Evaluation Of Rock Brittleness Research Articles

Research on multi-wave joint elastic modulus inversion based on improved quantum particle swarm optimization

Young's modulus and Poisson's ratio are crucial parameters for reservoir characterization and rock brittleness evaluation. Conventional methods often rely on indirect computation or approximations of the Zoeppritz equations to estimate Young's modulus, which can introduce cumulative errors and reduce the accuracy of inversion results. To address these issues, this paper introduces the analytical solution of the Zoeppritz equation into the inversion process. The equation is re-derived and expressed in terms of Young's modulus, Poisson's ratio, and density. Within the Bayesian framework, we construct an objective function for the joint inversion of PP and PS waves. Traditional gradient-based algorithms often suffer from low precision and the computational complexity. In this study, we address limitations of conventional approaches related to low precision and complicated code by using Circle chaotic mapping, Lévy flights, and Gaussian mutation to optimize the quantum particle swarm optimization (QPSO), named improved quantum particle swarm optimization (IQPSO). The IQPSO demonstrates superior global optimization capabilities. We test the proposed inversion method with both synthetic and field data. The test results demonstrate the proposed method's feasibility and effectiveness, indicating an improvement in inversion accuracy over traditional methods.

A New Brittleness Evaluation Index for Reservoir Rocks Based on Fuzzy Analytic Hierarchy Process and Energy Dissipation

Summary Accurate and quantitative evaluation of rock brittleness is essential for optimizing hydraulic fracturing parameter design. Specific aspects of rock brittleness can be characterized based on mineral composition, strength parameter, stress-strain, and elasticity parameter. However, in characterizing the effect of high confining pressure on rock brittleness, these advantages have not been shown for various reasons. The essence of rock fracture resides in the accrual and dissipation of energy, concomitant with the transformation of three distinct energy modalities: elastic energy, dissipated energy, and post-peak release energy. In this study, we proposed a new brittle index for reservoir rocks considering the interactions of the energy evolution process in different stages. The associated Bii values are computed through the analysis of energy evolution based on the fuzzy analytic hierarchy analysis process (FAHP) method. Tri-axial compression tests were conducted on granite, shale, and sandstone specimens under different confining pressures for the validation of the proposed brittleness evaluation (BE) index. The results indicate a decreasing trend in the brittleness index with increasing confining pressure. Comparative analysis with other existing brittleness evaluation indexes demonstrates that the new brittleness evaluation index offers a more precise evaluation of the brittleness of various rock types.

Evaluation Method for Determining Rock Brittleness in Consideration of Plastic Deformation in Pre-Peak and Failure Energy in Post-Peak

The assessment of rock brittleness holds significant importance for understanding and predicting the mechanical properties and engineering behavior of rocks. Due to the lack of a unified definition of rock brittleness, numerous evaluation methods for brittleness indexes have been proposed by scholars both domestically and internationally in recent decades, resulting in diverse evaluation outcomes. In this study, we first summarize the existing rock brittleness evaluation methods and highlight their respective advantages and disadvantages. Subsequently, considering the pre-peak plastic deformation of the rock mass, the pre-peak brittleness index factor is introduced. Furthermore, taking into account the total energy consumed by the rock mass for failure after the peak, the post-peak brittleness index factor is proposed. These two components of the brittleness index describe the characteristics of different stages of the stress-strain curve, leading to the development of a novel brittleness index. The proposed method is then applied to evaluate the brittleness of both red-bed sandstone and cyan sandstone, revealing the variation of rock brittleness under different working conditions. Finally, three existing evaluation methods are selected to validate the rationality of the proposed method. The results demonstrate that for red-bed sandstone, the proposed brittleness index exhibits maximum values under natural conditions at all confining pressures. The four brittleness indexes consistently characterize the brittleness of red-bed sandstone under natural conditions. Under saturated conditions, the brittleness indexes exhibit different patterns of variation. For cyan sandstone, the three brittleness indexes—B7, B9, and Bnew—exhibit a similar trend in characterizing the brittleness of cyan sandstone under natural conditions and freezing-thawing conditions, while the trend of B17 is essentially opposite to that of the previous three indexes. The research findings provide guidance for the assessment of sandstone brittleness.

Cutting energy characteristics for brittleness evaluation of rock using digital drilling method

The reliable evaluation of rock brittleness is essential in engineering geology for various applications, such as water conservancy, hydropower, transportation, energy exploration, and underground engineering development. This study proposes a new method for evaluating rock brittleness using a digital drilling approach. A cutting model was established to describe the relationship between the energy characteristics and mechanical parameters of the rock during the drilling process, accounting for the effects of friction and drilling fluid. Furthermore, a drilling-based index, the brittleness evaluation index (BEI), was developed to evaluate rock brittleness based on energy dissipation. We conducted drilling tests to investigate energy evolution, including cutting energy, friction energy, and liquid energy. The results indicated that cutting energy has a linear correlation with the strength parameters of rock, with a significant correlation coefficient. With increasing drilling depth, cutting energy and liquid energy exhibit a similar increasing trend, which initially remains constant and then increases linearly after a critical depth is reached. Friction energy rapidly increases with drilling depth at the beginning, and the growth rate decreases after reaching the critical point. Compared with the laboratory-determined brittleness index, our proposed method provides a reliable evaluation of rock brittleness. In summary, our study offers a practical approach to evaluating rock brittleness that could benefit various engineering applications.

Characterization and evaluation of brittleness of deep bedded sandstone from the perspective of the whole life-cycle evolution process

The quantitative determination and evaluation of rock brittleness are crucial for the estimation of excavation efficiency and the improvement of hydraulic fracturing efficiency. Therefore, a “three-stage” triaxial loading and unloading stress path is designed and proposed. Subsequently, six brittleness indexes are selected. In addition, the evolution characteristics of the six brittleness indexes selected are characterized based on the bedding effect and the effect of confining pressure. Then, the entropy weight method (EWM) is introduced to assign weight to the six brittleness indexes, and the comprehensive brittleness index Bc is defined and evaluated. Next, the new brittleness classification standard is determined, and the brittleness differences between the two stress paths are quantified. Finally, compared with the previous evaluation methods, the rationality of the proposed comprehensive brittleness index Bc is also verified. These results indicate that the proposed brittleness index Bc can reflect the brittle characteristics of deep bedded sandstone from the perspective of the whole life-cycle evolution process. Accordingly, the method proposed seems to offer reliable evaluations of the brittleness of deep bedded sandstone in deep engineering practices, although further validation is necessary.

Evaluation of Rock Brittleness Based on Complete Stress–Strain Curve

As a basic mechanical property of rocks, brittleness is closely related to the drillability, wellbore stability, and rockburst characteristics of reservoir rocks. Accurate evaluation of rock brittleness is of great significance for guiding oil and gas production and reservoir reconstruction. This paper systematically introduced the commonly used brittleness evaluation methods based on the stress–strain curve and analyzed their theoretical background and mathematical models. Combined with practical engineering application, the characterization effect of commonly used brittleness indexes in various rock samples is verified and optimized, and it is obtained that the brittleness index (B17 in this paper), based on the stress–strain curve and considering energy conversion, has the best characterization result for rock brittleness, which has good differentiation for different rock samples. At the same time, considering that the stress–strain curve under high confining pressure may result in a significant yield plateau phenomenon before and after the peak strength, the endpoint of the plastic yield plateau is used to replace the peak point as the starting point for the drop of bearing capacity. The revised brittleness index is consistent with the changing trend of the original curve, which verifies the reliability of the model. Finally, the method for characterizing the brittleness of Class II curves is supplemented, and the combined brittleness index of rock is established, which verifies the rationality and correctness of the index and provide a more general evaluation method for rock brittleness in engineering.

Comprehensive Characterization Integrating Static and Dynamic Data for Dynamic Fractures in Ultra-Low Permeability Reservoirs: A Case Study of the Chang 6 Reservoir of the Triassic Yanchang Formation in the Ordos Basin, China

The generation of dynamic fractures during the process of water injection in ultra-low permeability reservoirs aggravates the heterogeneity of the reservoir, resulting in a rapid rise of water cut and directional flooding of the producers, which affects waterflood sweep efficiency and recovery. A dynamic fracture, as the geological feature of ultra-low permeability reservoirs, has a complex genetic mechanism and is difficult to characterize. Taking the Chang 6 reservoir of the Triassic Yanchang Formation in the Ordos Basin of central China as an example, this paper presents the characterization method and workflow of dynamic fractures. On the one hand, through the analysis of triaxial-compression rock-mechanic experiments and the mineral composition of the core, we evaluated rock brittleness in order to identify the lithology that can easily generate new fractures. On the other hand, beginning with the ancient tectonic stress field and combining the fracture characteristics of core and geological outcrop, the multi-fractal method and the probabilistic neural network were applied to identify the natural fractures and to quantitatively predict the intensity of natural fractures. Based on the rock brittleness evaluation and the natural fracture feature, the intensity of dynamic fractures was characterized by integrating the analysis of the bottom hole pressure, fracture pressure, and production response characteristics. A dynamic fracture is a “double-edged sword” during the waterflooding development of ultra-low permeability reservoirs. The premature activation and generation of dynamic fractures could lead to a worse development status. Nevertheless, the rational control and utilization of dynamic fractures play a positive role in improving oil recovery. Dynamic fractures are of great significance to the optimization and adjustment of well patterns for ultra-low permeability reservoirs. This can provide a reference for similar reservoirs.

Rockburst tendency prediction based on an integrating method of combination weighting and matter-element extension theory: A case study in the Bayu Tunnel of the Sichuan-Tibet Railway

The Bayu Tunnel is the most problematic tunnel project in the Lhasa-Nyingchi section of the Sichuan-Tibet Railway. Rockbursts have occurred frequently during the construction process, which has seriously threatened the safety of construction workers and equipment. Aiming at the rockburst hazard prediction problem of the Bayu Tunnel, in this paper an integrating method of combination weighting and matter-element extension theory is proposed. This method introduces the rock brittleness evaluation index BICSS, Russenes criterion and rock mass integrity coefficient as the main rockburst evaluation indexes, which characterized the control factors of rock physical and mechanical property, surrounding rock stress and rock mass integrity on rockburst tendency, respectively. Then the rockburst tendency evaluation index system is constructed. The weight of each rockburst control factor is determined by the combination weighting algorithm of triangular fuzzy number analytic hierarchy process (TFN-AHP), criteria importance though intercriteria correlation (CRITIC), and determination of comprehensive weight (MDI). The correlation degree between the rockburst sample and rockburst grade can be calculated using the matter-element extension theory, after which the prediction of rockburst grade is carried out. Finally, the applicability, superiority and the necessity of selected rockburst control factors in this method are verified and discussed further. The research results provide a new analysis method and technical means for rockburst tendency assessment of deep tunnel.

Rock Brittleness Evaluation Index Based on Ultimate Elastic Strain Energy

Brittleness is an essential parameter to determine the deformation and failure behavior of rocks, and it is useful to quantify the brittleness of rocks in numerus engineering practices. A novel energy-based brittleness evaluation index is proposed in this study, which redefines the dissipated proportion of ultimate elastic strain energy relative to post-peak failure energy and residual elastic strain energy. A series of conventional triaxial compression (CTC) tests were performed on shale rock to verify the reliability and accuracy of the brittleness index. The results show that the proposed index can precisely reflect the deformation and failure characteristic of rocks under different confining pressures. Based on the testing data from six types of rocks in previous studies, the universality of the novel index was verified. According to comparison with existing brittleness indices, the new brittleness index can more precisely characterize the brittleness of rock.

P-wave reflectivity parameterization and nonlinear inversion in terms of Young’s modulus and Poisson ratio

Young’s modulus and Poisson ratio are essential parameters for rock brittleness evaluation. At present, prestack inversions of these parameters mostly adopt linear approximation, whereas the nonlinear inversion methods that are based on the exact Zoeppritz equation are seldomly conducted. Nevertheless, in the case of large incident angles and high-impedance contrast, the nonlinear equation has higher accuracy than the linear approximation. It is more in line with the nature of geophysical inversion problems, and it describes the variation of formations’ elastic parameters better than the linear approximation does. Therefore, based on the exact solution of the Zoeppritz equation, we have parameterized and derived a new nonlinear reflection coefficient equation using nonlinear Young’s modulus, Poisson ratio, and density (YPD) as variables. Subsequently, by combining the inverse operator algorithm with the YPD equation, we develop a new nonlinear amplitude-variation-with-offset inversion scheme. This approach can robustly extract Young’s modulus and Poisson ratio from seismic data and identify shale-gas sweet spot locations within the reservoir, as well as predict reservoir fluid properties. We have tested the accuracy and applicability of this approach with synthetic and real data examples.

An Evaluation Method of Rock Brittleness Based on the Prepeak Crack Initiation and Postpeak Stress Drop Characteristics

Recent research shows that the brittleness of rock is closely related to the initiation and propagation of internal microcracks, but there are few brittleness evaluation indices considering the characteristics of rock initiation. Based on the theoretical analysis of brittleness and the characteristics of rock initiation, this study proposes an evaluation method of rock brittleness based on the prepeak crack initiation and postpeak stress drop characteristics. First, based on the description and definition of brittleness by George Tarasov and Potvin et al., the feasibility of an evaluation method based on the prepeak crack initiation and postpeak stress drop is theoretically analyzed. Second, the component Bi representing the prepeak brittleness of rock and component Bii representing the prepeak brittleness of rock are constructed, and the product of the two is the brittleness index BI, representing the prepeak crack initiation and postpeak stress drop. Finally, experimental tests of granite and marble were conducted to evaluate the new index, and the brittleness indices of different methods are calculated and compared. The results show that, like other brittleness indices (B1∼B5), the brittleness index BI can effectively reflect the effects of different confining pressures and loading modes on rock brittleness. The brittleness of marble decreases with increasing confining pressure from 5 MPa to 35 MPa. At a confining pressure of 5 MPa, the brittleness of granite during a triaxial unloading test is greater than that during a triaxial compression test. The calculated results are consistent with the experimental results. By tests and comparison results, the reliability of this evaluation method was verified, which provides a way to evaluate rock brittleness from the perspective of crack initiation and is helpful to enrich the analysis and evaluation of rock brittleness in the laboratory.

Experimental Research on Brittleness and Rockburst Proneness of Three Kinds of Hard Rocks under Uniaxial Compression

Rockburst is a highly destructive geological disaster caused by excavation and unloading of hard and brittle rock mass under high geostress environment. Quantitative evaluation of rock brittleness and rockburst proneness is one of the important tasks in potential rockburst assessment. In this study, uniaxial compression and acoustic emission tests were carried out for basalt, granite, and marble, and their brittleness and rockburst proneness were quantitatively evaluated. The acoustic emission evolution characteristics of the three rocks during uniaxial compression were analyzed, and the differences of fracture mechanism of the three rocks were compared. The results show that (1) based on the brittleness evaluation index, basalt is the most brittle rock, followed by granite, and marble is the weakest; (2) based on the rockburst proneness evaluation index, combined with the macroscopic failure phenomenon and morphology of the samples, the rockburst proneness of basalt is the strongest, followed by granite, and marble is the weakest; (3) there exists a positive correlation between rockburst proneness and brittleness, and the fitting results show that they are approximately exponential; and (4) brittleness has an important influence on the rock fracture mechanism. Unlike marble, basalt and granite with strong brittleness continuously present high-energy acoustic emission signals in the stage of unstable crack propagation, and large-scale fracture events continue to occur; from the calculation results of the acoustic emission b value, the stronger the brittleness of rock, the larger the proportion of large-scale fracture events in the failure process.

Brittleness evaluation of Naparima Hill mudstones

The brittleness of unconventional source rock reservoirs is an essential parameter that controls the effectiveness of hydraulic fracturing operation. Brittleness can vary with effective pressure (equivalent to burial depth) and can be influenced by fluid saturation. Therefore, a detailed evaluation of rock brittleness is necessary prior to field applications. The late Cretaceous Naparima Hill Formation is the primary source for most of the 4 billion barrels of oil and 10 trillion cubic feet of gas produced in the last hundred years from conventional reservoirs in Trinidad, West Indies. This Formation is now being considered an unconventional reservoir in order to prolong and increase current productions. However, despite being a prolific Formation, there are no available geomechanical data, including brittleness. Brittleness index (BI) is often used to measure rock brittleness. In this study, we determined elastic-based BI from P- and S-wave velocity measurements, at effective pressures up to 130 MPa, for dry and water saturated outcrop mudstones from four lithofacies within the Naparima Hill Formation. The experimental results show that the BI for all four lithofacies, is independent of effective pressures in both dry and water saturated conditions. All four lithofacies can be considered brittle under dry conditions. However, when saturated, the BI reduces, which proves significant for softer lithofacies (siliceous calcareous mudstones and siliceous mudstones). In this saturated state, the harder lithofacies (calcareous mudstone interbedded with black chert and carbonate rich mudstone with nodular chert) are considered marginally brittle and the softer ones, are considered ductile. The results highlight that porosity is the main factor that controls the magnitude of reduction in the BI when the mudstones are fully saturated.

Application of rock brittleness evaluation in tight oil exploration—— Taking Lijiaweizi Fault Depression in Qijia Area as an Example

Brittle rock is one of the key factors in the use of unconventional oil and gas exploration technology to be considered, The stronger the brittleness of rock is, the easier to produce fractures to form fracture network in the over flushing of fracturing, the stock of oil and gas storage layer will be increased, and the production of single well will be increased. At present mainly based on rock elastic parameters, we use Young’s modulus and Poisson’s ratio of two ways to analyze brittle rock in earthquake prediction evaluation. However, since these two methods in the process, both the need for elastic parameters of rock normalized, leading to large errors, adaptability reduce. Based on this, we propose a Young’s modulus and Poisson’s ratio coefficient of utilization of compared to the brittle rock evaluation method to effectively improve the accuracy of prediction and evaluation of brittle rock and anti-jamming capability, while the brittle evaluation method based on the use of pre-stack seismic inversion of elastic parameters to Qijia area of Songliao basin compact oil brittle rock predict deployment guidance wells to obtain stable high level of industrial oil flow.

Rock brittleness evaluation based on energy dissipation under triaxial compression

Evaluating the brittleness of rock is significant for strategy determination of hydraulic fracturing, such as candidate selection and parameter optimization. A series of definitions and indices of brittleness have been proposed to characterize the mechanical behavior rock failure. However, these existing indices failed to consider the residual state and the confinement effect. Based on the analysis of energy evolution during the whole process of rock failure, the brittleness was redefined in this study as the comprehensive capability of dissipating little energy during the pre-peak stage and self-sustaining complete failure during the post-peak stage. Accordingly, a new energy-based brittleness index B was proposed in terms of the complete stress-strain curve to quantify this capability from following three aspects: the ratio of accumulated elastic strain energy and total absorbed energy during the pre-peak stage (B1); the proportion of elastic strain energy in all energy source consumed for sustaining rock failure (B2); and the dissipation extent of accumulated elastic strain energy during the post-peak stage (B3). To verify the reliability of the new method, uniaxial and triaxial compression tests were performed on different types of rock samples. The application and comparison of various indices showed that the new brittleness index precisely characterized the stress-strain curves and failure behavior of rock samples under different confinement levels. The variation trends of brittleness with confining pressure were obviously distinct among different rock types. Three independent brittleness indices B1, B2, and B3 were helpful for analyzing sensitivity difference of brittleness to confining pressure among different rock types. Accordingly, this new energy-based method can provide reliable evaluation of rock brittleness.

Quantitative evaluation of rock brittleness based on crack initiation stress and complete stress–strain curves

Brittleness is an important rock material property, and its accurate evaluation has guiding significance in construction as well as in disaster prevention and reduction. Considering the limitations of the existing brittleness indices, a new brittleness index based on the overall stress–strain process of a rock mass is established that considers both the stress growth rate between the peak stress and the crack initiation stress before the peak, as well as the stress descent rate after the peak. Uniaxial and triaxial compression tests were conducted to evaluate the new index. The results of the tests show that the new index can accurately determine the rock brittleness according to the prepeak stress–strain curve under uniaxial loading system conditions, which compensates for the limitation of inaccurate postpeak curves for brittle rock. Under triaxial compression conditions, the new index more clearly represents the influence of the confining pressure on the brittleness of marble. The reliability and comprehensiveness of the new index are verified, and these research results may improve the existing evaluation of rock brittleness.

Evaluation of rock brittleness based on the ratio of stress drop rate to strain drop rate and peak strength

Evaluation of rock brittleness based on the ratio of stress drop rate to strain drop rate and peak strength

A New Rock Brittleness Evaluation Index Based on the Internal Friction Angle and Class I Stress–Strain Curve

A New Rock Brittleness Evaluation Index Based on the Internal Friction Angle and Class I Stress–Strain Curve

Experimental analysis and application of the effect of stress on continental shale reservoir brittleness

Hydraulic fracturing is an effective measure of reservoir modification for the development of shale gas. The evaluation of rock brittleness can provide a basis for the optimization of fracturing. In this paper, the effect of stress on the brittleness of shale is systematically analyzed by designing triaxial mechanics tests. The strain analysis method was used to evaluate the shale brittleness. The research indicates that, with the increase of effective confining pressure, the value of the brittleness index (B1) decreases. There is a linear and positive correlation between the average reduction ratio of B1 and the buried depth. The stress has a significant effect on the shale brittleness. Therefore, the rock brittleness can be overestimated without considering the influence of the buried depth or the stress of formation when using the mineral composition method. Being affected by the stress, when the brittle mineral content of the shale reservoir is 70%, 65%, 60%, and 55%, the lower limit depth of the shale gas development is 5000 m, 4400 m, 3000 m, and 1800 m, respectively. However, when the brittle mineral content of the shale is less than 50%, the brittleness index is less than 50% in all of the buried depths. In this case, the shale will not have any commercial development potential. The logging interpretation results of the brittleness index conducted with stress correction are more consistent with the real situation, and thus, this method can be better used to help the optimization of the fracturing intervals of shale gas.

Quantitative evaluation of rock brittleness based on the energy dissipation principle, an application to type II mode crack

Rock brittleness is the key parameter in the evaluation of “sweet spot” in the shale reservoir which is characterized with low porosity and low permeability, and numerous methods are presented to qualitatively reveal the essence of rock brittleness. However, the quantitative evaluation of this property is still a tough question with continued attention. This paper proposes a new quantitative evaluation method based on the energy dissipation principle in fracture mechanics, which interprets the degree of rock brittle fracturing as energy efficiency contributing to the new generated surface energy of fractures. The fractal geometry and fractal dimension are introduced to describe fracture surface with intricate geometry. The experimentally constructed correlation between volume density of rock fracture and fractal dimension is incorporated into energy dissipation and fractal geometry of fracture. Finally, the energy-balance equation, which corresponds to the rock brittleness index with volume density of rock fracture, is established. The brittleness indexes of different rock are calculated and compared, and the influence of the confining pressure on the brittleness is also studied. Sichuan Longmaxi shale formation of X well in southwest China is analyzed, which verified the reliability of this quantitative evaluation method in pinpointing the fracturing candidate in hydraulic fracturing engineering.