A quantitative evaluation method for wellbore stability in horizontal sections of deep shale gas reservoirs

Abstract

The invention discloses a quantitative evaluation method for wellbore stability of a horizontal section of deep shale gas, comprising: step 1, quantitatively calculating formation collapse and fracture pressure; step 2, quantitatively characterizing the uncertainty of formation collapse and fracture pressure; step 3, calculating the ECD of the horizontal section of shale gas drilling, and quantitatively characterizing the uncertainty of the ECD of the horizontal section of shale gas drilling; step 4, based on the quantitative characterization of the uncertainty of formation collapse and fracture pressure in step 2 and the quantitative characterization of the uncertainty of the ECD of the horizontal section of shale gas drilling in step 3, quantitatively evaluating the wellbore stability of the horizontal section of shale gas, that is, establishing an analytical model for quantitatively evaluating the wellbore stability, thereby solving the problem of identifying the instability of the wellbore of the horizontal section of deep shale gas, realizing the quantitative evaluation of the wellbore stability, and effectively reducing the failure rate, improving the drilling efficiency, shortening the drilling cycle, and providing technical support for reducing the cost and speeding up the shale gas drilling construction.

Description

Technical Field

[0001] The invention relates to the technical field of deep shale gas complex formation drilling, and in particular to a quantitative evaluation method for wellbore stability of a horizontal section of deep shale gas. Background Art

[0002] In recent years, Sinopec has accelerated the exploration and development of shale gas resources and made major breakthroughs in the Sichuan and Chongqing regions. The southern Sichuan work area (Weiyuan, Changning, Zigong, etc.) is one of the areas with the richest shale gas resources and the greatest development potential in my country. The high formation pressure coefficient, complex drilling geological conditions, and poor wellbore stability in this work area lead to complex drilling and frequent failures. Based on the analysis of well data completed in 2018, the drilling was complex and the failure time reached 5029.79h, which seriously restricted the safe and efficient development of shale gas. Therefore, how to quantitatively evaluate the risk of wellbore instability in the horizontal section of deep shale gas is of great significance to ensure the safe drilling of the horizontal section of shale gas and reduce the frequency of complex failures in the well.

[0003] The existing wellbore stability analysis methods are all based on the formation collapse and fracture pressure profile composed of a single curve for evaluation, without considering the impact of formation pressure uncertainty on the wellbore stability analysis; at the same time, they ignore the impact of uncertainty factors such as high temperature and high pressure environment, irregular well diameter, and unstable displacement on the ECD calculation results. It is difficult to accurately calculate ECD, and its uncertainty needs to be described. For the above two reasons, it is impossible to quantitatively describe the probability and severity of the wellbore instability risk. Summary of the invention

[0004] In view of this, the present invention provides a quantitative evaluation method for the wellbore stability of the horizontal section of deep shale gas, which can solve the problem of identifying the wellbore instability of the horizontal section of deep shale gas, realize the quantitative evaluation of the wellbore stability, and can effectively reduce the failure rate, improve drilling efficiency, shorten the drilling cycle, and provide technical support for reducing costs and speeding up shale gas drilling construction.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A quantitative evaluation method for wellbore stability in horizontal sections of deep shale gas reservoirs, comprising:

[0007] Step 1, quantitatively calculating the formation collapse and fracture pressure;

[0008] Step 2: quantitatively characterize the uncertainty of formation collapse and fracture pressure;

[0009] Step 3, calculating the ECD of the horizontal drilling of shale gas, and quantitatively characterizing the uncertainty of the ECD of the horizontal drilling of shale gas;

[0010] Step 4: Based on the quantitative characterization of formation collapse and fracture pressure uncertainty in step 2 and the quantitative characterization of ECD uncertainty in shale gas horizontal section drilling in step 3, quantitatively evaluate the wellbore stability of the shale gas horizontal section.

[0011] Preferably, the quantitative characterization of the uncertainty of formation collapse and fracture pressure in step 2 includes: applying a Monte Carlo simulation method to perform a multi-factor uncertainty analysis on formation collapse and fracture pressure.

[0012] Preferably, the application of the Monte Carlo simulation method to perform multi-factor uncertainty analysis on formation collapse and fracture pressure includes:

[0013] (1) Determine the probability distribution of geostress and rock mechanics parameters

[0014] Assuming the probability density function of rock mechanics parameters and ground stress parameter X is f(x), the normal information diffusion estimate of the probability density function f(x) is:

[0015]

[0016] Where h is the diffusion coefficient, m is the rock mechanics parameter and ground stress parameter X in the target layer group ΔH = [H u ,H l ] is the maximum value x max , the minimum value is x min , then h is:

[0017]

[0018] Among them, the coefficient λ can be obtained according to Table 1:

[0019] (2n+1) λ (2n+1) λ (2n+1) λ 3 0.84932180 8 1.395189816 13 1.420698795 4 1.273982782 9 1.422962345 14 1.420669671 5 1.698643675 10 1.416278786 15 1.420693321 6 1.336252561 11 1.420835443 16 1.420692226 7 1.445461208 12 1.420269570 ≥17 1.420693101

[0020] Table 1

[0021] According to formula (1) and formula (2), the analysis sample library of the model input parameters at any depth h is constructed, and the probability distribution fitting function F is obtained based on the normal information diffusion estimation method. N ~f(x1),f(x2),···,f(x n );

[0022] (2) Constructing a random simulation sample set

[0023] Generate random numbers that conform to the probability distribution of key parameters (characteristic parameters, i.e., mean and standard deviation) of each model calculation, obtain random number samples of ground stress and rock mechanics parameters, and substitute them into the calculation model to obtain the pressure simulation calculation results at any depth position h;

[0024] (3) Constructing a sample set for formation collapse and fracture pressure analysis

[0025] Statistically analyze the pressure simulation calculation results, select the normal distribution form for fitting, and obtain the probability distribution and cumulative probability distribution function f of the formation collapse and fracture pressures at any depth h h (P t,f ), F h (P t,f );

[0026] (4) Quantitatively characterize the uncertainty of formation collapse and fracture pressures

[0027] Through the above method, the cumulative probabilities of formation collapse and fracture pressures at different depths are obtained, which can form a set:

[0028]

[0029] Denote the formation collapse and fracture pressure values with a cumulative probability of j at a depth of h i , take the same cumulative probability value j0, and form a new set:

[0030]

[0031] According to the elements in set (4), obtain the formation collapse and fracture pressure curves with cumulative probabilities of j1 and j2 (j1 < j2) The two curves form a formation collapse and fracture pressure interval with a confidence level of |j1 - j2|×100%.

[0032] Preferably, in step 4, the quantitative evaluation of the wellbore stability in the horizontal section of shale gas includes:

[0033] According to the wellbore instability risk mechanism, define the generalized stress as ECD, the generalized strength as the formation collapse and fracture pressures, and the risk function as the wellbore instability function, and establish an analytical model for quantitatively evaluating wellbore stability.

[0034] Preferably, the definition of the risk function as the wellbore instability function includes:

[0035] The parameters describing the function of the drilling fluid are random variables of reliability. Its reliability refers to the probability that the drilling fluid column can balance the formation collapse pressure, that is, the probability R that the equivalent circulating density of the drilling fluid is greater than the formation collapse pressure. The calculation formula is:

[0036] R = P(Q > S) = P(Q - S > 0) = P(Q / S > 1) (5)

[0037] In the formula, Q is the random variable of the equivalent circulating density of the drilling fluid, and S is the random variable of the formation collapse pressure;

[0038] When the random variable Q of drilling fluid equivalent circulating density and the random variable S of formation collapse pressure are both normally distributed, the interference random variable Z = QS is also normally distributed, and its probability density function is:

[0039]

[0040] In the formula, μ Z =μ S -μ Q ,

[0041] When Q>S or QS>0, the wellbore is stable, so the reliability R is expressed as:

[0042]

[0043] The probability of wellbore instability and reliability are inverse probabilities, that is, the probability F of wellbore instability is:

[0044]

[0045] Preferably, in step 3, calculating the ECD of shale gas horizontal drilling includes:

[0046] Equivalent Static Density (ESD) is the equivalent density of the liquid column pressure on any section of the wellbore, and is expressed by the formula:

[0047]

[0048] Where ESD is the equivalent static density of drilling fluid at the well depth H, g / cm 3 ;P o is the ground pressure, MPa; P esd is the hydrostatic column pressure at the well depth H, MPa; H is the well depth, m;

[0049] The equivalent circulating density (ECD) of drilling fluid is defined as the sum of the equivalent static density of drilling fluid and the annular pressure drop caused by the flow of drilling fluid. The expression of equivalent circulating density is as follows:

[0050]

[0051] Where ESD is the equivalent static density of drilling fluid at the well depth H, g / cm 3 ; ΔP f is the annular pressure loss at the well depth H, MPa; H is the well depth, m;

[0052] The ESD calculation process is as follows: firstly, a wellbore temperature field and a drilling fluid density prediction model are established, and an iterative numerical method is used to establish an ESD calculation model during the drilling fluid circulation period.

[0053] Preferably, in step 3, the uncertainty of the quantitative characterization of the ECD of the horizontal section drilling of shale gas includes at least high pressure environment, irregular well diameter and unstable displacement.

[0054] Preferably, the quantitative calculation of formation collapse and fracture pressure in step 1 includes:

[0055] Calculate collapse pressure

[0056]

[0057] In the formula, ρ c Collapse pressure, expressed as equivalent drilling fluid density, g / cm 3 ; H is the well depth, m; K = cot (45o-φ / 2), φ is the rock internal friction angle, °; C is the rock cohesion, MPa; ρ p Formation pore pressure, g / cm 3 ; σ H is the maximum horizontal principal stress, expressed as equivalent drilling fluid density, g / cm 3 ; σ h is the minimum horizontal principal stress, g / cm 3 ; η is the stress nonlinear correction coefficient, which is infinite; α is the effective stress Biot coefficient;

[0058] Calculate burst pressure

[0059] ρ f =3σ h -σ H -αρ p +S t / 0.00981×H (19)

[0060] In the formula, ρ f is the formation fracture pressure, expressed as equivalent drilling fluid density g / cm 3 ; S t is the tensile strength, MPa.

[0061] It can be seen from the above technical solutions that the quantitative evaluation method for wellbore stability in horizontal sections of deep shale gas provided by the present invention has the following beneficial effects:

[0062] 1. The present invention establishes and improves the existing deep shale gas formation collapse and fracture pressure calculation model, and quantitatively characterizes the formation collapse and fracture pressure. The result is no longer a single formation pressure curve, but a formation pressure interval profile with probability information;

[0063] 2. The present invention comprehensively considers the influence of uncertainty factors such as high temperature and high pressure environment, irregular well diameter, unstable displacement, etc. on the ECD calculation results, and establishes a quantitative characterization method for ECD uncertainty;

[0064] 3. The present invention establishes an analysis method that can quantitatively evaluate wellbore stability, solves the problem of identifying wellbore instability in the horizontal section of deep shale gas, and realizes quantitative evaluation of wellbore stability. It can effectively reduce the occurrence rate of failures, improve drilling efficiency, shorten the drilling cycle, and provide technical guarantee for reducing costs and speeding up shale gas drilling construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0066] Figure 1 A flow chart of a quantitative evaluation method for horizontal section wellbore stability in deep shale gas provided in an embodiment of the present invention;

[0067] Figure 2 A schematic diagram of a quantitative assessment model for wellbore instability risk provided by an embodiment of the present invention;

[0068] Figure 3 A probability interference distribution diagram of formation collapse pressure-circulating equivalent drilling fluid density provided by an embodiment of the present invention;

[0069] Figure 4 The probability distribution of the 3210m formation fracture pressure of the XX well provided in the embodiment of the present invention;

[0070] Figure 5 The probability distribution of formation collapse pressure at 3720m of Well XX provided in the embodiment of the present invention;

[0071] Figure 6 The probability distribution of equivalent circulating density of drilling fluid at 3210 m in Well XX provided by the embodiment of the present invention;

[0072] Figure 7 The probability distribution of equivalent circulating density of drilling fluid at 3720 m in Well XX provided by the embodiment of the present invention;

[0073] Figure 8 The risk assessment result of the well wall rupture at 3210 m of the XX well provided in the embodiment of the present invention;

[0074] Fig. 9This is the risk assessment result of wellbore collapse at 3720m in the XX well provided in the embodiment of the present invention. DETAILED DESCRIPTION

[0075] The present invention discloses a quantitative evaluation method for wellbore stability in horizontal sections of deep shale gas, establishes and improves the existing formation collapse and fracture pressure calculation model; on this basis, a quantitative characterization method for the uncertainty of formation collapse and fracture pressure is established; at the same time, considering the influence of uncertainty factors such as high temperature and high pressure environment, irregular well diameter, unstable displacement on the ECD calculation results, an ECD uncertainty quantitative characterization method is established based on uncertainty theory; on the basis of the above two aspects of research, an analytical model for quantitatively evaluating wellbore stability is established, which solves the problem of identifying wellbore instability in horizontal sections of deep shale gas, realizes quantitative evaluation of wellbore stability, can effectively reduce the occurrence rate of failures, improve drilling efficiency, shorten drilling cycle, and provide technical support for reducing costs and speeding up shale gas drilling construction.

[0076] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0077] The embodiment of the present invention provides a quantitative evaluation method for the stability of the horizontal section of a deep shale gas wellbore, such as Figure 1 As shown, including:

[0078] Step 1, quantitatively calculating the formation collapse and fracture pressure;

[0079] Step 2: quantitatively characterize the uncertainty of formation collapse and fracture pressure;

[0080] Step 3, calculating the ECD of the horizontal drilling of shale gas, and quantitatively characterizing the uncertainty of the ECD of the horizontal drilling of shale gas;

[0081] Step 4: Based on the quantitative characterization of the uncertainty of formation collapse and fracture pressure in step 2 and the quantitative characterization of the uncertainty of ECD during drilling of the shale gas horizontal section in step 3, the wellbore stability of the shale gas horizontal section is quantitatively evaluated.

[0082] It can be seen from the above technical solutions that the quantitative evaluation method for horizontal section wellbore stability of deep shale gas provided by the embodiment of the present invention has the following beneficial effects:

[0083] 1. The present invention establishes and improves the existing deep shale gas formation collapse and fracture pressure calculation model, and quantitatively characterizes the formation collapse and fracture pressure. The result is no longer a single formation pressure curve, but a formation pressure interval profile with probability information;

[0084] 2. The present invention comprehensively considers the influence of uncertainty factors such as high temperature and high pressure environment, irregular well diameter, unstable displacement, etc. on the ECD calculation results, and establishes a quantitative characterization method for ECD uncertainty;

[0085] 3. The present invention establishes a method for quantitatively evaluating wellbore stability, solves the problem of identifying wellbore instability in the horizontal section of deep shale gas, and realizes quantitative evaluation of wellbore stability. It can effectively reduce the occurrence rate of failures, improve drilling efficiency, shorten the drilling cycle, and provide technical guarantee for reducing costs and speeding up shale gas drilling construction.

[0086] In the quantitative evaluation method for wellbore stability of horizontal sections of deep shale gas provided in an embodiment of the present invention, the quantitative characterization of the uncertainty of formation collapse and fracture pressure in step 2 includes: applying the Monte Carlo simulation method to perform multi-factor uncertainty analysis on formation collapse and fracture pressure, and establishing a quantitative characterization method for uncertainty of formation collapse and fracture pressure on the basis of selecting similar structures. The result is no longer a single formation pressure curve, but a formation pressure interval profile with probability information, which helps to improve the accuracy of the quantitative evaluation method for wellbore stability of horizontal sections of deep shale gas.

[0087] Specifically, the application of the Monte Carlo simulation method to perform multi-factor uncertainty analysis on formation collapse and fracture pressure includes:

[0088] (1) Determine the probability distribution of geostress and rock mechanics parameters

[0089] In order to analyze the uncertainty of rock mechanical parameters and geostress and determine their probability distribution, it is necessary to first establish a sample library of rock mechanical parameters and geostress parameters. According to sequence stratigraphy, "in the same geological period and under the same sedimentary conditions, the formation rocks have the same lithology and will produce similar seismic or logging responses". Therefore, the logging interpretation results of rock mechanical parameters and geostress within a certain depth range of the same layer group are selected as samples to construct a sample library.

[0090] Assume that there are 2n+1 logging interpretation results of rock mechanical parameters and geostress within the range of well depth ΔH, which are taken as a set of measurement samples {x i-n ,x i-n+1 ,…,x i+n}; ΔH is the sample interval, and its value is twice the range of the theoretical variogram model in the sample stratigraphic group. Then, the probability distribution of rock mechanics and geostress parameters is obtained by normal information diffusion estimation, which is used as the probability distribution of parameters at the midpoint of the well depth range. Assuming that the probability density function of rock mechanics parameters and geostress parameters X is f(x), the normal information diffusion estimation of the probability density function f(x) is:

[0091]

[0092] Where h is the diffusion coefficient, m is the rock mechanics parameter and ground stress parameter X in the target layer group ΔH = [H u ,H l ] is the maximum value x max , the minimum value is x min , then h is:

[0093]

[0094] Among them, the coefficient λ can be obtained according to Table 1:

[0095] (2n+1) λ (2n+1) λ (2n+1) λ 3 0.84932180 8 1.395189816 13 1.420698795 4 1.273982782 9 1.422962345 14 1.420669671 5 1.698643675 10 1.416278786 15 1.420693321 6 1.336252561 11 1.420835443 16 1.420692226 7 1.445461208 12 1.420269570 ≥17 1.420693101

[0096] Table 1

[0097] According to formula (1) and formula (2), the analysis sample library of the model input parameters at any depth h is constructed, and the probability distribution fitting function F is obtained based on the normal information diffusion estimation method. N ~f(x1),f(x2),···,f(x n );

[0098] (2) Constructing a random simulation sample set

[0099] Generate random numbers that conform to the probability distribution of key parameters (characteristic parameters, i.e., mean and standard deviation) of each model calculation, obtain random number samples of ground stress and rock mechanics parameters, and substitute them into the calculation model to obtain the pressure simulation calculation results at any depth position h;

[0100] (3) Constructing a sample set for formation collapse and fracture pressure analysis

[0101] The pressure simulation calculation results are statistically analyzed, and the normal distribution form is selected for fitting to obtain the probability distribution of formation collapse and fracture pressure at any depth h and the cumulative probability distribution function f h (P t,f ), F h (P t,f );

[0102] (4) Quantitative characterization of formation collapse and fracture pressure uncertainty

[0103] Through the above method, the cumulative probabilities of formation collapse and fracture pressures at different depths are obtained, which can form a set:

[0104]

[0105] Denote the formation collapse and fracture pressure value with cumulative probability j at depth h i Take the same cumulative probability value j0 to form a new set:

[0106]

[0107] According to the elements in set (4), formation collapse and fracture pressure curves with cumulative probabilities j1 and j2 (j1 < j2) are obtained The two curves form a formation collapse and fracture pressure interval with a confidence level of |j1 - j2|×100%, which indicates that the actual value of the formation collapse and fracture pressure at the depth of this well falls within the confidence interval with a probability of |j1 - j2|×100%. When j1 = 0.05 and j2 = 0.95, a formation collapse and fracture pressure interval profile with a confidence level of 90% can be obtained

[0108] In order to further optimize the above technical solution, in step 4, the quantitative evaluation of the wellbore stability in the horizontal section of shale gas includes:

[0109] According to the risk mechanism of wellbore instability, the generalized stress is defined as ECD, the generalized strength is defined as the formation collapse and fracture pressure, the risk function is defined as the wellbore instability function, an analysis model for quantitatively evaluating wellbore stability is established, solving the problem of identifying wellbore instability in the horizontal section of deep shale gas, realizing the quantitative evaluation of wellbore stability, which can effectively reduce the failure rate, improve the drilling efficiency, shorten the drilling cycle, and provide technical support for cost reduction and speed increase in shale gas drilling construction

[0110] Specifically, the definition of the risk function as the wellbore instability function includes:

[0111] The parameter describing the function of drilling fluid is a random variable of reliability, and its reliability refers to the probability that the drilling fluid column can balance the formation collapse pressure, that is, the probability R that the equivalent circulating density of the drilling fluid is greater than the formation collapse pressure. The calculation formula is:

[0112] R = P(Q > S) = P(Q - S > 0) = P(Q / S > 1) (5)

[0113] In the formula, Q is the random variable of the equivalent circulating density of the drilling fluid, and S is the random variable of the formation collapse pressure; it should be noted that here the analysis process is specifically described taking the occurrence of wellbore collapse risk as an example. For example Figure 2The reason for the risk of wellbore collapse is that the pressure of the drilling fluid column in the wellbore is lower than the formation collapse pressure, which cannot maintain the stability of the wellbore, causing the wellbore safety barrier to fail;

[0114] In order to derive the risk probability calculation model, the drilling risk diagram Figure 2 The interference shadow part in is enlarged, such as Figure 3 When the random variable Q of drilling fluid equivalent circulating density and the random variable S of formation collapse pressure are both normally distributed, the interference random variable Z = QS is also normally distributed, and its probability density function is:

[0115]

[0116] In the formula, μ Z =μ S -μ Q ,

[0117] When Q>S or QS>0, the wellbore is stable, so the reliability R is expressed as:

[0118]

[0119] The probability of wellbore instability and reliability are inverse probabilities, that is, the probability F of wellbore instability is:

[0120]

[0121] In addition, the reason for the risk of wellbore rupture is that the pressure of the drilling fluid column in the wellbore is greater than the formation rupture pressure, which cannot maintain the stability of the wellbore, causing the wellbore safety barrier to fail; similarly, the probability value of the wellbore rupture risk can be obtained.

[0122] Preferably, in step 3, calculating the ECD of shale gas horizontal drilling includes:

[0123] Equivalent Static Density (ESD) is the equivalent density of the liquid column pressure on any section of the wellbore, and is expressed by the formula:

[0124]

[0125] Where ESD is the equivalent static density of drilling fluid at the well depth H, g / cm 3 ;P o is the ground pressure, MPa; P esd is the hydrostatic column pressure at the well depth H, MPa; H is the well depth, m;

[0126] The equivalent circulating density (ECD) of drilling fluid is defined as the sum of the equivalent static density of drilling fluid and the annular pressure drop caused by the flow of drilling fluid. The expression of equivalent circulating density is as follows:

[0127]

[0128] Where ESD is the equivalent static density of drilling fluid at the well depth H, g / cm 3 ; ΔP f is the annular pressure loss at the well depth H, MPa; H is the well depth, m;

[0129] The calculation of the ECD includes two parts: ESD and annular pressure loss. Among them, the calculation process of ESD is as follows: first, a prediction model of the wellbore temperature field and drilling fluid density is established, and an iterative numerical method is used to establish an ESD calculation model during the circulation of drilling fluid, so as to take into account the influence of unstable factors such as high-pressure environment, irregular well diameter and unstable displacement on the ESD calculation, so as to improve the accuracy of the ESD calculation model. Of course, in this scheme, the problem of friction pressure loss in the annulus during drilling circulation has been disclosed, and the present invention will not repeat it. This scheme discusses the source of ECD uncertainty: the uncertainty of ECD comes from the accuracy of the calculation model, the randomness and fuzziness of the parameters in the model; then, based on the uncertainty theory, the ECD uncertainty calculation formula is derived.

[0130] In order to further optimize the above technical solution, in step 3, the uncertainty of ECD of shale gas horizontal section drilling is quantitatively characterized, and its uncertainties include at least high-pressure environment, irregular well diameter and unstable displacement, so as to comprehensively consider the influence of uncertainty factors such as high temperature and high-pressure environment, irregular well diameter and unstable displacement on the ECD calculation results, and then establish an ECD uncertainty quantitative characterization method based on uncertainty theory to reduce the difficulty of accurately calculating ECD.

[0131] Furthermore, the main process of calculating formation fracture pressure and collapse pressure is: obtaining rock mechanical parameters, selecting appropriate geostress model to calculate geostress, selecting formation collapse and fracture pressure calculation model according to geological environment lithology and other characteristics, and finally calculating and obtaining formation collapse and fracture pressure. When calculating formation collapse and fracture pressure, many key calculation parameters affect the results, mainly including geostress and rock mechanical parameters, which can be calculated based on well logging data or seismic interpretation data.

[0132] Specifically, the quantitative calculation of formation collapse and fracture pressure in step 1 includes:

[0133] (1) Obtaining rock mechanics parameters; wherein the rock mechanics parameters are divided into rock mechanics physical parameters, rock elastic parameters and rock strength parameters. Obtaining rock mechanics parameters includes:

[0134] 1) Calculate the rock mechanical physical parameters

[0135] The rock longitudinal wave velocity Vp is:

[0136]

[0137] The rock shear wave velocity Vs is:

[0138]

[0139] Where, ΔT p is the rock longitudinal wave time difference, us / m; ΔT s is the rock shear wave time difference, us / m; ρ b is the rock density, g / cm 3 ; E d is the dynamic Young's modulus, MPa; u d is the dynamic Poisson’s ratio, dimensionless;

[0140] 2) Calculate the rock elastic parameters

[0141] The relationship between the various rock elastic parameters is:

[0142]

[0143] Where, E is Young's modulus, MPa; G is shear modulus, MPa; K b is the bulk elastic modulus, MPa; C b is the volume compression coefficient, dimensionless;

[0144] The rock elastic parameters are divided into dynamic elastic parameters and static elastic parameters:

[0145] 2.1. Obtaining the dynamic elastic parameters

[0146] According to formula (11), formula (12) and formula (13), the dynamic rock mechanics elastic parameter formula is derived, as shown in Table 2:

[0147]

[0148] Table 2

[0149] 2.2. Conversion between the dynamic elastic parameters and the static elastic parameters

[0150] The conversion relationship between the dynamic elastic parameter and the static elastic parameter is:

[0151] μ s =A1+K1μ d 、E s =A2+K2E d (14)

[0152] In the formula, A1 = a 11 +a 12 lg(σ1-σ3),A2=a 21 +a 22 lg(σ1-σ3), K1=k 11 +k 12 lg(σ1-σ3),K2=k 21 +k 22 lg(σ1-σ3), σ1 and σ3 are the maximum and minimum principal stresses respectively, a 11 、a 12 、a 21 、a 22 , k 11 , k 12 , k 21 , k 22 is the regression coefficient. The above conversion relationship needs to be obtained based on the regression of the core chamber experiment;

[0153] 3) Calculate the rock strength parameter, wherein the calculation formula of the rock strength parameter is shown in Table 3:

[0154]

[0155]

[0156] Table 3

[0157] In the table, ρ is the rock density, g / cm 3 ; V cl is the shale content, dimensionless; φ is the internal friction angle, °; M = ab × C; a and b are coefficients related to rock properties, obtained by reverse calculation from core test experiments;

[0158] (2) Calculate the ground stress

[0159] σ v =∫G o dH (15)

[0160] When the regional structure is more intense:

[0161] In the formula, σ v is the vertical stress, MPa; σ H is the maximum horizontal ground stress, MPa; σ his the minimum horizontal geostress, MPa; u is Poisson's ratio, dimensionless; E is elastic modulus, MPa; H is well depth, m; G p Formation pore pressure, MPa; β1 and β2 are tectonic stress coefficients, which are obtained by back-calculation from indoor core experiments or field formation fracture test data, namely:

[0162]

[0163] In the formula, the maximum and minimum horizontal geostress are measured by geostress measurement method, and the other parameters are calculated based on well logging data;

[0164] (3) Calculate the collapse pressure

[0165]

[0166] In the formula, ρ c is the collapse pressure (calculated according to the Mohr-Coulomb strength criterion), expressed as equivalent drilling fluid density, g / cm 3 ; H is the well depth, m; K = cot (45o-φ / 2), φ is the rock internal friction angle, °; C is the rock cohesion, MPa; ρ p Formation pore pressure, g / cm 3 ; σ H is the maximum horizontal principal stress, expressed as equivalent drilling fluid density, g / cm 3 ; σ h is the minimum horizontal principal stress, g / cm 3 ; η is the stress nonlinear correction coefficient, which is infinite; α is the effective stress Biot coefficient;

[0167] (4) Calculate the bursting pressure

[0168] ρ f =3σ h -σ H -αρ p +S t / 0.00981×H (19)

[0169] In the formula, ρ f is the formation fracture pressure, expressed as equivalent drilling fluid density g / cm 3 ; S t is the tensile strength, MPa. The quantitative calculation model of formation collapse and fracture pressure based on the above-mentioned formation collapse and fracture pressure is helpful to improve the existing calculation model of deep shale gas formation collapse and fracture pressure.

[0170] The present invention is described in detail below in conjunction with diagrams and embodiments:

[0171] Well XX is a deep shale gas exploration well in the southern Sichuan work area. During the drilling process of the fourth horizontal section of Well XX, a complex situation of leakage and collapse occurred, which seriously affected the normal and safe drilling. Therefore, Well XX was selected for case analysis. The minimum fracture pressure and maximum collapse pressure in the fourth section of Well XX are located at depths of 3210m and 3720m. The probability distribution form and characteristic parameters of the ground stress and rock mechanics parameters at 3210m and 3720m were calculated respectively. The results are shown in Table 4.

[0172]

[0173] Table 4

[0174] According to the probability distribution of geostress and rock mechanics parameters in Table 4, the random number is set to 4000 based on Monte Carlo simulation to generate a certain number of random values. The random values ​​are substituted into equations (18) and (19) to obtain a series of calculation results of formation collapse and fracture pressure. Based on probability statistics and normal information diffusion estimation method, the probability distribution of formation collapse and fracture pressure is obtained. The results are shown in Figure 4 The 3210m burst pressure is N (1.63, 0.005^2) and Figure 5 The 3720m formation collapse is shown as N(1.576,0.0085^2). Based on the established ECD uncertainty analysis method, the probability distribution of ECD at a well depth of 3720m is obtained. The results are shown in Figure 7 The 210m ECD-N (1.61, 0.008^2) and the probability distribution of ECD at a well depth of 3210m are shown in the following table. Figure 6 The 210m ECD-N (1.61, 0.008^2) is shown in the figure. According to the established wellbore stability evaluation method, the wellbore instability probability at the wellbore depths of 3210m and 3720m is quantitatively calculated and compared with the actual situation. The results are shown in the figure below. Figure 8 and Fig. 9 As shown. Figure 8 It can be seen that at a well depth of 3210m, the characteristic parameter of the probability distribution of the formation fracture pressure is: μ Q =1.63,σ Q =0.005; ECD probability distribution characteristic parameters are: μ S =1.61,σ S =0.008; the distribution interval of formation fracture pressure is [1.615, 1.645], and the distribution interval of ECD is [1.585, 1.635]. The interference between the two indicates the occurrence of wellbore instability risk; according to the risk quantitative calculation model and formula established by the present invention, and the application of Matlab programming to calculate the probability of wellbore instability risk is 63%. In the actual drilling process, the risk of wellbore fracture occurred at this location, and the evaluation result is consistent with the actual situation. Fig. 9 It can be seen that at a well depth of 3720m, the characteristic parameter of the probability distribution of formation collapse pressure is: μ Q =1.76,σ Q =0.0085; ECD probability distribution characteristic parameters are: μ S =1.59,σ S =0.0074; the distribution interval of formation collapse pressure is [1.505, 1.61], and the distribution interval of ECD is [1.568, 1.621]. The interference between the two indicates the occurrence of wellbore instability risk; according to the risk quantitative calculation model and formula, and the application of Matlab programming, the probability of wellbore instability is 78%. In the actual drilling process, the wellbore collapse risk occurred at this well depth, and the evaluation result is consistent with the actual situation. The example analysis verifies the reliability and accuracy of the method established in the present invention.

[0175] In summary, in the quantitative evaluation method for wellbore stability of horizontal sections of deep shale gas provided by the embodiment of the present invention, first, the logging data of key wells drilled in the area are collected, and the existing shale gas formation collapse and fracture pressure calculation model is established and improved; on this basis, the Monte Carlo simulation method is used to perform multi-factor uncertainty analysis on the prediction model, and a quantitative characterization method for formation collapse and fracture pressure uncertainty is established on the basis of drawing on and selecting similar structures; at the same time, considering the influence of uncertainty factors such as high temperature and high pressure environment, irregular well diameter, and unstable displacement on the ECD calculation results, a quantitative characterization method for ECD uncertainty is established based on uncertainty theory; on the basis of the above two aspects of research, based on the reliability theory of generalized stress and strength interference, the generalized stress is defined as the equivalent circulating density (ECD) of the wellbore drilling fluid, and the generalized strength is defined as the upper and lower limits of the safe drilling fluid density window for maintaining wellbore stability: that is, the lower limit of collapse pressure and the upper limit of fracture pressure, and the risk function is defined as the wellbore instability function; finally, a quantitative evaluation model for wellbore stability is established. The method is mainly divided into the following four steps: (1) quantitative calculation of formation collapse and fracture pressure; (2) quantitative characterization of uncertainty of formation collapse and fracture pressure; (3) ECD calculation and uncertainty analysis of shale gas horizontal section drilling; (4) quantitative evaluation of wellbore stability of shale gas horizontal section.

[0176] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0177] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A quantitative evaluation method for wellbore stability in horizontal sections of deep shale gas, characterized in that: include: Step 1, quantitatively calculating the formation collapse and fracture pressure; Step 2: quantitatively characterize the uncertainty of formation collapse and fracture pressure; Step 3, calculating the ECD of the horizontal drilling of shale gas, and quantitatively characterizing the uncertainty of the ECD of the horizontal drilling of shale gas; Step 4: Based on the quantitative characterization of formation collapse and fracture pressure uncertainty in step 2 and the quantitative characterization of ECD uncertainty in shale gas horizontal section drilling in step 3, quantitatively evaluate the wellbore stability of the shale gas horizontal section; including: According to the mechanism of wellbore instability, the generalized stress is defined as ECD, the generalized strength is defined as the formation collapse and fracture pressure, the risk function is defined as the wellbore instability function, and an analytical model for quantitatively evaluating wellbore stability is established. Defining the risk function as a wellbore instability function includes: The parameter describing the function of drilling fluid is a random variable of reliability. Its reliability refers to the probability that the drilling fluid column can balance the formation collapse pressure, that is, the probability R that the equivalent circulating density of the drilling fluid is greater than the formation collapse pressure. The calculation formula is: Where Q is the random variable of drilling fluid equivalent circulating density, and S is the random variable of formation collapse pressure; When the random variable Q of drilling fluid equivalent circulating density and the random variable S of formation collapse pressure are both normally distributed, the interference random variable Z=QS is also normally distributed, and its probability density function is: In the formula, , ; when Q>S or QS> At 0, the wellbore is stable, so the reliability R The expression is: The probability of wellbore instability and reliability are inverse probabilities, that is, the probability of wellbore instability F for: （8）。 2. The quantitative evaluation method for horizontal section wellbore stability of deep shale gas according to claim 1 is characterized in that: The quantitative characterization of the uncertainty of formation collapse and fracture pressure in step 2 includes: applying Monte Carlo simulation method to perform multi-factor uncertainty analysis on formation collapse and fracture pressure.

3. The quantitative evaluation method for horizontal section wellbore stability of deep shale gas according to claim 2 is characterized in that: The application of the Monte Carlo simulation method to perform multi-factor uncertainty analysis on formation collapse and fracture pressure includes: (1) Determine the probability distribution of geostress and rock mechanics parameters Assuming the probability density function of rock mechanics parameters and ground stress parameter X is f(x), the normal information diffusion estimate of the probability density function f(x) is: Where h is the diffusion coefficient, m is the rock mechanics parameter and ground stress parameter X in the target layer group ΔH=[H u ,H l ] is the maximum value x max , the minimum value is x min , then h is: Among them, the coefficient λ can be obtained according to Table 1: Table 1 According to the above formulas (1) and (2), an analysis sample library of the model input parameters at any depth h is constructed, and the probability distribution fitting function is obtained based on the normal information diffusion estimation method. ; (2) Constructing a random simulation sample set Generate random numbers that conform to the probability distribution of key parameters (characteristic parameters, i.e., mean and standard deviation) of each model calculation, obtain random number samples of ground stress and rock mechanics parameters, and substitute them into the calculation model to obtain the pressure simulation calculation results at any depth position h; (3) Constructing a sample set for formation collapse and fracture pressure analysis The pressure simulation calculation results are statistically analyzed, and the normal distribution form is selected for fitting to obtain the probability distribution of formation collapse and fracture pressure at any depth h and the cumulative probability distribution function f h (P t,f ), F h (P t,f ); (4) Quantitative characterization of formation collapse and fracture pressure uncertainty Through the above method, the cumulative probability of formation collapse and fracture pressure at different depths is obtained, which can be combined into a set: （3） Indicates the depth is h i The formation collapse and fracture pressure values ​​with cumulative probability j at location j are taken with the same cumulative probability value j0 to form a new set: （4） According to the elements in the set (4), obtain the formation collapse and fracture pressure curves with cumulative probabilities of j1 and j2 (j1 < j2). , , and the two curves form a formation collapse and fracture pressure interval with a confidence level of .

4. The quantitative evaluation method for horizontal section wellbore stability of deep shale gas according to claim 1 is characterized in that: In step 3, calculating the ECD of shale gas horizontal drilling includes: Equivalent Static Density (ESD) is the equivalent density of the liquid column pressure on the drilling fluid at any section of the wellbore, expressed by the formula: （9） Where ESD is the well depth H Equivalent static density of drilling fluid at g / cm 3 ; P o is the ground pressure, MPa; P esd Well depth H Hydrostatic column pressure at , MPa; H is the well depth, m; The equivalent circulating density (ECD) of drilling fluid is defined as the sum of the equivalent static density of drilling fluid and the annular pressure drop caused by the flow of drilling fluid. The expression of equivalent circulating density is as follows: （10） Where ESD is the well depth. H Equivalent static density of drilling fluid at , g / cm 3 ; Δ P f Well depth H Annular pressure loss, MPa; H is the well depth, m; The ESD calculation process is as follows: firstly, a wellbore temperature field and a drilling fluid density prediction model are established, and an iterative numerical method is used to establish an ESD calculation model during the drilling fluid circulation period.

5. The quantitative evaluation method for horizontal section wellbore stability of deep shale gas according to claim 1 is characterized in that: In step 3, the uncertainty of the ECD of the horizontal drilling of shale gas is quantitatively characterized, and its uncertainties include at least high pressure environment, irregular well diameter and unstable displacement.

6. The quantitative evaluation method for horizontal section wellbore stability of deep shale gas according to claim 1 is characterized in that: The quantitative calculation of formation collapse and fracture pressure in step 1 includes: Calculate collapse pressure （18） In the formula, ρ c Collapse pressure, expressed as equivalent drilling fluid density, g / cm 3 ; H is the well depth, m; K = cot (45o-ϕ / 2), ϕ is the rock internal friction angle, °; C is the rock cohesion, MPa; ρ p Formation pore pressure, g / cm 3 ; σ H is the maximum horizontal principal stress, expressed as equivalent drilling fluid density, g / cm 3 ; σ h is the minimum horizontal principal stress, g / cm 3 ; η is the stress nonlinear correction coefficient, which is infinite; α is the effective stress Biot coefficient; Calculate burst pressure （19） In the formula, ρ f is the formation fracture pressure, expressed as equivalent drilling fluid density g / cm 3 ; S t is the tensile strength, MPa.

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