A method for determining collapse pressure of a geothermal well

Abstract

The application provides a method for determining the collapse pressure of a geothermal well, and belongs to the technical field of drilling. The method comprises the following steps: collecting basic parameters of a target formation; determining a tensile strength reduction parameter of the rock of the target formation through a thermal cracking experiment, the tensile strength reduction parameter being used to represent the reduction degree of the tensile strength of the rock affected by thermal impact; and substituting the basic parameters into a collapse pressure prediction model to obtain the collapse pressure of the target formation, the expression of the collapse pressure prediction model being fused with the tensile strength reduction parameter. The reduction effect of high-temperature thermal impact on the strength of the rock is considered when the collapse pressure is calculated, so that the prediction result of the collapse pressure of the formation is more reasonable, the safe bearing upper limit can be obtained in combination with the engineering safety coefficient, and a scientific basis is provided for construction arrangement.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of drilling, in particular to a method for determining collapse pressure of geothermal well. BACKGROUND

[0002] In the development process of geothermal well, sticking of drill pipe often occurs due to collapse, which brings great loss. Therefore, wellbore stability is the biggest problem in the drilling process. Wellbore instability is caused by the change of ground stress near the borehole, stress concentration, and the failure of ground stress to establish a new balance with the drilling fluid pressure. When the formation collapse pressure is higher than the drilling fluid column pressure, the shear failure of wellbore rock will occur, which will lead to shrinkage if the wellbore is plastic rock, or collapse if the wellbore is brittle rock, resulting in expansion. The formation collapse pressure is an important parameter in the development process of geothermal well.

[0003] At present, the determination of collapse pressure is usually based on the calculation of wellbore stress distribution according to factors such as ground stress, borehole trajectory and wellbore pressure, and then the calculated stress is substituted into the rock strength criterion for calculation, so as to obtain the collapse pressure. However, for geothermal well, this method does not consider the influence of high temperature thermal shock on rock mechanics strength, so that the collapse pressure calculation is not accurate, which further affects the engineering design and construction. SUMMARY

[0004] The present application provides a method for determining the collapse pressure of geothermal well, which can make the prediction result of the formation collapse pressure more reasonable. The technical scheme is as follows:

[0005] Collecting basic parameters of the target formation, the basic parameters including maximum horizontal ground stress, minimum horizontal ground stress, rock cohesion, internal friction angle, rock Poisson's ratio, effective stress coefficient and stress nonlinearity correction coefficient;

[0006] Determining the tensile strength reduction parameter of the target formation rock by thermal fracture experiment, the tensile strength reduction parameter being used to represent the reduction degree of the rock tensile strength affected by thermal shock;

[0007] Substituting the basic parameters into the collapse pressure prediction model to obtain the collapse pressure of the target formation, the expression of the collapse pressure prediction model being fused with the tensile strength reduction parameter.

[0008] The technical scheme provided by the present application at least has the following beneficial effects:

[0009] The method provided by the embodiment of the application determines a tensile strength reduction parameter through a thermal rupture experiment, the parameter can represent the reduction degree of the tensile strength of the rock affected by thermal impact, and the tensile strength reduction parameter is fused in a collapse pressure prediction model, so that the reduction and loss effect of the rock strength caused by high-temperature thermal impact is considered when the collapse pressure is calculated, the prediction result of the formation collapse pressure is more reasonable, and the safe bearing upper limit can be obtained in combination with the engineering safety coefficient, thereby providing a scientific basis for construction arrangement. BRIEF DESCRIPTION OF DRAWINGS

[0010] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed to be used in the embodiment description will be briefly introduced.

[0011] Figure 1 is a flowchart of a method for determining the collapse pressure of a geothermal well provided by an example embodiment of the application;

[0012] Figure 2 is a flowchart of a method for determining the collapse pressure of a geothermal well provided by another example embodiment of the application;

[0013] Figure 3 is a flowchart of a method for determining the collapse pressure of a geothermal well provided by another example embodiment of the application. DETAILED DESCRIPTION

[0014] In order to make the objects, technical solutions and advantages of the application more clear, the embodiments of the application will be further described in detail below with reference to the drawings, and the described embodiments should not be regarded as limiting the application, and all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0015] In the following description, “some embodiments” are described, which describe a subset of all possible embodiments, but it can be understood that “some embodiments” can be the same subset or different subsets of all possible embodiments, and can be combined with each other without conflict.

[0016] If similar descriptions of “first\second\third” appear in the application file, the following description is added, in the following description, the terms “first\second\third” involved only distinguish similar objects, and do not represent a specific order of the objects, and it can be understood that “first\second\third” can be interchanged in a specific order or sequence as allowed, so that the embodiments of the application described here can be implemented in an order other than that illustrated or described here.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0018] Reference is made to Figure 1 which shows a flow chart of a method for determining a formation collapse pressure according to an example embodiment of the present application. The method comprises the following steps:

[0019] In step 101, basic parameters of the target formation are collected, including maximum horizontal stress, minimum horizontal stress, rock cohesion, internal friction angle, rock Poisson's ratio, effective stress coefficient, and stress nonlinearity correction coefficient.

[0020] Before predicting the collapse pressure of a geothermal well, a technician needs to collect and organize the basic parameters of the target formation.

[0021] The basic parameters include geomechanics parameters and rock mechanics parameters, including but not limited to maximum horizontal stress, minimum horizontal stress, rock cohesion, internal friction angle, rock Poisson's ratio, effective stress coefficient, and stress nonlinearity correction coefficient. Geostress is the stress existing in the earth's crust, i.e., the force per unit area within the medium due to rock deformation. Rock cohesion refers to the attractive force between the surface molecules of adjacent mineral particles within the rock. The internal friction angle refers to the inclination of the rock's shear strength line in the σ-τ coordinate plane. Poisson's ratio refers to the ratio of lateral normal strain to axial normal strain when a material is subjected to uniaxial tension or compression, and is an elastic constant that reflects the lateral deformation of the material. The effective stress coefficient, also known as the Biot coefficient, has a value between 0 and 1. The correction coefficient is a coefficient added to the calculation formula to make it as realistic as possible when there are deviations in data calculation, formula expression, etc. due to idealism and reality, reality and investigation, etc. It is generally represented by α. In step 102, the tensile strength reduction parameter of the target formation rock is determined through a thermal cracking experiment. The tensile strength reduction parameter is used to represent the degree of reduction of the rock's tensile strength under the influence of thermal impact.

[0022] Tensile strength refers to the maximum stress value that a material can withstand before it is pulled apart. The rock in the formation has a certain tensile strength, i.e., it can withstand a certain external force. However, geothermal wells have a special external force, thermal impact, which causes the rock's tensile strength to be reduced to some extent, thereby affecting its collapse pressure. Different rock types have different degrees of tensile strength reduction under the same thermal impact, so the technician needs to determine the tensile strength reduction parameter of the target formation rock through experiments.

[0023] In a possible implementation, the embodiment of the present application designs a tensile strength reduction parameter for characterizing the reduction degree of the tensile strength of rock affected by thermal impact, thereby assisting in calculating the actual tensile strength of the target formation.

[0024] Illustratively, the technician performs the laboratory test by using the Brazilian splitting method, combines the data measured in the experiment, and calculates the tensile strength reduction parameter of the rock of the target formation by using the parameter calculation formula constructed for the tensile strength reduction parameter.

[0025] Optionally, the technician can obtain multiple groups of parameters by multiple experiments, and determine the most reasonable tensile strength reduction parameter from the multiple groups of parameters.

[0026] In step 103, the basic parameters are substituted into the collapse pressure prediction model to obtain the collapse pressure of the target formation. The collapse pressure prediction model has the tensile strength reduction parameter fused in the expression of the model.

[0027] The technician constructs the collapse pressure prediction model based on the relationship among the collapse pressure, the tensile strength reduction parameter, and various basic parameters, thereby predicting the collapse pressure of the target formation.

[0028] In a possible implementation, the collapse pressure prediction model can be a neural network model. In the model training stage, the corresponding relationship between various parameter combinations and the collapse pressure is learned, and in the actual prediction, the collapse pressure of the geothermal well is predicted based on the basic parameters and the tensile strength reduction parameter.

[0029] By fusing the tensile strength reduction parameter in the collapse pressure prediction model, the reduction of the tensile strength of rock caused by thermal impact is considered, so that the predicted collapse pressure is closer to the actual value.

[0030] In summary, the method provided by the embodiment of the present application measures the tensile strength reduction parameter by the thermal cracking experiment. The parameter can characterize the reduction degree of the tensile strength of rock affected by thermal impact, and the tensile strength reduction parameter is fused in the collapse pressure prediction model. When the collapse pressure is calculated, the loss effect of the rock strength caused by high-temperature thermal impact is considered, so that the prediction result of the formation collapse pressure is more reasonable. The safe bearing upper limit can be obtained in combination with the engineering safety coefficient, thereby providing a scientific basis for the construction arrangement.

[0031] Please refer to Figure 2 which shows a flowchart of a method for determining the collapse pressure of a formation according to another example embodiment of the present application. The method includes the following steps:

[0032] In step 201, the basic parameters of the target formation are collected.

[0033] The specific implementation of step 201 can refer to step 101 described above, and will not be described herein again.

[0034] In a possible implementation, based on the collapse pressure prediction model provided in the embodiments of the present application, the basic parameters that the skilled person needs to collect include but are not limited to: maximum horizontal stress, minimum horizontal stress, cohesion of rock, internal friction angle, Poisson's ratio of rock, effective stress coefficient, and stress nonlinearity correction coefficient.

[0035] In step 202, the thermal shock coefficient of the rock is determined through a high-temperature impact experiment, and the thermal shock coefficient is used to characterize the damage ability of thermal shock to the rock.

[0036] The reduction degree of the tensile strength of the rock is related to not only the lithology of the rock itself but also the strength of the thermal shock effect. Therefore, a thermal shock coefficient is defined to construct the parameterized constitutive equation of the tensile strength and further construct the prediction model. The thermal shock coefficient characterizes the damage ability of thermal shock to the rock.

[0037] In a possible implementation, the thermal shock coefficient is the rate of change of the temperature gradient per unit time, and the expression can be written as:

[0038]

[0039] wherein, is the thermal shock coefficient, is the temperature gradient, is the heat exchange time.

[0040] The skilled person can obtain the above thermal shock coefficient through a high-temperature impact experiment. Specifically, step 202 includes the following steps:

[0041] In step 202a, a standard rock sample is processed, and the drilling position is arranged.

[0042] In step 202b, the thermal conductivity coefficient is inversely calculated based on the temperature change data of each measured point, and the thermal conductivity coefficient is used to characterize the heat conduction ability of the rock.

[0043] In step 202c, the thermal conductivity coefficient is substituted into the heat conduction equation to calculate the thermal shock coefficient.

[0044] Regarding the specific processing process of the sample, the above step 202a further includes the following steps:

[0045] In step one, the rock sample is processed into a cuboid sample of a preset size based on the international mechanics test standard.

[0046] In step two, a first drill hole is arranged at the intersection of the diagonal lines on the rectangular face of the cuboid sample, and a second drill hole and a third drill hole are arranged on both sides of the symmetry axis passing through the first drill hole, and the distance between the second drill hole and the third drill hole and the edge of the cuboid sample is a preset distance.

[0047] Illustratively, according to the international mechanical test standard with the ratio of height to bottom side length being 2:1, the rock sample is processed into a cuboid test piece with a height of 100 mm and a bottom side length of 50 mm; drill holes 1 are arranged at the intersection of two diagonal lines on the long face of the test piece, and drill holes 2 and 3 are arranged on the left and right of the drill hole 1 position at intervals of 25 mm, with the drill holes 2 and 3 being 25 mm away from the edge of the test piece.

[0048] The skilled person processes a standard rock cuboid test piece, reasonably arranges the drill hole positions, i.e., the probe embedding, based on the heat conduction equation, uses the temperature change data of each measured point to inversely calculate the heat conduction coefficient, and then calculates the temperature gradient in the whole rock test piece during the thermal shock process through the heat conduction coefficient, and finally obtains the thermal shock coefficient. The heat conduction equation is wherein Q is the heat flow rate, S is the heat conduction area, is the heat conduction coefficient.

[0049] Specifically, the above step 202c specifically further includes the following steps:

[0050] Step three, substituting the heat conduction coefficient into the heat conduction equation to obtain the temperature gradient.

[0051] Step four, based on the temperature gradient and the heat exchange time, the thermal shock coefficient is calculated

[0052] The thermal shock coefficient is the ratio of the temperature gradient to the heat exchange time, so the skilled person can measure the heat conduction coefficient, calculate the temperature gradient based on the heat conduction equation, and then calculate the thermal shock coefficient based on the temperature gradient and the heat exchange time.

[0053] Illustratively, the skilled person can perform multiple high-temperature impact experiments, and determine a thermal shock coefficient based on the results calculated from multiple experiments. For example, taking the average value, or fitting the results, etc.

[0054] Step 203, based on the thermal shock coefficient, the tensile strength reduction parameter of the rock is determined through the thermal cracking experiment.

[0055] There is a certain corresponding relationship between the tensile strength, the tensile strength reduction parameter and the thermal shock coefficient. Therefore, the skilled person first constructs a tensile strength reduction equation, and then measures the tensile strength of the rock through the thermal cracking experiment, and inversely calculates the tensile strength reduction parameter of the rock based on the tensile strength and the thermal shock coefficient.

[0056] Step 203 specifically includes the following steps:

[0057] Step 203a, constructing a parameterized constitutive equation of the tensile strength of the rock.

[0058] Since the thermal shock coefficient can better characterize the thermal shock damage ability, the greater the thermal shock coefficient, the stronger the thermal shock damage to the rock, and the weaker the mechanical strength, therefore, combined with the influencing factors of rock tensile strength and the damage of high temperature to rock, the constitutive equation model based on exponential equation is proposed.

[0059] In addition, since the heat transfer of thermal shock is a non-steady heat transfer, the linear constitutive equation does not meet the requirements, therefore, the parameterized constitutive equation expression of rock tensile strength is selected in the nonlinear function.

[0060] Illustratively, the expression of the parameterized constitutive equation of rock tensile strength is:

[0061]

[0062] wherein, is the thermal shock coefficient, A and B are the tensile strength reduction parameters, the values of A and B corresponding to different lithology of rock are different, is the tensile strength.

[0063] Step 203b, obtaining the core of the target stratum, and determining the tensile strength of the core under different high-temperature thermal shock through thermal fracture experiment.

[0064] The skilled person can process and manufacture multiple test pieces, and respectively use thermal shock of different temperatures to perform thermal fracture experiment to obtain multiple groups of tensile strength. For example, the skilled person obtains 20 tensile strength results based on 20 high-temperature thermal shock tests.

[0065] Step 203c, substituting the thermal shock coefficient and the tensile strength into the parameterized constitutive equation to calculate the tensile strength reduction parameter.

[0066] The skilled person substitutes the tensile strength obtained by the thermal fracture experiment and the thermal shock coefficient obtained by the above steps into the parameterized constitutive equation, and in the case of obtaining multiple groups of tensile strength, the tensile strength reduction parameter can be calculated by using the equation set.

[0067] In one possible implementation, step 203c specifically includes the following steps:

[0068] Step five, substituting the thermal shock coefficient and the tensile strength under at least two high-temperature thermal shocks into the parameterized constitutive equation to obtain at least two groups of experimental parameters.

[0069] Step six, simulating and checking at least two groups of experimental parameters by using physical field simulation software to obtain the tensile strength reduction parameter.

[0070] Since the calculation error, experimental error and other factors may cause deviation of the calculated experimental parameters, the experimental parameters need to be simulated and checked to obtain the final tensile strength reduction parameter.

[0071] The tensile strength reduction parameter is obtained by simulation and checking using COMSOL software.

[0072] In step 204, the basic parameters are substituted into the collapse pressure prediction model to obtain the collapse pressure of the target formation. The expression of the collapse pressure prediction model is fused with the tensile strength reduction parameter.

[0073] The calculation formula of the collapse pressure prediction model is as follows:

[0074]

[0075] wherein, Pc is the collapse pressure, σH is the maximum horizontal stress, σh is the minimum horizontal stress, C is the cohesion of the rock, K is the expression , φ is the internal friction angle of the rock, H is the well depth, β is the effective stress coefficient, Pp is the formation pore pressure, α is the stress nonlinear correction coefficient. The maximum horizontal stress and the minimum horizontal stress are calculated based on the collected Poisson's ratio.

[0076] In the embodiments of the present application, the thermal shock coefficient of the rock is determined through the high-temperature rock thermal cracking effect, and the tensile strength reduction parameter is calculated through the high-temperature rock thermal cracking test and the thermal shock coefficient. The collapse pressure is calculated by combining the tensile strength reduction parameter, the geomechanics parameter and the rock mechanics parameter, so that the collapse pressure of the high-temperature geothermal well can be better determined, and reasonable and effective guidance for the development of geothermal resources is provided.

[0077] In combination with the above embodiments, Figure 3 A flowchart for determining the collapse pressure of the geothermal well is shown. First, the geomechanics parameter and the rock mechanics parameter of the target formation are determined, and then on the basis of the conventional collapse pressure determination method, the reduction degree of the rock tensile strength affected by the thermal shock is determined by measuring the thermal shock coefficient, and a high-temperature geothermal well collapse pressure prediction model considering the reduction degree of the rock tensile strength is established.

[0078] Illustratively, the high-temperature granite body in a certain geothermal development area is subjected to thermal shock and collapses due to thermal shock. The value of the compressive strength of the granite during the thermal shock collapse process is predicted by using the above method, and the specific steps are as follows:

[0079] (1) Under the action of thermal shock, the temperature gradient of the high-temperature granite will generate thermal stress. When the thermal stress is greater than the cementation force inside the granite, the micro cracks will be penetrated and further lead to rock deterioration. According to the heat conduction theory, the thermal shock coefficient : the rate of change of temperature gradient per unit time during heat transfer, expressed as:

[0080]

[0081] where is the temperature gradient, dimensionless; is the heat transfer time, in seconds.

[0082] (2) Through the high-temperature rock thermal cracking effect, the thermal shock coefficient is obtained.

[0083] (3) Since the thermal shock coefficient can better represent the thermal shock damage capacity, the larger the thermal shock coefficient, the stronger the damage capacity of thermal shock on granite, and the weaker the mechanical strength. Combined with the influencing factors of granite tensile strength and the damage effect of high temperature on granite, an exponential equation-based constitutive equation model is proposed.

[0084] (4) Since thermal shock heat transfer is a non-steady heat transfer, linear constitutive equation does not meet the requirements, so a nonlinear function is selected. The parameterized constitutive equation expression of granite tensile strength is proposed:

[0085]

[0086] where is the granite tensile strength, in MPa; is the thermal shock coefficient, dimensionless; A and B are unknown parameters, and the values of A and B are different for different rock properties.

[0087] (5) Through the test, the thermal shock coefficient and the tensile strength value of granite under different thermal shock degrees are determined, the parameterized constitutive equation of tensile strength is substituted, the non-static equation expression is converted into a static expression and calculated, the unknown parameters A=231.6 and B=4×10-5 in the constitutive equation are determined, and the constitutive equation of granite tensile strength is obtained:

[0088]

[0089] where is the rock tensile strength, in MPa; is the thermal shock coefficient, dimensionless.

[0090] COMSOL software is used for simulation and verification.

[0091] Those skilled in the art should be aware that, in the above one or more examples, part of the functions described in the embodiments of the present application can be implemented in hardware, software, firmware or any combination thereof. When implemented in software, the functions can be stored in a computer readable storage medium or transmitted as one or more instructions or codes on a computer readable storage medium. The computer readable storage medium includes computer storage medium and communication medium, and the communication medium includes any medium that facilitates transfer of computer program from one place to another. The storage medium can be any available medium accessible by a general or special purpose computer.

[0092] The scope of protection of the present disclosure is not limited to the above-described embodiments, and it is obvious that those skilled in the art can make various modifications and changes to the present disclosure without departing from the scope and spirit of the present disclosure. If these modifications and changes belong to the scope of the claims of the present disclosure and their equivalents, the intention of the present disclosure also includes these modifications and changes.

Claims

1. A method for determining the collapse pressure of a geothermal well, characterized in that: The method comprises: Collecting basic parameters of the target formation, including maximum horizontal ground stress, minimum horizontal ground stress, rock cohesion, internal friction angle, Poisson's ratio of rock, effective stress coefficient, and stress nonlinear correction coefficient; Determine the tensile strength reduction parameter of the target formation rock through a thermal cracking experiment, wherein the tensile strength reduction parameter is used to indicate the degree of reduction of the rock tensile strength due to thermal shock; Substituting the basic parameters into a collapse pressure prediction model to obtain the collapse pressure of the target formation, wherein the expression of the collapse pressure prediction model incorporates the tensile strength reduction parameter; The method of determining the tensile strength reduction parameter of the target formation rock through a thermal cracking experiment includes: Determine the thermal shock coefficient of the rock through a high-temperature shock experiment, wherein the thermal shock coefficient is used to characterize the destructive capacity of the rock caused by thermal shock; Based on the thermal shock coefficient, determining the tensile strength reduction parameter of the rock through the thermal cracking test; The method of determining the tensile strength reduction parameter of rock through the thermal cracking test based on the thermal shock coefficient includes: Construct a parameterized constitutive equation for rock tensile strength; Obtaining a core of the target formation, and measuring the tensile strength of the core under different high-temperature thermal shocks through the thermal fracture experiment; Substituting the thermal shock coefficient and the tensile strength into the parameterized constitutive equation to calculate the tensile strength reduction parameter; The expression of the parameterized constitutive equation is: in, is the thermal shock coefficient, A and B are the tensile strength reduction parameters, and the values ​​of A and B are different for rocks of different lithologies. is the tensile strength; The calculation formula of the collapse pressure prediction model is: in, is the collapse pressure, is the maximum horizontal ground stress, is the minimum horizontal stress, C is the cohesion of rock, K is the expression , is the internal friction angle of rock, is the effective stress coefficient, is the formation pore pressure, is the stress nonlinear correction coefficient.

2. The method according to claim 1, characterized in that Substituting the thermal shock coefficient and the tensile strength into the parameterized constitutive equation to calculate the tensile strength reduction parameter includes: Substituting the thermal shock coefficient and the tensile strength under at least two high-temperature thermal shock conditions into the parameterized constitutive equation to obtain at least two sets of experimental parameters; The at least two groups of experimental parameters are simulated and checked using physical field simulation software to obtain the tensile strength reduction parameter.

3. The method according to any one of claims 1 to 2, characterized in that: The method of determining the thermal shock coefficient of rock by high temperature shock experiment includes: Process standard rock specimens and arrange drilling locations; The thermal conductivity coefficient is calculated based on the temperature change data of each measured point, and the thermal conductivity coefficient is used to characterize the thermal conductivity of the rock; The thermal conductivity coefficient is substituted into the heat conduction equation to calculate the thermal shock coefficient.

4. The method according to claim 3, characterized in that The thermal shock coefficient is the rate of change of the temperature gradient per unit time. The expression of the thermal shock coefficient is: in, is the thermal shock coefficient, is the temperature gradient, is the heat exchange time.

5. The method according to claim 4, characterized in that The heat conduction equation is , where Q is the heat flow rate, S is the heat conduction area, is the thermal conductivity coefficient; Substituting the heat conduction coefficient into the heat conduction equation to calculate the thermal shock coefficient includes: Substituting the heat conduction coefficient into the heat conduction equation to obtain the temperature gradient; The thermal shock coefficient is calculated based on the temperature gradient and the heat exchange time.

6. The method according to claim 3, characterized in that The processing of the standard rock specimen and the arrangement of the drilling positions include: Process the rock sample into a rectangular specimen of preset size based on international mechanical testing standards; A first drill hole is arranged at the intersection of diagonals on the rectangular surface of the rectangular specimen, and a second drill hole and a third drill hole are respectively set on both sides of the symmetry axis passing through the first drill hole. The distance between the second drill hole and the third drill hole and the edge of the rectangular specimen is a preset distance.

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