Keywords

1 Foreword

Sulige gas field belongs to the low permeability gas field and low water content of the gas wells is universal. More than thousands of gas wells cannot have normal production due to liquid loading from kinds of reasons, which has a serious influence on the normal production of the gas wells. According to the actual production status of the gas wells, it is particularly important to carry out the dewatering gas production measures to make normal and continuous production of the low production and low effectiveness gas wells.

In order to effectively ensure the continuous production and stimulation of gas wells, under the premise of the continuous production, the dewatering gas production measures should be taken in advance to ensure that gas wells can be continuously produced when the production capacity of gas wells reduce to a certain extent. Therefore, the dewatering gas production measures in advance are crucial to ensure the continuous production of gas wells. This paper carries outs a detailed research and discussion on the timing of dewatering gas production for water-producing gas wells from the aspects of critical liquid-carrying flow rate, liquid holdup and kinetic energy factor.

2 Critical Liquid-Carrying Flow Rate of Gas Wells

For the gas wells tubing of given specification, the gas wells will have the liquid holdup when the actual gas production capacity is more than or equal to the critical liquid carrying flow rate [1]. Li Min’s analysis show that there is certain differential pressure before and after the droplets when they are moving in high-speed gas flow. Under the influence of differential pressure, the droplets will change from sphere to ellipsoid and the incident flow area of ellipsoid is greater than that of the sphere. So the ellipsoid is more easily taken to the ground by the gas flow and the required liquid drainage speed is also lower. Therefore, Li Min considers that it is necessary to research on the influence of droplets deformation to the liquid holdup of gas wells when we research on the liquid holdup of gas wells. A new droplet model of liquid holdup of gas wells is provided according to the actual production of Sulige gas field.

Critical flow velocity of droplets is derived according to the new model and the force balance of droplets in the critical state.

(1)

The formula for the critical liquid-carrying flow rate for the new model is:

(2)

  • Where, v means the critical flow velocity of droplets (m/s);

  • g means the gravitational acceleration (m/s2);

  • d means inside diameter of tubing (m);

  • p means the tubing pressure at wellhead (MPa).

  • z means the compressibility factor (dimensionless);

  • T means temperature (K);

  • ρl means the density of the droplets (Kg/m3);

  • ρg means the gas density (Kg/m3);

  • σ means the surface tension of natural gas and droplets (N/m);

  • CD means the drag coefficient (dimensionless);

According to the principle of critical liquid-carrying flow rate of gas wells and the actual production of Sulige gas field, 1/2–2/3 (adjustable) of the theoretical critical liquid-carrying flow rate is selected to be as the boundary reference value to decide whether the water-producing wells have liquid holding [2]. Therefore, the dewatering gas production measures can be taken in advance to avoid the liquid loading of gas wells, to ensure the continuous liquid holdup of gas wells and to ensure continuous production when the gas wells have reached to the critical liquid-carrying flow rate (Table 1 and Fig. 1).

Table 1. Theoretical calculation sheet of critical liquid-carrying flow rate
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Fig. 1.
figure 1

Relation curve between critical liquid-carrying flow rate and wellhead tubing pressure of different tubing inside diameter

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3 Analysis on Liquid Holdup of Gas Wells

Hagedorn-Brown [1,2,3,4,5] experimental studies suggest that the theoretical liquid holdup is related to 4 dimensionless parameters and 3 related curves respectively as follows:

$$ {\text{Liquid}}\;{\text{velocity}}\;{\text{number:}}\;N_{lv} = v_{sl} \times \left( {\frac{{\rho_{l} }}{g \times \sigma }} \right)^{0.25} $$
(3)
$$ {\text{Gas}}\;{\text{velocity}}\;{\text{number:}}\;N_{gv} = v_{sg} \times \left( {\frac{{\rho_{l} }}{g \times \sigma }} \right)^{0.25} $$
(4)
$$ {\text{Liquid}}\;{\text{viscosity}}\;{\text{number:}}\;N_{l} = \mu_{l} \times \left( {\frac{g}{{\rho_{l} \times \sigma^{3} }}} \right)^{0.25} $$
(5)
$$ {\text{Tubing}}\;{\text{diameter:}}\;N_{d} = d \times \left( {\frac{{\rho_{l} \times g}}{\sigma }} \right)^{0.5} $$
(6)

The calculation formula of the actual liquid holdup is derived according to the definition of liquid holdup, described in Formulas (7) and (8).

$$ H_{actual} = \frac{{V_{w} }}{{V_{w} + GLR \times V_{w} \times \left( {\frac{{p_{sc} \times Z \times T}}{{T_{sc} \times p \times Z_{sc} }}} \right)}} $$
(7)

Simplified as:

$$ H_{actual} = \frac{1}{{1 + GLR \times \left( {\frac{{p_{sc} \times Z \times T}}{{T_{sc} \times p \times Z_{sc} }}} \right)}} $$
(8)

Where:

  • Hactual—actual liquid holdup

  • Hl—theoretical liquid holdup

  • GLR—gas water ratio, m3/m3

  • vsl—liquid apparent velocity m/s

  • vsg—gas apparent velocity m/s

  • ρl—density of water, kg/m3

  • g—the gravitational acceleration (m/s2)

  • μl—liquid viscosity, mPa.s

  • σ—liquid surface tension, mN/m

  • d—inside diameter of tubing, m

  • p—pressure, MPa

  • T—temperature, K

  • Psc—Standard case pressure, MPa

  • Tsc—Standard case temperature, MPa

  • Z—deviation factor of natural gas

  • Zsc—deviation factor of natural gas of standard case

It is clear that there is a certain relation between the theoretical liquid holdup and the actual liquid holdup of gas wells and wellbore: there is no liquid loading in the gas wells if the actual liquid holdup is less than the theoretical liquid holdup; there is liquid loading in the gas wells if the actual liquid holdup is higher than the theoretical liquid holdup. We can decide whether there is liquid loading and the timing of liquid loading in the gas wells, the timing of the dewatering gas production measures can be decided to ensure the continuous production of gas wells (Fig. 2).

Fig. 2.
figure 2

Discrimination board for the liquid loading timing of gas wells

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4 Analysis of Kinetic Energy Factor of Gas Wells

Kinetic energy factor reflects the flow features of gas and liquid in the tubing, represents the energy of gas wells [8, 9], and shows the liquid holdup capability of the producing gas wells. The critical liquid holdup models, represented by Turner [6, 7] model, are calculated based on the idealized fog flow model. Laboratory simulation experiment of gas and water phase has found that the liquid holdup capability has be existing in the loop fog flow and the slug flow, and the following relations exist between the kinetic energy (kinetic energy factor) of the unit fluid, the gas production, the relative density of natural gas and the flowing pressure.

$$ F = v_{s} \sqrt {\delta_{s} } = 2.9\frac{Q}{d}\sqrt {\frac{{rT_{S} Z_{S} }}{{P_{S} }}} $$
(9)

Liquid loading in the gas wells can be diagnosed through calculating the kinetic energy factor, and then the best timing to carry out the dewatering gas production measures will be decided (Fig. 3 and Table 2).

Fig. 3.
figure 3

Change cure of daily production of kinetic energy factors of different type wells

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Table 2. Calculation results of kinetic energy factors of different type wells
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By calculating the kinetic energy factors, it is found that kinetic energy factor shows a decreasing trend of two ends as a whole. The gas wells may be disturbed by the liquid loading if there is a turning point shows in the change of the kinetic energy factor, and the dewatering gas production measures should be taken in time to this type of gas wells in this stage.

5 Conclusion

According to the actual and theoretical calculations, it’s suggested that adopting the dewatering gas production in advance can help to ensure the continuous production of gas wells and it should be as one of the main means of the refined management of gas wells. The dewatering gas production measures should be taken in time when the following situations appear:

  1. (1)

    When the liquid holdup of gas wells is close to the critical liquid holdup;

  2. (2)

    When the actual liquid holdup is higher than the theoretical liquid holdup (according to the discrimination board for the liquid loading timing of gas wells);

  3. (3)

    When a turning point shows in the change of the kinetic energy factor, the turning point for Type I gas wells is about 3.5 with the daily gas production of 0.9 × 104 m3/d; the turning point for Type II gas wells is about 2.5 with the daily gas production of 0.7 × 104 m3/d; the turning point for Type III gas wells is about 1.5–2 with the daily gas production of 0.5 × 104 m3/d.