Ramesh Kannan K

A novel apparatus is used to quantify the filtration characteristics of water-based drilling fluids and rupture behaviour of external filter cake. Under static filtration, the filtration pressure (400 to 800 kPa), time (15 to 60 min), the particle size distribution of the weighing agent (D50: 11 to 136 µm) relative to the pore size distribution of the substrate (S50: 14 and 65 µm) and the type of weighing agent (CaCO3 or BaSO4) play an important role in governing the porosity, thickness, and the rupture resistance of filter cake. With an increase in D50 of mud, the normalised rupture resistance reduces by about 20% irrespective of the increase in the external filter cake thickness. Further, the viscosity of the filtrate increases with D50 indicating the retention of less xanthan gum in the filter cake. The denser barite particles form a thinner and tighter cake, which exhibits higher resistance to rupture.

Cohesion between grains in a geological system is perhaps the simplest and ideal representation of a range of material systems including soft rocks, structured soils, mudstones, cemented sands, powder compacts, and carbonate sands. This presence of inter granular cohesion is known to alter the ensemble mechanical response when subjected to varied boundary conditions. In this study, a hollow cylinder apparatus is used to investigate the mechanical behavior of weakly cemented sand ensembles by mapping the state boundary surfaces including the failure surface (locus of peak stress state) and the state of plastic flow (locus of final stress state). When these materials are sheared, the plastic deformation accumulates due to breakdown of cohesion between the grains, which introduces a lag in occurrence of peak stress ratio and maximum dilatancy, unlike a typical frictional granular material. This breakdown of cementation is affected by changes in the initial mean effective stress, initial reconstitution density, and intermediate principal stress ratio (stress path on the octahedral plane). The final state locus, emergent at large strains, was found to depend on the initial reconstitution density. Further, the parameters are extracted for calibration and prediction exercise using an elastic plastic constitutive model. In this and several other models, the effect of cementation is considered as an additional confinement to the ensemble. Such an approach predicts the stress state precisely but does not predict the volumetric response accurately, especially at large strains.

Laboratory studies of cavity initiation and propagation in weak or cohesionless materials rely on post-test observations to assess fracture geometry. The experimental setup in this work is a Hele-Shaw cell, which allows for visualization of cavity initiation and propagation within the sand pack, modified to apply differential confinement to a fully 3D specimen. Injection experiments with glycerine at different concentrations and at varying injection rates were conducted with anisotropic boundary conditions on loose sand. The fracture initiation and evolution were observed under various flow rates and viscosities. Infiltration and dislocation of particles were the main mechanisms observed in the tests. Fracture-like channels initially developed in a circular shape due to cavity expansion but then formed rod shapes (shear bands, material fluidization) where an anisotropic stress was applied. This transition from circular to elongated cavity appeared earlier in the tests with higher viscosity fluids, while higher injection rates produced wider openings as the larger volume of fluid was able to displace more particles. Although the cavity showed directionality in all cases, it became less confined to the propagating plane as the flow rate increased. For higher viscosities, the cavities tended to be more circular, whilst for lower viscosities, the cavities showed more directionality. Article Highlights.The experimental setup allows for visualization of cavity propagation in a 3D sandpackspecimen.Infiltration and dislocation of particles were the main mechanisms observed in theexperiments.Channels initially developed in

This paper aims to assess the various deterministic methods of liquefaction evaluation and compare them with probabilistic approaches in saturated silty sands. Deterministic liquefaction evaluating approaches as per Indian and European standard guidelines, and Boulanger and Idriss method are considered for the present study. Probabilistic liquefaction evaluation is carried out using Monte Carlo simulation (MCS) technique and preliminary estimation of the probability of liquefaction as per literature. Among the three deterministic approaches, Boulanger and Idriss approach gives more appropriate results as it is based on updated case histories and knowledge dataset. Probability approach by MCS technique gives more information about the liquefaction susceptibility as it considers uncertainty in the calculation of both cyclic resistance ratio and cyclic stress ratio. It can be concluded that even though, the deterministic approaches yield similar factors of safety for different boreholes at different depths, probabilities of failure values may be different.

Natural gas hydrate sediments are considered as an unconventional source of energy found in deep sea reservoirs and permafrost regions under specific equilibrium conditions. To efficiently extract the gas from these sediments, well-bores need to be installed in these reservoirs followed by adopting an appropriate hydrate dissociation technique. This study presents a custom-developed coupled THMC solver to realistically quantify the gas production and associated sediment deformation. The numerical scheme solves coupled equations describing hydrate phase change kinetics and non-isothermal multiphase flow in porous media. The proposed THMC solver incorporates the HISS-MH constitutive model, enabling it to capture the non-linear geomechanical behavior of gas hydrate sediments, including strain-softening and dilation characteristics. The solver is validated against existing experimental results and a benchmark problem. Finally, the effect of different horizontal well-bore positions on vertical settlement and cumulative gas production is studied. The results indicate that the well-bore in the middle of the reservoir yields the highest cumulative gas production while differential settlement is minimum, thus making it a preferable location for placing the horizontal well-bores. This study contributes towards the development of a custom THMC solver with advanced models in addition to identifying the ideal location of the horizontal well-bore for efficient gas extraction.

The poromechanical behaviour of granular materials are influenced by the rheological properties and stress state of the injected fluid in addition to the state of the porous media. Fluid injection through a granular continuum generally results in the elastic or plastic deformation of the material which reflects as the change in porosity due to particle rearrangement. This phenomenon results in fluid induced instabilities in the porous media which is commonly observed during CO2 sequestration, oil and hydrocarbon recovery, high pressure grouting, hydraulic fracturing. Solid-fluid interaction in porous media is a fundamental multi-physics problem encountered in civil engineering, petroleum, and mining industries. Injecting fluid into the porous materials results in the deformation of the existing solid skeleton, especially when the flow rate is greater than the ability of the media to permeate the fluid. Deformations can be considered as poroelastic when the storage of reversible elastic energy controls the process. Among the existing poroelastic models which predict the fluid induced deformation in porous media, a coupled non-linear continuum-based model proposed by MacMinn is used in this study. This model takes into account various fluid properties and predicts the material response under different boundary conditions. The efficiency of this model to capture the deformation characteristics of porous media under different flow rate, fluid viscosity, porosity of the media and other geometric parameters will be carried out in this study.

A comprehensive study on the stress-dilatancy behavior of cemented sand and its modeling is presented. The effect of confining pressure, relative density, and cement content on stress-dilatancy behavior are studied from the published experimental results and an additional series of experiments performed in this study. To facilitate a contrast and comparison of stress-dilatancy behavior between these datasets, a normalized stress ratio is proposed which removes the effect of mineralogy and morphology of parent sand. A set of key insights were obtained from this comparative study which aided in improving the stress-dilatancy relation; for example, the effect of initial conditions on stress-dilatancy behavior was found to be captured by the ratio of cohesion intercept (or tensile strength) and mean effective stress before shearing. The limitations of stress transformation, often used in modelling of cemented sand, were also systematically studied by a set of carefully designed experiments; it was found to be only applicable before gross yielding of cementation. After gross yielding, it is necessary to take in account of the breakage of bonds/cementation. The gross yield locus was identified from 70 experimental datasets and a cohesion/bond degradation model was formulated to model the stress-dilatancy behavior of cemented sand. The efficacy of stress-dilatancy relations (after including the gross yield locus and bond degradation behavior) is evaluated from the experimental results; the Rowe's stress-dilatancy relation was found to be most suitable with the proposed bond/cohesion degradation model.

The hydraulic fracturing technology is widely applied in the fields of groundwater hydraulics, hydrogeology, geo-environmental engineering and the oil and gas industry. Bio-cemented granular media were used as a proxy for weakly-cemented and poorly-consolidated sands in fluid injection experiments. The experiments were conducted in a testing apparatus which is capable of applying true triaxial stresses and allowed for visualisation of fracture propagation. Tests were conducted with bio-treated sands across various cementation levels at three stress states. In all tests, an opening was observed whose major principal direction was parallel to the maximum horizontal stress. A cavity was also developed with associated plastic deformation which in lower cementation levels reached even the boundaries of the specimen causing extensive ‘damage’ — disaggregation at the grain scale. At higher cementation levels, the cavity was developed near the injection point and then fractures made their presence in the expected direction. The peak pressures were approximately 10–15 times higher compared to the mean stress, while for lower cementations the pressure profiles were very noisy due to continuous breakdown of cementation. The fracture response is clearly dependant on the material properties (strength, hydraulic conductivity) and on the applied stresses. The combined effects of the applied stresses of the system, the strength and stiffness of the material were seen through the calculation of brittleness index derived from the Mohr–Coulomb and the Lade–Duncan failure criteria. The peak pressures agree better with the limit pressure calculated from the cavity expansion theory rather than the hydraulic fracturing theory.

Shear resistance in granular ensembles is a result of interparticle interaction and friction. However, even the presence of small amounts of cohesion between the particles changes the landscape of the mechanical response considerably. Very often such cohesive frictional (c-φ) granular ensembles are encountered in nature as well as while handling and storage of granular materials in the pharmaceutical, construction and mining industries. Modeling of these c-φ materials, especially in engineering applications have relied on the oft-made assumption of a "continua" and have utilized the popular tenets of continuum plasticity theory. We present an experimental investigation on the fundamental mechanics of c-φ materials specifically; we investigate if there exists a system size effect and any additional length scales beyond the continuum length scale on their mechanical response. For this purpose, we conduct a series of 1-D compression (UC) tests on cylindrical specimens reconstituted in the laboratory with a range of model particle-binder combinations such as sandcement, sand-epoxy, and glass ballotini-epoxy mixtures. Specimens are reconstituted to various diameters ranging from 10 mm to 150 mm (with an aspect ratio of 2) to a predefined packing fraction. In addition to the effect of the type of binder (cement, epoxy) and system size, the mean particle size is also varied from 0.5 to 2.5 mm. The peak strength of these materials is significant as it signals the initiation of the cohesive-bond breaking and onset of mobilization of the inter particle frictional resistance. For these model systems, the peak strength is a strong function of the system size of the ensemble as well as the mean particle size. This intriguing observation is counter to the traditional notion of a continuum plastic typical granular ensemble. Microstructure studies in a computed-tomograph have revealed the existence of a web patterned 'entangled-chain' like structure, we argue that this ushers an additional length scale as well as presents a system size effect.

Compaction plays a pivotal role in the formation of the aggregate skeleton of concrete mixtures, especially for stiff mixes such as Roller Compacted Concrete (RCC). RCC is compacted in the field with combinations of different energies (kneading/shear, impact, static & vibratory pressure), whereas various compactors are used, viz. modified Proctor (MP), vibratory hammer (VH), vibratory table (VT), and gyratory compactor (GY) in the laboratory to mimic the field compaction. Since the compaction mechanism is different in these compactors compared to field rollers, the behaviour of RCC is also distinct and warrants a fundamental study. In this study, the dominating parameters affecting the mesostructural arrangement of aggregates within the concrete skeleton when compacted with different mechanisms are comprehensively studied and compared with the field compacted specimens using several image processing techniques. The mesostructural parameters considered are interparticle spacing & distribution, aggregates segregation, orientation, morphology & breakage post-compaction, and crack length. The results indicate a higher possibility of getting better aggregate distribution and strength while a lower segregation potential when compacted with VH, followed by MP. However, the use of MP could alter the aggregates’ morphological characteristics due to the breakage of particles during the compaction process. On the other hand, VT and GY could exhibit similar interparticle distances to the field specimens but could not demonstrate similar performance in terms of strength and durability. The dominating parameters affecting aggregates’ spatial distribution are found to be aggregate-to-mortar ratio, interparticle distance, and the mechanism associated with each compaction technique.