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Deformational properties, such as, Young's modulus and Poisson's ratio are considered to be among the most critical parameters of coal measure rocks for coal bed methane (CBM) extraction, CO2 sequestration, and mining activities. The inherent heterogeneity of these rocks from basin to basin makes the accurate estimation of deformational properties very complex. Here, these properties were evaluated using artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS), and compared with the traditional multiple regression analysis (MRA). Multiple index geomechanical parameters, such as, compressive strength, tensile strength, shear strength, and P-wave velocity have been used as the inputs to build the prediction models. In total 69 data sets, developed from laboratory measurements were used for the analysis. First 57 datasets were used to train the soft computing models, and the rest 12 were used to measure the accuracy and validity. Performance of the each of the statistical and soft computing models were evaluated using correlation coefficient (R2), root mean square error (RMSE), variables accounted for (VAF), and mean absolute percentage error (MAPE). Results show that ANFIS model gives the best prediction model for both the Young's modulus and Poisson's ratio. Therefore, the present study shows that it is possible to build accurate and generalized soft computing model than can incorporate heterogeneity and predict deformational properties of coal measure rocks very well without resorting to time consuming test procedures. © 2019 Elsevier Ltd

Department of Civil & Infrastructure Engineering

World University root node

Measurement: Journal of the International Measurement Confederation

Predicting deformational properties of Indian coal: Soft computing and regression analysis approach

Mode-I fracture toughness (FT) is an important property to model fracture propagation in rocks and it has wide application in a plethora of rock mechanics problems such as hydraulic fracturing design, tunneling, geothermal energy extraction, CO2 sequestration, blasting and drilling activities, well-bore stability analysis etc. Determination of fracture toughness from different individual index geomechanical properties is an ongoing research area. Multiple past research have established simple regression relations between FT and individual mechanical parameters with various degree of success. But these were found to be rock and site specific and lack wide acceptance. In this paper, multiple regression analysis, and three types of soft computing methods (e.g. artificial neural network, fuzzy inference system, and adaptive neuro-fuzzy inference system) have been employed to predict mode-I fracture toughness from a set of common geomechanical properties. Based on Pearson's correlation coefficient, three index geomechanical properties, namely, tensile strength, P-wave velocity, and S-wave velocity were chosen to estimate the mode-I FT. As the results suggest, a single model is sufficient to estimate FT in all the sedimentary and crystalline rocks. Such model is capable of predicting fracture toughness irrespective of strength or compositional heterogeneity that is encountered in field. Statistical analysis of the predictions using correlation coefficient (R2), root mean square error (RMSE), mean absolute percentage error (MAPE), and variance accounted for (VAF) show that all the soft computing methods performed better than the multiple regression models. However, among the soft computing methods, adaptive neuro-fuzzy inference system, which incorporates elements of both the artificial neural network and fuzzy inference system, constructs the best model for prediction. This is the first ever reported instance, where soft computing has been used to predict mode-I fracture toughness of dry rocks from the mechanical properties, and the results confirmed that this is an easier, efficient but highly accurate tool for prediction. © 2018 Elsevier Ltd

Predicting mode-I fracture toughness of rocks using soft computing and multiple regression

Young's modulus of coal is a very important deformational property which dictates how the material will behave in presence of sub- and super-critical carbon dioxide during sequestration. But this is also a difficult property to measure due to the extensive instrumental requirements, and wide compositional and structural heterogeneity of the coal. Therefore, an indirect method to measure the saturated Young's modulus of coals has been developed in the present research using artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS). Low and high rank specimens from three different basins of India and Australia have been used for the analysis. Saturation pressure and the compressive strength (UCS) of the coal specimens have been used as the input parameters to build the models. The performance of the models were evaluated against the multivariate regression model using four different types of statistical parameters such as root mean square error (RMSE), coefficient of determination (R2), mean absolute error percentage (MAPE), and variables accounted for (VAF). Results show that ANFIS model is the best performing one with the least RMSE and MAPE, and highest VAF and R2. This research demonstrates that it is possible to develop soft computing models that can successfully estimate some of the critical rock mechanics parameters essential for the technical evaluation of the sequestration projects on coal. This generalized model is also excellent in incorporating the possible effect of heterogeneity in specimens and performs well for samples that show similar data distribution. If applied, this can potentially reduce the requirement of extensive, complex and expensive instruments that are required for similar investigations. © 2018 Elsevier Ltd

Regression and soft computing models to estimate young's modulus of CO2 saturated coals

For the effect of thermal treatment on the mode-I fracture toughness (FT), three crystalline rocks (two basalts and one tonalite) were experimentally investigated. Semi-circular bend specimens of the rocks were prepared following the method suggested by the International Society for Rock Mechanics (ISRM) and were treated at various temperatures ranging from room temperature (25 °C) to 600 °C. Mode-I FT was correlated with tensile strength (TS), ultrasonic velocities, and Young's modulus (YM). Additionally, petrographic and X-ray diffraction analyses were carried out to find the chemical changes resulting from the heat treatment. Further, scanning electron microscopy (SEM) was conducted to observe the micro structural changes when subjected to high temperatures. These experiments demonstrate that heat treatment has a strong negative impact on the FT and mechanical properties of the rocks. From room temperature to 600 °C, mode-I FT values of massive basalt, giant plagioclase basalt, and tonalite were reduced by nearly 52%, 68%, and 64%, respectively. Also, at all temperature levels, FT and mechanical properties are found to be exponentially correlated. However, the exact nature of the relationship mainly depends on rock type. Besides, TS was found to be a better indicator of degradation degree than the mode-I FT. SEM images show that micro crack density and structural disintegration of the mineral grains increase with temperature. These physical changes confirm the observed reduction in the stiffness of heat-treated crystalline rocks. © 2018 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences

Fulltext

Journal of Rock Mechanics and Geotechnical Engineering

Correlating mode-I fracture toughness and mechanical properties of heat-treated crystalline rocks

Rock discontinuities play a crucial and critical role on the deformational and failure behavior of the rock mass. In most investigations, both the surfaces of the rock joints are considered to have same roughness. But, in nature, the walls of a fresh joint is only expected to be complimentary and to have same roughness. Weathered and water percolating rock joint is most likely to develop different surface roughness on the two opposite walls. So, the shear strength and frictional response behavior derived from the single joint roughness coefficient (JRC) assumption cannot be used in such a condition. To address this shortcoming, we have prepared sandstone blocks with different surface roughness and conducted experiments in a tribometer. The static friction, shear stiffness and coefficient-of-friction of the joint surfaces were calculated and their changes with increasing normal load were noted. One of the major findings of this paper is that, shear strength of the joints may not have a direct correlation with the increasing JRC value of the individual joint walls. Hence, some of the joint walls having higher cumulative JRCs were found to show lower shear strength than those with lowers roughness. This is because, the opposing walls of such joints are not anymore complementary and the frictional resistance is completely controlled by the height and contact area of the asperites. © 2017, Geological Society of India.

Journal of the Geological Society of India

Effect of different wall roughness on the frictional behavior of rock joints

Fracture toughness (FT) and tensile strength (TS) indicate a rock's susceptibility to fracturing. Past studies showed that mechanical properties and FT of homogeneous rocks can be correlated well. In the present study, this capability has been extended to the jointed sedimentary rocks. Sandstone with a wide combination of analogue joints were tested in the laboratory, and the control of joint geometrical properties on the FT, TS, and development of fracture process zone (FPZ) were investigated. A FT prediction model was developed based on the TS of the jointed specimens. Multifractal scaling law (MFSL) was utilized to extend the laboratory results to the field scale. Further, the interaction of propagating crack with the already existing joints were investigated and post-peak behavior were studied. Results show that FT and TS decrease with decreasing joint spacing, but they are more sensitive at higher joint spacing. It is also possible to construct a linear prediction model between the FT and TS of rock. FPZ of jointed rock is less sensitive to joint orientation and more related to joint spacing. A negative correlation between the FPZ size and joint spacing is established in this study. Post-peak behavior of the jointed specimens indicate that crack-joint interaction increases the frictional resistance and dissipated fracture energy of the specimens. Further, comparison of mixed-mode fracture criteria with the experimental results show that Maximum Tangential Stress (MTS) criterion can successfully predict the mixed-mode fracture behavior of jointed sandstone. © 2017 Elsevier B.V.

Engineering Geology

Influence of joint anisotropy on the fracturing behavior of a sedimentary rock

Rock Mechanics and Rock Engineering

Correlating the Mechanical and Physical Properties with Mode-I Fracture Toughness of Rocks

Understanding the flow behavior through fractures is critically important in a wide variety of applications. In many situations, the fluid flow can be highly irregular and non-linear in nature. Numerical simulation can be employed to simulate such conditions which are difficult to replicate in laboratory experiments. Therefore, a parametric study has been conducted on the fluid flow through micro-fracture over a large range of inlet pressure, fluid density, fluid viscosity, temperature, joint roughness coefficient (JRC), and fracture using finite element analysis. Irregular fracture profiles were created using Barton’s joint roughness coefficient. The Navier-Stokes (NS) equation was used to simulate the flow of water in those micro-fractures. The result showed that the fracture, fluid, and ambient conditions have a wide and varied effect on the fluid flow behavior. The interrelationship between these parameters was also studied. The model simulation provided result in the form of velocity and pressure drop profile, which can be used to determine the behavior of flow under different condition. The volumetric flow was calculated for each condition and has been plotted against the corresponding parameter to study the interrelationship. © 2017, Saudi Society for Geosciences.

Arabian Journal of Geosciences

Parametric study of factors affecting fluid flow through a fracture

Fracture and mechanical properties of the water saturated sedimentary rocks have been experimentally investigated in the present paper. Three types of sandstones and one type of shale were saturated in water for different periods of time. They were then tested for their index geomechanical properties such as Brazilian tensile strength (BTS), Young’s modulus (YM), P-wave velocity and all pure and mixed-mode fracture toughness (FT). FT was measured using semicircular bend specimens in a three-point bend set-up. All the geomechanical and fracture properties of the saturated rocks were compared together to investigate their interrelations. Further, statistical methods were employed to measure the statistical significance of such relationships. Next, three types of fracture criteria were compared with the present experimental results. Results show that degree of saturation has significant effect on both the strength and fracture properties of sedimentary rock. A general decrease in the mechanical and fracture toughness was noticed with increasing saturation levels. But, t-test confirmed that FT, BTS, P-wave velocity and YM are strongly dependent on each other and linear relationships exist across all the saturation values. Calculation of the ‘degradation degree’ (DD) appeared to be a difficult task for all types of sedimentary rocks. While in sandstone, both the BTS and mode-I FT overestimated the DD calculated by YM method, in shale BTS was found to give a closure value. © 2017, Springer-Verlag GmbH Austria.

Effect of Water Saturation on the Fracture and Mechanical Properties of Sedimentary Rocks

The knowledge of fluid flow through rock fractures is directly related to hydrocarbon migration, waste disposal, and carbon dioxide sequestration. The hydraulic nature and response of the fractures are directly controlled by the roughness of the fracture surfaces. However, this parameter is hard to understand because it can behave differently under different ambient conditions. The prevalent controlling parameters are the fracture inflow pressure, aperture of the fracture, and shearing displacement during flow. To understand the influence of these parameters, a systematic study was carried out numerically on different fracture geometries. In this paper, two-dimensional fractures with different surface roughness were simulated in a finite-element modeling (FEM) program, and the fluid-flow parameters were evaluated. The Navier-Stokes (NS) equation was used to model the fluid flow through the roughness profiles generated using Barton's joint roughness coefficient. By simulating the laminar fluid flow through the NS equation and predicting the particle transport using a streamline particle-tracking method, the flow-velocity profiles, outlet-pressure distribution, Reynolds number, shear rates, and particle transmissivity were measured. The parameters at different locations along the length of the fractures were compared to identify changes in the fluid flow. The models show that local undulations have considerable effect on the fluid flow. The velocity and shear-rate evolution, pore-pressure distribution, and Reynolds number of the flow indicate the presence of a strong wall effect on the fluid flow. The aperture and shearing displacements of the fracture walls also have significant control over the wall effect. © 2015 American Society of Civil Engineers.

International Journal of Geomechanics

Fluid flow through rough rock fractures: Parametric study

The effect of heat treatment and the layer orientation on the tensile properties of granitic gneiss were studied under the unconfined stress condition. The tensile strength of the samples was studied using a Brazilian configuration, and the geochemical and microstructural properties were studied using the X-ray diffraction technique as well as scanning electron microscopy (SEM), respectively. The fracture pattern and the geometrical analyses were performed using the digital photographs. The results show that both the heat treatment and layer orientation have strong control on the tensile strength, force-parallel and layer-parallel strains, and on the tensile fracture geometry. A general decrease in the tensile strength of the rock was documented with the increasing heat treatment. Although, in the heat-treated samples, X-ray diffraction study do not reveal any major change in the mineral composition, but the SEM shows the development of several micro-cracks in the grains. In the samples with different layer orientation, along with the changes in the tensile strength and layer-parallel to force-parallel strain ratio, the layer activation under shear stress is also noticed. Here, the ratio between the tensile to shear stress, acting along the layers is thought to be the major controlling factor of the tensile properties of rocks, which has many applications in mining, civil constructions, and waste disposal work. © 2015, Springer-Verlag Wien.

Effect of Heat Treatment and Layer Orientation on the Tensile Strength of a Crystalline Rock Under Brazilian Test Condition

Carbon dioxide sequestration is considered to be an efficient method of curbing the release of carbon dioxide emissions into the atmosphere and to mitigating corresponding effects on climate change. In this regard, basaltic rocks are among the potential repositories due to their ability to trap carbon dioxide in form of carbonate minerals. A series of laboratory-scale saturation experiments was conducted on three types of basalts that were collected from different horizons of the Deccan Volcanic Province (DVP), India. The basalt cores were treated in a saturation chamber for different exposure periods under low-pressure and room-temperature conditions. A set of key mechanical, physical, and chemical properties of the pre- and post-treated samples were analyzed to identify saturation-related changes. The results show that exposure of the basalts to carbon dioxide has a strong effect on their strength. However, the extent of the effect is different for different types of basalts and is strongly controlled by the exposure time, the mineralogical composition, and rock texture. X-ray diffraction and scanning electron microscopy (SEM) show that saturation leads to dissolution of the mafic minerals, and precipitation of new carbonate minerals in the cracks and vesicles of the host rock. These changes in the mineralogical content and the development of micro-cracks in the samples are interpreted as being the primary factors that affect the loss of integrity in the rock. © 2016, Springer-Verlag Berlin Heidelberg.