A system and method for inverting the volume of formation karst caverns during well gas invasion.

Abstract

This invention relates to a system and method for inverting formation cavity volume during well gas invasion, belonging to the field of petroleum drilling engineering technology. The system includes a wellbore throttling and circulation pressure control section, a throttling response parameter acquisition section, and multiphase flow simulation software. Using this system, three wellbore throttling response parameters—wellhead back pressure, inlet flow rate, and outlet flow rate—can be acquired. Then, based on a multiphase flow model, bottom hole pressure, gas dissolution rate, and gas invasion flow rate are predicted. Finally, the formation cavity volume is inverted using a reservoir model and gas state equations. This method fully utilizes field-obtainable parameters, eliminating the need for extensive data training, and is simple, efficient, and real-time. Furthermore, the method acquires relevant parameters during pressure control measures, which helps to curb gas invasion development. This invention enables the calculation of formation cavity volume after gas invasion occurs in the wellbore, providing theoretical support for decision-making regarding pressure-controlled drilling and well control measures in such formations.

Description

Technical Field

[0001] This invention relates to a system and method for inverting the volume of formation karst caverns during drilling gas invasion, belonging to the field of petroleum drilling engineering technology. Background Technology

[0002] With the sustained and rapid development of my country's economy, the total demand for energy has been increasing year by year, and the development of oil and gas resources is gradually moving towards deeper formations. Carbonate reservoirs in regions such as the Tarim Basin and Sichuan Basin in my country have developed typical cavernous reservoirs, characterized by large reservoir space and high permeability, possessing significant oil and gas development value. However, the distribution of cavernous formations is extremely uneven, exhibiting strong heterogeneity, and these formations are typically structurally complex, posing higher requirements for drilling operations. Encountering enclosed caverns during drilling can easily lead to complex downhole accidents such as gas intrusion and well kicks, posing a significant challenge to well control safety. Accurately determining the size of the cavernous formation is crucial for rationally formulating drilling fluid density and optimizing well control measures and plans, and is extremely important for guiding safe drilling in cavernous formations.

[0003] Currently, the main methods for assessing the volume of karst caves in the industry include pre-drilling seismic data interpretation, well logging data analysis, and empirical models based on drilling time-torque parameters. Seismic data interpretation roughly estimates the volume of karst caves from seismic exploration images; however, the seismic identification accuracy is only around 15m, making this technique unsuitable for calculating the volume of karst caves with a diameter less than 15m, and the volume calculated using seismic data interpretation has a large error. Conventional well logging data can only reflect the distribution of karst caves within a limited area around the wellbore, resulting in significant errors in predicting the spatial distribution of large karst cave systems. Empirical models based on parameters such as drilling time and torque are easily affected by changes in formation pressure and drill string vibration, leading to excessive deviations in karst cave volume estimation. Therefore, there is an urgent need to develop an efficient dynamic calculation method for karst cave volume to solve the well control safety challenges in drilling karst formations.

[0004] This invention proposes a system and method for inverting the volume of formation karst caverns during drilling by acquiring wellbore throttling response parameters after gas intrusion and combining them with multiphase flow models and gas state equations. This invention can effectively utilize wellbore response parameters to rapidly invert the volume of formation karst caverns, providing theoretical and technical support for safe drilling in karst-type formations. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a system and method for inverting the volume of formation karst caverns during drilling gas invasion. This system and method do not require shutting in the well; they only need to take throttling measures and record the wellbore response parameters. By combining multiphase flow models, reservoir models, and gas state equations, the volume of karst caverns can be inverted. This method is not only simple to operate but also has a certain inhibitory effect on on-site gas invasion.

[0006] The technical solution of the present invention is as follows:

[0007] This invention provides a system for inverting the volume of formation caverns during drilling gas invasion. The system includes a throttling response parameter acquisition section, a wellbore throttling circulation pressure control section, and a multiphase flow simulation software section.

[0008] The throttling response parameter acquisition section includes an inlet flow meter, an outlet flow meter, and a pressure sensor for measuring wellhead back pressure. The inlet flow meter and outlet flow meter measure the real-time flow at the drill string inlet and annulus outlet, respectively, and the pressure sensor measures the wellhead back pressure. When gas intrusion occurs at the bottom of the well or the wellhead throttling valve is adjusted, the above devices acquire response parameters in real time.

[0009] The wellbore throttling and circulation pressure control system includes a gas intrusion monitoring device at the wellhead, a throttling pipeline, and a throttling valve. The gas intrusion monitoring device relies on data from the inlet and outlet flow meters. When the outlet flow rate is greater than the inlet flow rate, the gas intrusion monitoring device will detect the occurrence of gas intrusion and issue a warning. Subsequently, the wellhead back pressure is changed by adjusting the throttling valve, thereby reducing the outlet flow rate and inhibiting the development of gas intrusion.

[0010] In addition to the aforementioned hardware, the system also includes multiphase flow simulation software, which mainly handles information input, model calculation, and output of calculation results. Throttling response parameters collected by the outlet flow meter, inlet flow meter, and pressure sensor, as well as relevant parameters from the model calculation, are input into the software. The multiphase flow model built into the software can calculate the volume of the karst cave and display the results.

[0011] This invention also provides a method for inverting the volume of formation caverns during well gas invasion, comprising the following steps:

[0012] Step 1: After detecting air intrusion, implement throttling and circulation measures, performing the following at intervals of Δt: t 1. t 2. t Record the wellhead back pressure, inlet flow rate, and outlet flow rate at 3:00 a.m.; calculate the mud pit increment using the inlet and outlet flow rates; and calculate the assumed gas intrusion mass flow rate based on the mathematical relationship between the mud pit increment, gas dissolved volume, and gas intrusion volume, combined with the gas state equation.

[0013] Step 2: Obtain the basic parameters for calculating multiphase flow in the wellbore, establish a transient multiphase flow model and solution method for the wellbore after gas intrusion, and determine the solution conditions for the model.

[0014] Step 3: Using the wellhead back pressure and the assumed gas influx mass flow rate as boundary conditions, substitute them into the multiphase flow model and iteratively solve for 0~ t 1. t 1~ t 2. t 2~ tThe actual mass flow rate of air intrusion within 3 time periods, and its inversion. t 1. t 2. t Bottom hole pressure at time 3; according to t 1. t 2. t Using the gas state equation and reservoir model at time 3, the volume of the formation cavern is calculated.

[0015] According to the present invention, preferably, in step one, it is assumed that when gas intrusion is detected, the gas has just entered the wellbore, at which point... t At time =0, wellhead back pressure P c At atmospheric pressure; when gas intrusion is detected at the bottom of the well, t 1. t Adjust the throttle valve opening at 2 o'clock, and at t 1. t 2. t Record wellhead back pressure and inlet flow rate at 3 o'clock. Q in and export flow Q out The time interval for collecting throttling response parameters is Δt. A smaller Δt can prevent drastic gas expansion within the wellbore. 0~ t 1. t 1~ t 2. t 2~ t The increase in mud volume within a 3-hour period can be achieved through... t 1. t 2. t The difference between the outflow and inflow at time 3 is multiplied by the time interval for calculation.

[0016] Gas from formation cavities enters the wellbore annulus through fractures, causing a change in the annulus fluid volume and consequently increasing the mud pool volume. The gas undergoes dissolution and expansion, but the expansion rate is extremely small in the early stages of gas intrusion, and the expansion volume is negligible. Based on the principle of constant wellbore volume, the mathematical relationship between the bottom hole gas intrusion volume and the increase in the wellhead mud pool volume can be derived:

[0017] V G = V L + V dis (1)

[0018] In the formula, V G For gas intrusion volume, m 3 ; V dis m is the amount of gas dissolved. 3 ;V L The volume added to the mud pit, m 3 ;

[0019] Based on the gas law, Δ can be obtained t The gas intrusion mass flow rate over the time interval is:

[0020] (2)

[0021] Among them, M g The molar mass of the gas is expressed in kg / mol. P wf Z represents the bottom hole pressure, in Pa; g is the gas compressibility factor, dimensionless; R is the gas constant (8.314), J / (mol·K); T is the gas temperature, K;

[0022] Before performing multiphase flow model calculations, Δ t The volume of gas dissolved within a given time is unknown; we can initially assume the volume of gas dissolved is... Meanwhile, Δ t The average bottom hole pressure over a given time period is also unknown; we can assume it to be the initial bottom hole pressure. This gives us the value 0~ t 1. t 1~ t 2. t 2~ t The assumed gas influx mass flow rate over time 3 is:

[0023] (3)

[0024] In the formula, The assumed gas intrusion mass flow rate is kg / s; Let m be the assumed gas dissolution volume. 3 .

[0025] According to the present invention, preferably, in step two, the basic parameters for calculating multiphase flow in the wellbore are obtained, including wellbore trajectory, wellbore structure, drill string assembly, fluid properties, initial temperature distribution, and drilling pump parameters. A transient multiphase flow model after gas intrusion in the drilling wellbore is established, and the wellbore pressure field, gas-liquid distribution, and solubility distribution are calculated. The model includes the gas phase mass conservation equation, the liquid phase mass conservation equation, the gas-liquid two-phase momentum conservation equation, and corresponding auxiliary equations. The auxiliary equations include a gas compressibility factor calculation model, a gas viscosity calculation model, a gas density calculation model, a gas solubility calculation model, and a frictional pressure drop calculation model.

[0026] The mass conservation equation for the gas phase in the gas intrusion section is:

[0027] (4)

[0028] The mass conservation equation for the gas phase in the non-intrusive section is:

[0029] (5)

[0030] The mass conservation equation for the liquid phase is:

[0031] (6)

[0032] The momentum conservation equation for a gas-liquid two-phase system is:

[0033] (7)

[0034] In the formula, R s The solubility of gas in drilling fluid, m 3 / m 3 ; A , cross-sectional area of ​​the annulus, m 2 ; q = q gas / Δ H q Mass flow rate of air intrusion per unit length, kg / (s·m); Δ H q Depth of the gas-producing section, m; g, acceleration due to gravity, m / s² 2 ; r gsc Gas density under standard conditions, kg / m³ 3 ; r g Gas density, kg / m³ 3 ; r l Drilling fluid density, kg / m³ 3 ; u g Gas flow rate, m / s; u l Drilling fluid flow rate, m / s; E g Gas volume fraction, dimensionless; E l Integral number of drilling fluid, dimensionless; Frictional pressure drop, Pa / m; B l Z is the drilling fluid volume factor, %; P is the pressure, Pa; z is the vertical length, m;

[0035] The finite difference method is used to discretize and solve the above multiphase flow model. The difference scheme is as follows:

[0036] The finite difference scheme for the gas phase continuity equation in the gas intrusion section is as follows:

[0037] (40)

[0038] The finite difference scheme for the gas phase continuity equation in the non-intrusive section is as follows:

[0039] (41)

[0040] The finite difference scheme for the liquid phase continuity equation is as follows:

[0041] (42)

[0042] The difference scheme for the momentum equation is:

[0043] (43)

[0044] (44)

[0045] (45)

[0046] (46)

[0047] j is a spatial node, dimensionless; n is a time node, dimensionless.

[0048] Based on the actual engineering situation, determine the model. t =0、 t 1. t Initial and boundary conditions at time 2.

[0049] According to the present invention, preferably, in step three, 0~ t Taking the actual gas influx mass flow rate calculation process as an example, the wellhead back pressure and assumed gas influx mass flow rate in step one are used as boundary conditions. The calculation is performed using a multiphase flow model to obtain 0~ t Bottom hole pressure at each time step within 1 time period Calculate 0~ according to the trapezoidal rule t Average bottom hole pressure over 1 time period ;

[0050] (52)

[0051] In the formula, i This represents the number of time nodes; The bottom hole pressure at time node 0 is in Pa; for i Bottom hole pressure at the specified time point, in Pa; dt Let be the time step, in seconds; N 1 is t Number of time points within a time period. N 1=t 1 / dt;

[0052] Solubility calculated using a multiphase flow model R s The distribution of the data is calculated. t The height of drilling fluid entering the annulus within 1 time period, i.e., the length of the liquid column where gas dissolution has occurred. H R1 The amount of gas dissolved is obtained according to the following formula. V dis,1 ;

[0053] (53)

[0054] In the formula, j This represents the number of nodes in the space. V dis,1 0~ t Gas dissolution amount in 1 time, m 3 ; H R1 0~ t The height of drilling fluid entering the annulus within 1 time period, in meters; R s,j for j Solubility at spatial nodes; A a Let m be the cross-sectional area of ​​the annulus. 2 ; dz Let m be the spatial step size.

[0055] It is calculated by the following formula:

[0056] (54)

[0057] V L,1 0~ t The increase in volume of the mud pit within 1 time period, m 3 ;

[0058] Compare the calculated gas intrusion mass flow rates q gas,1 And assumed gas intrusion mass flow rate The error between them, if the error satisfies:

[0059] (55)

[0060] The calculation ends, and the output is 0~ t Real gas intrusion mass flow rate over 1 time period q gas,1 and t Bottom hole pressure at moment 1 P wf,1 Otherwise, qgas,1 The value assigned to The multiphase flow model was recalculated;

[0061] t 1~ t 2. t 2~ t Real gas intrusion mass flow rate over 3 time periods q gas,2 , q gas,3 ,and t 2. t Bottom hole pressure at time 3 P wf,2 , P wf,3 The calculation is the same as the steps above;

[0062] Before gas intrusion occurs, the initial gas mass in the cave is: m ,0~ t 1. t 1~ t 2. t 2~ t The masses of gas flowing out within 3 time periods were respectively q gas,1 Δ t , q gas,2 Δ t , q gas,3 Δ t From this, it can be calculated t 1. t 2. t The remaining mass of gas in the cave at time 3 m 1. m 2. m 3. Let t 1. t 2. t The pressure in the cave at time 3 were respectively P v1 , P v2 , P v3 ;

[0063] The reservoir model uses a linear gas production model:

[0064] q gas = J ( P v - P wf (56)

[0065] J The reservoir model productivity index is expressed in kg / (Pa·s). P v The pressure in the cavern is Pa;

[0066] Based on the gas law and reservoir model, the following set of equations is obtained:

[0067] (61)

[0068] In the equation, R, T, Mg, Δ t It is a constant; q gas,1 、q gas,2 , q gas,3 , Z g,1 , Z g,2 , Z g,3 , P wf,1 , P wf,2 , P wf,3 All of these can be solved using a multiphase flow model; the solution can be obtained by simultaneously solving the above 6 equations. P v1 , P v2 , P v3 , J , V , m Thus, the pressure of the karst cave is obtained. P v Reservoir model productivity index J and the volume of karst caves V .

[0069] This system and method can acquire three wellhead throttling response parameters: wellhead back pressure, inlet flow rate, and outlet flow rate. Then, based on a multiphase flow model, it predicts bottomhole pressure, gas dissolution rate, and gas intrusion rate. Finally, it uses a reservoir model and gas state equations to invert the formation cavity volume. This method fully utilizes field-obtainable parameters, eliminating the need for extensive data training, and is simple, efficient, and real-time. Furthermore, acquiring relevant parameters during pressure control measures helps to curb gas intrusion development.

[0070] The beneficial effects of this invention are as follows:

[0071] The system and method for calculating the volume of formation karst cavern during drilling gas invasion provided by this invention can easily obtain the wellhead throttling response parameters, use the multiphase flow model to invert the bottom hole parameters that are difficult to obtain, and calculate the karst cavern volume through the reservoir model and the gas equation of state. The main advantages of this method are as follows: (1) By adjusting the throttling valve at the wellhead, the wellbore response parameters can be obtained, which is simple to operate and has a certain inhibitory effect on gas invasion and overflow in the field; (2) The multiphase flow model considers the dissolved volume of gas in the drilling fluid, which can more accurately determine the gas invasion volume, thereby reducing the calculation error of the karst cavern volume; (3) The non-ideal gas equation of state that considers the influence of the gas compressibility factor is adopted, which is more consistent with the actual gas properties and improves the calculation accuracy. Other features and advantages of this invention will be described in detail in the following specific implementation section. Attached Figure Description

[0072] To more clearly illustrate the technical solutions of the specific embodiments of the present invention, the accompanying drawings used in the description of the specific embodiments will be briefly introduced below, so as to be used together with the following specific embodiments to explain the embodiments of the present invention, but not to limit the embodiments of the present invention. In the drawings:

[0073] Figure 1 This is a schematic diagram of the formation cavern volume inversion system during well gas invasion.

[0074] Figure 2 This is a flowchart of the method for inverting the volume of formation karst caves during well gas invasion.

[0075] Figure 3 This is a schematic diagram illustrating the mathematical relationship between the gas intrusion volume at the bottom of the well and the increase in the mud pit at the wellhead;

[0076] Figure 4 This is a flowchart of the solution method for the transient multiphase flow model of the wellbore after gas intrusion;

[0077] Figure 5 This is a schematic diagram of the principle for solving the volume of a karst cave based on the gas law.

[0078] Among them, 1-inlet flow meter; 2-outlet flow meter; 3-gas intrusion monitoring device; 4-pressure sensor; 5-mud pump; 6-throttle valve; 7-throttle pipeline; 8-mud pit; 9-information input and storage equipment; 10-model calculation software; 11-result output interface; 12-annulus; 13-drill pipe; 14-formation; 15-formation intrusion gas; 16-drill bit; 17-cavity; 18-fracture; 19-gas in the cavity; 20-intrusion annulus gas; 21-gas dissolved in drilling fluid; 22-mud pit increment; 23-initial mud pit volume; 24-annulus channel; 25-drill pipe channel. Detailed Implementation

[0079] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0080] Example 1

[0081] A system for inverting the volume of formation karst caverns during drilling gas invasion, such as Figure 1 As shown, the system includes a wellbore throttling and circulation pressure control section ( Figure 1 Sections 3, 6, and 7), throttling response parameter acquisition section ( Figure 1 Parts 1, 2, and 4) and the multiphase flow simulation software ( Figure 1 (See sections 9, 10, and 11). The wellbore throttling and circulation pressure control section includes a gas intrusion monitoring device at the wellhead, a throttling valve, and a throttling pipeline. The gas intrusion monitoring device relies on data from the inlet and outlet flow meters. When the outlet flow rate exceeds the inlet flow rate, the device detects gas intrusion and issues a warning. Subsequently, it adjusts the throttling valve to change the wellhead back pressure, thereby reducing the outlet flow rate and suppressing the development of gas intrusion. The throttling response parameter acquisition section includes an inlet flow meter, an outlet flow meter, and a pressure sensor that measures the wellhead back pressure. The inlet and outlet flow meters measure the real-time flow at the drill string inlet and annular outlet, respectively, while the pressure sensor measures the wellhead back pressure. When gas intrusion occurs at the bottom of the well or the wellhead throttling valve is adjusted, the above equipment acquires response parameters in real time. The multiphase flow simulation software includes information input and storage devices, model calculation software, and result output interface. It mainly performs information input, model calculation, and calculation result output. Throttling response parameters collected by the outlet flow meter, inlet flow meter, and pressure sensor, as well as relevant parameters from the model calculation, are input into the software. The multiphase flow model built into the software can perform karst cave volume calculation and output and display the results.

[0082] The specific workflow of the gas invasion process and system is as follows: During normal drilling, the drilling fluid in the mud pit (8) is pumped into the drill pipe (13) by the mud pump (5). The inlet flow meter (1) can measure and record the drilling fluid inlet flow rate. The drilling fluid flows through the drill pipe and drill bit (16), and flows out of the wellbore through the annulus (12). It returns to the mud pit through the choke line (7). The outlet flow meter (2) on the line can measure and record the drilling fluid outlet flow rate, and the pressure sensor (4) can measure and record the wellhead back pressure. If a cavern (17) is encountered during drilling, the gas in the cavern enters the wellbore annulus through the cracks (18), thereby affecting the bottom hole pressure and the annulus liquid volume, and further affecting the drilling fluid outlet flow rate. The gas invasion monitoring device (3) at the wellhead determines the occurrence of gas invasion by analyzing the difference between the inlet flow meter and the outlet flow meter. At this time, the opening of the choke valve (6) is adjusted to control the outlet flow rate, and the gas invasion is handled by choke circulation. Starting from the moment gas intrusion is detected, the throttle valve opening is adjusted every Δt time interval, and the inlet flow rate, outlet flow rate, and wellhead back pressure are recorded three times using the inlet flow meter, outlet flow meter, and pressure sensor. The recorded data is transmitted to the information input and storage device (9), the formation cavity volume is calculated using the model calculation software (10), and the results are displayed in real time through the result output interface (11).

[0083] Example 2

[0084] A method for inverting formation cavity volume during drilling gas invasion is disclosed. Based on the system and workflow described in Example 1, this system can intermittently collect throttling response parameters such as drilling fluid outlet flow rate, inlet flow rate, and wellhead back pressure, and calculate the formation cavity volume using model software. The specific calculation method for formation cavity volume is as follows:

[0085] Figure 2 The flowchart illustrates the method for inverting the volume of formation caverns during well gas invasion, including the following steps:

[0086] Step 1: After detecting air intrusion, implement throttling and circulation measures, performing the following at intervals of Δt: t 1. t 2. t Record the wellhead back pressure, inlet flow rate, and outlet flow rate at 3 pm. Calculate the mud pit increment using the inlet and outlet flow rates. Based on the mathematical relationship between the mud pit increment, gas dissolved volume, and gas intrusion volume, and combined with the gas state equation, calculate the assumed gas intrusion mass flow rate.

[0087] Step 2: Obtain the basic parameters for calculating multiphase flow in the wellbore, establish a transient multiphase flow model and solution method for the wellbore after gas intrusion, and determine the solution conditions for the model.

[0088] Step 3: Using the wellhead back pressure and the assumed gas influx mass flow rate as boundary conditions, substitute them into the multiphase flow model and iteratively solve for 0~ t 1. t 1~ t 2. t 2~ t The actual mass flow rate of air intrusion within 3 time periods, and its inversion. t 1. t 2. t The bottom hole pressure at time 3. According to... t 1. t 2. t Using the gas state equation and reservoir model at time 3, the volume of the formation cavern is calculated.

[0089] In step one, assuming that gas intrusion is detected just as gas enters the wellbore, at this point... t At time =0, the wellhead back pressure ( P c (The pressure is) atmospheric pressure. When gas intrusion is detected at the bottom of the well, the drilling fluid inlet flow rate is not changed, and no well shut-in measures are taken. Instead, throttling and circulation are implemented by adjusting the throttle valve opening. For a period of time after the gas intrusion is detected (0~... t 3) Adjust the throttle valve opening at regular intervals (Δt) and collect wellbore gas intrusion response parameters during this period, mainly including wellhead back pressure and drilling fluid inlet flow rate (Δt). Q in ) and drilling fluid outlet flow rate ( Q out The above parameters can be obtained separately through... Figure 1 The pressure sensor (4), inlet flow meter (1), and outlet flow meter (2) are used to collect data. To avoid drastic expansion of the gas inside the wellbore, it is recommended that the time interval for collecting the throttling response parameters be 5 minutes, and the recording times for each time are as follows: t 1 = 5min t 2 = 10 min t 3 = 15 min.

[0090] pass t 1. t 2. t The drilling fluid inlet flow rate and drilling fluid outlet flow rate collected at time 3 can be used to obtain 0~ t 1. t 1~ t 2. t 2~ t Increase in mud pit volume within 3 hours ( V L ): V L =( Q out -Q in )Δt, where, V L Mud pit increment, m 3 ; Q out Drilling fluid outlet flow rate, m 3 / s; Q in Drilling fluid inlet flow rate, m 3 / s; Δt, the data acquisition time interval, in seconds.

[0091] Figure 3 This is a schematic diagram illustrating the mathematical relationship between the gas intrusion volume at the bottom of the well and the increase in the mud pool at the wellhead. Initially, the cavern contains a portion of gas that has not yet intruded into the wellbore (19), with a volume of... V G After gas intrusion occurs, gas from the formation karst caverns rapidly enters the wellbore annulus through fractures (24), causing a change in the annulus fluid volume, which is reflected in changes in wellhead parameters, such as the mud pit increase (22). The increased volume in the mud pit is... V L However, some of the gas (21) will dissolve into the drilling fluid, and the volume of this portion is... V dis The actual volume occupied by the annular fluid is only the remaining intruding gas (20). Simultaneously, the gas expands after entering the wellbore, with an expansion volume of... V exp Based on the principle of constant wellbore volume, the mathematical relationship between the gas intrusion volume at the bottom of the well and the increase in the mud pit at the wellhead can be obtained: V L = V G + V exp - V dis In the formula, V G Gas intrusion volume, m 3 ; V exp Gas expansion, m 3 ; V dis Gas dissolved amount, m 3 .

[0092] Considering that the gas expansion rate in the early stage of gas invasion is extremely small and can be ignored, we only consider the dissolved volume of gas in the drilling fluid. The mathematical relationship between the bottom hole gas invasion volume and the increase in the wellhead mud pit is as follows:

[0093] V G = VL + V dis (1)

[0094] The relationship between the intruding gas mass and the intruding mass flow rate is as follows: m g = q gas ·Δt; Based on the gas law, the following equation can be obtained: In the formula, P wf Bottom hole pressure, Pa; Z g , gas compressibility factor, dimensionless; m g Mass of intruding gas, kg; M g , gas molar mass, kg / mol; R, gas constant (8.314), J / (mol·K); T Gas temperature, K; q gas , gas intrusion mass flow rate, kg / s.

[0095] Combining the above equations, we can obtain:

[0096] (2)

[0097] Assuming the gas temperature is the formation temperature, it can be calculated using the surface temperature, local geothermal gradient, and current well depth. T = T sur +Δ T · H In the formula, T sur Ground temperature, K; Δ T Geothermal gradient, K / m; H , well depth, m.

[0098] Since the multiphase flow model calculation has not yet been performed, therefore Δ t The gas dissolution volume and average bottom-hole pressure over time are unknowns. Reasonable assumptions can be made about them as conditions for the multiphase flow model. Through iterative model calculations, the actual gas dissolution volume can be obtained, and then the gas influx mass flow rate can be calculated. Assuming the gas dissolution volume is... The average bottom-hole pressure is the initial bottom-hole pressure. The assumed gas influx mass flow rate calculated in this step is:

[0099] (3)

[0100] In the formula, Assuming the gas intrusion mass flow rate, kg / s; Assuming the gas dissolution volume, m 3 .

[0101] In step two, the basic parameters for multiphase flow calculation in the wellbore, such as wellbore trajectory, wellbore structure, drill string assembly, and fluid properties, are obtained. These basic parameters specifically include: depth measurement, well inclination angle, casing size for different openings, casing running depth, open hole depth, drill string size, drill bit diameter, drilling fluid density, drilling fluid rheological parameters, current well depth, and temperature distribution data.

[0102] A transient multiphase flow model for the drilling wellbore after gas intrusion is established. This model can calculate the wellbore pressure field, gas-liquid distribution, etc., at different times. The model includes the gas phase mass conservation equation, the liquid phase mass conservation equation, the gas-liquid two-phase momentum conservation equation, and corresponding auxiliary equations.

[0103] In the gas intrusion section, the gas phase mass conservation equation is:

[0104] (4)

[0105] Similarly, the gas phase mass conservation equation for the non-intrusive section is:

[0106] (5)

[0107] The mass conservation equation for the liquid phase is:

[0108] (6)

[0109] The momentum conservation equation for a gas-liquid two-phase system is:

[0110] (7)

[0111] In the formula, R s The solubility of gas in drilling fluid, m 3 / m 3 ; A , cross-sectional area of ​​the annulus, m 2 ; q Mass flow rate of air intrusion per unit length, kg / (s·m). q = q gas / Δ H q Δ H q Depth of the gas-producing section, in meters; r gsc Gas density under standard conditions, kg / m³ 3 ; r g Gas density, kg / m³3 ; r l Drilling fluid density, kg / m³ 3 ; u g Gas flow rate, m / s; u l Drilling fluid flow rate, m / s; E g Gas volume fraction, dimensionless; E l Integral number of drilling fluid, dimensionless; g, acceleration due to gravity, m / s² 2 ; Frictional pressure drop, Pa / m; B l is the drilling fluid volume factor, %; P is the pressure, Pa; z is the vertical length, m.

[0112] In addition to the multiphase flow control equations mentioned above, the model also includes several auxiliary equations:

[0113] (1) Gas compressibility factor calculation model

[0114] The gas compressibility factor is calculated using the PR equation of state:

[0115] (8)

[0116] (9)

[0117] (10)

[0118] (11)

[0119] (12)

[0120] (13)

[0121] (14)

[0122] In the formula, Deviation factor, dimensionless; P c Critical pressure, Pa; T c Critical temperature, K.

[0123] (2) Gas viscosity calculation model

[0124] The viscosity of the gas was calculated using the Lee model:

[0125] (15)

[0126] (16)

[0127] In the formula, m g , gas viscosity, Pa·s.

[0128] (3) Gas density calculation model

[0129] The gas density is derived from the equation of state:

[0130] (17)

[0131] In the formula, P sc Pressure under standard conditions, Pa; Z sc The compressibility factor under standard conditions is dimensionless. T sc Temperature under standard conditions, in K; r gsc Gas density under standard conditions, kg / m³ 3 ; Z g , gas compressibility factor, dimensionless.

[0132] (4) Gas solubility calculation model

[0133] The solubility of formation intrusion gases in drilling fluid can be calculated using the following formula:

[0134] (18)

[0135] SG g = r g / r air (19)

[0136] APIs =141.5 / SG l -131.5 (20)

[0137] SG l = r l / r w (twenty one)

[0138] The amount of gas dissolved in the annular atmosphere can be calculated using solubility:

[0139] (twenty two)

[0140] In the formula, R s Gas solubility, m 3 / m 3 ; SG g The relative density of a gas is dimensionless. SG l The relative density of drilling fluid is dimensionless. APIs Drilling fluid APIs Specific gravity, dimensionless; P Gas pressure, Pa; T Gas temperature, K; r w Water density (1000), kg / m³ 3 ; r l Drilling fluid density, kg / m³ 3 ; r g Gas density, kg / m³ 3 ; r air The air density under standard conditions is 1.293 kg / m³. 3 V s Drilling fluid volume, m 3 .

[0141] (5) Frictional pressure drop calculation model

[0142] The single-phase flow friction voltage drop is calculated using the following formula:

[0143] (twenty three)

[0144] (twenty four)

[0145] (25)

[0146] In the formula, Re is the Reynolds number, which is dimensionless; D a , , annular diameter, m; D pout drill pipe outer diameter, m; u l Drilling fluid flow rate, m / s; r l Drilling fluid density, kg / m³ 3 ; or p Plastic viscosity, Pa·s; t 0, yield value, Pa; f r , coefficient of friction, dimensionless.

[0147] The calculation of frictional pressure drop in multiphase flow requires distinguishing between different flow patterns, mainly categorized as bubbly flow, slug flow, and annular flow.

[0148] The frictional pressure drop in bubbly flow is calculated using the following formula:

[0149] (26)

[0150] (27)

[0151] (28)

[0152] (29)

[0153] In the formula, Re m The Reynolds number of the mixed fluid is dimensionless. u m Apparent flow velocity of the mixed phase, m / s; r m Mixed density, kg / m³ 3 ; m m Δ is the mixed viscosity, Pa·s; Δ is the effective annular roughness, m.

[0154] The apparent flow rate, mixing density, and mixing viscosity of the mixed phase can be calculated using the following formula:

[0155] u m = u g E g + u l E l (30)

[0156] r m = r g E g + r l E l (31)

[0157] m m = m g E g + m l E l (32)

[0158] In the formula, m l Drilling fluid viscosity, Pa·s.

[0159] The frictional pressure drop in slug flow is calculated using the following formula:

[0160] (33)

[0161] (34)

[0162] (35)

[0163] (36)

[0164] The frictional pressure drop in annular bubbly flow is calculated using the following formula:

[0165] (37)

[0166] (38)

[0167] (39)

[0168] A solution method for the transient multiphase flow model of the wellbore after gas intrusion is established, and the solution conditions are determined. The finite difference method is used to numerically discretize and iteratively solve the aforementioned transient multiphase flow model of the wellbore after gas intrusion, employing a four-point difference scheme. The difference schemes for the gas-phase and liquid-phase continuity equations, as well as the gas-liquid two-phase momentum equations, are as follows:

[0169] (1) Gas phase continuity equation

[0170] The finite difference scheme for the gas phase continuity equation in the gas intrusion section is as follows:

[0171] (40)

[0172] The finite difference scheme for the gas phase continuity equation in the non-intrusive section is as follows:

[0173] (41)

[0174] (2) Liquid phase continuity equation

[0175] The finite difference scheme for the liquid phase continuity equation is as follows:

[0176] (42)

[0177] (3) Momentum equation

[0178] The difference scheme for the momentum equation is:

[0179] (43)

[0180] (44)

[0181] (45)

[0182] (46)

[0183] j is a spatial node, dimensionless; n is a time node, dimensionless.

[0184] Based on the actual engineering situation, determine the initial and boundary conditions of the equations.

[0185] t =0、 t 1. t The initial conditions at time 2 are different. t When the initial condition is 0, the formation gas has not yet invaded the wellbore, and the flow in the wellbore annulus is single-phase. The initial conditions are set as follows:

[0186] (47)

[0187] t = t 1. t At point 2, the model calculation results can be used as initial conditions. In the bottom hole pressure calculation, the change in wellhead back pressure must be considered. The initial conditions are set as follows:

[0188] (48)

[0189] In the formula, P f Friction loss, Pa; u sl Drilling fluid apparent velocity, m / s; u sg , apparent gas velocity, m / s.

[0190] The outlet boundary condition for wellbore pressure is the wellhead back pressure:

[0191] P (0, t )= P c (49)

[0192] The fluid velocity boundary condition is:

[0193] (50)

[0194] Figure 4 This is a flowchart illustrating the solution method for the transient multiphase flow model of a drilling wellbore after gas intrusion. The specific solution steps are as follows:

[0195] (1) Assumption t At time +1, the pressure at the bottom of the well is The physical properties (density, viscosity, compressibility factor, etc.) of each phase of the gas and liquid are determined based on the auxiliary equation.

[0196] (2) Assumption t +1 time to the circular position node j The wellbore pressure at the location is The assumed value is usually referenced t Time-loop position node j Wellbore pressure Then determine the physical property values ​​of the gas and liquid phases at the annular location node.

[0197] (3) Assumption t +1 time to the circular position node j The gas content at that location is Assuming values ​​are usually referenced t Time-loop position node j gas content at Calculate the apparent flow rates of each phase (gas and liquid) and the solubility of the gas phase.

[0198] (4) Calculate the new gas content using the drift flow model and the mass conservation equation. And check whether the new gas content meets the requirements. If the condition is not met, return to step (3) to recalibrate until... Established.

[0199] (5) Calculate the position nodes of the annulus using the mixed momentum conservation equation. j New wellbore pressure And check whether the new wellbore pressure meets the requirements. If the condition is not met, return to step (2) to recalibrate until... Established.

[0200] (6) In t The wellbore pressure is calculated iteratively at time +1 until the wellhead pressure is obtained. If the calculated wellhead pressure meets the following conditions... This indicates that in t The calculated value of the entire wellbore pressure at time +1 is accurately predicted. Otherwise, it is necessary to return to step (1) for recalibration until... Established.

[0201] In step three, step one P c As a wellhead pressure boundary condition. Assuming the gas influx mass flow rate, substitute it into the model and iteratively solve for 0~ t 1. t 1~ t 2. t 2~ t The actual gas intrusion mass flow rate over a 3-hour period ( q gas,1 、q gas,2 , q gas,3 ), and inverse t 1. t 2. t Bottom hole pressure at time 3 ( P wf,1 , P wf,2 , P wf,3 ).

[0202] With 0~ t Taking the actual gas intrusion mass flow rate calculation process as an example, the wellhead back pressure is: P c,1 ( t =0), the mud pit increment is V L,1 ( t = t 1) Assume the amount of gas dissolved is =0, initial bottom hole pressure is P wf ( t =0). The assumed gas intrusion mass flow rate can be calculated according to formula (3):

[0203] (51)

[0204] Calculations were performed using a multiphase flow model to obtain 0~ t The bottom hole pressure at each time step within a time period is calculated using the trapezoidal rule. t Average bottom hole pressure over 1 time period Simultaneously, the amount of gas dissolved was obtained. V dis,1 The calculation yields:

[0205] (52)

[0206] In the formula, i This represents the number of time nodes; The bottom hole pressure at time node 0 is in Pa; for i Bottom hole pressure at the specified time point, in Pa; dt Let be the time step, in seconds; N 1 ist Number of time points within a time period. N 1= t 1 / dt.

[0207] (53)

[0208] In the formula, j This represents the number of nodes in the space. V dis,1 0~ t Gas dissolution amount in 1 time, m 3 ; H R1 0~ t The height of drilling fluid entering the annulus within 1 time period, in meters; R s,j for j Solubility at spatial nodes; A a Let m be the cross-sectional area of ​​the annulus. 2 ; dz Let m be the spatial step size.

[0209] (54)

[0210] V L,1 0~ t The increase in volume of the mud pit within 1 time period, m 3 .

[0211] Compare the newly calculated gas intrusion mass flow rate q gas,1 And assumed gas intrusion mass flow rate The error between them, if the error satisfies:

[0212] (55)

[0213] The iterative calculation then ends, and the output is... t Real gas intrusion mass flow rate at 1 moment q gas,1 and bottom hole pressure P wf,1 Otherwise, q gas,1 The value assigned to Then, perform the multiphase flow model calculation again until it meets the formula (55).

[0214] Similarly, t The wellhead back pressure at time 2 is P c,2 ( t = t 1) The increase in mud pit volume is VL,2 ( t = t 2) Assuming the amount of gas dissolved is = V dis ( t = t 1) The initial bottom hole pressure is P wf ( t = t 1). t The wellhead back pressure at time 3 is P c,3 ( t = t 2) The increase in mud pit volume is V L,3 ( t = t 3) Assuming the amount of gas dissolved is = V dis ( t = t 2) The initial bottom hole pressure is P wf ( t = t 2). t 1~ t 2. t 2~ t The actual gas intrusion mass flow rate over a 3-hour period ( q gas,2 , q gas,3 )and t 2. t Bottom hole pressure at time 3 ( P wf,2 , P wf,3 The calculation is the same as the steps above.

[0215] according to t 1. t 2. t The gas state equation and reservoir model within the karst cave at time 3 were used to solve for parameters such as the productivity index and karst cave pressure, and then the volume of the karst cave in the formation was calculated.

[0216] For ease of calculation, the following assumptions are made regarding the solution process:

[0217] (1) The gas in the karst cavern, during its intrusion into the wellbore, conforms to the linear flow equation of the reservoir:

[0218] q gas = J ( Pv - P wf (56)

[0219] In the formula, J Reservoir model productivity index, kg / (Pa·s); P v , cave pressure, Pa.

[0220] (2) 0~ t The gas temperature remains constant over a period of 3 hours, and is the temperature T at the depth of the stratum where the cave is located.

[0221] (3) The cave is connected to the wellbore only through cracks, and the formation fluid will not enter the cave.

[0222] Figure 5 This diagram illustrates the principle of solving for the volume of a cavern based on the gas law. Before gas intrusion, the initial gas mass in the cavern is... m ,0~ t 1. t 1~ t 2. t 2~ t The masses of gas flowing out within 3 time periods were respectively q gas,1 Δ t , q gas,2 Δ t , q gas,3 Δ t .so t 1. t 2. t The remaining mass of gas in the cave at time 3 is as follows:

[0223] (57)

[0224] correspond t 1. t 2. t The pressure in the cave at time 3 were respectively P v1 , P v2 , P v3 .

[0225] by t Taking the gas state equation inside the cave at time 1 as an example:

[0226] (58)

[0227] Simplifying the equation, we get:

[0228] (59)

[0229] Simultaneously, from formula (56), we obtain:

[0230] (60)

[0231] Will t 2. t The equations at time 3 are written out, resulting in the following system of equations:

[0232] (61)

[0233] In the equation, R, T, Mg, Δ t Since it is a constant, no solution is required. q gas,1 、q gas,2 , q gas,3 0~ t 1. t 1~ t 2. t 2~ t Gas intrusion mass flow rate over time 3. Z g,1 , Z g,2 , Z g,3 , P wf,1 , P wf,2 , P wf,3 for t 1. t 2. t The gas compressibility factor and bottom hole pressure at time 3, and all nine parameters mentioned above, can be solved using a multiphase flow model.

[0234] P v1 , P v2 , P v3 , J , V , m The parameters to be solved can be obtained by solving the above six equations simultaneously, thus obtaining the cavern pressure. P v And reservoir model productivity index J and the volume of karst caves V .

Claims

1. A method for inverting the volume of formation karst caverns during drilling gas invasion, characterized in that, Includes the following steps: Step 1: After detecting air intrusion, implement throttling and circulation measures, performing the following at intervals of Δt: t 1. t 2. t Record the wellhead back pressure, inlet flow rate, and outlet flow rate at 3:00 a.m.; calculate the mud pit increment using the inlet and outlet flow rates; and calculate the assumed gas intrusion mass flow rate based on the mathematical relationship between the mud pit increment, gas dissolved volume, and gas intrusion volume, combined with the gas state equation. Step 2: Obtain the basic parameters for calculating multiphase flow in the wellbore, establish a transient multiphase flow model and solution method for the wellbore after gas intrusion, and determine the solution conditions for the model; Step 3: Using the wellhead back pressure and the assumed gas influx mass flow rate as boundary conditions, substitute them into the multiphase flow model and iteratively solve for 0~ t 1. t 1~ t 2. t 2~ t The actual mass flow rate of air intrusion within 3 time periods, and its inversion. t 1. t 2. t Bottom hole pressure at time 3; according to t 1. t 2. t Using the gas state equation and reservoir model at time 3, the volume of the formation cavern is calculated. With 0~ t Taking the actual gas influx mass flow rate calculation process as an example, the wellhead back pressure and assumed gas influx mass flow rate in step one are used as boundary conditions. The calculation is performed using a multiphase flow model to obtain 0~ t Bottom hole pressure at each time step within 1 time period Calculate 0~ according to the trapezoidal rule t Average bottom hole pressure over 1 time period ; （52） In the formula, i This represents the number of time nodes; The bottom hole pressure at time node 0 is in Pa; for i Bottom hole pressure at the specified time point, in Pa; dt Let be the time step, in seconds; N 1 is t Number of time points within a time period. N 1= t 1 / dt; Solubility calculated using a multiphase flow model R s The distribution of the data is calculated. t The height of drilling fluid entering the annulus within 1 time period, i.e., the length of the liquid column where gas dissolution has occurred. H R1 The amount of gas dissolved can be obtained according to the following formula. V dis,1 ; （53） In the formula, j This represents the number of nodes in the space. V dis,1 0~ t Gas dissolution amount in 1 time, m 3 ; H R1 0~ t The height of drilling fluid entering the annulus within 1 time period, in meters; R s,j for j Solubility at spatial nodes; A a Let m be the cross-sectional area of ​​the annulus. 2 ; dz Let m be the spatial step size. It is calculated by the following formula: （54） Z g is the gas compressibility factor, dimensionless; R is the gas constant (8.314), J / (mol·K); T is the gas temperature, K; V L,1 0~ t The increase in volume of the mud pit within 1 time period, m 3 ; Compare the calculated gas intrusion mass flow rates q gas,1 And assumed gas intrusion mass flow rate The error between them, if the error satisfies: （55） The calculation ends, and the output is 0~ t Real gas intrusion mass flow rate over 1 time period q gas,1 and t Bottom hole pressure at moment 1 P wf,1 Otherwise, q gas,1 value assigned to The multiphase flow model was recalculated. t 1~ t 2. t 2~ t Real gas intrusion mass flow rate over 3 time periods q gas,2 , q gas,3 ,and t 2. t Bottom hole pressure at time 3 P wf,2 , P wf,3 The calculation is the same as the steps above; Before gas intrusion occurs, the initial gas mass in the cave is: m ,0~ t 1. t 1~ t 2. t 2~ t The masses of gas flowing out within 3 time periods were respectively q gas,1 Δ t , q gas,2 Δ t , q gas,3 Δ t From this, it can be calculated t 1. t 2. t The remaining mass of gas in the cave at time 3 m 1. m 2. m 3. Let t 1. t 2. t The pressure in the cave at time 3 were respectively P v1 , P v2 , P v3 ; The reservoir model uses a linear gas production model: q gas = J ( P v - P wf )（56） J The reservoir model productivity index is expressed in kg / (Pa·s). P v The pressure in the cavern is Pa; Based on the gas law and reservoir model, the following set of equations is obtained: （61） In the equation, R, T, Mg, Δ t It is a constant; q gas,1 、q gas,2 , q gas,3 0~ t 1. t 1~ t 2. t 2~ t Gas intrusion mass flow rate over time 3. Z g,1 , Z g,2 , Z g,3 , P wf,1 , P wf,2 , P wf,3 for t 1. t 2. t The gas compressibility factor and bottom hole pressure at time 3 were both solved using a multiphase flow model; the above six equations were solved simultaneously. P v1 , P v2 , P v3 , J , V , m Thus, the pressure of the karst cave is obtained. P v Reservoir model productivity index J and the volume of karst caves V .

2. The method for inverting the volume of formation karst caverns during drilling gas invasion according to claim 1, characterized in that, In step one, assuming that gas intrusion is detected just as gas enters the wellbore, at this point... t At time =0, wellhead back pressure P c At atmospheric pressure; when gas intrusion is detected at the bottom of the well, t 1. t Adjust the throttle valve opening at 2 o'clock, and at t 1. t 2. t Record wellhead back pressure and inlet flow rate at 3 o'clock. Q in and export flow Q out The time interval for collecting throttling response parameters is Δt, 0~ t 1. t 1~ t 2. t 2~ t The increase in mud volume within a 3-hour period, through t 1. t 2. t The difference between the outflow and inflow at time 3 is multiplied by the time interval for calculation. Based on the principle of constant wellbore volume, the mathematical relationship between the gas intrusion volume at the bottom of the well and the increase in the mud pit at the wellhead is obtained: V G = V L + V dis （1） In the formula, V G For gas intrusion volume, m 3 ; V dis m is the amount of gas dissolved. 3 ; V L The volume added to the mud pit, m 3 ; Based on the gas law, we obtain Δ t The gas intrusion mass flow rate over the time interval is: （2） Among them, M g The molar mass of the gas is expressed in kg / mol. P wf Z represents the bottom hole pressure, in Pa; g is the gas compressibility factor, dimensionless; R is the gas constant (8.314), J / (mol·K); T is the gas temperature, K; Before performing multiphase flow model calculations, it is assumed that the gas dissolution volume is... Meanwhile, Δ t The average bottom-hole pressure over a given time period is assumed to be the initial bottom-hole pressure; thus, 0~ t 1. t 1~ t 2. t 2~ t The assumed gas influx mass flow rate over time 3 is: （3） In the formula, The assumed gas intrusion mass flow rate is expressed in kg / s. Let m be the assumed gas dissolution volume. 3 .

3. The method for inverting the volume of formation karst caverns during drilling gas invasion according to claim 1, characterized in that, In step two, the basic parameters for calculating multiphase flow in the wellbore are obtained, including wellbore trajectory, wellbore structure, drill string assembly, fluid properties, initial temperature distribution, and drilling pump parameters. A transient multiphase flow model after gas intrusion in the wellbore is established to calculate the wellbore pressure field, gas-liquid distribution, and solubility distribution. This model includes the gas phase mass conservation equation, the liquid phase mass conservation equation, and the gas-liquid two-phase momentum conservation equation.

4. The method for inverting the volume of formation karst caverns during drilling gas invasion according to claim 3, characterized in that, The mass conservation equation for the gas phase in the intrusion section is: （4） The mass conservation equation for the gas phase in the non-intrusive section is: （5） The mass conservation equation for the liquid phase is: （6） The momentum conservation equation for a gas-liquid two-phase system is: （7） In the formula, R s The solubility of gas in drilling fluid, m 3 / m 3 ; A , cross-sectional area of ​​the annulus, m 2 ; q = q gas / Δ H q Mass flow rate of air intrusion per unit length, kg / (s·m); Δ H q Depth of the gas-producing section, in meters; g, acceleration due to gravity, m / s² 2 ; ρ gsc Gas density under standard conditions, kg / m³ 3 ; ρ g Gas density, kg / m³ 3 ; ρ l Drilling fluid density, kg / m³ 3 ; u g Gas flow rate, m / s; u l Drilling fluid flow rate, m / s; E g Gas volume fraction, dimensionless; E l Integral number of drilling fluid, dimensionless; Frictional pressure drop, Pa / m; B l Z is the drilling fluid volume factor, %; P is the pressure, Pa; z is the vertical length, m; The finite difference method is used to discretize and solve the above multiphase flow model. The difference scheme is as follows: The finite difference scheme for the gas phase continuity equation in the gas intrusion section is as follows: （40） The finite difference scheme for the gas phase continuity equation in the non-intrusive section is as follows: （41） The finite difference scheme for the liquid phase continuity equation is as follows: （42） The difference scheme for the momentum equation is: （43） （44） （45） （46） j is a spatial node, dimensionless; n is a time node, which is dimensionless; Based on the actual engineering situation, determine the model. t =0、 t 1. t Initial and boundary conditions at time 2.

5. A system for inverting the volume of formation karst caverns during drilling gas invasion, wherein the system is applied to the method for inverting the volume of formation karst caverns during drilling gas invasion as described in claim 1, characterized in that, The system includes a throttling response parameter acquisition section, a wellbore throttling circulation pressure control section, and a multiphase flow simulation software section; The throttling response parameter acquisition section includes an inlet flow meter, an outlet flow meter, and a pressure sensor for measuring wellhead back pressure. The inlet flow meter and outlet flow meter measure the real-time flow at the drill string inlet and annulus outlet, respectively, and the pressure sensor measures the wellhead back pressure. When gas intrusion occurs at the bottom of the well or the wellhead throttling valve is adjusted, the above devices acquire response parameters in real time. The wellbore throttling circulation pressure control system includes a gas intrusion monitoring device at the wellhead, a throttling pipeline, and a throttling valve. The gas intrusion monitoring device relies on data from the inlet and outlet flow meters. When the outlet flow rate is greater than the inlet flow rate, the gas intrusion monitoring device will detect the occurrence of gas intrusion and issue a warning. Subsequently, the wellhead back pressure is changed by adjusting the throttling valve. The system also includes multiphase flow simulation software, which performs information input, model calculation, and output of calculation results. Throttling response parameters collected by the outlet flow meter, inlet flow meter, and pressure sensor, as well as relevant parameters from the model calculation, are input into the software. The software then calculates the volume of the karst cave and displays the results.

Images