Geothermal well underbalanced drilling, system, and prediction and method

Abstract

The present application relates to the technical field of geothermal well, particularly to a geothermal well underbalanced drilling, system and prediction and installation method, wherein the geothermal well underbalanced drilling comprises a casing; a drill pipe coaxially arranged with the casing, an outer diameter of the drill pipe being smaller than an inner diameter of the casing, and an annulus existing between the drill pipe and the casing; and a gas injection pipe arranged outside the casing and in communication with the annulus, the gas injection pipe being used for injecting variable flow of gas to mix with drilling fluid in the annulus to form gas-liquid two-phase flow. The gas injection pipe is in communication with the annulus of the casing, the mixed density of the liquid in the annulus is changed by injecting gas into the casing, so that the bottom hole pressure is less than the formation pressure, thereby achieving the effect of underbalanced drilling, improving the drilling speed, and preventing a large amount of drilling fluid from leaking into the formation fracture.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geothermal wells, in particular to a geothermal well underbalanced drilling, system and prediction and method. BACKGROUND

[0002] In the drilling process of geothermal wells, as the drill bit continuously penetrates into the underground, the hydrostatic pressure generated by the circulating drilling fluid column in the wellbore will increase. When this hydrostatic pressure exceeds the natural pore pressure of the drilled formation, the wellbore is in a "overbalanced" state. Under the action of this high pressure difference, the drilling operation will face many challenges.

[0003] Firstly, the excessively high bottom hole pressure will exert a huge reaction force on the rock below the drill bit, making it difficult to effectively break and cut the rock, which is known as "pressure holding effect". This directly leads to a significant reduction in the cutting efficiency of the drill bit, thereby significantly slowing down the rate of penetration. The decrease in drilling speed not only prolongs the drilling cycle, but also increases the overall operation cost.

[0004] Secondly, the overbalanced state can also cause a series of downhole complex problems. The high-pressure drilling fluid and the solid particles carried by it may be forced into the formation, blocking the reservoir pores and causing irreversible damage to the natural permeability of the geothermal reservoir, thereby affecting the production capacity of geothermal resources. In addition, if the bottom hole pressure exceeds the fracture pressure of the formation, the drilling fluid will be lost in large quantities into the formation cracks, resulting in serious circulation loss, which not only wastes valuable drilling fluid, but also may cause well control risks. In some cases, the filter cake formed by the drilling fluid on the well wall may also cause the drill pipe to be stuck by differential pressure, further increasing the drilling risk and non-productive time. Therefore, effective management of the bottom hole pressure and avoidance of the negative effects of overbalance are the key to ensuring efficient and safe geothermal drilling. The purpose of the present application is to derive a calculation method for predicting and adjusting the gas injection flow rate based on the data relationship between the bottom hole pressure and the formation pressure. SUMMARY

[0005] Some simplification or omission may be made in this part as well as the abstract of the specification and the invention name to avoid obscuring the purpose of this part, the abstract of the specification and the invention name, and such simplification or omission cannot be used to limit the scope of the present application.

[0006] To solve the problems of the prior art, one object of the present application is to provide a geothermal well underbalanced drilling, which comprises a casing; a drill pipe coaxially arranged with the casing, the outer diameter of the drill pipe being smaller than the inner diameter of the casing, and an annulus existing between the drill pipe and the casing; and a gas injection pipe arranged outside the casing and in communication with the annulus, the gas injection pipe being used for injecting variable flow of gas to mix with the drilling fluid in the annulus to form a gas-liquid two-phase flow.

[0007] As a preferred scheme of the geothermal well underbalanced drilling of the present application, wherein: further comprising a base including an inlet and an outlet; the casing is provided with a through hole, the through hole is communicated with the annulus in the casing, the outlet is communicated with the through hole, and the inlet is used for connecting the gas injection pipe.

[0008] As a preferred scheme of the geothermal well underbalanced drilling of the present application, wherein: further comprising a fixing member arranged on the casing, the fixing member is provided with a limiting cavity, the inner wall of the limiting cavity is in abutment with the outer peripheral wall of the gas injection pipe, and the limiting cavity exerts a force on the gas injection pipe in a direction towards the central axis of the gas injection pipe.

[0009] Another object of the present application is to provide a geothermal well underbalanced drilling system, comprising a collection module for collecting the bottom layer depth and the water head depth of the drilling hole in real time and generating a depth signal; a calculation module for calculating the formation pressure and the bottom hole pressure in real time according to the depth signal and generating a pressure signal; a control module for judging the quantitative relationship between the bottom hole pressure and the formation pressure according to the pressure signal, generating an overbalanced state signal if the bottom hole pressure > the formation pressure, and feeding back the overbalanced state signal to the calculation module; the calculation module continues to calculate the target mixed density according to the liquid column pressure in the balanced state, and the control module generates a gas flow control signal according to the target mixed density again; and a gas injection module for adjusting the gas injection flow in real time according to the gas flow control signal.

[0010] As a preferred scheme of the geothermal well underbalanced drilling system of the present application, wherein: comprising m open depths, m is an integer greater than or equal to 2; the casing is provided with n, n casings are arranged one by one corresponding to the m open depths; the n casings are coaxial in the horizontal direction and arranged in order from outside to inside, the n casings are arranged in order from top to bottom along the vertical direction, and there is an overlapping section in the vertical direction between adjacent two casings; the annulus exists between the drill pipe and the n casings, the n annuli are communicated; and the gas injection pipe is communicated with the annulus in the casing of the first open depth.

[0011] Another object of the present application is to provide a geothermal well underbalanced drilling method, comprising drilling a first open depth and installing a matched casing in the first open depth; continuing the drilling action of a second open depth, collecting the bottom layer depth and the water head depth of the drilling hole in real time, and calculating the formation pressure and the bottom hole pressure in real time; judging the quantitative relationship between the bottom hole pressure and the formation pressure, and injecting gas into the casing in the first open depth if the bottom hole pressure > the formation pressure; adjusting the gas injection flow in real time; drilling a second open depth and installing a matched casing in the second open depth; repeating the above steps and drilling m open depths, m is an integer greater than or equal to 2.

[0012] As a preferred solution of the geothermal well underbalanced prediction and installation method, the target mixed density is calculated respectively , the gas density , the liquid volume fraction , the annular area , the flow of the drilling fluid in the annular space , the static pressure at the gas injection point and the downhole gas flow .

[0013] As a preferred solution of the geothermal well underbalanced prediction and installation method, the downhole gas flow is converted into the standard state gas flow , which is expressed as:

[0014] ;

[0015] wherein, is the standard state gas flow, is the downhole gas flow, is the average pressure, is the pressure at the wellhead standard atmospheric pressure, is the temperature at the wellhead standard atmospheric pressure, is the average temperature.

[0016] As a preferred solution of the geothermal well underbalanced prediction and installation method, the equivalent average pressure is further corrected, which is expressed as:

[0017] ;

[0018] wherein, is the average pressure, is the static pressure at the gas injection point, is the pressure at the wellhead standard atmospheric pressure.

[0019] As a preferred solution of the geothermal well underbalanced prediction and installation method, the following calculation steps are further included:

[0020] ;

[0021] ;

[0022] ;

[0023] ;

[0024] ;

[0025] ;

[0026] wherein, is the downhole gas flow rate, is the annular flow rate of the drilling fluid, is the liquid volume fraction, is the annular area, is the annular inverse speed, is the inner diameter of the casing in the open hole depth, is the outer diameter of the drill pipe, is the target mixed density, is the drilling fluid density, is the gas density, is the gas injection depth, is the bottom depth, is the formation pressure, is the water specific weight, is the water head depth.

[0027] The beneficial effects of the present application: the gas injection pipe is connected with the annulus of the casing, by injecting gas into the casing, the mixed density of the liquid in the annulus is changed, so that the bottom hole pressure is less than the formation pressure, so as to achieve the effect of underbalanced drilling, which can improve the drilling speed, and can prevent the drilling fluid from leaking into the formation cracks. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0029] Figure 1 is the structure diagram of the underbalanced drilling of the geothermal well of the present application.

[0030] Figure 2 is the structure diagram of the underbalanced drilling of the geothermal well of the present application.

[0031] Figure 3 is the matching diagram of the parts of the underbalanced drilling of the geothermal well of the present application.

[0032] Figure 4 is the sectional view of the parts of the underbalanced drilling of the geothermal well of the present application.

[0033] Figure 5 is the connection diagram of the underbalanced drilling system of the geothermal well of the present application.

[0034] Figure 6 is the structure diagram of the underbalanced drilling system of the geothermal well of the present application.

[0035] In the figure: 100, casing; 101, overlapping section; 102, annulus; 103, through hole; 200, drill pipe; 300, gas injection pipe; 400, base; 401, inlet; 402, outlet; 500, fixing piece; 501, limiting cavity; 600, collection module; 700, calculation module; 800, control module; 900, gas injection module. DETAILED DESCRIPTION

[0036] In order to make the objects, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0037] In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, and those skilled in the art can make similar generalizations without departing from the concept of the present application, therefore the present application is not limited to the specific embodiments disclosed below.

[0038] Secondly, "one embodiment" or "embodiment" referred to herein means that specific features, structures or characteristics can be included in at least one implementation of the present application. "In one embodiment" appearing in different places in the specification does not mean the same embodiment, nor is it an embodiment that is separate or alternative to other embodiments.

[0039] Embodiment 1, reference Figure 1 This embodiment is the first embodiment of the application, comprising a casing 100, a drill pipe 200 and a gas injection pipe 300.

[0040] Specifically, the drill pipe 200 is drilled in the vertical direction, first drilling a one-kilometer borehole, then setting a casing 100 in the borehole, it is worth noting that one casing 100 here refers to the combination of segmented casings 100, not a whole one-kilometer long pipe, and segmented installation can be achieved by prior art, which will not be described here.

[0041] When drilling at subsequent depths, the drill pipe 200 is coaxially arranged with the casing 100, the outer diameter of the drill pipe 200 is smaller than the inner diameter of the casing 100, and there is an annulus 102 between the drill pipe 200 and the casing 100. Due to the hollow inside of the drill pipe 200, drilling fluid is injected into the borehole, and after flowing out from the bottom of the drill pipe 200, the drilling fluid then flows from bottom to top along the vertical annulus 102.

[0042] The gas injection pipe 300 is arranged outside the casing 100, the axis of the gas injection pipe 300 is parallel to the axis of the casing 100, the annulus 102 between the drill pipe 200 and the casing 100 is communicated with the gas injection pipe 300, and the gas injection pipe 300 is used to inject variable flow of gas into the casing 100 during drilling at a subsequent depth, the gas is mixed with the drilling fluid in the annulus 102 to form a gas-liquid two-phase flow, the mixed density is reduced through such mixing, and finally the bottom hole pressure is less than the formation pressure, so that the underbalanced drilling effect is achieved, thereby the drilling speed can be improved, and the risk of drilling fluid loss can be reduced.

[0043] Embodiment 2, refer to Figures 1-4 This embodiment is a second embodiment of the application, and is based on embodiment 1.

[0044] Specifically, the base 400 is arranged on the casing 100, the base 400 comprises an inlet 401 and an outlet 402; the inlet 401 is arranged vertically, the side wall of the casing 100 is provided with a coupling, the coupling is provided with a through hole, the side wall of the casing 100 is provided with a through hole 103, the through hole 103 is communicated with the annulus 102 in the casing 100, the outlet 402 is communicated with the through hole 103, and the inlet 401 is used to connect the gas injection pipe 300; the distance between the through hole 103 and the ground surface is 500 m; a single gas injection pipe 300 (length 1 m, outer diameter 42 mm) is connected through a female joint and a male joint, the male joint is screwed into the base 400 to be fixed, forming the starting point of the gas injection channel, and the hexagonal outer wall of the female joint of the gas injection pipe 300 is convenient for tightening with a wrench, so as to ensure the sealing property.

[0045] Further, the fixing member 500 is arranged on the casing 100, the fixing member 500 is provided with a limiting cavity 501, the inner wall of the limiting cavity 501 abuts against the outer peripheral wall of the gas injection pipe 300, and the force applied by the limiting cavity 501 to the gas injection pipe 300 is directed towards the central axis of the gas injection pipe 300; after each gas injection pipe 300 is installed, the fixing member 500 of the gas injection pipe 300 is welded at a position 25 cm away from the female joint. The fixing member 500 of this embodiment is a U-shaped steel strip, which rigidly fixes the gas injection pipe 300 and the outer wall of the casing 100, so as to prevent vibration and falling off; this step is repeated, and the gas injection pipes 300 are extended one by one to a position 1 m above the ground surface, and the top end of the gas injection pipe 300 is reserved for an air compressor interface.

[0046] It is worth mentioning that the inner diameter of the casing 100 is Φ339.7mm; the collar of the casing 100 has a 30mm through hole; the base 400 of the gas injection pipe 300 is welded on the collar, and the outlet 402 is 30mm; the male joint of the gas injection pipe 300 has an outer diameter of 36mm and an inner diameter of 30mm, and is connected with the base by threads; the gas injection pipe 300 has an outer diameter of 42mm and an inner diameter of 30mm; the fixing member 500 of the gas injection pipe 300 (U-shaped steel strip, welded on the outer wall of the casing 100); the female joint of the gas injection pipe 300 has an outer diameter of 54mm and an inner diameter of 36mm, and is connected with the gas injection pipe 300 by threads; the ground air compressor interface is connected with the top end of the gas injection pipe 300 through a hose.

[0047] Embodiment 3, refer to Figure 5 This embodiment is the third embodiment of the application, and is based on embodiment 1 or embodiment 2; this embodiment provides an underbalanced drilling system for geothermal wells, which comprises a collection module 600, a calculation module 700, a control module 800 and a gas injection module 900.

[0048] Specifically, the collection module 600 is used to collect the bottom depth and the water head depth of the borehole in real time and generate a depth signal; the calculation module 700 is used to calculate the formation pressure and the bottom hole pressure in real time according to the depth signal and generate a pressure signal; the control module 800 is used to determine the quantitative relationship between the bottom hole pressure and the formation pressure according to the pressure signal, and if the bottom hole pressure > the formation pressure, a overbalanced state signal is generated and fed back to the calculation module 700; the calculation module 700 continues to calculate the target mixed density according to the liquid column pressure in the balanced state, and the control module 800 generates a gas flow control signal according to the target mixed density; the gas injection module 900 adjusts the gas injection flow in real time according to the gas flow control signal.

[0049] Specifically, the drill pipe 200 can gradually drill a borehole with m opening depths, where m is an integer ≥2; in this embodiment, m=2, and the drill pipe 200 of this embodiment can drill a borehole with two opening depths, and the casing 100 is provided with n casings, and the n casings are arranged one-to-one corresponding to the m opening depths; the casing 100 of this embodiment is provided with two casings, one casing 100 is arranged in the borehole with one opening depth after the borehole with one opening depth is completed, and one casing 100 is also arranged in the borehole with two opening depths after the borehole with two opening depths is completed.

[0050] Preferably, the n casings 100 are coaxial in the horizontal direction and arranged from outside to inside, the n casings 100 are arranged from top to bottom along the vertical direction, and there is an overlapping section 101 between two adjacent casings 100 in the vertical direction; and in the embodiment, two casings 100 are arranged in the drill hole of the first opening depth and the drill hole of the second opening depth respectively, the casing 100 in the first opening depth and the casing 100 in the second opening depth are coaxially arranged, the inner diameter of the casing 100 in the first opening depth is greater than the inner diameter of the casing 100 in the second opening depth, and there is an overlapping section 101 between the upper and lower casings 100 in the vertical direction, and the length of the overlapping section 101 is 30 meters.

[0051] Preferably, there is an annulus 102 between the drill pipe 200 and the n casings 100, and the n annuli 102 are connected; in the embodiment, there is an annulus 102 between the drill pipe 200 and the two casings 100, and the upper and lower annuli 102 are connected, so that the drilling fluid released from the bottom of the drill pipe 200 flows from bottom to top in the two annuli 102; by using the characteristic of the overlapping section 101, the upper and lower annuli 102 are connected, the continuous penetration of the gas injection pipe 300 in the drill hole of the m opening depth is realized, the gas injection process is simplified and the reliability is improved, and it is not necessary to connect each sleeve 100 in the drill hole of the m opening depth to the gas injection pipe 300 respectively; the multi-opening-time penetration is suitable for the adaptability of the geothermal well, and when the casing 100 is drilled in the subsequent opening time, no new pipeline is needed, and the gas injection can be directly drilled.

[0052] Preferably, the gas injection pipe 300 is connected to the annulus 102 in the casing 100 of the first opening depth. During the drilling process, if the bottom hole pressure is greater than the formation pressure, only by injecting gas into the annulus 102 in the casing 100 of the first opening depth through the gas injection pipe 300, the mixed density of the drilling fluid and the gas after mixing in the annulus 102 in the casing 100 of the first opening depth can be reduced, and finally the bottom hole pressure is less than the formation pressure, so that the underbalanced drilling effect is achieved.

[0053] When the drill pipe 200 is circulating drilling, compressed air (pressure 0.5-2 MPa) is injected into the gas injection pipe 300 through the gas injection module 900, and the gas enters the annulus 102 in the casing 100 of the first opening depth through the base 400.

[0054] After the gas is mixed with the drilling fluid, the liquid column density in the annulus 102 in the casing 100 of the first opening depth is reduced from 1.2 g / cm3 to 0.75-1.0 g / cm3, and the bottom hole pressure is reduced by 1.5-3 MPa, forming an underbalanced state, and the drilling speed is increased by 20%-35%.

[0055] Embodiment 4 provides a geothermal well underbalanced prediction and installation method.

[0056] S100: complete a drilling hole of a first depth and install a suitable casing 100 in the first depth.

[0057] S200: continue the drilling hole of a second depth, collect the bottom depth and the water head depth of the bottom layer of the drilling hole in real time, and calculate the formation pressure and the bottom hole pressure in real time, at this time, the bottom hole pressure is first equal to the liquid column pressure;

[0058] ;

[0059] ;

[0060] the formation pressure, the water bulk density, the water head depth, the liquid column pressure, the target mixed density (when collected for the first time = ), the gas injection depth (when collected for the first time, no gas injection, then 0 is taken for calculation), the bottom depth.

[0061] S300: judge the quantitative relationship between the bottom hole pressure and the formation pressure, if the bottom hole pressure > the formation pressure, then inject gas into the casing 100 in the first depth.

[0062] S400: adjust the gas injection flow in real time.

[0063] S401: calculate the target mixed density , the gas density , the liquid volume fraction , the annular area , the drilling fluid flow in the annulus , the static liquid pressure at the gas injection point and the downhole gas flow ;

[0064] ;

[0065] ;

[0066] ;

[0067] ;

[0068] the downhole gas flow, the drilling fluid flow in the annulus, the liquid volume fraction, the annular area, is annulus reverse speed, is inner diameter of casing 100 in the first open depth, is outer diameter of drill pipe, is target mixed density, is drilling fluid density, is gas density.

[0069] S402: The gas injection flow model includes a correction to the equivalent average pressure, expressed as:

[0070] ;

[0071] wherein, is average pressure, is static pressure at the gas injection point, is pressure at the wellhead under standard atmospheric pressure.

[0072] S403: Finally, the downhole gas flow rate is converted to the standard state gas flow rate , expressed as:

[0073] ;

[0074] wherein, is standard state gas flow rate, is downhole gas flow rate, is average pressure, is pressure at the wellhead under standard atmospheric pressure, is temperature at the wellhead under standard atmospheric pressure, is average temperature.

[0075] S500: Complete the drilling of the second open depth and install the appropriate casing 100 in the second open depth.

[0076] S600: Repeat the above steps and complete the drilling of the mth open depth, m is an integer ≥ 2.

[0077] Example 5, this example is the fifth embodiment of the invention, this example is based on example 4. This example is an embodiment of example 4 taking the calculation process of specific parameters.

[0078] S100: Complete the drilling of the first open depth of 1000 meters and install the appropriate casing 100 in the first open depth.

[0079] S200: Continue the drilling action of the second open depth, at this time the second open depth is drilled to 1100 meters, the water head is 100m below the ground surface, then the formation pressure calculation formula is: ;

[0080] wherein, This is the specific gravity of water, taken as 9.81 kN / m³. This represents the vertical distance from the groundwater level to the target point, which is 1100 meters - 100 meters = 1000 meters.

[0081] therefore: =9.81 kN / m³×1000 m=9810 kPa.

[0082] Bottom hole pressure calculation: The diameter of the casing in the first drilling section of the geothermal well is 339.7 mm, and the first drilling depth is 1000 m. The first drilling depth has been completed and 100 mm of casing has been run. The second drilling section has been drilled to a depth of 1100 m, with a drill bit diameter of 311.2 mm. The drilling fluid density is 1.2 g / cm³, and the drilling fluid density is considered to be the same at all points within the wellbore. Let's assume the bottom hole pressure equals the fluid column pressure. The formula for calculating the bottom hole pressure is: ;

[0083] in, For the target mixing density (at the time of initial acquisition) = ), The drilling fluid density is taken as 1.2 g / cm³. The depth of the gas injection port (if no gas has been injected during the first data collection). (Calculate using 0) The bottom depth is set at 1100m;

[0084] therefore: .

[0085] S300: Determine the quantitative relationship between bottom hole pressure and formation pressure. At this point, the bottom hole pressure... If the pressure is greater than the formation pressure u, the well is in an overbalanced state. Adjusting the gas injection and changing the bottom hole pressure are necessary to achieve an underbalanced drilling state. Less than the formation pressure u.

[0086] Now, let's assume the liquid column pressure at equilibrium is... ,but,

[0087] ;

[0088] The calculation yielded: ;

[0089] At this point, a critical equilibrium is reached, and further reducing the density will achieve a sub-equilibrium state.

[0090] However, when Although it did not reach underbalance, it still significantly reduced the bottom hole pressure, which helped to increase the drilling speed.

[0091] S400: Real-time adjustment of gas injection flow rate, calculation of specific parameters.

[0092] (1) Annular geometric parameters, annular area:

[0093] ;

[0094] (2) Drilling fluid flow rate and flow rate, assuming annular return speed = 1 m / s, then the flow rate of drilling fluid in the annulus is: ;

[0095] (3) Gas injection point pressure, static pressure at 500 meters, ;

[0096] (4) Due to the target mixed density: ;

[0097] (5) Gas density calculation (pressure 58.86 bar, temperature 299.4 K):

[0098] ;

[0099] In the formula: P (pressure) is the absolute pressure (unit: Pa or bar) of the gas at the gas injection point; M (molar mass) is the molar mass of air (unit: kg / mol); R (universal gas constant) is 8.314 J / (mol·K); T (temperature) is the absolute temperature (unit: K) at the gas injection point;

[0100] Value: P takes the pressure at the gas injection point (500 meters deep), which is 5.886 MPa; R takes the international standard value; T is assumed to be a geothermal gradient of 3 ℃ / 100 m, with a surface temperature of 25 ℃;

[0101] Then the temperature at the gas injection point is:

[0102] ;

[0103] Let the liquid volume fraction be , solve for , substitute into the formula to get:

[0104] ;

[0105] The solution is = 43.59%, the gas volume fraction = 56.41%;

[0106] (6) Downhole gas flow rate calculation ,

[0107] ;

[0108] (7) Equivalent average pressure correction: assuming that the gas expansion is approximately linear pressure reduction (from 5.886 MPa to 0.1 MPa), then

[0109]

[0110] (8) Convert to standard state gas flow, downhole gas flow

[0111] Convert to standard conditions,

[0112]

[0113] (ground standard temperature: 20℃),

[0114]

[0115] (ground standard atmospheric pressure).

[0116] When the gas injection amount is ≥83.2 m³ / min, the bottom hole pressure is lower than the formation pressure (9.81 MPa), and underbalanced pressure is achieved; under the underbalanced state, the mechanical drilling speed can be improved by 30%~50%, and the leakage amount is reduced by more than 70%.

[0117] The purpose of this calculation is to estimate the gas injection amount required to achieve the downhole underbalanced state. According to the calculation, the theoretical gas injection amount is about 83.2 m³ / min (standard working condition). This value is in the middle interval of the conventional air compressor unit working range (0-200 m³ / min), confirming that the existing gas injection equipment can meet the process requirements. The calculation result verifies the technical path of achieving underbalanced drilling through wellhead gas injection from the perspective of engineering feasibility.

[0118] It should be noted that the above examples are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present application, and they should be covered in the scope of the claims of the present application.​​​​​

Claims

1. An underbalanced drilling system for geothermal wells, characterized in that: include, Sleeve (100); A drill rod (200) is coaxially arranged with the casing (100), the outer diameter of the drill rod (200) is smaller than the inner diameter of the casing (100), and there is an annular space (102) between the drill rod (200) and the casing (100). The gas injection pipe (300) is located outside the casing (100) and connected to the annulus (102). The gas injection pipe (300) is used to inject variable flow rate gas to mix with drilling fluid in the annulus (102) to form a gas-liquid two-phase flow. Acquisition module (600) is used to acquire the bottom depth and water head depth of the borehole in real time; The calculation module (700) is used to calculate the formation pressure and bottom hole pressure in real time based on the bottom depth and water head depth. ； ； in, Formation pressure, For the specific gravity of water, For the bottom layer depth, The depth of the water head. This refers to the pressure at the bottom of the well. This refers to the pressure of the liquid column. For the target mixing density, For drilling fluid density, during the first sampling = , This refers to the depth of the gas injection port. No gas was injected during the initial sampling. Calculate using 0; The control module (800) is used to determine the quantitative relationship between the bottom hole pressure and the formation pressure. If the bottom hole pressure is greater than the formation pressure, it generates an over-equilibrium state signal and feeds it back to the calculation module (700). The calculation module (700) sets the liquid column pressure at equilibrium and sets the... ； Calculate the target mixing density; The control module (800) generates an airflow control signal again based on the target mixing density; The gas injection module (900) adjusts the gas injection flow rate in real time according to the airflow control signal; ； ； ； ； ； in, This refers to the downhole gas flow rate. The drilling fluid flow rate in the annulus (102) is given. It is the liquid volume fraction. Let P be the gas density, M be the absolute pressure of the gas at the injection point, R be the molar mass of air, T be the universal gas constant, and T be the absolute temperature at the injection point. Let the area of ​​the annulus (102) be the area of ​​the annulus. For the annular (102) reverse velocity, Let (100) be the inner diameter of the sleeve at one opening depth. The outer diameter of the drill pipe (200); Downhole gas flow rate Converted to standard state gas flow rate , is represented as: ； ； in, This is the standard gas flow rate. This refers to the downhole gas flow rate. Average pressure, This refers to the pressure at the wellhead under standard atmospheric pressure. The standard surface temperature The average temperature. The hydrostatic pressure at the injection point; It also includes the open depth m, where m is an integer ≥ 2; There are n sleeves (100), and each of the n sleeves (100) is set in a one-to-one correspondence with the m opening depth; The n sleeves (100) are coaxial in the horizontal direction and arranged sequentially from the outside to the inside. The n sleeves (100) are arranged sequentially from top to bottom in the vertical direction. There is an overlapping section (101) between two adjacent sleeves (100) in the vertical direction. There are annular spaces (102) between the drill pipe (200) and each of the n casings (100), and the n annular spaces (102) are connected to each other; The gas injection pipe (300) is connected to the annulus (102) within the sleeve (100) at an opening depth.

2. The underbalanced drilling system for geothermal wells as described in claim 1, characterized in that: It also includes, The base (400) includes an inlet (401) and an outlet (402); The sleeve (100) is provided with a through hole (103), the through hole (103) is connected to the annulus (102) inside the sleeve (100), the outlet (402) is connected to the through hole (103), and the inlet (401) is used to connect to the gas injection pipe (300).

3. The underbalanced drilling system for geothermal wells as described in claim 2, characterized in that: It also includes, A fixing member (500) is provided on the sleeve (100). The fixing member (500) is provided with a limiting cavity (501). The inner wall of the limiting cavity (501) abuts against the outer peripheral wall of the air injection pipe (300). The direction of the force exerted by the limiting cavity (501) on the air injection pipe (300) is toward the central axis of the air injection pipe (300).

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