A method for predicting cold and hot cycle damage of a granular rock

Abstract

This invention relates to the field of thermal cycling damage treatment technology, and particularly to a method for predicting thermal cycling damage in granular rocks. The method includes: establishing a particle-based numerical model using discrete element method (DEM) software; setting initial temperature and confining pressure for the rock sample, and constructing heating, equilibration, and cooling experimental processes; setting different preset temperature values ​​and heating / cooling step sizes; obtaining elastic modulus damage and crack quantity development; calculating thermal damage using elastic modulus damage and crack quantity development, obtaining the relationship between volumetric thermal strain and heating-cooling damage of rocks under different confining pressures, and constructing a damage prediction model for rocks under different confining pressures under thermal cycling conditions. This invention addresses the rock damage problem caused by heating expansion and cooling contraction, constructing a thermal damage and volumetric thermal strain model that considers both heating and cooling, and the model has strong versatility.

Description

Technical Field

[0001] This invention relates to the field of thermal cycling damage treatment technology, and in particular to a method for predicting thermal cycling damage in granular rocks. Background Technology

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

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

[0004] The patent with publication number CN117473716A describes a method for predicting thermal damage in rocks of different compositions based on PFC numerical research. It uses the relationship between thermal damage and volumetric thermal strain, but the model lacks theoretical support and has poor compatibility. Summary of the Invention

[0005] To address the shortcomings of existing methods, this invention constructs a thermal damage and volumetric thermal strain model that takes into account both heating and cooling to address rock damage caused by heating expansion and cooling contraction. The model is highly versatile and can predict the damage state and mechanical properties of formation rocks at a certain depth during oil and gas field drilling, thus providing a guarantee for oil and gas field development.

[0006] The technical solution adopted in this invention is: a method for predicting thermal cycling damage in granular rocks, comprising the following steps:

[0007] Step 1: Collect the quartz content of the rock core and establish a particle-based numerical model using discrete element method software;

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

[0009] In a preferred embodiment of the present invention, step two specifically includes:

[0010] During the first cycle at the first preset temperature, the temperature is increased by 1°C every 100 steps from the initial temperature. After reaching the first preset temperature, the temperature is maintained for 5000 steps. Then, the temperature is decreased by 1°C every 100 steps until it returns to the initial temperature and stops.

[0011] In 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] In 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 to obtain the relationship between volumetric 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 thermal 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 the thermal damage of rock by reducing the elastic modulus after cycling;

[0016] The heating and cooling damage during each cycle are solved in reverse.

[0017] In a preferred embodiment of the present invention, the formula for temperature-induced damage is as follows:

[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 heating cycle to the total number of micro-elements before rock destruction. T3 It is the ratio of the number of micro-elements destroyed after the second heating cycle to the total number of micro-elements before the rock was destroyed.

[0020] In 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 cooling cycle to the total number of micro-elements before rock destruction. T4 It is the ratio of the number of micro-elements destroyed after the second cooling cycle to the total number of micro-elements before the rock was destroyed.

[0023] In a preferred embodiment of the present invention, the formula for the damage prediction model is as follows:

[0024]

[0025] Among them, D T It is damage to rocks caused by temperature changes; ε v It is the volumetric thermal strain; K and m are the Weibull distribution scale parameter and shape parameter, respectively.

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

[0027] As a preferred embodiment of the present invention, a computer-readable medium storing computer program code implements a method for predicting thermal cycling damage in granular rocks when executed by a processor.

[0028] The beneficial effects of this invention are:

[0029] 1. This invention constructs a thermal damage (heating-cooling) and volumetric thermal strain model. By applying heat to granite and then conducting uniaxial mechanical experiments, the thermal cycle damage of granite can be easily determined. This model can accurately predict the damage state and mechanical properties of formation rocks at a certain depth during oil and gas field drilling. Attached Figure Description

[0030] Figure 1 This is a flowchart of the method for predicting thermal cycling damage in granular rocks according to the present invention;

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

[0032] Figure 3 This is the initial disk fill diagram;

[0033] Figure 4 It generates rigid block diagrams;

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

[0035] Figure 6 It involves filling in smaller disks to generate the final model diagram;

[0036] Figure 7 This is a schematic diagram of a circulating heating / cooling system;

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

[0038] Figure 9 These are diagrams showing crack development after hot and cold cycling loading with quartz contents of 5% and 70%.

[0039] Figure 10 These are stress-strain curves after heating / cooling cycles for quartz contents of 5% and 70%.

[0040] Figure 11 The peak compressive strength diagrams are for quartz contents of 5% and 70% after heating / cooling cycles.

[0041] Figure 12 This is a graph showing the volumetric thermal strain changes during heating / cooling cycles with different confining pressures for quartz contents of 5% and 70%.

[0042] Figure 13 The graph shows the relationship between volumetric thermal strain and rock damage caused by temperature rise.

[0043] Figure 14 This is a graph showing the relationship between volumetric thermal strain and rock cooling damage. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and embodiments. The drawings are simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, and therefore only show the components related to the present invention.

[0045] The problem to be solved by this invention is how to provide a model based on grains using discrete element method software to simulate the microcrack propagation and mechanical response of rocks under different confining pressures under thermal cycling; to analyze the relationship between volumetric thermal strain and rock heating-cooling damage under different confining pressures under thermal cycling; and to establish a damage prediction model for rocks with different compositions.

[0046] like Figure 1 As shown, a method for predicting thermal cycling damage in granular rocks includes the following steps:

[0047] Step 1: Establish a particle-based numerical model using discrete element method (DEM) software;

[0048] A numerical model of the particles was constructed by collecting the quartz content of the rock core.

[0049] For the numerical model of the particles, please refer to step S1 in patent CN117473716A;

[0050] Figures 2-6 This invention simulates the process of establishing a numerical core, from individual discrete element spheres to a complete numerical core model.

[0051] By uniformly heating and cooling the rock sample under different confining pressures, and then performing mechanical experiments on the rock, axial loading was directly applied to obtain stress-strain curves and microcrack propagation.

[0052] Step 2: The initial temperature of the rock sample is set at 25℃. Under different confining pressure conditions, during the first cycle at the first preset temperature, the temperature is increased by 1℃ every 100 steps until the first preset temperature value is reached. After reaching the first preset temperature value, the temperature is maintained for 5000 steps to achieve equilibrium. Then, the temperature is decreased by 1℃ every 100 steps until the initial temperature is reached. The first cycle at the first preset temperature is now complete.

[0053] The next iteration will proceed according to the above process, with a total iteration count > 1.

[0054] Similarly, the heating, balancing, and cooling cycle at the second preset temperature value is executed;

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

[0056] Figure 7 The first preset temperature is cycled twice, and there are three preset temperature values: 150°, 300°, and 600°. Both the number of cycles and the number of preset temperature values ​​are custom parameters.

[0057] like Figure 7 In the process, the heating and cooling rates are the same. Each group of samples is heated to 150℃, 300℃ and 600℃ under different confining pressures, and then cooled back to the initial temperature. The heating-cooling cycle is repeated twice.

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

[0059] Step 3: The elastic modulus damage and crack number development are obtained through the heating and cooling process in Step 2. The thermal damage is calculated based on the elastic modulus damage and crack number development. The relationship between the volumetric thermal strain of rocks under different confining pressures and the heating-cooling damage of rocks is analyzed. A damage prediction model for rocks under different confining pressures under thermal cycling conditions is constructed.

[0060] The relationship between volumetric thermal strain and rock heating-cooling damage was analyzed, and a damage prediction model for rocks with different mineral compositions was established.

[0061] Among them, thermal damage D is calculated by considering elastic modulus damage and crack number development. T The process is as follows:

[0062] 1. Calculate the thermal damage D of rock 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 TnLet E0 be the elastic modulus of the rock after the nth cooling cycle, and n be the elastic modulus of the rock at room temperature; 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 destruction;

[0065] 2. Because of 1-E in Formula 1 Tn / E0 is a known value calculated theoretically. The thermal damage during each cycle is obtained by reverse calculation using Formula 1, 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] Where N is the total number of damaged micro-elements, N T1 N represents the number of damaged micro-elements during the first heating cycle. T2 The number of damaged micro-elements during the first cooling cycle;

[0072] 3. Taking two cycles with n=2 as an example, the damage to the rock caused by heating and cooling respectively under the influence of no thermal stress can be calculated using the following 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 These are heating damage and cooling damage to the rock, respectively; heating damage and cooling damage are collectively referred to as thermal damage.

[0076] As volumetric thermal strain increases, thermal damage gradually increases.

[0077] In the development and extraction of high-temperature deep energy, the heat exchange between drilling fluid and surrounding rock causes irregular temperature changes, leading to rock damage. During heating, the internal particles of the rock expand and compress against each other, inducing the formation of cracks. During cooling, the contraction of the internal particles of the rock causes tensile stress, thereby triggering crack development. Since thermal expansion cracking and cooling contraction cracking are two independent processes, lock-on models for these two processes were studied separately. Compared with the damage model of CN117473716A, which only considers the heating process, the thermal cycle damage model of this invention considers conditions that are more consistent with the actual drilling fluid circulation in the formation and can more accurately simulate the thermal cycle damage of the rock.

[0078] The thermal damage and volumetric thermal strain model is constructed using the following formula:

[0079]

[0080] Among them, D T Damage to rocks caused by temperature changes, i.e., D T D H D C ;ε v It is the volumetric thermal strain; K and m are the Weibull distribution scale parameter and shape parameter, respectively.

[0081] Damage to rocks with different quartz contents can be predicted using equation (2);

[0082] Statistical damage models for rocks typically assume that the element strength of the rock follows a Weibull distribution. The shape 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 peak strength and residual strength), thereby obtaining the evolution law of the damage factor.

[0083] Compared to the rock damage formula in CN117473716A, this invention uses the Weibull function to analyze rock damage. Since the rock micro-element failure damage conforms to statistical damage theory, the Weibull damage model can fit the experimental results better.

[0084] like Figure 8 Numerical models are provided for quartz content of 5% and 70%; numerical cores with quartz content of 5% and 70% can be obtained by setting the mineral composition parameters of the core.

[0085] Figure 9These are diagrams showing crack development after thermal cycling with quartz contents of 5% and 70%. Under no confining pressure, when the rock is first heated to 150℃, intergranular tensile cracks gradually develop. When the temperature rises to 300℃, in addition to tensile cracks, a small number of intergranular shear cracks also appear, but intergranular tensile cracks still dominate. When the rock is first heated to 600℃, the quartz grains fracture, resulting in significant shear cracks within the grains, while tensile cracks develop less. During the initial cooling process at 150℃ or 300℃, secondary damage occurs within the sample due to tensile stress, forming additional intergranular tensile cracks. At the initial cooling temperature of 600℃... During cooling, since the grain boundaries or intergranular bonding are almost completely destroyed after the initial heating, only a small number of intergranular tensile cracks develop, and the number of other types of cracks does not change significantly. In secondary heating-cooling cycles at different temperatures, only a small number of tensile cracks are generated with increasing cycle number. Under confining pressure, microcrack development is inhibited. Overall, crack development is mainly concentrated in the initial temperature cycle stage, with subsequent cycles having a smaller impact. With increasing confining pressure, the total number of cracks decreases, while the proportion of shear cracks and tensile cracks continues to rise. Granite under confining pressure is more susceptible to fracture due to the thermal cycling effect. Figure 9 We get N.

[0086] Figure 10 These are stress-strain curves after heating / cooling cycles for rock samples with 5% and 70% quartz content. Numerical simulations of uniaxial and triaxial compression tests were used to investigate the effect of heating / cooling cycles on the mechanical properties of the rock. The results show that with increasing cycle number and temperature, the rock failure mode gradually shifts from brittle to ductile. Overall, under lower confining pressure and higher temperature conditions, rock samples with higher quartz content develop more cracks and exhibit more significant ductile failure characteristics. Figure 10 Get E Tn .

[0087] Figure 11 This is a graph showing the peak compressive strength after heating / cooling cycles for rock samples with 5% and 70% quartz content. The evolution trend of peak rock strength shows that during heating / cooling cycles, significant random microcracks develop in the rock sample due to thermal expansion and contraction. The density and orientation of these microcracks directly affect the initiation location and type of macrocracks during compression, thus significantly altering the rock's compressive strength. This results in more complex fluctuations in peak strength data compared to the elastic modulus, especially under unconfined pressure conditions, where more microcracks are generated inside the rock sample, and their random distribution has a more significant impact on strength, leading to greater fluctuations in test results. Under confined pressure conditions, the overall trend of peak rock strength is basically consistent with that under unconfined pressure conditions. Compared to the original rock sample, during the first and second heating processes, the peak strength shows a decreasing trend due to the combined effects of crack development and thermal expansion stress, with the decrease becoming more significant at higher temperatures.

[0088] Figure 12 The figures show the volumetric thermal strain changes under different confining pressure heating / cooling cycles for quartz contents of 5% and 70%. Under no confining pressure, the rock exhibits the highest volumetric thermal strain during the initial heating process. After the initial heating, numerous microcracks have formed within the rock mass, providing ample space for the expansion / contraction of mineral particles in subsequent thermal cycles. More importantly, the initial heating process has disrupted a large number of mineral boundary bonds, significantly weakening the displacement coordination and stress transfer capacity between particles, resulting in a decrease in volumetric thermal strain in subsequent cycles with a significantly reduced fluctuation range. For the sample with a quartz content of 70%, the crack development characteristics are similar to those under no confining pressure conditions, and a large number of cracks can still be generated. Therefore, the volumetric thermal strain continues to decrease in subsequent cycles. This difference is mainly due to the fact that samples with high quartz content are more prone to irreversible cracks under thermal stress, and the inhibitory effect of confining pressure on cracks between quartz particles weakens with increasing quartz content.

[0089] Figure 13 The graph shows the relationship between volumetric thermal strain and rock thermal damage. Within the confining pressure range of 0-120 MPa, as the volumetric thermal strain increases, the thermal damage of the rock 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 The graph shows the relationship between volumetric thermal strain and rock cooling damage. Based on Equation 4, the relationship between cooling shrinkage thermal damage and strain in the sample without confining pressure does not conform to the Weibull statistical distribution. However, under confining pressure, this relationship better conforms to the Weibull damage model. Without confining pressure, cooling damage mainly originates from the initial heating process (as confirmed by observation of the number of microcracks), and rock damage is concentrated in the initial heating stage. Due to the lack of confining pressure constraint, existing cracks will not close, and the 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 damage to the bonding force, and stress transfer is more complete; therefore, a larger amount of cooling shrinkage can still cause significant damage. Figure 14 All rock samples under confining pressure showed similar trends, and the curves shifted to the right as the confining pressure increased.

[0091] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for predicting thermal cycling damage in granular rocks, characterized in that, Includes the following steps: Step 1: Collect the quartz content of the rock core and establish a particle-based numerical model using discrete element method software; Step 2: Set the initial temperature and confining pressure of the rock sample, and construct the heating, equilibration, and cooling experimental process; set different preset temperature values ​​and heating and cooling step sizes; 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 volumetric 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 thermal cycling conditions. Calculations of thermal damage using elastic modulus damage and crack number development include: Calculate the thermal damage of rock by reducing the elastic modulus after cycling; Inversely calculate the heating and cooling damage during each cycle; The formula for heat-induced damage is: D H =1- ( 1-D T1 )( 1-D T3 ) ； (2) in, D T1 This is the ratio of the number of micro-elements destroyed after the first heating cycle to the total number of micro-elements before rock destruction. D T3 The ratio of the number of micro-elements destroyed after the second heating cycle to the total number of micro-elements before rock destruction; The formula for cooling damage is: D C =1- ( 1-D T2 )( 1-D T4 ) ； (3) in, D T2 The ratio of the number of micro-elements destroyed after the first cooling cycle to the total number of micro-elements before rock destruction. D T4 It is the ratio of the number of micro-elements destroyed after the second cooling cycle to the total number of micro-elements before the rock was destroyed.

2. The method for predicting thermal cycling damage in granular rocks according to claim 1, characterized in that, Step two specifically includes: During the first cycle at the first preset temperature, the temperature is increased by 1°C every 100 steps from the initial temperature. After reaching the first preset temperature, the temperature is maintained for 5000 steps. Then, the temperature is decreased by 1°C every 100 steps until it returns to the initial temperature and stops.

3. The method for predicting thermal cycling damage in granular rocks according to claim 1, characterized in that, The formula for the damage prediction model is: (4) in, It is damage to rocks caused by temperature changes; It is volumetric thermal strain; K , m These are the scale parameter and shape parameter of the Weibull distribution, respectively.

4. The method for predicting thermal cycling damage in granular rocks according to claim 2, characterized in that, The initial temperature is 25℃, and the confining pressure is 0, 15, 45, or 120 MPa.

5. The method for predicting thermal cycling damage in granular rocks according to claim 2, characterized in that, The preset temperature values ​​include: 150°, 300°, and 600°.

6. A system for predicting thermal cycling damage in granular rocks, characterized in that, include: Memory is used to store instructions that can be executed by the processor; A processor for executing instructions to implement the method for predicting thermal cycling damage in granular rocks as described in any one of claims 1-5.

7. A computer-readable medium storing computer program code, characterized in that, The computer program code, when executed by a processor, implements the method for predicting thermal cycling damage in granular rocks as described in any one of claims 1-5.

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