Plug drilling and sand washing method for low-pressure leakage horizontal well

Abstract

The invention relates to the technical field of plug drilling and sand washing construction methods, in particular to a plug drilling and sand washing method for a low-pressure leakage horizontal well, which comprises the following steps of: calculating bottom hole pressure according to a proportion calculation model of nitrogen and circulating liquid; determining the displacement and proportion of nitrogen and circulating liquid for implementing the plug drilling and sand washing operation; according to a solid phase carrying capacity calculation model, calculating the capacity of carrying shaft sediments under the corresponding circulating liquid and nitrogen displacement; according to a ground construction pressure calculation model, the construction pumping pressure under the corresponding nitrogen and circulating liquid displacement conditions is calculated; calculating an outlet flow velocity according to an outlet flow velocity calculation model; and plug drilling and sand washing operation is carried out according to the calculated parameters. According to the plug drilling and sand washing method for the low-pressure leakage horizontal well, the problems of shaft foreign matter cleaning and gas reservoir protection after bridge-shooting combined volume fracturing of the low-pressure leakage horizontal well are solved, the shaft foreign matter of the low-pressure leakage horizontal well can be removed, a full-bore production channel of a gas well can be recovered, and the yield of the gas well can be increased.

Description

Technical Field

[0001] The invention relates to the technical field of drilling plug sand flushing construction methods, in particular to a drilling plug sand flushing method for a low-pressure leakage horizontal well. Background Art

[0002] The original formation pressure coefficient of the Upper Paleozoic natural gas reservoir in the Ordos Basin is generally between 0.8 and 0.85. After the implementation of bridge-shot combined volume fracturing transformation, the newly drilled horizontal wells have different degrees of proppant spitting, and the small drillable bridge plug channel, soluble bridge plug or incomplete dissolution have caused the gas well to fail to achieve full-bore production. In the early stage of gas well production, due to the good gas phase permeability and low or even no water content, the impact on the gas well production is relatively small. However, with the production of the gas well, the formation pressure gradually decreases and the water content gradually increases. Due to the blockage of the wellbore by sand deposition and bridge plug residues, the impact on the gas well production capacity becomes more and more prominent. The conventional drilling plug sand flushing process either cannot establish circulation or the circulating fluid displacement, viscosity, and chip carrying performance do not meet the requirements due to the low gas reservoir pressure coefficient and serious leakage. In addition, the leakage of circulating fluid into the gas layer can easily cause water lock and water sensitivity damage, resulting in a sharp drop in gas well production. Therefore, it is urgent to research and develop a process technology that can protect gas reservoirs and safely drill plug sand flushing to restore gas well production capacity. Summary of the invention

[0003] The present invention provides a drilling plug sand flushing method for a low-pressure leaking horizontal well, which overcomes the shortcomings of the above-mentioned prior art and can effectively solve the problems of wellbore foreign matter cleaning and gas reservoir protection after bridge-shot volume fracturing in a low-pressure leaking horizontal well.

[0004] The technical scheme of the present invention is realized by the following measures: a method for drilling plug sand flushing of a low-pressure leaking horizontal well, comprising calculating the bottom hole pressure according to a nitrogen and circulating fluid ratio calculation model, and when the bottom hole pressure and the formation pressure reach a balanced or underbalanced state, determining the displacement and ratio of nitrogen and circulating fluid for implementing the drilling plug sand flushing operation; calculating the ability to carry wellbore sediments under the corresponding circulating fluid and nitrogen displacement according to a solid phase carrying capacity calculation model; calculating the construction pump pressure under the corresponding nitrogen and circulating fluid displacement conditions according to a ground construction pressure calculation model; calculating the outlet flow rate according to an outlet flow rate calculation model; and implementing the drilling plug sand flushing operation according to the calculated displacement and ratio of nitrogen and circulating fluid, the ability to carry wellbore sediments, the construction pump pressure, and the outlet flow rate construction parameters.

[0005] The following are further optimizations and / or improvements to the above technical solutions:

[0006] The above nitrogen and circulating fluid ratio calculation model. The purpose of optimizing the nitrogen and circulating fluid gas-liquid ratio is to fill the coiled tubing and annulus with a certain proportion of nitrogen through the coiled tubing circulation drill plug sand flushing, so as to reduce the density of the mixed gas-liquid column in the coiled tubing and annulus, thereby reducing the pressure of the liquid column in the coiled tubing and annulus. Under the joint action of the annular liquid column pressure and the wellhead back pressure, the pressure is balanced or under-balanced with the formation pressure, avoiding the invasion of the working circulating fluid into the gas layer due to leakage, causing water lock and water sensitivity damage, and reducing or seriously affecting the gas well production.

[0007] Nitrogen is a compressible gas, and its density is closely related to pressure and ambient temperature. The compressibility coefficient of clean water is 0.0485, which is generally regarded as an incompressible fluid, meaning that its density does not change with changes in ambient temperature and pressure. Whether it is a bridge plug (insoluble residue), a proppant returned from the formation, or other solid particles, their density is also considered constant. Therefore, this model only needs to consider the relevant factors that affect the density of nitrogen to establish a mathematical model. To reduce the amount of calculation, we regard the curve from the inclination point to the target point A in the wellbore trajectory as 1 / 4 of the arc.

[0008] 1) For the vertical well section, according to the first law of thermodynamics, the calculus equation of the correlation between the wellbore flow pressure (actually the bottom hole pressure) and the well depth can be obtained:

[0009]

[0010] The density of the mixture can be expressed as:

[0011]

[0012] Solving the calculus equation, we can get the relationship between the surface throttle pressure and the hydrostatic column pressure:

[0013]

[0014] The relationship between the pressure loss from the surface to the bottom of the well and the well depth is obtained. The pressure loss is caused by friction resistance:

[0015]

[0016]

[0017]

[0018] 2) For horizontal wells, the pressure loss is related to the inclination depth of the well, that is, the sum of the inclination depth of the vertical well section + the pressure loss from the inclination point to any well depth. Considering the arc from the inclination point to the target point A as 1 / 4, solving the calculus can obtain the relationship between the pressure loss of the arc section (caused by friction resistance) and the inclination depth and pump pressure:

[0019]

[0020]

[0021]

[0022] In formulas (1) to (9), P is pressure, MPa; γ mx —Density of mixed fluid, kg / m 3 ; f—friction coefficient, dimensionless; v—flow velocity, m / s; D H —Hydraulic diameter of the flow channel, mm; g—gravitational acceleration, 9.81 N / kg; w s —Solid mass flow rate, kg / min; w l —Liquid mass flow rate, kg / min; w g —Gas mass flow rate, kg / min; Q s —Volume flow rate of solids, m 3 / min; Q l —Volume flow rate of liquid, m 3 / min; Q g —Volume flow rate of gas, m 3 / min; P hy —Hydrostatic column pressure, MPa; P s —ground throttle pressure, MPa; H—vertical depth, m; P fr1 —Friction pressure loss 1, MPa; P fr2 —Friction pressure loss 2, MPa; P fr3 ——Friction pressure loss 3, MPa; R—radius of curvature, m; I m —Maximum well inclination angle (bottom of well), radians.

[0023] When calculating the bottom hole pressure (when the value corresponding to the well depth is the bottom hole depth, then the P value corresponds to the bottom hole pressure value), prepare an Excel table and use Excel's macro command function to perform a loop calculation until the deviation of the obtained value is less than 1%.

[0024] In the above calculation model of the ratio of nitrogen to circulating liquid:

[0025]

[0026]

[0027]

[0028]

[0029]

[0030] Among them, d b—Diameter of drill bit or grinding shoe, mm; S s —Relative density of solid to air; R P —Sand blasting speed, m / h; W m —Density of solid-liquid mixture, kg / m 3 ;Q m —Volume flow rate of solid-liquid mixture, m 3 / min; S l —relative density of liquid relative to air; Q f —Formation fluid invasion flow rate, m 3 / h; S g —Relative density of gas relative to air; Q go —Gas volume flow rate under standard conditions, m 3 / min; T—temperature, K.

[0031] The above-mentioned solid-carrying capacity calculation model: When the gas, liquid and solid mixed phase flows, the solid-carrying capacity refers to the sedimentation rate of the solid phase in the gas and liquid phases. If the flow rate of the gas and liquid phases is greater than the natural sedimentation rate of the solid phase, theoretically we call it the ability to carry the solid phase. In engineering practice, for vertical wells, the flow rate of the gas and liquid phases is greater than or equal to 1.6 times the natural sedimentation rate of the solid phase, and for horizontal wells, the flow rate is 6 times the natural sedimentation rate of the solid phase, which is considered safe and feasible. Therefore, in the present invention, a mathematical model is established according to engineering practice.

[0032] For turbulent flow, the critical settling velocity of cuttings in a vertical well can be estimated using formula (10):

[0033]

[0034] The critical settling velocity of cuttings in a horizontal well can be estimated using formula (11):

[0035]

[0036] In other words, the critical solid particle diameter that a vertical well can carry is:

[0037]

[0038] The critical solid particle diameter that a horizontal well can carry is:

[0039]

[0040] In formulas (10) to (13), v sl ——critical sedimentation velocity, m / s; ρ s —Solid particle density, kg / m 3 ; Ds—equivalent diameter of solid particles, mm; ρ f=-Foam liquid density (gas and liquid mixed fluid), kg / m 3 , ρ f The value is related to the bottom hole pressure value of the calculation model of the gas-liquid ratio and the ratio of nitrogen to circulating fluid (when the value corresponding to the well depth is the bottom hole depth, then the P value corresponds to the bottom hole pressure value); when the diameter of the solid phase particle is smaller than the calculated value of formula (12) and formula (13), it is considered that it can be safely carried to the ground.

[0041] The above-mentioned ground construction pressure calculation model: the ground construction pressure is related to safety, the pressure bearing capacity of equipment and facilities, and is one of the keys to whether the process can be implemented. The ground construction pressure (construction pump pressure) is calculated in three sections in this model, namely the vertical well section, the arc section (from the inclination point to the target point A) and the horizontal section, which is closely related to factors such as the inner diameter of the casing, the outer diameter of the coiled tubing, the inner diameter of the coiled tubing, the length of the coiled tubing, the gas displacement, the liquid displacement, the solid content of the annulus, the well depth (vertical depth and inclined depth), the viscosity of the gas-liquid two-phase, and the roughness of the pipe wall. In the present invention, the above correlation factors are used to establish a mathematical model, which is also the most complex mathematical model in the present invention.

[0042] The Mohs friction coefficient is estimated using the Nikuradse friction coefficient correlation formula. To simplify the calculation, it is assumed that the roughness of the inner wall of the coiled tubing is consistent with that of its outer wall and the inner wall of the casing.

[0043]

[0044] According to the calculation model of the ratio of nitrogen to circulating liquid, the calculus equation can be obtained:

[0045]

[0046] In the formula

[0047]

[0048]

[0049] When H = 0, solving the calculus equation yields:

[0050]

[0051] From formula (18), the relationship between the wellhead back pressure in the vertical well section and the pressure at the deflection point can be obtained:

[0052]

[0053] The relationship between the top pressure and the bottom pressure between the inclination point and the target point A of the horizontal well is:

[0054]

[0055] For the horizontal segment:

[0056]

[0057] In formulas (14) to (21), d H —Hydraulic diameter of the flow channel, mm; G—geothermal gradient, ℃ / m; T s —ground temperature, K; d b —Diameter of drill bit or grinding shoe, mm; S x —Relative density of atomized fluid relative to clean water; Q x —Physical fluid flow rate, m 3 / min; e—absolute roughness of the pipe wall, mm; A—flow area, mm 2 ;P s —ground throttle pressure, MPa; P top —Top pressure, MPa; P up —Downstream pressure, MPa; P bot — Bottom hole pressure, MPa; T av —Average temperature, K; P dn —Downstream pressure, MPa; T dn —Wellbore temperature, K; S—Wellbore length of vertical directional well, m.

[0058] When calculating the ground construction pump pressure, the P in formula (21) should be calculated first. up , and then calculate the value (at this time P up =P bot ), substitute into formula (20) to calculate P top , P top The value is equal to the pressure P at the deflection point, and the construction pump pressure value can be obtained.

[0059] The above outlet velocity calculation model: The faster the gas, liquid and solid mixed phase velocity, the more serious the erosion of the wellhead facilities. The vertical jet test data show that when the velocity exceeds 20m / s, the erosion of the mixed phase flow on the steel is significantly aggravated. The gas velocity is closely related to the gas displacement, flow area and gas density (ambient pressure and temperature). This model is similar to the nitrogen and circulating liquid ratio calculation model. In engineering implementation, the mixed phase flow outlet velocity is generally controlled by controlling the back pressure and flow area to ensure construction safety.

[0060] The value of the mixture outlet velocity is equal to the mixture flow rate corresponding to the outlet temperature and pressure (back pressure), and its expression is:

[0061]

[0062] In formula (22), vo is the outlet flow rate, m / s.

[0063] The drilling plug sand flushing method for low-pressure leaking horizontal wells of the present invention solves the difficult problems of cleaning foreign matter in the wellbore and protecting the gas reservoir after volume fracturing of the low-pressure leaking horizontal well by bridge shooting. It can remove foreign matter in the wellbore of the low-pressure leaking horizontal well (drillable bridge plugs and insoluble matters of soluble bridge plugs in the wellbore, and remove formation sand, spit-back proppant and other solid phases), restore the full-diameter production channel of the gas well, and increase the gas well production. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Attached Figure 1 Schematic diagram of the inclination point, target point A and target point B in the horizontal wellbore structure. DETAILED DESCRIPTION

[0065] The present invention is not limited by the following embodiments, and the specific implementation method can be determined according to the technical solution of the present invention and the actual situation. In the process of implementing the drilling plug sand flushing method of the present invention, except for calculating the displacement and ratio of gas and circulating fluid, the ability to carry wellbore sediments, the construction pump pressure, and the outlet flow rate according to the model, the rest are implemented according to the conventional drilling plug sand flushing operation process.

[0066] The present invention will be further described below in conjunction with embodiments:

[0067] Embodiment 1: The method for drilling and sand flushing of low-pressure leaking horizontal wells comprises calculating the bottom hole pressure according to a nitrogen and circulating fluid ratio calculation model, and when the bottom hole pressure and the formation pressure reach a balanced or underbalanced state, determining the displacement and ratio of nitrogen and circulating fluid for implementing the drilling and sand flushing operation; calculating the capacity for carrying wellbore sediments under the corresponding circulating fluid and nitrogen displacement according to a solid phase carrying capacity calculation model; calculating the construction pump pressure under the corresponding nitrogen and circulating fluid displacement conditions according to a ground construction pressure calculation model; calculating the outlet flow rate according to the outlet flow rate calculation model; and implementing the drilling and sand flushing operation according to the calculated displacement and ratio of nitrogen and circulating fluid, the capacity for carrying wellbore sediments, the construction pump pressure, and the outlet flow rate construction parameters.

[0068] Embodiment 2: As an optimization of the above embodiment, a calculation model for the ratio of nitrogen to circulating liquid: Optimizing the gas-liquid ratio of nitrogen to circulating liquid aims to fill the coiled tubing and annulus with a certain proportion of nitrogen through the coiled tubing circulation drill plug sand flushing, so as to reduce the density of the mixed gas-liquid column in the coiled tubing and annulus, thereby reducing the pressure of the liquid column in the coiled tubing and annulus. Under the joint action of the annular liquid column pressure and the wellhead back pressure, a balanced or under-balanced state is reached with the formation pressure, thereby avoiding the invasion of the working circulating fluid into the gas layer due to leakage, causing water lock and water-sensitive damage, and reducing or seriously affecting the gas well production.

[0069] The circulating fluid is clean water or an aqueous solution or fracturing flowback fluid with 0.5% conventional clay stabilizer added.

[0070] Nitrogen is a compressible gas, and its density is closely related to pressure and ambient temperature. The compressibility coefficient of clean water is 0.0485, and it is generally regarded as an incompressible fluid, which means that its density does not change with changes in ambient temperature and pressure. Whether it is a bridge plug (insoluble residue), a proppant or other solid particles in the formation, its density is also considered constant. Therefore, this model only needs to consider the relevant factors that affect the density of nitrogen to establish a mathematical model. In order to reduce the amount of calculation, we regard the curve from the inclination point to the target point A in the wellbore trajectory as 1 / 4 of the arc (see Figure 1 ).

[0071] 1) For the vertical well section, according to the first law of thermodynamics, the calculus equation of the correlation between the wellbore flow pressure (actually the bottom hole pressure) and the well depth can be obtained:

[0072]

[0073] The density of the mixture can be expressed as:

[0074]

[0075] Solving the calculus equation, we can get the relationship between the ground throttling pressure and the hydrostatic column pressure (without considering the friction resistance):

[0076]

[0077] The relationship between the pressure loss from the ground to the bottom of the well (caused by friction resistance) and the well depth is obtained:

[0078]

[0079]

[0080]

[0081] 2) For horizontal wells, the pressure loss is related to the inclination depth of the well, that is, the sum of the inclination depth of the vertical well section + the pressure loss from the inclination point to any well depth. Considering the arc from the inclination point to the target point A as 1 / 4, solving the calculus can obtain the relationship between the pressure loss of the arc section (caused by friction resistance) and the inclination depth and pump pressure:

[0082]

[0083]

[0084]

[0085] In formulas (1) to (9), P is pressure, MPa; γ mx —Density of mixed fluid, kg / m 3; f—friction coefficient, dimensionless; v—flow velocity, m / s; D H —Hydraulic diameter of the flow channel, mm; g—gravitational acceleration, 9.81 N / kg; w s —Solid mass flow rate, kg / min; w l —Liquid mass flow rate, kg / min; w g —Gas mass flow rate, kg / min; Q s —Volume flow rate of solids, m 3 / min; Q l —Volume flow rate of liquid, m 3 / min; Q g —Volume flow rate of gas, m 3 / min; P hy —Hydrostatic column pressure, MPa; P s —ground throttle pressure, MPa; H—vertical depth, m; P fr1 —Friction pressure loss 1, MPa; P fr2 —Friction pressure loss 2, MPa; P fr3 ——Friction pressure loss 3, MPa; R—radius of curvature, m; I m —Maximum well inclination angle (bottom of well), radians.

[0086] When calculating the bottom hole pressure (when the value corresponding to the well depth is the bottom hole depth, then the P value corresponds to the bottom hole pressure value), prepare an Excel table and use Excel's macro command function to perform a loop calculation until the deviation of the obtained value is less than 1%.

[0087] In the calculation model of the ratio of nitrogen to circulating liquid:

[0088]

[0089]

[0090]

[0091]

[0092]

[0093] Among them, d b —Diameter of drill bit or grinding shoe, mm; S s —Relative density of solid to air; R P —Sand blasting speed, m / h; W m —Density of solid-liquid mixture, kg / m 3 ;Q m —Volume flow rate of solid-liquid mixture, m 3 / min; S l—relative density of liquid relative to air; Q f —Formation fluid invasion flow rate, m 3 / h; S g —Relative density of gas relative to air; Q go —Gas volume flow rate under standard conditions, m 3 / min; T—temperature, K.

[0094] Embodiment 3: As an optimization of the above embodiment, the calculation model of the ability to carry solid phase: When the gas, liquid and solid mixed phases flow, the ability to carry solid phase refers to the sedimentation rate of the solid phase in the gas and liquid phases. If the flow rate of the gas and liquid phases is greater than the natural sedimentation rate of the solid phase, theoretically we call it the ability to carry solid phase. In engineering practice, for vertical wells, the flow rate of the gas and liquid phases is greater than or equal to 1.6 times the natural sedimentation rate of the solid phase, and for horizontal wells, the flow rate is 6 times the natural sedimentation rate of the solid phase, which is considered safe and feasible. Therefore, in the present invention, a mathematical model is established according to engineering practice.

[0095] For turbulent flow, the critical settling velocity of cuttings in a vertical well can be estimated using formula (10):

[0096]

[0097] The critical settling velocity of cuttings in a horizontal well can be estimated using formula (11):

[0098]

[0099] In other words, the critical solid particle diameter that a vertical well can carry is:

[0100]

[0101] The critical solid particle diameter that a horizontal well can carry is:

[0102]

[0103] In formulas (10) to (13), v sl ——critical sedimentation velocity, m / s; ρ s —Solid particle density, kg / m 3 ; Ds—equivalent diameter of solid particles, mm; ρ f =-Foam liquid density (gas and liquid mixed fluid), kg / m 3 , ρ f The value is related to the bottom hole pressure value of the calculation model of the gas-liquid ratio and the ratio of nitrogen to circulating fluid (when the value corresponding to the well depth is the bottom hole depth, then the P value corresponds to the bottom hole pressure value); when the diameter of the solid phase particle is smaller than the calculated value of formula (12) and formula (13), it is considered that it can be safely carried to the ground.

[0104] Embodiment 4: As an optimization of the above embodiment, the ground construction pressure calculation model: the ground construction pressure is related to safety, the pressure bearing capacity of equipment and facilities, and is one of the keys to whether the process can be implemented. The ground construction pressure (construction pump pressure) is calculated in three sections in this model, namely the vertical well section, the arc section (from the inclination point to the target point A) and the horizontal section, which is closely related to the inner diameter of the casing, the outer diameter of the continuous pipe, the inner diameter of the continuous pipe, the length of the continuous pipe, the gas displacement, the liquid displacement, the solid content of the annulus, the well depth (vertical depth and inclined depth), the viscosity of the gas-liquid two-phase, the roughness of the pipe wall and other factors. In the present invention, the above correlation factors are used to establish a mathematical model, which is also the most complex mathematical model in the present invention.

[0105] The Mohs friction coefficient is estimated using the Nikuradse friction coefficient correlation formula. To simplify the calculation, it is assumed that the roughness of the inner wall of the coiled tubing is consistent with that of its outer wall and the inner wall of the casing.

[0106]

[0107] According to the calculation model of the ratio of nitrogen to circulating liquid, the calculus equation can be obtained:

[0108]

[0109] In the formula

[0110]

[0111]

[0112] When H = 0, solving the calculus equation yields:

[0113]

[0114] From formula (18), the relationship between the wellhead back pressure in the vertical well section and the pressure at the deflection point can be obtained:

[0115]

[0116] The relationship between the top pressure and the bottom pressure between the inclination point and the target point A of the horizontal well is:

[0117]

[0118] For the horizontal segment:

[0119]

[0120] In formulas (14) to (21), d H —Hydraulic diameter of the flow channel, mm; G—geothermal gradient, ℃ / m; T s —ground temperature, K; d b—Diameter of drill bit or grinding shoe, mm; S x —Relative density of atomized fluid relative to clean water; Q x —Physical fluid flow rate, m 3 / min; e—absolute roughness of the pipe wall, mm; A—flow area, mm 2 ;P s —ground throttle pressure, MPa; P top —Top pressure, MPa; P up —Downstream pressure, MPa; P bot — Bottom hole pressure, MPa; T av —Average temperature, K; P dn —Downstream pressure, MPa; T dn —Wellbore temperature, K; S—Wellbore length of vertical directional well, m.

[0121] When calculating the ground construction pump pressure, the P in formula (21) should be calculated first. up , and then calculate the value (at this time P up =P bot ), substitute into formula (20) to calculate P top , P top The value is equal to the pressure P value at the deflection point (Formula 19), and the construction pump pressure value can be obtained.

[0122] Example 5: As an optimization of the above example, the outlet flow rate calculation model: the faster the gas, liquid and solid mixed phase flow rate, the more serious the erosion of the wellhead facilities. The vertical jet experimental data show that when the flow rate exceeds 20m / s, the erosion of the mixed phase flow on the steel is significantly aggravated. The gas flow rate is closely related to the gas displacement, flow area and gas density (ambient pressure and temperature). This model is similar to the nitrogen and circulating liquid ratio calculation model. In engineering implementation, the mixed phase flow outlet flow rate is generally controlled by controlling the back pressure and flow area to ensure construction safety.

[0123] The value of the mixture outlet velocity is equal to the mixture flow rate corresponding to the outlet temperature and pressure (back pressure), and its expression is:

[0124]

[0125] In formula (22), vo is the outlet flow rate, m / s.

[0126] The low-pressure leakage horizontal well drilling plug sand flushing method of the present invention was tested and applied for 5 wells in the area of ​​Sulige gas field, with a corresponding pressure coefficient of 0.5 to 0.85, and 3.6-6.5m3 / single well of spitted-back proppant was cleaned out, effectively relieving wellbore blockage, restoring or increasing gas well production by 10,000 cubic meters / day to 26,000 cubic meters / day, achieving good economic benefits.

[0127] Through the test and application of 5 wells, the 4 theoretical calculation models of the low-pressure leakage horizontal well drilling plug sand flushing method of the present invention are accurate and reliable, and the configuration of downhole tools and ground equipment and facilities meets the process requirements. The following will be explained with one of the test cases.

[0128] Test Case

[0129] (1) Collect core data of the target well: current formation pressure or current formation pressure coefficient, oil, gas and water production, maximum solid density, maximum solid diameter, casing data, gas well body structure and wellbore trajectory, whether there are casing and other wellbore integrity defects that affect the passage of tools, etc.

[0130] (2) Calculate the nitrogen and circulating fluid displacement and ratio based on the target well core data and the nitrogen and circulating fluid ratio calculation model; verify whether the conditions for safely carrying the solid phase in the wellbore to the ground are met based on the solid phase carrying capacity calculation model; predict and calculate the construction pump pressure under the corresponding nitrogen and circulating fluid displacement conditions based on the ground construction pressure calculation model; calculate the outlet flow rate based on the outlet flow rate calculation model, and verify whether the conditions for controlling the outlet flow rate within 20m / s through back pressure and flow area are met. Finally, determine the reasonable nitrogen and circulating fluid displacement and the corresponding construction pump pressure, the ability to carry wellbore sediments, and the outlet liquid velocity range.

[0131] (3) According to the estimated test parameters calculated in (2), match the downhole tools and ground equipment and facilities. The downhole tools mainly include grinding shoes, air screws, low-speed rotary flushing heads, one-way valves, safety joints and continuous pipe connectors that match the inner diameter of the casing and the test parameters; the surface equipment includes continuous pipe vehicles, liquid circulation pump vehicles, nitrogen pump vehicles, wellheads and accessories (flowback process), ground throttling and well killing manifolds, combustion tanks, sedimentation tanks, liquid storage tanks, etc. that are suitable for the inner diameter of the casing, well depth and test parameters. The connection method of the downhole tools and ground equipment and facilities is connected in accordance with the requirements of conventional drilling and sand flushing operations.

[0132] ① Low-speed rotating flushing head: SC287 flushing head produced by Stoneage Company of the United States;

[0133] ② Air screw: K5LZ73×7.0 air screw produced by Beijing Petroleum Machinery Co., Ltd.

[0134] ③ Grinding shoe: non-standard parts, Φ91mm flat-bottomed grinding shoe produced by Tuha Underground Company is selected;

[0135] ④ Other downhole tools: one-way valves, safety joints and coiled tubing connectors are all manufactured by Siji Saiwa Company;

[0136] ⑤ Continuous pipe truck: equipped with Φ50.8mm×6000m continuous pipe;

[0137] ⑥ Circulation pump truck: 2 2000-type fracturing trucks, minimum displacement 0.3m 3 / min, maximum displacement 1m 3 / min, 1 in use and 1 in reserve to prevent emergencies;

[0138] ⑦ Nitrogen pump truck: Use 50MPa nitrogen generator truck produced by Shandong Kerui Group, with nitrogen displacement of 10m 3 / min to 20m 3 / min;

[0139] ⑧Wellhead: main diameter 130mm, pressure rating 105MPa, double-side diameter 65mm;

[0140] ⑨Throttle and well-killing manifold: 35MPa, 12 valves, double nozzles, 65mm diameter, meeting the requirements of double inlet and double outlet;

[0141] ⑩ Combustion tanks, sedimentation tanks, liquid storage tanks, etc. rely on and cooperate with the on-site resources of the gas testing team;

[0142] ⑾Circulating fluid: Prepare 200m3 of fracturing flowback fluid with drag reducer added. 3 , reduce construction pressure and protect nitrogen production equipment.

[0143] Field test and effect analysis

[0144] The XX-XX-7H2 well in the Sulige gas field in the Ordos Basin was selected for field testing. This well is a development horizontal well in the Sulige gas field. The well was drilled in the stratum box 8, with a vertical depth of 2860m and an oblique depth of 4976m. In 2023, 14 stages of fracturing transformation were completed, with a total liquid volume of 10212.9m 3 , spraying and draining 367m 3 , gas lift drainage 345m 3 , cumulative discharge 712m 3 , the return rate is 6.97% and no natural gas is found. The measured current formation pressure coefficient is 0.80 to 0.82, and the wellbore is suspected to be blocked. In order to avoid the secondary contamination of the reservoir caused by the large amount of leakage of the conventional drilling plug sand flushing circulating fluid, it was decided to test the continuous pipe mixed nitrogen drilling plug sand flushing process (the drilling plug sand flushing method of the present invention) in this well. Considering that the well was not completed for a long time due to fracturing, the soluble bridge plug was not completely dissolved, and the downhole tool combination of air screw + grinding shoe was used to speed up the test progress.

[0145] (1) Test process: From April 23 to April 27, 2023, the drilling and removal of 13-level soluble bridge plugs in the entire well was completed, and 6.5m of proppant spitted out of the formation was cleared 3 The gas test results showed that the well had a stable gas production of 26,600 cubic meters per day, a water production of 40 cubic meters per day, and an unobstructed natural gas flow of 432,000 cubic meters per day.

[0146] (2) Effect analysis

[0147] ①Gas-liquid ratio: Designed liquid displacement 0.32m 3 / min to 0.33m 3 / min, nitrogen displacement 123m 3 / min to 13m 3 / min, actual liquid displacement 0.31min to 0.33m 3 / min, nitrogen displacement 13min to 14m 3 / min, the outlet gas and liquid are continuous phases, intermittent proppant spitting is seen, and the liquid displacement is stable at 0.33min to 0.34m 3 / min, slightly larger than the inlet liquid displacement, indicating that the bottom hole pressure is basically balanced with the current formation pressure. The experiment proves that the calculation model of nitrogen and circulating fluid ratio is accurate and reliable;

[0148] ② Ground construction pressure: The predicted construction pump pressure is 39MPa to 42MPa (without drag reducer), which is close to the pressure limit of the nitrogen generator. In order to ensure that the nitrogen generator can operate for a long time, 0.1% to 0.2% drag reducer is added to the circulating fluid. The actual construction pump pressure is 32MPa to 35MPa, which means that even if the gas and liquid in the continuous pipe are dispersed, there is still a certain drag reduction effect after adding the drag reducer. It also shows that the ground construction pressure calculation model is relatively accurate;

[0149] ③ Solid-carrying capacity: The intermittent sand discharge at the outlet proves that there is a slug-like blockage of proppant in the wellbore. The carrying capacity of ceramsite (density 2.9 g / cm 3 The diameter of the ceramsite is 1.26mm to 1.45mm, which means that any diameter smaller than this value can be safely returned to the surface. The test confirmed that the ceramsite of 20 to 40 mesh (particle size of 0.425mm to 0.812mm) used in the well fracturing was safely returned to the surface, totaling 6.5m 3 The experiment confirmed that the calculation model of the solid phase carrying capacity is accurate and reliable.

[0150] ④ Outlet flow rate: Based on the wellhead back pressure of 0.5MPa and the flow channel diameter of 65mm single-channel outlet, the outlet flow rate is calculated to be 15.97m / s to 16.25m / s, and the flow rate that does not exceed 20m / s causes obvious erosion of steel. During the test, the wellhead back pressure was 0.7MPa to 1.0MPa, which was in line with the expected value. After the well was completed, the wellhead drainage four-way, the inner wall of the outlet pipeline, and the inner wall of the elbow were checked for obvious signs of erosion. Due to the lack of corresponding technical means to measure the outlet flow rate, the control effect of the outlet flow rate is only intuitively judged by comparing the erosion of the inner wall of the outlet pipeline and other facilities, which is also the fundamental purpose of controlling the outlet flow rate. This experiment proves that the outlet flow rate calculation model is accurate and reliable.

[0151] The above technical features constitute the embodiments of the present invention, which have strong adaptability and implementation effect. Non-essential technical features can be added or reduced according to actual needs to meet the requirements of different situations.

Claims

1. A method for drilling and sand flushing of low-pressure leakage horizontal wells, characterized in that It includes calculating the bottom hole pressure based on the nitrogen and circulating fluid ratio calculation model, and when the bottom hole pressure and the formation pressure reach a balanced or underbalanced state, determining the displacement and ratio of nitrogen and circulating fluid for implementing the drilling plug sand flushing operation; calculating the ability to carry wellbore sediments under the corresponding circulating fluid and nitrogen displacement based on the solid phase carrying capacity calculation model; calculating the construction pump pressure under the corresponding nitrogen and circulating fluid displacement conditions based on the ground construction pressure calculation model; calculating the outlet flow rate based on the outlet flow rate calculation model; implementing the drilling plug sand flushing operation based on the calculated displacement and ratio of nitrogen and circulating fluid, the ability to carry wellbore sediments, the construction pump pressure, and the outlet flow rate construction parameters.

2. The method for drilling and sand flushing of low-pressure leakage horizontal wells according to claim 1, characterized in that Calculation model for the ratio of nitrogen to circulating liquid: 1) For the vertical well section, according to the first law of thermodynamics, the calculus equation of the correlation between wellbore flow pressure and well depth is obtained: The density of the mixture is expressed as: Solving the calculus equation, we can get the relationship between the surface throttle pressure and the hydrostatic column pressure: The relationship between the pressure loss from the surface to the bottom of the well and the well depth is obtained. The pressure loss is caused by friction resistance: 2) For horizontal wells, the pressure loss is related to the inclination depth of the well, that is, the sum of the inclination depth of the vertical well section + the pressure loss from the inclination point to any well depth; for the distance from the inclination point to the target point A, consider it as 1 / 4 of the arc, and solve the calculus to get the relationship between the pressure loss of the arc section and the inclination depth and pump pressure: In formulas (1) to (9), P is pressure, MPa; γ mx —Density of mixed fluid, kg / m 3 ; f—friction coefficient, dimensionless; v—flow velocity, m / s; h—well depth, m; D H —Hydraulic diameter of the flow channel, m; g—gravitational acceleration, 9.81 N / kg; w s —Solid mass flow rate, kg / min; w l —Liquid mass flow rate, kg / min; w g —Gas mass flow rate, kg / min; Q s —Volume flow rate of solids, m 3 / min; Q l —Volume flow rate of liquid, m 3 / min; Q g —Volume flow rate of gas, m 3 / min; P hy —Hydrostatic column pressure, MPa; P s —ground throttle pressure, MPa; H—vertical depth, m; P fr1 —Friction pressure loss 1, MPa; P fr2 —Friction pressure loss 2, MPa; P fr3 ——Friction pressure loss 3, MPa; R—radius of curvature, m; I m —Maximum well inclination angle at the bottom of the well, in radians.

3. The method for drilling and sand flushing of a low-pressure leakage horizontal well according to claim 1 or 2, characterized in that In the calculation model of the ratio of nitrogen to circulating liquid: Among them, d b —Diameter of drill bit or grinding shoe, mm; S s —Relative density of solid to air; R P —Sand blasting speed, m / h; W m —Density of solid-liquid mixture, kg / m 3 ;Q m —Volume flow rate of solid-liquid mixture, m 3 / min; S l —relative density of liquid relative to air; Q f —Formation fluid invasion flow rate, m 3 / h; S g —Relative density of gas relative to air; Q go —Gas volume flow rate under standard conditions, m 3 / min; T—temperature, K.

4. The method for drilling and sand flushing of a low-pressure leakage horizontal well according to claim 1 or 2, characterized in that Calculation model of solid phase carrying capacity: For turbulent flow, the critical settling velocity of cuttings in a vertical well is estimated using formula (10): The critical settling velocity of cuttings in a horizontal well is estimated using formula (11): In other words, the critical solid particle diameter that a vertical well can carry is: The critical solid particle diameter that a horizontal well can carry is: In formulas (10) to (13), v sl ——critical sedimentation velocity, m / s; ρ s —Solid particle density, kg / m 3 ; Ds—equivalent diameter of solid particles, mm; ρ f =-Foam liquid density (gas and liquid mixed fluid), kg / m 3 , ρ f The value, gas-liquid ratio and bottom hole pressure value of the calculation model of the ratio of nitrogen to circulating fluid are related; when the diameter of the solid phase particle is smaller than the calculated value of formula (12) and formula (13), it is considered to be safely carried to the surface.

5. The method for drilling and sand flushing of low-pressure leakage horizontal wells according to claim 3 is characterized in that Calculation model of solid phase carrying capacity: For turbulent flow, the critical settling velocity of cuttings in a vertical well is estimated using formula (10): The critical settling velocity of cuttings in a horizontal well is estimated using formula (11): In other words, the critical solid particle diameter that a vertical well can carry is: The critical solid particle diameter that a horizontal well can carry is: In formulas (10) to (13), v sl —critical sedimentation velocity, m / s; ρ s —Solid particle density, kg / m 3 ; Ds—equivalent diameter of solid particles, mm; ρ f =-Foam liquid density (gas and liquid mixed fluid), kg / m 3 , ρ f The value, gas-liquid ratio and bottom hole pressure value of the calculation model of the ratio of nitrogen to circulating fluid are related; when the diameter of the solid phase particle is smaller than the calculated value of formula (12) and formula (13), it is considered to be safely carried to the surface.

6. The method for drilling and sand flushing of a low-pressure leakage horizontal well according to claim 1, 2 or 5, characterized in that Ground construction pressure calculation model: In this model, the ground construction pressure is calculated in three sections, namely the vertical well section, the arc section and the horizontal section. The arc section is from the inclination point to the A target point. The Mohs friction coefficient is estimated by using the Nikuradse friction coefficient correlation formula. To simplify the calculation, it is assumed that the roughness of the inner wall of the coiled tubing is consistent with that of its outer wall and the inner wall of the casing. The calculus equation is obtained based on the calculation model of the ratio of nitrogen to circulating liquid: In the formula When H = 0, solving the calculus equation yields: The relationship between the wellhead back pressure in the vertical well section and the pressure at the deflection point is obtained from formula (18): The relationship between the top pressure and the bottom pressure between the inclination point and the target point A of the horizontal well is: For the horizontal segment: In formulas (14) to (21), d H —Hydraulic diameter of the flow channel, mm; G—geothermal gradient, ℃ / m; T s —ground temperature, K; d b —Diameter of drill bit or grinding shoe, mm; S x —Relative density of atomized fluid relative to clean water; Q x —Physical fluid flow rate, m 3 / min; e—absolute roughness of the pipe wall, mm; A—flow area, mm 2 ;P s —ground throttle pressure, MPa; P top —Top pressure, MPa; P up —Downstream pressure, MPa; P bot — Bottom hole pressure, MPa; T av —Average temperature, K; P dn —Downstream pressure, MPa; T dn —Wellbore temperature, K; S—Wellbore length of vertical directional well, m. When calculating the ground construction pump pressure, the P in formula (21) should be calculated first. up , at this time P up =P bot , and then substitute the calculated value into formula (20) to calculate P top , P top The value is equal to the pressure P at the deflection point, and the construction pump pressure value can be obtained.

7. The method for drilling and sand flushing of low-pressure leakage horizontal wells according to claim 3 is characterized in that Ground construction pressure calculation model: In this model, the ground construction pressure is calculated in three sections, namely the vertical well section, the arc section and the horizontal section. The arc section is from the inclination point to the A target point. The Mohs friction coefficient is estimated by using the Nikuradse friction coefficient correlation formula. To simplify the calculation, it is assumed that the roughness of the inner wall of the coiled tubing is consistent with that of its outer wall and the inner wall of the casing. The calculus equation is obtained based on the calculation model of the ratio of nitrogen to circulating liquid: In the formula When H = 0, solving the calculus equation yields: The relationship between the pump pressure in the vertical well section and the pressure at the deflection point is obtained from formula (18): The relationship between the top pressure and the bottom pressure between the inclination point and the target point A of the horizontal well is: For the horizontal segment: In formulas (14) to (21), d H —Hydraulic diameter of the flow channel, mm; G—geothermal gradient, ℃ / m; T s —ground temperature, K; d b —Diameter of drill bit or grinding shoe, mm; S x —Relative density of atomized fluid relative to clean water; Q x —Physical fluid flow rate, m 3 / min; e—absolute roughness of the pipe wall, mm; A—flow area, mm 2 ;P s —ground throttle pressure, MPa; P top —Top pressure, MPa; P up —Downstream pressure, MPa; P bot — Bottom hole pressure, MPa; T av —Average temperature, K; P dn —Downstream pressure, MPa; T dn —Wellbore temperature, K; S—Wellbore length of vertical directional well, m. When calculating the ground construction pump pressure, the P in formula (21) should be calculated first. up , and then calculate the value, at this time P up =P bot , substitute into formula (20) to calculate P top , P top The value is equal to the pressure P at the deflection point, and the construction pump pressure value can be obtained.

8. The method for drilling and sand flushing of low-pressure leakage horizontal wells according to claim 4, characterized in that Ground construction pressure calculation model: In this model, the ground construction pressure is calculated in three sections, namely the vertical well section, the arc section and the horizontal section. The arc section is from the inclination point to the A target point. The Mohs friction coefficient is estimated by using the Nikuradse friction coefficient correlation formula. To simplify the calculation, it is assumed that the roughness of the inner wall of the coiled tubing is consistent with that of its outer wall and the inner wall of the casing. The calculus equation is obtained based on the calculation model of the ratio of nitrogen to circulating liquid: In the formula When H = 0, solving the calculus equation yields: The relationship between the pump pressure in the vertical well section and the pressure at the deflection point is obtained from formula (18): The relationship between the top pressure and the bottom pressure between the inclination point and the target point A of the horizontal well is: For the horizontal segment: In formulas (14) to (21), d H —Hydraulic diameter of the flow channel, mm; G—geothermal gradient, ℃ / m; T s —ground temperature, K; d b —Diameter of drill bit or grinding shoe, mm; S x —Relative density of atomized fluid relative to clean water; Q x —Physical fluid flow rate, m 3 / min; e—absolute roughness of the pipe wall, mm; A—flow area, mm 2 ;P s —ground throttle pressure, MPa; P top —Top pressure, MPa; P up —Downstream pressure, MPa; P bot — Bottom hole pressure, MPa; T av —Average temperature, K; P dn —Downstream pressure, MPa; T dn —Wellbore temperature, K; S—Wellbore length of vertical directional well, m. When calculating the ground construction pump pressure, the P in formula (21) should be calculated first. up , and then calculate the value, at this time P up =P bot , substitute into formula (20) to calculate P top , P top The value is equal to the pressure P at the deflection point, and the construction pump pressure value can be obtained.

9. The method for drilling and sand flushing of a low-pressure leaking horizontal well according to claim 1, 2, 5, 7 or 8, characterized in that Outlet velocity calculation model: The outlet velocity of the mixture is equal to the mixture flow rate corresponding to the outlet temperature and back pressure, and its expression is: In formula (22), vo is the outlet flow rate, m / s.

10. The method for drilling and sand flushing of low-pressure leakage horizontal wells according to claim 6, characterized in that Outlet velocity calculation model: The outlet velocity of the mixture is equal to the mixture flow rate corresponding to the outlet temperature and back pressure, and its expression is: In formula (22), vo is the outlet flow rate, m / s.

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