируют теоретическое расчетное значение температуры гидратообразования t_гт по когнитивной модели.This goal is achieved due to the fact that in the method of preventing hydrate formation in field gas collection systems, mainly in the Far North, in which a hydrate inhibitor is fed into the plume, and to determine the beginning of the hydrate formation process, measure the gas temperature at the wellhead t ₁ , the actual gas temperature at the outlet of the loop t ₂ and the ambient temperature t ₀ , some calculated value of the gas temperature is calculated according to the deterministic model, it is taken as the base value and in p in real time mode, the actual temperature value at the outlet of the plume is compared with the base value, in addition, in real time, the pressure at the wellhead p ₁ and at the output of the plume p ₂ are measured, the theoretical calculated value of hydration temperature t _{gt is} taken as the base temperature value, with a decrease the actual temperature at the outlet of the loop t ₂ to values below the base value t _gm compare the current value of the pressure at the outlet of the loop

with the value of p ₂ obtained in the previous measuring cycle, and the current value of the pressure at the wellhead

with the value of p ₁ obtained in the previous measuring cycle, and if this pressure at the mouth increased by a certain value Δ ₁ , and the pressure at the outlet of the loop at the same time decreased by a certain value Δ ₂ , the specific values of which are determined by the cognitive model for this loop, and this dynamics is maintained for a time τ, also determined by the cognitive model, then the onset of hydrate formation is diagnosed and the inhibitor is fed (increased) to the plume, and if the pressure at the wellhead and the plume exit and do not go beyond the limits set by the cognitive model changes, adjust the theoretical calculated value of hydrate formation temperature t _rm on the cognitive model.

In this case, the theoretical calculated value of the temperature of hydrate formation is determined by a deterministic model of the form

,

,

- длина шлейфа, G - массовый расход газа, С_р - теплоемкость газа.where K _T is the theoretical calculated heat transfer coefficient of the loops, D _i is the Joule-Thompson coefficient,

is the length of the loop, G is the mass flow rate of gas, C _p is the heat capacity of the gas.

The cognitive model includes the diameter, length and spatial location of the loop, the electrical resistance of its insulation, the direction and speed of the wind, the thickness of the snow cover on the loop, volumetric flow rate, moisture content and gas composition.

The technical and economic feasibility of the proposed method is to increase the reliability of preventing hydrate formation in loops while saving a hydrate inhibitor, in particular methanol, by optimizing its consumption. Reducing the consumption of methanol, in addition to direct savings, can also reduce the cost of cleaning and disposal of spent water-methanol solution.

The technical result of the invention:

- optimization of the flow rate of the hydrate inhibitor, in particular methanol;

- improving the reliability of the operation of the gas collection system. The claimed technical result is provided as follows.

Optimization of methanol consumption is ensured by the fact that:

- to diagnose the onset of hydrate formation, they use, in addition to reducing the temperature at the outlet of the plume, an additional feature, namely, they analyze the dynamics of pressure changes at the wellhead and at the exit of the plume, and begin to apply a hydrate inhibitor (or increase its flow) only if both criteria for hydrate formation are met;

- the theoretical temperature of hydrate formation, calculated according to the deterministic model, if necessary, clarified by the cognitive model of a particular loop.

Improving the reliability of the operation of the gas collection system is achieved by the fact that the beginning of the hydrate formation process in the proposed method is diagnosed at an earlier stage, because the change in gas flow pressure occurs more dynamically compared to a change in its temperature.

In FIG. 1 shows an algorithm for implementing the method, FIG. 2 and FIG. Figure 3 shows the trends in pressure at the wellhead and at the shelf outlet in real time, respectively, for cases of the absence of hydrate formation and its presence.

The method is implemented as follows (Fig. 1).

Information such as the theoretical value of the heat transfer coefficient of the plumes from gas to the environment is recorded in the database of the control system for supplying a hydrate formation inhibitor, for example methanol, in the normative and reference information section; loop length; mass gas flow; gas heat capacity; the concentration of the inhibitor supplied to the wellhead and coming from the loop; the amount of produced water from the wells; gas density, etc. The results of temperature and pressure measurements at the wellhead by means of telemetry installed there (for example, RTP-4 recorders with a radio communication channel) and temperature and pressure at the loop output measured by measuring instruments included in the composition are recorded in the operational information section of the database ACS TP UKPG.

These data are used to calculate the theoretical temperature of hydrate formation according to a deterministic model of the form

,

,

- длина шлейфа, G - массовый расход газа, С_р - теплоемкость газа.where K _T is the theoretical calculated heat transfer coefficient of the loops, D _i is the Joule-Thompson coefficient,

is the length of the loop, G is the mass flow rate of gas, C _p is the heat capacity of the gas.

A cognitive knowledge base is formed by extracting information from expert surveys in the form of a cognitive model that establishes patterns between the loop heat transfer coefficient and its characteristics such as diameter, length and location in space, insulation state, snow cover thickness, as well as wind direction and speed, etc. .P. The cognitive model in the process of work is periodically refined and supplemented.

According to the results of temperature and pressure measurements and the data stored in the database, the theoretical hydrate formation temperature t _{gm is} calculated using the deterministic model given above and the corresponding nominal inhibitor flow rate q _{nom is} determined.

The process of preventing hydrate formation in the loops is controlled by monitoring pressure at the wellhead p ₁ , temperature t ₂ and pressure p ₂ at the end of the loop. These parameters are measured in real time with a given frequency and entered into the database.

Diagnosis of a possible start of the hydrate formation process in the input gas treatment stubs of the gas treatment plant is carried out by constantly comparing the theoretical calculated temperature of hydrate formation t _gt with the measured temperature t ₂ at the outlet of the cable (at the input of the gas treatment plant). While the condition t _{rm 2} ≥t inhibitor, such as methanol, in the plume is not fed (or supplied with a nominal flow rate q _nom). If the measured temperature falls below the value of t _gt , then theoretically this may mean the beginning of the hydrate formation process. However, this may be due to the fact that the deterministic model for calculating t _gt does not take into account factors not directly measured, such as, for example, the terrain on which the train is laid, its isolation state, snow cover thickness, affecting the heat transfer coefficient, etc. P.

со значением p₁, полученным в предыдущем измерительном цикле. Одновременно сравнивают значения давления на выходе шлейфа

со значением р₂, полученным в предыдущем измерительном цикле.Therefore, a further comparison is made of the pressure at the wellhead

with the value of p ₁ obtained in the previous measuring cycle. At the same time, the pressure values at the loop output are compared

with the value of p ₂ obtained in the previous measuring cycle.

If at the wellhead a positive dynamics of pressure changes is observed (it has increased by a certain amount, the specific value of which is established by the cognitive model, for example, by 10%), and negative dynamics is observed at the outlet of the loop (pressure over the same period of time decreased by some of the cognitive model, the value, for example, also by 10%), and this position persists for a certain time (for example, at least 30 minutes, the time period is also set by the cognitive model), make an unambiguous conclusion about the beginning of the hydrate formation process and supply methanol to the loop (or increase the amount of methanol supplied to the value corresponding to the elimination of the hydrate plug and determined by the technological regulations of the gas treatment plant).

If the pressure has not changed or the change is within the acceptable limits for this loop (they are determined from the cognitive model), the value of the theoretical hydrate formation temperature t _rm is corrected using the cognitive model.

An example of a specific implementation of the method.

The proposed method was tested on the loop of the integrated gas treatment unit UKPG-5 of the Yamburg gas condensate field. The length of the loop is 5 km. The pressure at the wellhead at the beginning of the plume is 2.8 MPa, and at the inlet of the gas treatment facility - 0.9 MPa. The field is in the final stage of operation, therefore methanol is fed into the loop constantly with a certain nominal flow rate q _nom .

The theoretical hydrate formation temperature t _gt was calculated according to the deterministic model given above for the initial data given in table 1. The value of t _gt was minus 16.6 ° C, and the concentration of water-methanol solution corresponding to this temperature was 40%.

At the wells connected to the loop, RTP-4 technological parameters recorders were installed, measuring pressure and temperature values and transmitting them over the air to ACS TP UKPG.

In FIG. Figure 2 shows a piece of data on recording pressure values at the wellhead (upper graph) and at the loop exit (lower graph) obtained on 02.10.2016. As can be seen from the graphs, at about 12 o’clock the pressure begins to change, the temperature at the loop exit at this point in time was minus 17.8 ° C. An analysis of the results of pressure measurements at the wellhead and at the loop exit after about 30 minutes (Table 2) shows that the pressure at the wellhead increased by 1.8% (ΔP ₁ ), and the pressure at the loop outlet decreased by 16.7% (ΔP ₂ ). Thus, since the pressure at the outlet of the loop decreased by more than 10%, then in accordance with the algorithm (see Fig. 1) after 30 minutes, a repeated analysis of the dynamics of pressure changes should be carried out. An analysis of the pressure values corresponding to a time stamp of 12.58 shows that the pressure at the wellhead increased by 4.3% (ΔP ₁ ), then stabilized, and the pressure at the outlet of the loop decreased by 27.8% (ΔP ₂ ) and also stabilized. Therefore, according to the algorithm (see Fig. 1), the condition for the occurrence of hydrate formation is not fulfilled and a correction of the theoretical temperature of hydrate formation according to the cognitive model to minus 18.3 ° C is required.

In FIG. Figure 3 shows the graphs of pressure changes taken on 02/14/2016. The pressure begins to change at about 21.51, the temperature at the outlet of the loop at this point in time was minus 18.6 ° C. An analysis of the results of pressure measurements at the wellhead and at the loop exit at 22.21 shows that the pressure at the wellhead increased by 10.5% (ΔP1), and the pressure at the inlet to the gas treatment facility decreased by 49.2% (ΔP2). Thus, since both pressure values at the wellhead and at the inlet of the gas treatment facility have changed by more than 10%, in accordance with the algorithm (see Fig. 1), a reanalysis of the dynamics of pressure changes should be carried out after 30 minutes. Measurement taken at 22.51 shows that the pressure at the mouth increased by 13.3% (ΔP1), and the pressure at the outlet of the loop decreased by 52% (ΔP2). Therefore, according to the algorithm (see Fig. 1), the condition for hydrate formation occurs and it is necessary to increase the supply of methanol.

Thus, the proposed method provides the following positive results:

- increasing the reliability of determining the moment of hydrate formation by introducing additional criteria and checking and adjusting them in real time;

- reducing the flow rate of the hydrate inhibitor due to the fact that the supply starts only in the event of a real occurrence of hydration conditions;

- increasing the flow rate of the hydrate inhibitor due to the early detection of conditions for the occurrence of hydrate plugs;

- improving the accuracy of determining the temperature of hydrate formation through the use of a cognitive model.

Claims (5)

1. A method of preventing hydrate formation in field gas collection systems, mainly in the Far North, where a hydrate inhibitor is fed into the plume, and to determine the beginning of the hydrate formation process, measure the gas temperature at the wellhead t ₁ , the actual gas temperature at the outlet of the plume t ₂ and ambient temperature t ₀ is calculated by the deterministic model a calculated value of the gas temperature, taking for its base value and in real time comparing actual The values of the outlet temperature of the plume with the base value, characterized in that an additional real-time measured pressure wellhead p ₁ and at the outlet of the loop p _2, the base temperature value is taken theoretically calculated hydrate formation temperature value t _zm, while reducing the actual outlet temperature t ₂ loop to a value below the reference value t _rm compare current pressure value at the outlet of the loop p ₂ ^'with the value p _2, obtained in the previous measurement cycle, and the current znach of pressure at the wellhead p ₁ ^'to the value p _1, obtained in the previous measurement cycle, and if the pressure at the wellhead increased by some amount Δ _1, and the pressure at the exit of the loop at the same time decreased by a certain value Δ _2, specific values are determined according to the cognitive model for this loop, and this dynamics is maintained for a time τ, also determined by the cognitive model, then the onset of the hydrate formation process is diagnosed and the flow of the inhibitor into the loop is started (increased), and if the values are pressured At the wellhead and the loop exit, they do not go beyond the limits of change established by the cognitive model; they correct the theoretical calculated value of hydrate formation temperature _tmr according to the cognitive model. 2. A method of preventing hydrate formation in commercial gas collection systems according to claim 1, characterized in that the theoretical calculated value of the temperature of hydrate formation is determined by a deterministic model of the form

where K _T is the theoretical calculated heat transfer coefficient of the loops, D _i is the Joule-Thompson coefficient, l is the length of the loop, G is the mass flow of gas, C _p is the heat capacity of the gas. 3. A method of preventing hydrate formation in field gas collection systems according to any one of paragraphs. 1, 2, characterized in that the cognitive model includes the diameter, length and spatial location of the loop, the electrical resistance of its insulation, the direction and speed of the wind, the thickness of the snow cover on the loop, volumetric flow rate, moisture content and gas composition.

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