Method for predicting cold and hot cycle damage of granular rock

Abstract

The invention relates to the technical field of thermocycling damage treatment, in particular to a granular rock thermocycling damage prediction method, which comprises the following steps: establishing a particle-based numerical model by using discrete element software; setting initial temperature and confining pressure of the rock sample, and constructing a heating, balancing and cooling experiment process; setting different preset temperature values and temperature rising and cooling step lengths; elastic modulus damage and crack number development are obtained; thermal damage is calculated through elastic modulus damage and crack number development, the relation between rock volume thermal strain and rock heating-cooling damage under different confining pressures is obtained, and a damage prediction model of rocks under different confining pressures under the cold and heat cycle condition is constructed. Aiming at the rock damage problem caused by rock heating expansion and cooling shrinkage, the thermal damage and volume thermal strain model considering heating and cooling is constructed, and the universality of the model is high.

Description

Technical Field

[0001] The present invention relates to the technical field of thermal cycle damage treatment, and in particular to a thermal cycle damage prediction method for granular rocks. Background Art

[0002] Hot dry rock energy has attracted much attention as a new renewable energy source. It has great potential, but its development and utilization still faces a series of challenges.

[0003] During hot dry rock drilling, timely analysis of the damage state and mechanical properties of the formation rock has a profound impact on the mining efficiency and safety of hot dry rock.

[0004] Patent publication number CN117473716A is a method for predicting thermal damage of rocks of different compositions based on PFC numerical research. It uses the relationship between thermal damage and volumetric thermal strain. However, the model has no theoretical support and poor model compatibility. Summary of the Invention

[0005] To address the shortcomings of existing methods, the present invention addresses the problem of rock damage caused by rock expansion upon heating and contraction upon cooling, constructing a thermal damage and volumetric thermal strain model that takes both heating and cooling into account. The model has strong versatility and can predict the damage state and mechanical properties of rocks at a certain depth during oil and gas field drilling, providing protection for oil and gas field exploitation.

[0006] The technical solution adopted by the present invention is: a method for predicting thermal cycle damage of granular rock includes the following steps:

[0007] Step 1: Collect the quartz content of the core and use discrete element software to build a particle-based numerical model;

[0008] Step 2: Set the initial temperature and confining pressure of the rock sample, and construct the heating, equilibrium, and cooling experimental process; set different preset temperature values, heating and cooling step lengths; and obtain the elastic modulus damage and crack number development;

[0009] As a preferred embodiment of the present invention, step 2 specifically includes:

[0010] During the first cycle of the first preset temperature, the temperature is increased by 1°C every 100 steps starting from the initial temperature. After reaching the first preset temperature value, the temperature cycle is maintained for 5000 steps; then the temperature is decreased by 1°C every 100 steps, and the cycle stops after returning to the initial temperature.

[0011] As a preferred embodiment of the present invention, the initial temperature is 25° C., and the confining pressure is 0, 15, 45, or 120 MPa.

[0012] As a preferred embodiment of the present invention, the preset temperature values ​​include: 150°, 300°, and 600°.

[0013] Step 3: Calculate thermal damage using elastic modulus damage and crack number development, obtain the relationship between volume thermal strain of rocks under different confining pressures and rock heating-cooling damage, and construct a damage prediction model for rocks under different confining pressures under hot-cold cycling conditions;

[0014] As a preferred embodiment of the present invention, calculating thermal damage using elastic modulus damage and crack number development includes:

[0015] Calculate rock thermal damage by reducing elastic modulus after cycling;

[0016] Reversely solve the heating damage and cooling damage during each cycle.

[0017] As a preferred embodiment of the present invention, the formula for temperature rise damage is:

[0018] D H =1-(1-D T1 )(1-D T3 ); (2)

[0019] Among them, D T1 D is the ratio of the number of micro-elements destroyed after the first cycle of heating to the total number of micro-elements before rock failure. T3 It is the ratio of the number of microelements destroyed after the second cycle of heating to the total number of microelements before rock failure.

[0020] As a preferred embodiment of the present invention, the formula for cooling damage is:

[0021] D C =1-(1-D T2 )(1-D T4 ); (3)

[0022] Among them, D T2 D is the ratio of the number of micro-elements destroyed after the first cycle of cooling to the total number of micro-elements before rock failure. T4 It is the ratio of the number of microelements destroyed after the second cycle cooling to the total number of microelements before rock failure.

[0023] As a preferred embodiment of the present invention, the damage prediction model formula is:

[0024]

[0025] Among them, D T is the damage caused by temperature change on rock; v is the volume thermal strain; K and m are the scale parameter and shape parameter of the Weibull distribution, respectively.

[0026] As a preferred embodiment of the present invention, a system for predicting thermal cycle damage of granular rock includes: a memory for storing instructions executable by a processor; and a processor for executing the instructions to implement a method for predicting thermal cycle damage of granular rock.

[0027] As a preferred embodiment of the present invention, a computer readable medium stores computer program code, and the computer program code implements a method for predicting thermal cycle damage of granular rock when executed by a processor.

[0028] Beneficial effects of the present invention:

[0029] 1. The present invention constructs a thermal damage (heating-cooling) and volumetric thermal strain model. By directly performing uniaxial mechanical experiments on granite after thermal loading, the thermal and cooling cycle damage of granite can be easily determined. This can accurately predict the damage state and mechanical properties of rocks at a certain depth during oil and gas field drilling. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of the method for predicting thermal cycle damage of granular rocks according to the present invention;

[0031] Figure 2 This is a diagram of a numerical model of granite based on grains;

[0032] Figure 3 is the initial disk filling diagram;

[0033] Figure 4 is to generate a rigid block diagram;

[0034] Figure 5 It is a polygonal grain structure diagram;

[0035] Figure 6 is to fill in the smaller disks to produce the final model diagram;

[0036] Figure 7 It is a schematic diagram of cyclic heating / cooling;

[0037] Figure 8 It is a numerical model with a quartz content of 5%;

[0038] Figure 9 This is the crack development diagram after hot and cold cycle loading when the quartz content is 5% and 70%;

[0039] Figure 10 is the stress-strain curve after heating / cooling cycle for quartz contents of 5% and 70%;

[0040] Figure 11 is a graph of peak compressive strength after heating / cooling cycles for quartz contents of 5% and 70%;

[0041] Figure 12 is the volumetric thermal strain variation diagram for heating / cooling cycles at different confining pressures with quartz contents of 5% and 70%;

[0042] Figure 13 is the relationship between volume thermal strain and rock heating damage;

[0043] Figure 14 This is a diagram showing the relationship between volumetric thermal strain and rock cooling damage. DETAILED DESCRIPTION

[0044] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. This figure is a simplified schematic diagram, which only illustrates the basic structure of the present invention in a schematic manner, and therefore only shows the components related to the present invention.

[0045] The problem to be solved by the present invention is how to provide a grain-based model constructed using discrete element software to simulate the microcrack propagation and mechanical response of rocks under different confining pressure conditions under the action of thermal cycling; analyze the relationship between the volumetric thermal strain of rocks under different confining pressures under the action of thermal cycling and the heating-cooling damage of rocks, and establish a damage prediction model for rocks with different components.

[0046] like Figure 1 As shown, a method for predicting thermal cycle damage of granular rock includes the following steps:

[0047] Step 1: Use discrete element software to establish a particle-based numerical model;

[0048] A numerical model of the grains was constructed by sampling the quartz content of the cores;

[0049] The numerical model of the particles refers to step S1 in CN117473716A patent;

[0050] Figure 2-Figure 6 The present invention simulates the numerical core establishment process, from a single discrete element ball to the overall core numerical model.

[0051] By applying temperature to the rock sample for uniform heating and cooling, and conducting mechanical experimental simulation of the rock under different confining pressure conditions, axial loading is directly performed to obtain the stress-strain curve and microcrack propagation.

[0052] Step 2: The rock sample is set to have an initial temperature of 25°C and different confining pressure conditions. During the first cycle of the first preset temperature, the temperature is first increased by 1°C per 100 steps until it reaches the first preset temperature value. After reaching the first preset temperature value, the temperature is maintained for 5000 steps, i.e., equilibrium is achieved. The temperature is then lowered by 1°C per 100 steps until it reaches the initial temperature. The first cycle of the first preset temperature ends.

[0053] Execute the next cycle according to the above process, and the number of cycles is >1.

[0054] Similarly, a heating, balancing and cooling cycle of the second preset temperature value is performed;

[0055] A second preset temperature value may also be implemented; wherein the number of preset temperature values ​​is greater than 1.

[0056] Figure 7 The number of cycles for the first preset temperature is 2, and the number of preset temperature values ​​is 3, namely 150°, 300°, and 600°; the number of cycles and the number of preset temperature values ​​are both custom parameters.

[0057] like Figure 7 During the heating and cooling process, the heating and cooling rates were the same. Each group of samples were heated to 150℃, 300℃, and 600℃ under different confining pressure conditions, and then cooled back to the initial temperature for two heating-cooling cycles.

[0058] The first heating, the first cooling, the second heating, and the second cooling are respectively referred to as heating 1, cooling 1, heating 2, and cooling 2 (the same below). The specific steps of the hot and cold cycle are as shown in the attached figure. Figure 7 shown.

[0059] Step 3: The elastic modulus damage and crack number development are obtained through the temperature increase and decrease process in step 2. The thermal damage is calculated based on the elastic modulus damage and crack number development. The relationship between the volume thermal strain of rocks under different confining pressures and the rock heating-cooling damage is analyzed, and a damage prediction model for rocks under different confining pressures under hot-cold cycling conditions is constructed.

[0060] Analyze the relationship between volumetric thermal strain and rock heating-cooling damage, and establish a damage prediction model for rocks with different mineral compositions;

[0061] Among them, the thermal damage D is calculated by the elastic modulus damage and the number of cracks. T The process is as follows:

[0062] 1. Calculate rock thermal damage D by reducing the elastic modulus after cycling T , the formula is as follows:

[0063] D T =1-E Tn / E0=1-(1-D T1 )(1-D T2 )(1-D T3 )(1-D T4 )……(1-D T2n ) (1)

[0064] Among them, E Tnis the elastic modulus of the rock after the nth cooling, E0 is the elastic modulus of the rock at room temperature, and n is the nth heating-cooling cycle; D T1 、D T2 It is expressed as the ratio of the number of micro-elements destroyed after heating and cooling to the total number of micro-elements before rock failure;

[0065] 2. Since 1-E in formula 1 Tn / E0 is a known value calculated theoretically. The thermal damage during each cycle is obtained by reversely using Formula 1. The formula is as follows:

[0066] D T1 =N T1 / N;

[0067] D T2 =N T2 / (NN T1 );

[0068] D T3 =N T3 / (NN T1 -N T2 );

[0069] …

[0070] D Tn =N Tn / (NN T1 -N T2 -……N T(n-1) );

[0071] Among them, N is the total amount of damaged micro-element, N T1 is the number of damaged micro-elements during the first cycle heating process, N T2 is the number of damaged microelements during the first cooling cycle;

[0072] 3. Taking two cycles with n=2 as an example, the damage caused to the rock by heating and cooling without the influence of thermal stress can be calculated using the formula:

[0073] D H =1-(1-D T1 )(1-D T3 ); (2)

[0074] D C =1-(1-D T2 )(1-D T4 ); (3)

[0075] Among them, D H With D C They are respectively the temperature increase damage and temperature decrease damage suffered by rocks; temperature increase damage and temperature decrease damage are collectively referred to as thermal damage.

[0076] With the increase of volumetric thermal strain, thermal damage gradually increases.

[0077] During the development and exploitation of high-temperature deep-layer energy, the heat exchange between drilling fluid and surrounding rock will cause irregular temperature changes, leading to rock damage and destruction; during the heating process, the particles inside the rock expand and squeeze each other, inducing the formation of cracks; during the cooling process, the contraction of the particles inside the rock causes tensile stress, thereby triggering the development of cracks; since thermal expansion cracking and cooling contraction cracking are two independent processes, the locking models of these two processes are studied separately; compared with the damage model of CN117473716A that only has a heating process, the conditions considered in the hot and cold cycle damage model of the present invention are more consistent with the actual drilling conditions of the drilling fluid circulation in the formation, and can more accurately simulate the hot and cold cycle damage of rocks.

[0078] Construct the thermal damage and volumetric thermal strain model, the formula is:

[0079]

[0080] Among them, D T It is the damage caused by temperature change on rock, namely D T D H 、D C ; ε v is the volume thermal strain; K and m are the scale parameter and shape parameter of the Weibull distribution, respectively.

[0081] The damage of rocks with different quartz contents is predicted by formula (2);

[0082] Rock statistical damage models usually assume that the unit strength of rock satisfies the Weibull distribution. The shape parameters and size parameters of the Weibull distribution can be determined based on certain characteristic points of the rock stress-strain curve (such as the points corresponding to the peak strength and residual strength), thereby obtaining the evolution law of the damage factor.

[0083] Compared with the rock damage formula of CN117473716A, the present invention uses the Weibull function to analyze rock damage. Since the damage of rock micro-element destruction conforms to the statistical damage theory, the Weibull damage model can better fit the test results.

[0084] like Figure 8 It is a numerical model when the quartz content is 5% and 70%. By setting the mineral composition parameters of the core, the numerical core with quartz content of 5% and 70% can be obtained.

[0085] Figure 9Figure 3 is a crack development diagram after hot-cold cyclic loading of quartz with a quartz content of 5% and 70%; under no confining pressure conditions, when the rock is first heated to 150°C, intergranular tensile cracks gradually develop; when the temperature rises to 300°C, in addition to tensile cracks, a small amount of intergranular shear cracks will also be generated, but intergranular tensile cracks are still the main type; when first heated to 600°C, the quartz grains break, resulting in significant shear cracks inside the grains, while tensile cracks develop less; during the first cooling process at 150°C or 300°C, secondary damage occurs inside the sample due to tensile stress, forming additional intergranular tensile cracks; and at the first cooling process at 600°C, the sample is damaged by tensile stress, forming additional intergranular tensile cracks. During cooling, since the grain boundaries or the bonds between particles have been basically destroyed after the first heating, only a small number of intergranular tensile cracks develop, and the number of other types of cracks does not change significantly. In the secondary heating-cooling cycle at different temperatures, only a small number of tensile cracks are generated as the number of cycles increases. Under confining pressure conditions, the development of microcracks is inhibited. In general, crack development is mainly concentrated in the first temperature cycle stage, and the subsequent cycles have little effect. As the confining pressure increases, the total number of cracks decreases, while the ratio of shear cracks to tensile cracks continues to increase. Granite under confining pressure is more susceptible to the thermal cycling effect and cracks. Figure 9 Get N.

[0086] Figure 10 The stress-strain curves after heating / cooling cycles for samples with quartz contents of 5% and 70% are shown. The effects of heating / cooling cycles on the mechanical properties of rocks were studied through numerical simulations of uniaxial and triaxial compression tests. The results show that as the number of cycles and temperature increase, the rock failure mode gradually changes from brittle to ductile. In general, under conditions of lower confining pressure and higher temperature, rock samples with higher quartz contents develop more cracks and exhibit more significant ductile failure characteristics. Figure 10 Get E Tn .

[0087] Figure 11 Figure 3 is the peak compressive strength diagram after heating / cooling cycles with quartz content of 5% and 70%; the evolution trend of rock peak strength, during the heating / cooling cycle, the rock sample will produce significant random microcracks due to thermal expansion and contraction; the density, orientation and other characteristics of these microcracks will directly affect the initiation position and type of macro cracks during compression, thereby significantly changing the compressive strength of the rock; making the peak strength data show more complex fluctuation characteristics than the elastic modulus, especially under no confining pressure conditions, more microcracks are generated inside the rock sample, and their random distribution characteristics have a more significant impact on the strength, resulting in a larger fluctuation in the test results; under confining pressure conditions, the overall change trend of rock peak strength is basically consistent with the no confining pressure condition; compared with the original rock sample, during the first and second heating processes, due to the combined effect of crack development and thermal expansion stress, the peak strength shows a downward trend, and the higher the temperature, the more significant the decline.

[0088] Figure 12 Figure 3 is a graph showing the volume thermal strain variation under different confining pressure heating / cooling cycles for quartz contents of 5% and 70%. Under no confining pressure conditions, the rock exhibits the highest volume thermal strain during the first heating process. After the first heating, a large number of microcracks have formed inside the rock mass, which provides ample space for the expansion / contraction behavior of mineral particles in subsequent thermal cycles. More importantly, the initial heating process has destroyed a large number of mineral boundary bonds, significantly weakening the displacement coordination and stress transfer capabilities between particles, resulting in a decrease in volume thermal strain in subsequent cycles, and a significantly reduced fluctuation amplitude. For the sample with a quartz content of 70%, its crack development characteristics are similar to those under no confining pressure conditions, and a large number of cracks can still be generated, so the volume thermal strain continues to decrease in subsequent cycles. This difference is mainly due to the fact that high quartz content samples are more likely to produce irreversible cracks under the action of thermal stress. The inhibitory effect of confining pressure on cracks between quartz particles weakens with increasing quartz content.

[0089] Figure 13 This is a graph showing the relationship between volumetric thermal strain and rock heating damage. Within the confining pressure range of 0-120 MPa, as the volumetric thermal strain increases, rock thermal damage continues to accumulate. Under the same volumetric thermal strain conditions, the greater the confining pressure, the lower the degree of thermal damage.

[0090] Figure 14 Figure 4 shows the relationship between volumetric thermal strain and rock cooling damage. The relationship between thermal damage and strain during cooling shrinkage of the specimen does not conform to the Weibull statistical distribution under no confining pressure. However, under confining pressure, the relationship better conforms to the Weibull damage model. Without confining pressure, cooling damage primarily originates from the initial heating process (as confirmed by observations of the number of microcracks), and rock damage is concentrated in the initial heating stage. Due to the lack of confining pressure, formed cracks will not close, and an increase in volumetric shrinkage during cooling shrinkage does not lead to a significant increase in the number of cracks or damage. In contrast, under confining pressure, the initial heating causes limited bond damage and more complete stress transfer, so a larger cooling shrinkage can still induce significant damage. Figure 14 All rock samples under confining pressure show similar changing trends, and as the confining pressure increases, the curve shifts to the right as a whole.

[0091] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A method for predicting thermal cycle damage of granular rocks, characterized in that: The following steps are involved: Step 1: Collect the quartz content of the core and use discrete element software to build a particle-based numerical model; Step 2: Set the initial temperature and confining pressure of the rock sample, and construct the heating, equilibrium, and cooling experimental process; set different preset temperature values, heating and cooling step lengths; and obtain the elastic modulus damage and crack number development; Step 3: Calculate thermal damage using elastic modulus damage and crack number development to obtain the relationship between volume thermal strain of rocks under different confining pressures and rock heating-cooling damage, and construct a damage prediction model for rocks under different confining pressures under hot-cold cycling conditions.

2. The method for predicting thermal cycle damage of granular rock according to claim 1, characterized in that: Step 2 specifically includes: During the first cycle of the first preset temperature, the temperature is increased by 1°C every 100 steps starting from the initial temperature. After reaching the first preset temperature value, the temperature cycle is maintained for 5000 steps; then the temperature is decreased by 1°C every 100 steps, and the cycle stops after returning to the initial temperature.

3. The method for predicting thermal cycle damage of granular rock according to claim 1, characterized in that: Calculation of thermal damage using elastic modulus damage and crack number development includes: Calculate rock thermal damage by reducing elastic modulus after cycling; Reversely solve the heating damage and cooling damage during each cycle.

4. The method for predicting thermal cycle damage of granular rock according to claim 3, characterized in that: The formula for heating damage is: D H =1-(1-D T1 )(1-D T3 ); (2) Among them, D T1 D is the ratio of the number of micro-elements destroyed after the first cycle of heating to the total number of micro-elements before rock failure. T3 It is the ratio of the number of microelements destroyed after the second cycle of heating to the total number of microelements before rock failure.

5. The method for predicting thermal cycle damage of granular rock according to claim 3, characterized in that: The formula for cooling damage is: D C =1-(1-D T2 )(1-D T4 ); (3) Among them, D T2 D is the ratio of the number of micro-elements destroyed after the first cycle of cooling to the total number of micro-elements before rock failure. T4 It is the ratio of the number of microelements destroyed after the second cycle cooling to the total number of microelements before rock failure.

6. The method for predicting thermal cycle damage of granular rock according to claim 4 or 5, characterized in that: The damage prediction model formula is: Among them, D T is the damage caused by temperature change on rock; v is the volume thermal strain; K and m are the scale parameter and shape parameter of the Weibull distribution, respectively.

7. The method for predicting thermal cycle damage of granular rock according to claim 2, characterized in that: The initial temperature was 25 °C, and the confining pressures were 0, 15, 45, and 120 MPa.

8. The method for predicting thermal cycle damage of granular rock according to claim 2, characterized in that: The preset temperature values ​​include: 150°, 300°, 600°.

9. Granular rock thermal cycle damage prediction system, characterized by: include: a memory for storing instructions executable by the processor; A processor, configured to execute instructions to implement the method for predicting thermal cycle damage of granular rock according to any one of claims 1 to 8.

10. A computer-readable medium storing computer program code, characterized in that When the computer program code is executed by a processor, the computer program code implements the method for predicting thermal cycle damage of granular rock according to any one of claims 1 to 8.

Images