Gas-liquid interface control method based on salt cavern gas storage nitrogen dissolution-resistant cavity construction

Abstract

The invention relates to the technical field of gas storage cavity construction, in particular to a gas-liquid interface control method based on salt cavern gas storage nitrogen dissolution-resistant cavity construction. The gas-liquid interface control method comprises a first stage of cavity construction, cavity top reaming and a second stage of cavity construction. Wherein cavity top reaming comprises the steps that part of nitrogen is discharged till the depth of a gas-liquid interface is pushed upwards to a second set depth, and then water injection cavity making is conducted till the average diameter of a borehole between the top of a cavity formed in the first stage of cavity making and the second set depth is expanded by a preset width. Wherein the gas-liquid interface is determined by the following mode: establishing a gas-liquid interface prediction model based on the nitrogen pressure, the brine discharge pressure and the brine discharge flow at the wellhead, and predicting to obtain the depth of the gas-liquid interface. The problems that the fluctuation range of a gas-liquid interface is large and the gas-liquid interface is difficult to control during nitrogen dissolution-resistant cavity making can be solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gas storage caverns, and in particular to a gas-liquid interface control method for nitrogen blocking dissolution of salt cavern gas storage. BACKGROUND

[0002] Salt cavern gas storage refers to a cavity formed by dissolving salt through water injection in a salt layer underground, used for storing natural gas. When the cavity is formed, a cavity inner pipe and a cavity outer pipe are lowered into the production casing, and water dissolution of the cavity can be completed through two circulation modes, i.e., positive circulation and reverse circulation. When water dissolution of the cavity is performed, water first enters the bottom of the cavity and gradually dissolves from bottom to top, and the salt water concentration in the upper region is lower than that in the lower region, so the upward dissolution speed of the salt layer is higher than the lateral dissolution speed. Too fast upward dissolution is not conducive to controlling the shape and volume of the cavity during the construction of the salt cavern gas storage, so a blocking agent needs to be added to the annulus of the production casing. By using the density difference between the blocking agent and the salt water, a certain thickness of insoluble layer is formed between the salt water and the top plate of the cavity, so as to prevent water from dissolving upward, protect the top of the cavity, and promote the lateral expansion of the cavity. Nitrogen is often used as a blocking agent for water dissolution of the salt cavern gas storage because of its low cost and environmental friendliness.

[0003] Currently, the conventional nitrogen blocking dissolution process for cavity formation is as follows: after drilling is completed, a cavity string is lowered into the small hole, and nitrogen is injected as a blocking agent to form a cavity. After the designed volume is reached, the depth of the cavity string is adjusted through a maintenance operation, the blocking agent is re-injected to a new interface depth to continue the cavity formation, and the subsequent cavity formation is completed according to the stage design. During the process, the interface measurement mode of optical fiber + neutron or acoustic wave + neutron is used, and the gas-liquid interface is completely monitored by the instrument, and nitrogen is timely withdrawn and supplemented to control the position of the gas-liquid interface. SUMMARY

[0004] In order to solve the problem of large fluctuation amplitude of the gas-liquid interface and difficult control of the gas-liquid interface when nitrogen is used for blocking dissolution of the cavity, the present application proposes a gas-liquid interface control method for nitrogen blocking dissolution of the salt cavern gas storage. The following technical solutions are adopted:

[0005] A gas-liquid interface control method for nitrogen blocking dissolution of the salt cavern gas storage, comprising:

[0006] Cavity formation first stage: lowering a cavity inner pipe and a cavity outer pipe into a production casing, injecting nitrogen from the annulus between the production casing and the cavity outer pipe, and after the depth of the gas-liquid interface between the nitrogen and the brine is lowered to a first set depth, performing water injection to form a cavity until the volume of the cavity is expanded to the designed volume;

[0007] The cavity top reaming stage: part of the nitrogen is discharged until the depth of the gas-liquid interface is pushed to the second set depth, then water injection is performed to form a cavity until the average diameter of the wellbore between the top of the cavity formed in the first cavity forming stage and the second set depth is expanded by a preset width;

[0008] The cavity forming second stage: all the nitrogen is discharged, the positions of the cavity inner tube and the cavity outer tube are adjusted upwards, then nitrogen is injected until the depth of the gas-liquid interface is pushed to the third set depth, and water injection is performed to form a cavity;

[0009] The third set depth is between the first set depth and the second set depth.

[0010] The gas-liquid interface is determined by the following method:

[0011] A gas-liquid interface prediction model is established based on the nitrogen pressure, the brine discharge pressure and the brine discharge flow rate at the wellhead to predict the depth of the gas-liquid interface.

[0012] By using the above technical solution, after the first cavity forming stage, the position of the gas-liquid interface is first adjusted, then water injection is performed to form a cavity in the region between the top of the cavity in the first stage and the adjusted gas-liquid interface to achieve reaming, and the wellbore between these regions is expanded by a preset width, and then the cavity forming process in the subsequent stage is performed. In the subsequent cavity forming stage, since the diameter of the wellbore where the gas-liquid interface is located is further expanded, the up and down fluctuation amplitude of the nitrogen is reduced when the nitrogen is compressed. Thus, it is beneficial to accurately control the gas-liquid interface, slow down the dissolution speed of the salt cavity, and speed up the lateral expansion speed of the salt cavity. At the same time, since the diameter of the wellbore in the cavity forming region of the whole stage is expanded at one time, the reamed open hole section covers the second stage to the last stage. Thus, in the subsequent cavity forming stage, the positions of the cavity inner tube and the cavity outer tube only need to be adjusted once, thereby greatly saving the cavity forming process, improving the cavity forming efficiency, and reducing the cavity forming cost. In the above process, the nitrogen pressure, the brine discharge pressure and the brine discharge flow rate at the wellhead are obtained, the depth of the gas-liquid interface is predicted based on the related calculation model, the gas-liquid interface is monitored in real time, then nitrogen is supplemented or discharged according to the change of the depth of the gas-liquid interface, and the gas-liquid interface is effectively controlled.

[0013] Optionally, the gas-liquid interface prediction model is established based on the nitrogen pressure, the brine discharge pressure and the brine discharge flow rate at the wellhead to predict the depth of the gas-liquid interface, which includes:

[0014] The gas-liquid interface prediction model established based on the nitrogen pressure, the brine discharge pressure and the brine discharge flow rate at the wellhead is as follows:

[0015]

[0016] wherein, P gis the nitrogen pressure at the wellhead, M is the molar mass of nitrogen, R is the gas Planck constant, h is the gas-liquid interface depth, T is the average temperature of the gas in the wellbore, Z is the average compressibility factor of the gas in the wellbore, P wh is the brine discharge pressure at the wellhead, p b is the density of the brine, P f is the hydraulic frictional resistance of the fluid in the pipe string, and g is the acceleration of gravity.

[0017] Optionally, in the gas-liquid interface prediction model, the hydraulic frictional resistance of the fluid in the pipe string is determined by formula 3 as follows:

[0018] P f = p b gh f , formula 3

[0019] where h f is the head loss.

[0020] Optionally, in formula 4, when water injection cavity forming is performed in a positive circulation manner, the head loss is determined by formula 4 as follows:

[0021]

[0022] When water injection cavity forming is performed in a reverse circulation manner, the head loss is determined by formula 6 as follows:

[0023]

[0024] where l is the hydraulic friction factor, Q is the brine discharge flow rate at the wellhead, d i1 is the outer diameter of the cavity-forming inner pipe, d0 is the outer diameter of the cavity-forming outer pipe, and d i2 is the inner diameter of the cavity-forming inner pipe.

[0025] By adopting the above technical solution, the nitrogen pressure, the brine discharge pressure, and the brine discharge flow rate at the wellhead are obtained by setting relevant measuring instruments at the wellhead and are transmitted to the computer system in real time. In combination with the above calculation model, the gas-liquid interface depth is predicted, which is conducive to real-time monitoring of the gas-liquid interface, so as to further supplement nitrogen injection or discharge according to the change of the gas-liquid interface depth.

[0026] Optionally, the cavity volume is determined in the following manner:

[0027] Based on the volume and concentration of the brine discharged at the wellhead, the cavity volume is calculated according to formula 1 as follows:

[0028]

[0029] where V e is the cavity volume, and V bvolume of brine discharged from the wellhead, C b concentration of brine discharged from the wellhead, p s density of salt rock in the salt layer section for cavity formation, a

[0030] Optionally, the distance between the second set depth and the bottom of the production casing is 30-40 m.

[0031] By adopting the technical scheme, all the areas between the second set depth and the top of the cavity formed in the first stage are reamed. Thus, the wellbore area in the second stage and subsequent cavity formation stages is reamed at one time, avoiding repeated adjustment of the cavity formation string in subsequent reaming and cavity formation, and saving procedures.

[0032] Optionally, the preset width is 1-2 m.

[0033] By adopting the technical scheme, the diameter of the wellbore between the cavity top and the second set depth is expanded by 1-2 m compared with the original, before the cavity is formed in the second stage, so as to increase the distance between the cavity formation string and the well wall. Thus, the subsequent cavity formation stages are all located in a large-size wellbore, achieving the effect that the gas-liquid interface does not fluctuate greatly.

[0034] Optionally, the first cavity formation stage further comprises: correcting the gas-liquid interface by using neutron logging.

[0035] By adopting the technical scheme, in the first cavity formation stage, since the gas-liquid interface is located in a small-size wellbore, the optical fiber for monitoring the gas-liquid interface is easily damaged when being lowered. Therefore, the first cavity formation stage uses neutron logging for non-continuous monitoring to check and correct the gas-liquid interface.

[0036] Optionally, in the cavity top reaming stage, when water is injected for cavity formation in the positive circulation mode, the brine discharge flow rate is controlled to be not more than 60 m 3 / h.

[0037] By adopting the technical scheme, in the cavity top reaming stage, the cavity is formed in the positive circulation low water discharge mode, which is beneficial to controlling the uniform increase of the wellbore diameter, so as to ensure that the cavity expands in the subsequent stages towards the designed shape.

[0038] Optionally, the second cavity formation stage further comprises: monitoring the gas-liquid interface by using an optical fiber measuring device.

[0039] By adopting the technical scheme, when the optical fiber measuring device is used to monitor the gas-liquid interface, the optical fiber measuring device needs to be installed on the outer sidewall of the cavity formation outer pipe. Therefore, by reaming after the first cavity formation stage, the distance between the optical fiber and the well wall is increased, which significantly reduces the risk of damage to the optical fiber measuring device caused by the collapse of the insoluble interlayer, and ensures the continuous measurement of the gas-liquid interface by the optical fiber.

[0040] Based on the above technical solution, the beneficial effects of the present application compared with the prior art are:

[0041] 1、The method for controlling the gas-liquid interface by cavity top hole expansion provided in the embodiments of the present application widens the wellbore diameter at the position where the gas-liquid interface is located, and provides a buffer space for the interface fluctuation. In the subsequent cavity forming stage, since the diameter at the wellbore where the gas-liquid interface is located is further expanded, the up-down fluctuation amplitude is reduced when the nitrogen gas is compressed. Thus, it is beneficial to precisely control the gas-liquid interface, slow down the salt cavity dissolution speed, speed up the lateral expansion speed of the salt cavity, help the cavity shape expand towards the designed shape, and ensure the safety and stability of the cavity. In the whole process of water injection cavity forming, the specific depth of the gas-liquid interface is monitored in real time by the prediction model built in the computer system, which has the advantages of lower cost than the conventional monitoring instrument, and is not easy to be damaged, can be continuously monitored, and has high precision. Thus, the problems of frequent fluctuation of the gas-liquid interface and easy damage of the optical fiber are effectively solved, the problems of too fast dissolution of the salt cavity, difficult opening of the volume, low utilization rate of the salt layer, and abnormal shape of the cavity are avoided, and the application of the nitrogen gas dissolution resistance cavity forming is facilitated.

[0042] 2、The method for controlling the gas-liquid interface by cavity top hole expansion provided in the embodiments of the present application does not lower the optical fiber in the first cavity forming stage, but uses the neutron logging technology to check the depth of the gas-liquid interface. The optical fiber measuring device is lowered again to monitor the gas-liquid interface in the second cavity forming stage after the cavity top hole expansion, the distance between the optical fiber measuring device and the well wall is increased, the risk of collapse and falling of the insoluble interlayer to damage the optical fiber measuring device is significantly reduced, and the continuous measurement of the gas-liquid interface by the optical fiber is ensured. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 is a flowchart of the gas-liquid interface control method in the embodiments of the present application;

[0044] Figure 2 is a structural schematic diagram at the beginning of the first cavity forming stage in the embodiments of the present application;

[0045] Figure 3 is a structural schematic diagram at the end of the first cavity forming stage in the embodiments of the present application;

[0046] Figure 4 is a structural schematic diagram at the beginning of the cavity top hole expansion in the embodiments of the present application;

[0047] Figure 5 is a structural schematic diagram at the end of the cavity top hole expansion in the embodiments of the present application;

[0048] Figure 6 is a structural schematic diagram at the beginning of the first cavity forming stage in the embodiments of the present application.

[0049] BRIEF DESCRIPTION OF DRAWINGS

[0050] 1, wellbore; 2, inner cavity-creating tube; 3, outer cavity-creating tube; 4, nitrogen; 5, first set depth; 6, production casing; 7, second set depth; 8, third set depth; 9, brine; 10, insoluble residue; 11, pre-reaming wellbore; 12, post-reaming wellbore; 13, fiber-optic measuring device; 14, fiber-optic cable. DETAILED DESCRIPTION

[0051] Exemplary embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings; however, they are not limited to the examples shown in the drawings, but can be implemented in various forms. The embodiments are described in such a manner that one skilled in the art can fully understand the present disclosure, and the range of the present disclosure can be complete.

[0052] The exemplary embodiments will be described in detail with reference to the drawings. In the following description, the same drawing reference numerals are used to designate elements having the same functions in different drawings. The embodiments described in the following exemplary embodiments are not representative of all embodiments consistent with the present disclosure. Rather, they are merely examples of devices and methods consistent with some aspects of the present disclosure, as detailed in the appended claims.

[0053] In the description of the present disclosure, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are merely for the purpose of facilitating the description of the present disclosure and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present disclosure. In addition, the terms "first", "second", "third" are only for the purpose of description and cannot be understood as indicating or implying relative importance.

[0054] In the description of the present disclosure, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present disclosure can be understood according to the specific circumstances.

[0055] The inventor finds that nitrogen has compressibility, unlike the interface stability formed by diesel and brine, the gas-liquid interface formed by nitrogen and brine has fluctuation, and the fluctuation amplitude can reach more than ten meters. In addition, the salt layer of the domestic salt cavern generally has a difficult-to-dissolve interlayer, and during the cavity forming process, the difficult-to-dissolve interlayer will collapse and fall to the bottom of the cavity to form residues. Under such a complex salt layer, in order to obtain a safe, stable and large capacity cavity shape and realize the maximum economic benefit of the salt cavern gas storage, the position of the gas-liquid interface needs to be accurately controlled (preferably not fluctuating) when the nitrogen dissolution is blocked to form the cavity.

[0056] Among the three interface measurement methods currently adopted, the neutron logging has the characteristics of high precision (≤0.1 m), high cost, need to stop well operation and cannot realize continuous measurement; the optical fiber logging needs to be pre-installed on the outer wall of the cavity forming outer pipe, has low cost, meets the precision requirement (≤0.5 m), can realize continuous monitoring, but is easy to be damaged by the collapse of the interlayer; the acoustic wave measurement can also realize continuous monitoring, but has poor precision (the error can reach 7 m). Therefore, the conventional nitrogen dissolution blocking cavity forming technology completely relies on monitoring instruments to control the gas-liquid interface, has problems of high cost, easy to be damaged, discontinuous, poor precision and the like, cannot effectively solve the problems of large and frequent fluctuation of the gas-liquid interface and easy damage of the optical fiber, and leads to that the position of the gas-liquid interface is difficult to be efficiently controlled, the salt cavity is dissolved too fast, the volume is difficult to be opened, the salt layer utilization rate is low, the cavity shape is deformed and the like, which seriously restricts the application of the nitrogen dissolution blocking cavity forming.

[0057] In order to solve the above problems, the inventor further researches and develops to make the present application.

[0058] The embodiment of the present application provides a gas-liquid interface control method based on nitrogen dissolution blocking cavity forming of a salt cavern gas storage. Figure 1 The method comprises the following steps.

[0059] S1: cavity forming first stage: lowering the cavity inner pipe and the cavity outer pipe in the production casing, injecting nitrogen from the annulus between the production casing and the cavity outer pipe, until the depth of the gas-liquid interface between nitrogen and brine is lowered to a first set depth, then water injection cavity forming is performed until the cavity volume expands to the design volume.

[0060] S2: cavity top reaming stage: discharging part of the nitrogen, until the depth of the gas-liquid interface is raised to a second set depth, then water injection cavity forming is performed until the average diameter of the wellbore between the top of the cavity formed in the cavity forming first stage and the second set depth is expanded by a preset width.

[0061] S3: cavity forming second stage: discharging all the nitrogen, adjusting the positions of the cavity inner pipe and the cavity outer pipe upwards, then injecting nitrogen, until the depth of the gas-liquid interface is lowered to a third set depth, and water injection cavity forming is performed, wherein the third set depth is between the first set depth and the second set depth.

[0062] The gas-liquid interface is determined by:

[0063] Based on the nitrogen pressure, brine drainage pressure and brine drainage flow rate at the wellhead, a gas-liquid interface prediction model was established to predict the depth of the gas-liquid interface.

[0064] Specifically, such as Figures 2-3 As shown, in step S1, the first stage of cavity creation is carried out as follows: After drilling is completed, production casing 6 is first lowered into the wellbore 1. Inner and outer cavity creation pipes 2 and 3 are then lowered into the production casing 6 to the designed position for the first stage of cavity creation. Optionally, the distance between the designed position for the first stage of cavity creation and the bottom of the production casing 6 is determined based on the stratum and height of the cavity creation. The cavity height varies for different formation thicknesses and can be between 100 and 250 meters. For example, in this embodiment, the distance can be 110 meters. Then, nitrogen 4 is injected into the annulus between the production casing and the outer cavity creation pipe 3. The nitrogen 4 forms a gas-liquid interface with the brine 9. Nitrogen 4 injection is continued until the gas-liquid interface is pushed down to a first set depth 5. Finally, water injection is performed to create the cavity until the cavity volume expands to the designed volume for the first stage.

[0065] Specifically, the cavity volume can be calculated according to the following formula 1:

[0066]

[0067] Among them, V e is the cavity volume, m 3 ; V b is the volume of brine discharged from the wellhead, m 3 ; C b is the brine concentration discharged from the wellhead, g / L; ρ s is the density of salt rock in the cavity salt layer, kg / m 3 ; α is the mineral content of salt rock in the cavity-forming salt layer, %; β is the residue expansion coefficient.

[0068] Among them, the density of salt rock in the cavity-forming salt layer section can be obtained by density logging during drilling, the mineralization rate of salt rock in the cavity-forming salt layer section can be obtained by logging during drilling, and the residual expansion coefficient can be obtained through indoor experiments.

[0069] It should be noted that water injection and cavity creation include two methods: positive circulation water injection and reverse circulation water injection. In the first stage of cavity creation, positive circulation is used for water injection and cavity creation. Figure 3 As shown, when water is injected to form the cavity, the middle and upper parts of the cavity are brine 9, and the bottom is insoluble residue 10.

[0070] Optionally, in step S1 , the distance between the first set depth 5 and the bottom of the production casing 6 is also determined according to the layer and cavity height of the cavity. In this embodiment, the distance may be 100 m.

[0071] Further, such as Figures 4-5 As shown, in step S2, the cavity top reaming stage is carried out as follows: after completing the first stage of cavity making, part of the nitrogen 4 is discharged until the gas-liquid interface is pushed up to the second set depth 7. Then, the positive circulation method is continued to be used for water injection and cavity making. At this time, the brine discharge flow rate is controlled to no more than 60m 3 / h, low-displacement cavity making is carried out to increase the cavity diameter uniformly at a low speed until the average diameter of the wellbore 1 between the top of the cavity formed in the first stage and the second set depth 7 is expanded to a preset width, which can be 1 to 2 meters.

[0072] In an optional embodiment, in step S2, referring to Figure 4 The distance between the second set depth 7 and the bottom of the production casing can be 30 to 40 meters. As an option, it can be 30 meters.

[0073] It should be noted that in step S2, when the cavity top is expanded, the average diameter of the wellbore between the top of the cavity and the second set depth 7 is not strictly expanded into a rectangle. Since water dissolves slowly from bottom to top, the actual expansion will be trapezoidal. This embodiment shows the wellbore after expansion under ideal conditions, such as Figure 5 As shown, the average diameter of the wellbore 12 after expansion is increased by 1 to 2 meters compared with the wellbore 11 before expansion.

[0074] like Figure 6 As shown, in step S3, the second stage of cavity creation can be carried out as follows: After completing cavity top reaming, all nitrogen 4 is exhausted, and the bottom positions of the inner and outer cavity tubes 2 and 3 are first raised to 100 m. At this point, the bottoms of the inner and outer cavity tubes 2 and 3 are located at the wellbore 12 after reaming. Nitrogen 4 is then injected, pushing the gas-liquid interface between nitrogen 4 and brine 9 down to a third set depth 8. The distance between the third set depth 8 and the bottom of the production casing 6 is also determined by the stratum and cavity height of the cavity. In this embodiment, the distance can be 80 to 90 m, specifically 90 m. Water injection is then used to create the cavity using a reverse circulation method until the cavity volume reaches the second stage design volume. Continuing to create the cavity based on the cavity formed in the first stage helps expand the cavity shape toward the design shape and ensures the safety and stability of the cavity.

[0075] In an optional embodiment, a series of measuring instruments can be arranged to collect the nitrogen pressure at the wellhead, the brine discharge pressure and flow rate, and the average temperature of the gas in the wellbore, and the like, during the entire process of steps S1-S3, to establish a gas-liquid interface prediction model. Specifically, according to the equality of the pressure at the inner annular gas-liquid interface and the pressure at the outer annular gas-liquid interface, the gas-liquid interface depth can be calculated in real time based on the following formula 2:

[0076]

[0077] wherein P g is the nitrogen pressure at the wellhead, Pa; M is the molar mass of nitrogen; R is the gas Planck constant, generally taking an empirical value of 8.314 J / (mol.K); h is the gas-liquid interface depth, m; T is the average temperature of the gas in the wellbore, K; Z is the average compressibility factor of the gas in the wellbore, which can be obtained from an empirical formula; P wh is the brine discharge pressure at the wellhead, Pa; p b is the density of the brine, Kg / m 3 , and P f is the hydraulic frictional resistance of the fluid in the pipe string, Pa.

[0078] When the water injection cavity-creating is performed in a positive circulation mode, the hydraulic frictional resistance P f of the fluid in the pipe string is determined according to the following formulas 3-5:

[0079] P f = p b gh f , formula 3

[0080]

[0081]

[0082] When the water injection cavity-creating is performed in a reverse circulation mode, the hydraulic frictional resistance P f of the fluid in the pipe string is determined according to formula (3) and the following formulas 6-7:

[0083]

[0084]

[0085] wherein h f is the head loss, m; l is the hydraulic friction coefficient; Q is the brine discharge flow rate at the wellhead, m 3 / h; d i1 is the inner diameter of the outer pipe for cavity-creating, m; d i2 is the inner diameter of the inner pipe for cavity-creating, m; d0 is the outer diameter of the inner pipe for cavity-creating, m; Re is the Reynolds number; and m is the dynamic viscosity coefficient, Pa.s.

[0086] In the above formula 3 to formula 7, under the premise that the sizes of the cavity-internal pipe and the cavity-external pipe are known, since the dynamic viscosity coefficient can be obtained by an empirical formula, the Reynolds number value can be calculated by formula 5 or formula 7 first, and the hydraulic friction coefficient λ can be determined by the Reynolds number value in the following specific manner:

[0087] If Re≤2000, then

[0088] If then

[0089] If then

[0090] If then

[0091] wherein ε is the relative roughness of the pipe string; Δ is the absolute roughness of the pipe surface, the value of which is 0.14mm-0.15mm for a new pipe string and 1.0-1.8mm for an old pipe string; d is the difference between the inner diameter of the cavity-external pipe and the outer diameter of the cavity-internal pipe when the water is injected in the positive circulation mode, and the value of d is the same as the inner diameter of the cavity-internal pipe when the water is injected in the reverse circulation mode.

[0092] Therefore, based on the Reynolds number and the hydraulic friction coefficient, and in combination with formula 2-formula 7, the gas-liquid interface depth in the positive circulation mode and the reverse circulation mode can be predicted respectively.

[0093] In an optional embodiment, in step S1, the neutron logging can be used to correct the gas-liquid interface periodically during the first cavity-formation stage. The neutron logging is to obtain the formation information by measuring the scattering and absorption of neutrons in the formation. When the neutrons are emitted from the source probe and pass through the formation, they will scatter with the atomic nuclei in the formation. The intensity and energy distribution of the scattering signal can provide information about the scattering materials in the formation. Near the gas-liquid interface, the characteristics of the scattering signal will change. The neutron logging for the gas-liquid interface has high accuracy and short measurement time.

[0094] In an optional embodiment, in step S3, the fiber-optic measuring device 13 can be used to monitor the gas-liquid interface during the second cavity-formation stage. Specifically, as shown in Figure 6 the fiber-optic measuring device 13 is installed on the outer wall of the cavity-external pipe 3 before the cavity-formation pipe string is lowered, so that the fiber-optic measuring device 13 and the optical cable 14 are lowered into the production casing together with the cavity-external pipe 3. Since the wellbore width at the position of the gas-liquid interface in the second cavity-formation stage is increased during the cavity-top reaming stage, the distance between the subsequently lowered fiber-optic measuring device 13 and the wellbore 1 is increased, and the risk of the interlayer collapse damaging the optical cable 14 is reduced, thereby realizing the continuous fiber-optic measurement of the gas-liquid interface in each subsequent cavity-formation stage. Meanwhile, the neutron logging can be carried out irregularly for measurement and verification.

[0095] The gas-liquid interface control method in the embodiments of the application widens the wellbore diameter at the position where the gas-liquid interface is located by using cavity top hole expansion after the first stage of salt cavern gas storage construction, and since the larger the diameter is, the smaller the amplitude of the up-and-down fluctuation of nitrogen gas when compressed is, the fluctuation amplitude is reduced from tens of meters to within one meter, thus providing a buffer space for the gas-liquid interface fluctuation, and the gas-liquid interface control precision can be significantly improved. After the first stage, the hole is expanded and dissolved by adjusting the primary gas-liquid interface, and in the second stage and the subsequent cavity construction stage, the cavity is constructed according to the normal procedure, so that the purpose of saving the procedure and reducing the cost is achieved. In the whole process, the gas-liquid interface is monitored and controlled in real time by using a prediction model, which has the advantages of lower cost, less damage, continuous monitoring and high precision compared with conventional monitoring instruments. Thus, the problems of large and frequent fluctuation of the gas-liquid interface and easy damage of the optical fiber are effectively solved, and the problems of too fast salt cavity dissolution, difficult opening of the volume, low utilization rate of the salt layer, and abnormal shape of the cavity are avoided, which is conducive to the application of nitrogen gas blocking dissolution cavity construction.

[0096] The above description includes examples of one or more embodiments. Of course, it is impossible to describe all possible combinations of components or methods for describing the above embodiments, but those of ordinary skill in the art should recognize that various embodiments can be further combined and arranged. Therefore, the embodiments described herein are intended to cover all such changes, modifications and variations falling within the scope of the appended claims. In addition, with respect to the term "comprising" used in the specification or claims, the scope of the term is similar to the term "including", as explained by the term "including" used as a conjunction in the claims. In addition, the use of any one term "or" in the claims and the specification is intended to mean "non-exclusive or".

Claims

1. A gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage, characterized in that: include: The first stage of cavity creation: an inner cavity tube and an outer cavity tube are lowered into the production casing, nitrogen is injected from the annulus between the production casing and the outer cavity tube until the depth of the gas-liquid interface between the nitrogen and the brine is pushed down to a first set depth, and then water is injected into the cavity until the cavity volume reaches the designed volume; Cavity top reaming stage: exhaust part of the nitrogen until the depth of the gas-liquid interface is pushed up to the second set depth, and then water injection is performed to create a cavity until the average diameter of the wellbore between the top of the cavity formed in the first stage of cavity creation and the second set depth expands to a preset width; The second stage of cavity creation: exhaust all nitrogen, adjust the positions of the inner and outer cavity creation tubes upward, and then inject nitrogen until the depth of the gas-liquid interface is pushed down to the third set depth, and then perform water injection to create the cavity; wherein the third set depth is between the first set depth and the second set depth; The gas-liquid interface is determined by: Based on the nitrogen pressure, brine drainage pressure and brine drainage flow rate at the wellhead, a gas-liquid interface prediction model is established to predict the depth of the gas-liquid interface.

2. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 1 is characterized in that: The gas-liquid interface prediction model is established based on the nitrogen pressure, brine drainage pressure and brine drainage flow rate at the wellhead to predict the depth of the gas-liquid interface, including: Based on the nitrogen pressure, brine discharge pressure and brine discharge flow rate at the wellhead, the following gas-liquid interface prediction model is established: Among them, P g is the nitrogen pressure at the wellhead, M is the molar mass of nitrogen, R is the Planck constant of gas, h is the depth of gas-liquid interface, T is the average temperature of gas in the wellbore, Z is the average compressibility coefficient of gas in the wellbore, P wh is the brine discharge pressure at the wellhead, ρ b is the density of brine, P f is the hydraulic friction resistance of the fluid in the pipe string, and g is the acceleration due to gravity.

3. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 2 is characterized in that: In the gas-liquid interface prediction model, the hydraulic friction along the fluid in the pipe string is determined by the following formula 3: P f =ρ b gh f , formula 3 Among them, h f is the head loss along the way.

4. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 3 is characterized in that: In Formula 3, when water injection and cavity creation are performed in a positive circulation mode, the head loss along the process is determined by the following Formula 4: When water injection and cavity creation are performed in a reverse circulation manner, the head loss along the process is determined by the following formula 6: Among them, λ is the hydraulic friction coefficient, Q is the brine discharge flow at the wellhead, d i1 is the inner diameter of the outer tube of the cavity, d0 is the outer diameter of the inner tube of the cavity, d i2 is the inner diameter of the lumen tube.

5. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 1 is characterized in that: The cavity volume is determined by: Based on the volume and concentration of brine discharged from the wellhead, the cavity volume is calculated according to the following formula 1: Among them, V e is the cavity volume, V b is the volume of brine discharged from the wellhead, C b is the brine concentration discharged from the wellhead, ρ s is the density of salt rock in the cavity-forming salt layer, α is the mineral content of salt rock in the cavity-forming salt layer, and β is the residual expansion coefficient.

6. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 1 is characterized in that: The distance between the second set depth and the bottom of the production casing is 30 to 40 meters.

7. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 1 is characterized in that: The preset width is 1 to 2 meters.

8. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 3 is characterized in that: The first stage of cavity creation, the stage of cavity top expansion and the second stage of cavity creation also include: Neutron logging is used to verify the gas-liquid interface.

9. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 3 is characterized in that: The second stage of cavity creation further includes: monitoring the gas-liquid interface using an optical fiber measuring device.

10. The gas-liquid interface control method based on nitrogen dissolution cavitation in salt cavern gas storage according to claim 1 is characterized in that: During the cavity top expansion stage, when water injection is carried out to create the cavity, the brine discharge flow rate is controlled to no more than 60m 3 / h.

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