RU2778763C1 - Method for preventing hydrate formation in a gas collection system of gas condensate fields - Google Patents

Abstract

FIELD: gas industry.

SUBSTANCE: invention relates to the field of gas production, namely, to methods for preventing hydrate formation in the process of collecting and transporting gas in gas condensate fields. Method for inhibiting the hydrate formation of gas collection systems of gas condensate fields consists in supplying methanol to the gas collection system periodically for the time T₁, followed by terminating the supply of methanol for the time T₀. The time T₁ of supplying methanol to the gas collecting line and the time T₀ after which the methanol supply is terminated are determined based on the proposed mathematical expressions.

EFFECT: reduced consumption of methanol in the process of inhibiting the hydrate formation in a gas collection system of gas condensate fields and possibility of automating the process of controlling the supply of methanol for the purpose of minimisation thereof on standard computing capacities of a gas production enterprise.

1 cl, 3 dwg, 1 tbl

Description

The invention relates to the field of gas production, and in particular to methods for preventing hydrate formation in the process of collecting and transporting gas in gas condensate fields.

In the process of gas and gas condensate production, there is a problem of hydrate formation. Gas hydrates are solid crystalline compounds with cavities filled with gas. Gas hydrates can be formed throughout the entire technological chain of gas production and treatment from the bottom of the well to the point of supply to the gas transmission network. Most often, hydrate deposits are formed in gas gathering reservoirs due to a significant length, up to several tens of kilometers, as well as an increased water content in the composition of natural gas.

The operation of the field and gas transmission system under conditions of hydrate formation may lead to the risk of deposits of natural gas crystalline hydrates, which may disrupt the operating mode of the field gas treatment plant. In the worst case, clogging of pipelines, field equipment, blocking of pipeline valves can occur. Therefore, the formation of hydrates is a significant problem in ensuring the safe and efficient operation of the gas collection system and its individual elements.

The generally accepted solution to this problem is the complete prevention of the hydrate formation process. There are various ways to prevent this process, but the most effective and reliable way remains inhibition with thermodynamic inhibitors, such as methanol, which, when added, can reduce the temperature at which natural gas hydrates.

At the gas and gas condensate fields of the Far North of Russia, practically only methanol is used for the following reasons:

относительно низкая стоимость и широкая промышленная база;

relatively low cost and broad industrial base;

высокая технологичность процесса ввода и распределения метанола;

high manufacturability of the process of input and distribution of methanol;

наивысшая антигидратная активность, сохраняющаяся даже при низких температурах;

the highest anti-hydrate activity, which remains even at low temperatures;

очень низкая температура замерзания растворов метанола и их малая вязкость;

very low freezing point of methanol solutions and their low viscosity;

сравнительно низкая растворимость метанола в нестабильном конденсате;

relatively low solubility of methanol in unstable condensate;

некоррозионность метанола и его водных растворов;

non-corrosiveness of methanol and its aqueous solutions;

возможности использования технических сортов метанола;

the possibility of using technical grades of methanol;

наличие простых технологических схем регенерации отработанных растворов;

the presence of simple technological schemes for the regeneration of waste solutions;

проработанность вопросов утилизации и захоронения промстоков, содержащих метанол;

elaboration of the issues of utilization and disposal of industrial effluents containing methanol;

высокая эффективность ликвидации несплошных гидратных пробок.

high efficiency of elimination of discontinuous hydrate plugs.

Despite considerable experience in preventing hydrate formation, there are still unresolved problems in the practice of field gas treatment. In practice, the actual consumption of methanol at gas production enterprises is often overestimated due to irrational use.

The method of automated distribution of methanol in the process control system of gas treatment units of Gazprom PJSC is based on the principle of determining such an amount of methanol, which, when introduced into the pipeline, will achieve a concentration in the flow sufficient for a hydrate-free regime. (VRD 39-1.13-051-2001 "Instruction on the regulation of consumption and calculation of methanol emissions for Gazprom facilities") - prototype. Consumption is calculated using the following formulas:

The optimal consumption of methanol is determined by the formula:

where:

q _met - methanol consumption, kg/hour

Q - gas consumption in the gas collection manifold under standard conditions, thousand m ³ /hour;

G _pl.v - specific consumption of formation water, g/st.m ³ ;

W _yct , W _GTP - gas moisture content at the wellhead and at the inlet to the GTP, respectively (determined according to VRD 39-1.13-051-2001), g/st.m ³ ;

X is the concentration of the hydrate formation inhibitor (usually 95%), wt %;

G, M - the concentration of methanol in the gas and liquid hydrocarbon phases, respectively (determined according to VRD 39-1.13-051-2001), g/m ³ .

The required concentration of methanol at the protected point, sufficient to dissolve the hydrates, is determined by the formula:

where:

T _hydr is the temperature of hydrate formation (determined according to VRD 39-1.13-051-2001), °C;

T - gas temperature at the protected point, °C.

The calculated flow rate of methanol is brought to the control device and continuously around the clock is fed into the protected pipeline.

The disadvantage of this method is the high consumption of methanol in the process of protection against hydrate formation and the need for constant operation of the methanol supply equipment due to the fact that the technique does not take into account the duration of the formation of a hydrate plug in the pipeline.

The objective of the present invention is to eliminate this drawback. The technical result consists in reducing the consumption of methanol in the process of inhibiting hydrate formation in the gas collection system of gas condensate fields and the ability to automate the process of controlling the supply of methanol in order to minimize it using standard computing power of a gas production enterprise.

The technical result is achieved in the following way. The method for inhibiting hydrate formation in gas gathering systems of gas condensate fields consists in the fact that methanol is periodically fed into the gas gathering system over a period of time determined by the formula:

по прошествии которого подачу метанола отключают на время Т₀, определяемое по формуле:

after which the methanol supply is turned off for a time T ₀ determined by the formula:

where T ₁ is the time for supplying methanol to the gas collection loop at a flow rate q _met to achieve the methanol concentration value from C _min to C _eff , sec, T ₀ is the methanol supply shutdown time at which the methanol concentration decreases from C _eff to C _min , sec, L - length of the pipeline, m, D - inner diameter of the pipeline, mm, Q - gas flow rate in the gas collection manifold under standard conditions, thousand m ³ / hour, Z - coefficient of supercompressibility, P - gas pressure at the end of the gas collection manifold, MPa, T - temperature at the end of the gas-gathering header, °C, C _eff - concentration of methanol in the formation mixture sufficient for conditions of hydrate-free regime, % mass, C _min - concentration of methanol in the pipeline, to which the concentration is allowed to decrease when methanol supply is stopped (if unknown, it is taken equal to 0), % mass, α ₁ =0.661, β ₁ =0.00052, δ ₁ =1, α ₀ =0.687, β ₀ =0.00052, δ ₀ =1.

The process of formation of a hydrate plug does not occur instantly. In particular, the time of formation of hydrate in the "raw" gas can take from minutes to several hours, depending on the physical parameters of the flow. At the same time, the hydrate crystal itself does not pose a danger, since it can move driven by the gas flow. The danger is hydrate plugs, which are formed due to the deposition of crystals on the walls of the pipeline for a long time. The formation of a "dead" hydrate plug can take from several days to a month, depending on the concentration of methanol in the stream. According to the experience of operating the Urengoy gas condensate field, some plumes with a length of 5 to 10 km could be operated in the hydrate regime for about 28 days before the formation of a hydrate plug.

To prevent hydrate blockage, it is not necessary to maintain a constant hydrate-free concentration of methanol in the pipeline. It is enough to periodically reach it at the protected point by supplying methanol with a certain period of time. Despite the periodic operation in the hydrate mode, the formation of a hydrate plug is excluded, since the value of the time to "freeze" is much longer than the time to re-achieve the state of "no hydrate" at the protected point. With increasing methanol concentration, the time to hydrate plug increases up to infinity when the required concentration is reached. With a decrease - the time to traffic jam is reduced. But even at “zero” concentration, the formation of a plug will not occur instantly, but only after some time has passed. This cycle is shown in the graph of Fig. one.

With the help of dynamic modeling of the movement of gas condensate reservoir gas through the gas gathering pipeline, the dependence of the change in the concentration of methanol at the end of the plume on the gas velocity (Fig. 2) in the pipeline was revealed. Based on this dependence, the rate of increase in the concentration of methanol during the supply and the rate of decrease in the concentration when the supply is turned off are determined in significance from the gas velocity. As a result, we received:

- formula (1), which allows you to calculate the time during which, with a constant supply of methanol, its concentration in the formation gas of the gas-gathering reservoir will increase from the minimum value to the required value for the hydrate-free regime;

- formula (2), which allows you to calculate the time during which the concentration of methanol will decrease from optimal to acceptable in the absence of methanol supply,

Thanks to these dependencies, a method for dosing methanol into gas-gathering collectors of wells of gas condensate fields is carried out.

Until now, there has been no simple calculation method by which it would be possible to determine the frequency of supply of the inhibitor - methanol to the gas collection headers. This is due to the fact that, usually, to determine the periodicity of the inhibitor supply, a multi-iteration calculation of the flow of a three-phase reservoir mixture for the pipeline is required. Moreover, the result will depend not only on the operating mode of the wells, but also on the profile of the pipeline. The calculation method will be individual for each pipeline, and there may be several dozen of them in the gas collection system. An automated system that can make these calculations in real time would require a lot of computing power, which is very expensive. It is easier for production personnel to use the old approach of maintaining a constant supply of methanol than to install an expensive computer system.

The proposed method solves this problem due to the fact that it allows you to determine the optimal frequency depending on the well mode in a simple way, without iterative calculations, and, accordingly, do it automatically. As a result, the control of the methanol supply can be carried out on standard computing power of the gas production enterprise. At the same time, the system can automatically change the frequency of methanol supply depending on the well mode, that is, when the temperature, pressure and gas flow change, the mode of the methanol supply frequency changes automatically.

The method of inhibition of hydrate formation of gas gathering systems of gas condensate fields is carried out as follows. The optimal consumption of methanol q _meth is determined according to formula I known from the prototype.

The required concentration of methanol at the protected point, sufficient for the conditions of the hydrate-free regime C _eff , is determined according to the formula II known from the prototype.

Next, the time T ₁ is determined, during which methanol is supplied until the concentration C _eff is reached with the optimal flow rate q _met , according to the formula:

where:

T ₁ - the time of supply of methanol in the gas collection loop with a flow rate q _met to achieve the value of the concentration of methanol from C _min to C _max , sec;

L is the length of the pipeline;

D - internal diameter of the pipeline, mm;

P - gas pressure at the end of the gas collection manifold, MPa;

T is the temperature at the end of the gas collection manifold, °С;

Q - gas consumption in the gas collection manifold under standard conditions, thousand m ³ /hour;

Z is the supercompressibility factor;

C _eff is the concentration of methanol in the reservoir mixture, sufficient for the conditions of the hydrate-free regime, wt %.;

C _min is the concentration of methanol in the loop, to which the concentration is allowed to decrease when the supply of methanol is stopped (if unknown, it is assumed to be 0), wt %;

α ₁ \u003d 0.661, β ₁ \u003d 0.00052, δ ₁ \u003d 1

After the time T ₁ , the methanol supply is turned off for the time T ₀ determined by the formula:

where:

T ₀ - time to stop the supply of methanol to the gas collection manifold, sec; C _min is the concentration of methanol, to which it is allowed to reduce the concentration of methanol in the loop when the methanol supply is turned off, wt %. α ₀ \u003d 0.687, β ₀ \u003d 0.00052, δ ₀ \u003d 1

After the time T ₀ the supply of methanol is resumed with a flow rate q _met for the time T ₁ determined by the formula (1). Next, the cycle is repeated.

According to the presented method, the consumption of methanol was determined for the gas-gathering collector of the cluster of gas wells 107 of the gas treatment plant No. 102 of the Kovykta gas condensate field. The length of the gas collection manifold is L=7200 m, the inner diameter of the pipeline is D=159 mm. Gas consumption Q=16.5 thousand m ³ /hour. The gas pressure P varies from 17.2 MPa to 16.8 MPa along the length of the pipeline. Temperature T - from 24°C to 14°C. According to the method, the optimal concentration of methanol for hydrate-free mode With _eff =8.9% of the mass. The optimal consumption of methanol, at which the required concentration will be achieved, is Qmet=18.39 kg/h. The results of calculating the consumption of methanol are presented in table 1.

According to the calculation, the time of supply of methanol T ₁ during which the concentration of methanol in the loop will increase from the initial concentration of 0% to 8.9% of the mass, is 2.75 hours. When the methanol supply is turned off, the time T ₀ during which the concentration will decrease to the value C _min =4.5% of the mass. is 1.17 hours. Further, when the methanol supply is turned on, the concentration will increase to the value C _eff =8.9% of the mass during the time T ₁ =1.37 hours. Further, methanol is supplied with a given frequency. The methanol feed mode is shown in the graph of FIG. 3.

With the usual constant method of supplying methanol, the consumption will be 441.3 kg/day or 1.107 g/m ³ . With periodic supply of methanol according to the presented Method, the daily consumption of methanol will be 261 kg/hour or 0.656 g/m ³ , 40% less compared to the Prototype.

Claims (20)

A method for inhibiting hydrate formation in gas gathering systems of gas condensate fields, which consists in the fact that methanol is periodically fed into the gas gathering system over a period of time determined by the formula:

after which the methanol supply is turned off for a time T ₀ determined by the formula:

where T ₁ - the time of supply of methanol in the gas collection loop with a flow rate q _met to achieve the value of the concentration of methanol from C _min to C _eff , sec; T ₀ - time off the supply of methanol, at which the concentration of methanol will decrease from the value With _eff to C _min , sec; L - pipeline length, m; D - internal diameter of the pipeline, mm; P - gas pressure at the end of the gas collection manifold, MPa; Q - gas consumption in the gas collection manifold under standard conditions, thousand m ³ /hour; T is the temperature at the end of the gas collection manifold, °С; z - coefficient of supercompressibility; C _eff is the concentration of methanol in the reservoir mixture, sufficient for the conditions of the hydrate-free regime, wt.%; C _min is the methanol concentration in the loop, to which the concentration is allowed to decrease when the methanol supply is stopped, wt.%; α ₁ =0.661; β ₁ =0.00052; δ ₁ =1; α ₀ =0.687; β ₀ =0.00052; δ0 = ₁ .

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