Ph.D Student, Indian Institute of Technology Bombay
Correlating fracture and mechanical parameters of rocks under different temperature and saturation
Though India is the fourth largest energy consumer in the world, we hold huge reserves of shale gas across several sedimentary basins which can help meet our energy requirements. The key to tapping this energy is the proper and efficient use of hydraulic fracturing. Development of hydraulic fracturing in the hydro-carbon bearing sedimentary basins of India requires in-depth knowledge of the fracture toughness and key geomechanical properties of rocks. These properties are strongly controlled by the fluid content, pressure, temperature and presence of natural fractures at the great of operation (1.5-3Km below the earth surface). Additionally, measurement of these critical parameters are technologically and economically very difficult. Therefore, the present research aims to develop empirical and soft computing techniques to indirectly measure fracture parameters under the above mentioned conditions. Parameters evaluated from this study will be used further to develop numerical models of the hydraulic fractures to evaluate large-scale rock behaviour and hydrocarbon production volume. The three main contributions of this projects are: (1) it will contribute to the development of technical knowledge of hydraulic fracturing in India. In future, this research work can be used as the benchmark to design an efficient well-drilling and shale gas exploration programme, (2) the advanced numerical models that we will use to simulate several possible geological scenarios of hydraulic fracturing will help companies design more technically and economically efficient hydraulic fracturing programmes, and (3) a better understanding of the subject will ensure that propagating fractures do not damage groundwater-bearing zones and remain within the reservoir zone. This mitigates the chance of groundwater contamination, which is perceived as a major environmental threat resulting from hydraulic fracturing.
Abstract: Understanding the aspect of enhanced energy extraction through an approach of rock mechanics requires a detailed interpretation of rock fracture mechanism. Common examples are hydraulic fracturing of gas shales and geothermal energy systems. Resolving rock fracture behavioural patterns and its properties such as fracture toughness and energy-release rate during fracturing is important for the successful implementation of such projects. These properties are a function of different environmental factors such as temperature, humidity, water vapour, pressure, and strain rate. In this study, the effects of various strain rates on the fracture toughness as well as the energy-release rate of gas shales were investigated. The three-point bending method was applied using notched semicircular bending shale specimens that were prepared as per the international standards. The fracture toughness and the energy-release rates were measured for three different modes, namely, mode I, mixed mode (I–II) and mode II. In addition, X-ray diffraction analysis was carried out to identify the composition of the selected shales. Finally, scanning electron microscope (SEM) analyses were performed in order to acquire an insight into the effects of strain rate on fractures at microstructural scales. The experimental results indicate that the fracture toughness and the energy-release rate for all the three modes are a function of strain rate. At lower strain rates, the fracture toughness and the strain-energy-release rates for all the modes are comparable but vary significantly at higher strain rates. At high strain rates, the strength and stiffness of the shale increases, which in turn increases the fracture toughness and, eventually, the energy-release rate of the shale. For all the strain rates, mode I requires the minimum application of energy, while mode II requires maximum energy for the onset of crack growth. The energy-release rate in mode I is maximum, in comparison with the two other modes. The findings of this investigation will be useful in achieving a better and comprehensive understanding of some aspects such as initiation, propagation and failure of shales during hydraulic fracturing for the extraction of hydrocarbons.
Pub.: 28 Dec '16, Pinned: 27 Jul '17
Abstract: Direct tensile strength and fracture toughness of rock and concrete, important properties for many applications, are cumbersome to measure directly. In this study, granite is chosen as an example to show how the tensile strength and fracture toughness can be measured from small three-point-bend samples of a single size but with different notches. An existing fracture mechanics model has been extended to include the stable fictitious crack growth before peak loads, which is then linked to the granite grain size. Both tensile strength and fracture toughness of granite can be estimated by the maximum load measurements from those notched three-point-bend samples. In total, 72 three-point-bend granite samples with different notches have been tested, and the estimated tensile strength and fracture toughness are compared with those available in the literature. The modified fracture mechanics model is then used to predict the fracture behaviour of smaller samples of the same granite. The theoretical prediction is confirmed by the experimental results of those smaller samples. Finally, the fracture model and its relation with the American Society for Testing and Materials (ASTM) standard on fracture toughness are discussed.
Pub.: 03 Oct '16, Pinned: 27 Jul '17
Abstract: The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to measure the mode I fracture toughness of rocks, and has been widely adopted in laboratory tests. Nevertheless, a certain discrepancy has been observed in results when compared with those derived from methods using straight through cracked specimens, which might be due to the fact that the fracture profiles of rock specimens cannot match the straight through crack front as assumed in the measuring principle. In this study, the progressive fracturing of the CCNBD specimen is numerically investigated using the discrete element method (DEM), aiming to evaluate the impact of the realistic cracking profiles on the mode I fracture toughness measurements. The obtained results validate the curved fracture fronts throughout the fracture process, as reported in the literature. The fracture toughness is subsequently determined via the proposed G-method originated from Griffith’s energy theory, in which the evolution of the realistic fracture profile as well as the accumulated fracture energy is quantified by DEM simulation. A comparison between the numerical tests and the experimental results derived from both the CCNBD and the semi-circular bend (SCB) specimens verifies that the G-method incorporating realistic fracture profiles can contribute to narrowing down the gap between the fracture toughness values measured via the CCNBD and the SCB method.
Pub.: 23 Apr '16, Pinned: 27 Jul '17
Abstract: Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens.
Pub.: 28 Apr '16, Pinned: 27 Jul '17