Rock Mechanical Characteristics Research Articles

Damage and reliability analysis of double-arch tunnel without a middle pilot tunnel under blast load

In this study, a new type of multi-arch tunnel construction method is proposed. This effort is undertaken due to the many disadvantages of the traditional multi-arch tunnel construction method. Furthermore, this method omits the construction of a middle pilot tunnel, and it has the advantages of safety, high efficiency, and being economical. When using the method of continuous arch tunneling without a middle pilot tunnel, the blasting of the first tunnel and the following tunnel has a greater impact on the surrounding rock damage, as well as on the supporting structure of the same cross-section. Therefore, this study uses LS-DYNA finite element software to construct a three-dimensional numerical model. In addition, the perimeter rock damage law and mechanical response characteristics of the supporting structure in the same cross-section of the first tunnel, as well as the following tunnel after blasting without a middle pilot tunnel, are analyzed. At the same time, the results of the study are based on optimizing the blasting program, and these are then applied to the field. Through the results, it is found that, after blasting a continuous arch tunnel without a medial pilot tunnel, the surrounding rock damage in the arch cross-region of the double-arch tunnel (where the arch top and the arch shoulder are more significant) and the effective stress of the supporting structure exceed the strength design value. In addition, the maximum adequate pressure is distributed in the medial diaphragm wall. With the optimized blasting scheme, the range of the peripheral rock damage is reduced by a maximum of 67%, and the effective stress in the supporting structure is reduced by 25.9–64.8%. The research results are of great significance in terms of improving construction safety, economic efficiency, and project quality, as well as in promoting the research and development of new work methods for double-arch tunnels.

Study on the influence of pore water pressure on shear mechanical properties and fracture surface morphology of sandstone

To further investigate the weakening effect of pore water pressure on intact rock mechanics properties and characteristics of fracture surface after failure, direct shear tests of sandstone were conducted under different pore pressure. A 3D scanner was employed to digitize the morphology of the post-shear fracture surface. The variogram function was applied to quantify the anisotropic characteristics of post-shear fracture surface. The relationship between deformation during shear failure of intact rock and quantitative parameters of fracture surface after shear failure was initially established. It can be found that amplitudes of the sinusoidal surface determine the maximum value of variogram, and period affect lag distance that reach the maximum value of variogram. Test results revealed that the increase of pore pressure has obvious weakening effect on shear strength and deformation of rock. Moreover, the increase of pore pressure makes the shear fracture surface flatter. It can be obtained that both Sillmax and Rangemax are positively related to shear strain, but negatively related to normal strain.

Simulation of Gas Fracturing in Reservoirs Based on a Coupled Thermo-Hydro-Mechanical-Damage Model

Gas fracturing technology for enhancing rock permeability is an area with considerable potential for development. However, the complexity and variability of underground conditions mean that a variety of rock physical parameters can affect the outcome of gas fracturing, with temperature being a critical factor that cannot be overlooked. The presence of a temperature field adds further complexity to the process of gas-induced rock fracturing. To explore the effects of temperature fields on gas fracturing technology, this paper employs numerical simulation software to model the extraction of shale gas under different temperature conditions using gas fracturing techniques. The computer simulations monitor variations in the mechanical characteristics of rocks during the process of gas fracturing. This analysis is performed both prior to and following the implementation of a temperature field. The results demonstrate that gas fracturing technology significantly improves rock permeability; temperature has an impact on the effectiveness of gas fracturing, with appropriately high temperatures capable of enhancing the fracturing effect. The temperature distribution plays a crucial role in influencing the results of gas fracturing. When the temperature is low, the fracturing effect is diminished, resulting in a lower efficiency of shale gas extraction. Conversely, when the temperature is high, the fracturing effect is more pronounced, leading to a higher shale gas production efficiency. Optimal temperatures can enhance the efficacy of gas fracturing and consequently boost the efficiency of shale gas extraction. Changes in the parameters of the rock have a substantial impact on the efficiency of gas extraction, and selecting suitable rock parameters can enhance the recovery rate of shale gas. This paper, through numerical simulation, investigates the influence of temperature on gas fracturing technology, with the aim of contributing to its improved application in engineering practices.

Hybrid PSO with tree-based models for predicting uniaxial compressive strength and elastic modulus of rock samples

The mechanical characteristics of rocks, specifically uniaxial compressive strength (UCS) and elastic modulus (E), serve as crucial factors in ensuring the integrity and stability of relevant projects in mining and civil engineering. This study proposes a novel hybrid PSO (particle swarm optimization) with tree-based models, such as gradient boosting regressor (GBR), light gradient boosting machine (LightGBM), random forest (RF), and extreme gradient boosting (XGBoost) for predicting UCS and E of rock samples from Block IX of the Thar Coalfield in Pakistan. A total of 122 datasets were divided into training and testing sets, with an 80:20 ratio, respectively, to develop the predictive models. Key performance metrics, including the coefficient of determination (R2), mean absolute error (MAE), and root mean square error (RMSE), were employed to assess the model’s predictive performance. The results indicate that the PSO-XGBoost model demonstrated the highest accuracy in predicting UCS and E, outperforming the other models, which exhibited inferior predictive performance. Furthermore, this study utilized the SHAP (Shapley Additive exPlanations) machine learning method to enhance our understanding of how each input feature variable influences the output values of UCS and E. In conclusion, the proposed framework offers significant advantages in evaluating the strength and deformation of rocks at Thar Coalfield, with promising applications in the field of mining and rock engineering.

The Influence of Acid on the Rock Mechanical Characteristics of Deep Shale in the Wujiaping Formation

The Influence of Acid on the Rock Mechanical Characteristics of Deep Shale in the Wujiaping Formation

Fractured reservoirs: An extensive geomechanical integrity approach

There is no more challenging task for today oil and gas production companies than simulating the production behavior of naturally fractured reservoirs fields. These reservoirs are highly prosperous at their early stage of production, only to decline afterward. Previous behavior has risen awareness of the geomechanics role in controlling deliverability of those reservoirs. Hasdrubal is a naturally fractured reservoir field located in offshore Tunisia. Recently, the field has manifested serious integrity and water coning issues. An integrity study was necessary to accurately determine the ever-changing stress settings and properly assimilate their impact on the production trend. Thus, former geomechanical investigations of the field were revisited to acquire a strong understanding of the reservoir and so develop an appropriate Mechanical Earth Models that assimilate the current stresses state and mechanical properties. The rock-mechanical characteristics was subsequently up-scaled to an Intact Three-Dimensional Geomechanical Model that incorporates the geomechanical properties along with the faults network. The homogenization concept was then adopted to generate the homogenized rock properties that were then populated over the latest model in order to establish the Three-Dimensional Homogenized Geomechanical Model (3D-HGM). The 3D-HGM was deployed to investigate the invasive impact of the fractures on field deliverability and ultimately generate a reactivation potential map of the faults at current and future conditions, which showed again the reactivation of most of faults intercepted by the well notably the ones responsible of water production. A justification that the former investigation studies were not in position to offer it as they were grounded only on the 1D modelling approaches (i.e. intact model). Previous observation was confirmed through the built of a two dimensional finite model for the problematic well A1ST thus allowing us to compute displacement of the fault's walls and accordingly interpret the new values of permeability associated to the different faults of the field. The up-to-date permeabilities were introduced to the dynamic reservoir model hence to estimate production trend of the A1ST1 well. A fair matching with field production data was achieved, proving again the reliability of the integral approach developed in this study in modelling the Hasdrubal field with high accuracy.

Research on the Capacity of Shale Fracture Network based on Fractal Dimension

The quantitative evaluation method of fracture network capacity after shale fracturing is crucial for optimizing hydraulic fracturing of shale gas reservoirs and improving shale gas recovery. In order to quantitatively evaluate the shape of the shale fracture network after fracturing, the mud shale cores from the Longmaxi Formation were taken to carry out the constant-rate strain method fracturing pressure experiment. The fracture information of the two-dimensional lateral development surface after the core is compressed, and the surface fracture rate, concentrative value and fractal dimension of the fractures after compression are calculated. Through multiple linear regression, it is found that the fractal dimension is positively correlated with the surface fracture rate, which describes the number of fractures, and the concentrative value, which describes the spatial distribution of fractures, and the former has a greater impact on the size of the fractal dimension. Considering the above parameters comprehensively, a fracture network capacity evaluation model is established, which is in good agreement with the rock mechanical characteristics reflected by the strain-stress curve and the reconstructed three-dimensional fracture morphological characteristics. It has a certain guiding significance for shale brittleness research and fractability evaluation.

Study on the mechanical properties of sandstone-shale composite continental shale gas based on the discrete element method

China has abundant continental shale oil resources, and multiple shale oil exploration and development demonstration zones have been established, making it a key field for oil and gas exploration and development. However, drilling and fracturing treatment often encounter vertically interlaced and superimposed sandstone and shale reservoirs, which significantly affect the stability of the reservoir and the propagation of fractures. Therefore, it is of great significance to study the rock mechanics characteristics of sandstone-shale composite. Using the orthogonal experimental method for microscale parameter calibration, this study established the relationship between the macroscopic and microscopic parameters of sandstone-shale in the parallel bond model. The results of the orthogonal experimental scheme were analyzed using multi-factor variance analysis and linear regression analysis to determine the influence and ranking of each microscale parameter on the macroscopic parameters. Based on the results of uniaxial and triaxial compression experiments, the particle flow simulation of sandstone-shale composite rock samples was studied using the discrete element method. The study investigated the influence of the thickness ratio, combination form, and confining pressure of sandstone-shale on the mechanical properties of composite rock samples. The results showed that: (1) Under uniaxial loading conditions, for the combination form of sandstone on top and shale on the bottom (M-S type), the peak stress of the composite rock sample increased first and then decreased and then increased again with the increase of the thickness of sandstone; for the combination form of shale on top and sandstone on the bottom (S-M type), the peak stress of the composite sample decreased first and then increased and then decreased again with the increase of the thickness of shale. (2) After applying confining pressure, the compressive strength of the M-S type composite sample decreased with the increase of the thickness ratio of sandstone, while the compressive strength of the S-M type composite sample increased with the increase of the thickness ratio of shale as well as the compressive strength of the composite sample was lower than that of any complete rock sample. (3) Under uniaxial loading, the fracture mode of the composite sample was related to the combination form and the thickness ratio of different lithologies. For the M-S type sample, when the thickness ratio of sandstone was <30%, the sample was mainly fractured by tension, and when the thickness ratio of sandstone was >30%, it was mainly fractured by shear mode. For the S-M type sample, when the thickness ratio of shale was <70%, the sample had the most internal shear cracks, and when the thickness ratio of shale was >70%, it had the most tension cracks.

A New Method for Evaluating Fracability Based on Cluster Analysis and Well Logging Petrophysical Facies Division

The fracability of rock is a crucial parameter in determining the efficiency of fracturing in tight reservoirs. However, current methods for evaluating fracability have significant limitations. This paper proposes a new method for evaluating reservoir fracability based on cluster analysis of logging petrophysical facies. This method considers rock mechanics characteristics, petrophysical response characteristics, and reservoir physical parameters, among other factors. The Well X of the block was used as the research subject. Using the brittleness index and permeability evaluation index, the reservoir fracability of the target block is classified into five categories: high brittleness and low permeability rock, high brittleness and super low permeability rock, low brittleness and low permeability rock, low brittleness and super low permeability rock, and clay/shale. Geolog software is used to generate fracability prediction profiles for representative wells based on petrophysical facies. To further validate this evaluation method, the fracture construction data and microseismic monitoring interpretation results of Well Y in a tight block of an oil field were synthesized and analyzed to predict its fracability. The overall prediction accuracy reached 81.8%, indicating that the new fracability evaluation method has good application effect.

A Constitutive Model Study of Chemical Corrosion Sandstone Based on Support Vector Machine and Artificial Bee Colony Algorithm

The mechanical characteristics of rock are greatly influenced by hydrochemical corrosion. The chemical corrosion impact and deformation properties of the meso-pore structure of rock under the action of different hydrochemical solutions for the stability evaluation of rock mass engineering are of high theoretical relevance and applied value. Based on actual data, a support vector machine (SVM) rock constitutive model based on artificial bee colony algorithm (ABC) optimization is constructed in this article. The impact of porosity (chemical deterioration), confining pressure, and other aspects is thoroughly examined. It is used to mimic the triaxial mechanical behavior of rock under various hydration conditions, with high nonlinear prediction ability. Simultaneously, the statistical damage constitutive model and the ABC-SVM constitutive model are used to forecast the sample’s stress–strain curve and compare it to the experimental data. The two models’ correlation coefficients (R2), root mean square error (RMSE), and mean absolute percentage error (MAPE) are computed and examined. The correlation coefficient between the ABC-SVM constitutive model calculation results and the experimental results is found to be larger (R2 = 0.998), and the error is smaller (RMSE = 0.7730, MAPE = 1.51), indicating that it has better prediction performance on the conventional triaxial constitutive relationship of rock. It is a highly promising new way of describing the rock’s constitutive connection.

Infrared radiation constitutive model of sandstone during loading fracture

The significant increase in energy demand will inevitably boost the exploitation of natural resources. The exploitation of resources including coal, oil, geothermal, natural gas, shale gas, and other energy has to be extracted with deep mining to fulfill the industry energy demand. With the increase of mining depth and the utilization of deep rock mass, the problem of high ground stress has become increasingly prominent. During high ground stress conditions, the mechanical behavior of hard rock will change significantly. Constructing a reasonable stress–strain damage constitutive model of hard rock is the basis for the design of a resource mining construction scheme, numerical simulation analysis, more accurate acquisition of rock mechanical characteristics, and evaluation of engineering rock stability. This is very crucial that needs to be addressed in a better way to solve the scientific problems of rock engineering. In this research, the infrared radiation observation experiments of sandstone biaxial under different lateral stresses are carried out. It is found that the infrared radiation energy has a near power function relationship with the effective stress in loading and fracture process. The quantitative characterization method of infrared radiation of the first invariant of stress and the second invariant of deviator stress is established. The cumulative high-temperature point scale factor amplitude is proposed to characterize the plastic volumetric strain of sandstone. Based on the effective stress and plastic strain, the plastic and damage models are established respectively. The three-dimensional plastic damage constitutive model of loaded sandstone based on infrared radiation is constructed. The model has clear input parameters having physical significance, and the compaction stage of sandstone is considered. The stress prediction of sandstone during biaxial loading is realized through the secondary development of a finite element software subroutine. The research results can lay a theoretical foundation for the analysis and evaluation of the mechanical characteristics of hard rock in the process of deep energy exploitation.

Research on transversely isotropic in-situ stress logging prediction of transitional shale reservoir: a case study of Daji Block, Eastern Ordos Basin, North China

Reservoir in-situ stress is crucial in oil and gas exploration and development. With the transitional shale as the research object, the ultrasonic transmission test and uniaxial compression test of rock samples from different sampling angles were performed. Further, the transversely isotropic in-situ stress logging calculation method for transitional shale reservoirs was established based on the transversely isotropic constitutive relationship. Finally, the in-situ stress profile of the transitional shale reservoir in the study area was analyzed, and the reliability of the results was verified. The results indicated that the mechanical characteristics of rock and its acoustic velocity varied significantly depending on the sample angle, with some regularity in the changes between vertical and horizontal mechanical properties. The logging profile revealed that the transverse Young modulus was greater than the vertical, and the vertical Poisson’s ratio was typically greater than the transverse. The suggested transversely isotropic model performed better than the conventional in-situ stress model at predicting the in-situ stress of transitional shale reservoirs, with an error of less than 2%. This demonstrated the applicability of the model’s in-situ stress calculation of the transitional shale reservoir.

A new model for evaluating rock drillability considering the rock plasticity and chip hold down effect caused by hydrostatic column pressure under high confining pressure

The Yingqiong Basin in the northern part of the South China Sea is a region with large natural gas reserves. However, the high formation pressure and hydrostatic column pressure of the extra-thick mudstone section in this area present significant challenges to drilling operations. The mudstone has high plasticity and is very hard, and its thickness accounts for only 40% of the total footage, which results in pure drilling time accounting for over 75% of the total drilling time. To accelerate the rate of penetration (ROP) in this area, this study investigates the rock mechanics characteristics of mudstone under high confining pressure through experimental methods. The study reveals that the strength of the mudstone increases rapidly with increasing confining pressure, and the critical transformation stress of bottom-hole mudstone is around 45 MPa. Moreover, a cylindrical-cutter micro bit was developed, and the drillability of mudstone was tested under high confining pressure and hydrostatic column pressure, which further confirms that 45 MPa is the critical stress for a sudden increase in mudstone's drillability. Based on multiple regression analysis, the acoustic interval transit time, hydrostatic column pressure, and argillaceous content were identified as strong correlation parameters for the drillability of plastic mudstones. A new model for evaluating rock drillability considering rock plasticity and chip hold down effect caused by hydrostatic column pressure was developed, which provides effective guidance for the optimal design of bits and drilling fluid density in the extra-thick mudstone formations of the Yingqiong Basin. This study provides theoretical and technical support for the economic and efficient development of high-pressure gas fields in this area.

Геомеханическое обоснование формирования зон хрупкого разрушения пород в окрестности сопряжений горных выработок рудников АО «Апатит»

Introduction. Prediction of excavations stability is an important step towards ensuring geo-mechanical safety during their construction. A reliable prediction of the stability of excavations includes such factors as the initial stress state, physical and mechanical characteristics of rocks, rock mass fracturing, and a number of others. These factors should be considered when performing calculations. The geo-mechanical conditions of the Apatite deposits are different, rock pressure can manifest itself in the form of brittle fracture of rocks, limited displacement of the rock contour, the formation of local rock failures or arching. In this paper, we consider only areas of the rock mass, where the loss of stability of the excavation contour is associated with the formation of brittle fracture zones. An analysis of methods for predicting brittle fracture zones suggests that they are devoted to their formation in the vicinity of single excavation and do not consider the formation of brittle fracture zones in the areas of excavation junctions. Despite the fact that a significant number of research papers are devoted to the formation of plastic deformation zones and the assessment of the excavation contour stability, they mainly consider two forms of geo-mechanical processes - rock failure along fractures and rock contour deformations. At the same time, little attention is paid to the formation of brittle fracture zones in the areas of excavation junctions in rocks predisposed to the development of such geo-mechanical processes. Considering the conditions of the Apatite deposits, it can be noted that the values of deformational and strength characteristics of rocks are very high. The rock mass is different in fracturing characteristics, however, the volume of rocks with excavations, which can be classified as weakly fractured, is very significant. The initial stress state of rock mass is characterized by the tectonic nature, where the magnitudes of the largest horizontal stresses are several times greater than the vertical stress component. All this makes it possible to classify the conditions of Apatite deposits as very difficult and rock burst hazardous. Methods and materials. To assess the correctness of the numerical modeling using model of brittle fracture of rocks, the following typical problems are considered: forecasting the development of brittle fracture zones in the vicinity of excavations of research laboratory in Canada; forecast of zones of brittle fracture of rocks for mining-geological and geo-mechanical conditions of the Apatite enterprises. Numerical modeling and the stress-strain state prediction in the vicinity of excavations was carried out in a generalized plane-strain formulation (three-dimensional formulation with the assignment of appropriate boundary conditions). An analysis of numerical simulation results suggests that the increase in the size of the brittle fracture zone is that greater, the larger the junction span. This effect was obtained for all four variants of excavation junction. The following results of the stress-strain state formation in rock mass in the vicinity of excavation junctions were obtained: the size of the brittle fracture zone as a whole tends to increase in the areas of excavation junctions, while the type of junction plays a significant role in its size; the higher the value of the σmax index, the more clearly the influence of the junction type on the size of the brittle fracture zone is manifested, while at the ratio σmax/σc &lt; 0.8, the difference in the sizes of the zones in the junction and the extended section of the excavation is not significant. It was noted that the excavation junction has an impact not only on the size of the brittle fracture zone, but also on the nature of this zone formation. There is a “flow” of the brittle fracture zone from the junction area directly to the single excavation, the direction of which coincides with the direction of the maximum principal stresses. Thus, it is possible to form rock failure in the excavation located in a moderately stressed rock mass; The proposed model for predicting brittle fracture zones based on setting the functions of hardening/softening of the medium, makes it possible to effectively predict brittle fracture zones in the vicinity of excavation junctions. In general, the research results confirm the patterns of brittle fracture zones development in the vicinity of excavation junctions obtained by the authors, however, an improved model for describing the development of geo-mechanical processes in rocks prone to brittle fracture made it possible to significantly increase the reliability of predicting the shape of the brittle fracture zone near the excavation.

Application of Non-Destructive Test Results to Estimate Rock Mechanical Characteristics—A Case Study

Accurately determining rock elastic modulus (EM) and uniaxial compressive strength (UCS) using laboratory methods requires considerable time and cost. Hence, the development of models for estimating the mechanical properties of rock is a very attractive alternative. The current research was conducted to predict the UCS and EM of sandstone rocks using quartz%, feldspar%, fragments%, compressional wave velocity (PW), the Schmidt hardness number (SN), porosity, density, and water absorption via simple regression, multivariate regression (MVR), K-nearest neighbor (KNN), support vector regression (SVR) with a radial basis function, the adaptive neuro-fuzzy inference system (ANFIS) using the Gaussian membership (GM) function, and the back-propagation neural network (BPNN) based on various training algorithms. The samples were categorized as litharenite and feldspathic litharenite. By increasing the feldspar% and quartz% and decreasing the fragments%, the static properties increased. The results of the statistical analysis showed that the SN and porosity have the greatest effect on the UCS and EM, respectively. Among the Levenberg–Marquardt (LM), Bayesian regularization, and Scaled Conjugate Gradient training algorithms using the BPNN method, the LM achieved the best results in forecasting the UCS and EM. The ideal obtained BPNN, using a trial-and-error process, contains four neurons in a hidden layer with eight inputs. All five models attained acceptable accuracy (correlation coefficient greater than 70%) for estimating the static properties. By comparing the methods, the ANFIS showed higher precision than the other methods. The UCS and EM of the samples can be determined with very high accuracy (R2 &gt; 99%).

Forecasting failure load of Sandstone under different Freezing-Thawing cycles using Gaussian process regression method and grey wolf optimization algorithm

The stability analysis of rock is an important basis to ensure the safe exploitation of underground resources and the reliable operation of space engineering. Failure load is one of the most important parameters on which the stability analysis of rocks can be performed. Rocks are constantly subjected to the freezing-thawing (F-T) weathering phenomena. F-T cycles can significantly affect the failure load of rocks and cause geological disasters in cold regions. As a result, the effects of F-T cycles on rock mechanical characteristics should be carefully investigated. Weak, heavily fractured, worn, and thinly bedded rocks, on the other hand, can never provide enough high-quality cores. Aside from being difficult, time-consuming, and costly, this test also entails destructive testing. Therefore, the prediction of failure load under different F-T cycles utilizing simplified indirect test procedures has been emphasized. In this study, a Gaussian process regression (GPR) technique was developed to predict the failure load of rock samples, utilizing 300 datasets obtained from the laboratory tests for Sandstone under different F-T cycles (0,1,4,8,16,32). Seven input parameters effective on the failure load were considered in the datasets. The grey wolf optimization (GWO) algorithm was used to fine-tune the GPR’s hyperparameters. The evaluation indices results showed that the GPR model developed by this study has a high potential ability in the prediction of failure load. The stepwise method was used for feature selection purposes. Confining pressure and porosity parameters were selected as the most effective features on the failure load. Considering only the confining pressure and porosity parameters in the model, the accuracy of the model was increased as the RMSE and R2 were obtained by about 5.57 and 0.9885, respectively. Also, the performance of the GPR model was compared to the other machine learning methods. Finally, the GPR model was recommended to predict the failure load of rock samples. This work’s significance is that it allows geotechnical engineers to accurately estimate the failure load rock samples under F-T cycles. In this way, instead of preparing more samples and waiting a long time to prepare these samples for the same laboratory test, we can use the trained model and minimize the time and costs required for laboratory tests.

Thermal Behavior of Saturated Sand at Different Relative Densities

Abstract With the proposal of a dual carbon goal in China, shallow geothermal energy as a kind of clean energy has been gradually promoted and applied. At the same time, more and more geotechnical workers have gradually paid attention to the influence of temperature on the mechanical characteristics of rock and soil mass. Existing experimental research has shown that the thermodynamic behavior for cohesive soil such as clay and silt is relatively mature but is relatively less mature for noncohesive soil, especially for sand. Based on the hollow cylinder triaxial specimen, a series of temperature-controlled triaxial tests have been carried out on saturated Fujian sand under different initial relative density and temperature conditions to capture the change of axial and volumetric strains for saturated sand specimen with increasing temperature. In addition, a bulk volumetric coefficient of thermal expansion of the sand specimen has been put forward, and the relationship between this coefficient and initial relative density also has been established. Then, the thermal deformation mechanism of saturated sand specimen has been revealed. After that, based on the test data during undrained shearing, the stress–strain relationship, deviatoric stress, and pore water pressure at peak state for loose and medium-dense saturated sand specimens have been explored, which can be used to provide some theoretical guidance for shallow geothermal energy and other temperature-related engineering applications.

Study on mechanical properties and wellbore stability of deep sandstone rock based on variable parameter M-C criterion

With the depletion of shallow oil and gas resources and the increase in fossil energy consumption, deep oil and gas resources have become the main driving force of economic development. However, the physical and chemical properties and rock mechanics characteristics of the deep sandstone stratum are remarkable, and there is little research on the rock failure law of the deep sandstone stratum. Therefore, a batch of sandstone with similar homogeneity was selected, and several triaxial mechanical experiments with variable confining pressure were conducted for such sandstone. The mechanical constitutive model of rock samples under different confining pressures, the macroscopic failure characteristics of rock samples, and the evolution law of wave velocity and internal energy of rock samples during mechanical loading were analyzed. According to the experimental results of triaxial mechanics, a calculation model of wellbore collapse pressure in deep sandstone formation was established based on the variable parameter Mohr–Coulomb criterion. The results show that the crack propagation process of rock samples under mechanical loading can be divided into four stages. The characteristic strength, peak strain, elastic modulus, and deformation modulus of rock samples were positively correlated with confining pressure and showed obvious nonlinearity in high confining pressure areas. The failure mode of the rock sample changed from brittle failure to plastic brittle failure with the increase in confining pressure. According to the characteristics of the stress-strain curve, the variation of acoustic wave velocity during triaxial mechanical loading can be divided into three different types. The variation trend of acoustic wave amplitude of rock samples is consistent with the stress-strain curve. With the loading of axial stress, the elastic energy curve was a concave function, the curve of dissipated energy started increasing at the cracking stress of rock samples. After destroying the rock sample, the decline slope of the dissipated energy curve decreased with the increase in confining pressure. Considering the limited failure of the borehole wall, an evaluation model of borehole wall stability in deep sandstone formation based on variable parameter M–C criterion was established. The equivalent density of borehole collapse pressure predicted using the M–C strength criterion method with variable parameters was smaller than that predicted with traditional methods. The research results provide a theoretical basis for safe and efficient drilling in a deep sandstone formation.

A Study of Constitutive Model of Rock Damage under the Joint Effect of Load and Moisture

To study the mechanical damage characteristics of rock under the effect of subversion, a series of mechanical experiments, including both uniaxial and triaxial mechanical compression experiments under various levels of water content were performed. In this study, researchers investigate the impact of water content on the mechanical characteristics of rock, based on the compliance of the rock damage variants to the Weibull statistic distribution, and Drucker–Prager strength rule, aiming to construct a constitutive model under the joint effect of load and moisture. In addition, the established constitutive model is tested in the experiment. According to the test results, during the initial phase of the submersion, the water content in the rock increases following the exponential function until reaching the threshold. The water content remains stable after the threshold. Under the uniaxial and triaxial loads, the damage detected in the rock and the elasticity modulus decreases linearly as the water content increases. The rock’s mechanical parameters and the damage evolution rate are significantly impacted by the surrounding pressure. As the surrounding pressure increases, the weakening effect of the water in the rock decreases. The theoretic curves developed to describe the rock damage under the joint effect of the water and load are consistent with the curves drafted based on the test, indicating that the constitutive model can accurately describe the stress and strain behaviors of rock under various water contents and loading conditions.

Experimental Study on the Influence of Different Loading Rates on Fatigue Mechanical Properties of Sandstone

Underground rock engineering often encounters long-term cyclic loading and unloading. Under the influence of this effect, the mechanical characteristics of rocks will inevitably change, which will affect the stability and safety of underground engineering. Therefore, it is necessary to study the fatigue characteristics of rocks under a certain period of action. With an RDL series electronic creep relaxation testing machine, fatigue loading and unloading tests of sandstone at different loading rates were carried out, followed by uniaxial compression on the samples. The study shows that the stress–strain curves of the uniaxial compression specimens have three stages: a compaction pore fracture stage, an elastic deformation stage, and an unstable fracture developing to failure stage. The stress–strain curves of the samples with a certain number of cycles of loading and unloading give the thinning and dense phenomenon, and the axial upper limit strain and axial cumulative residual strain gradually decrease as the loading rate increases. With the increase, the uniaxial compressive strength of the reloaded samples increases gradually, which is higher than the ordinary uniaxial compressive strength. In the process of cyclic loading and unloading, the internal particles of the sample present fracture and reorganization of the fragile structure and, at the same time, compaction stability.