Govindarajan Suresh Kumar

Bio: Govindarajan Suresh Kumar is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Fracture (geology) & Mass transfer. The author has an hindex of 4, co-authored 10 publications receiving 34 citations.

Influence of transient porosity in a coupled fracture-skin-matrix system at the scale of a single fracture

Abstract: The subsurface leaching of soluble chemicals in a fractured porous medium poses long-term risk of groundwater contamination. Tracing the occurrence, movement and consequences of such hydro-geo-chemical interactions is the fundamental process for an effective remediation plan. However, the complexity of geomorphology and mass transfer mechanisms makes it challenging while addressing these issues in a real field scale. The present study focuses on simulating the concentration profile of nitrate elution in a pseudo two-dimensional coupled fracture-skin-matrix system under active biodegradation using an implicit finite difference numerical technique. The interface between the fracture and rock matrix is assumed to possess a skin with time-varying porosity imitating the effect of bio-clogging. The results indicate that denitrification is significant in reducing the dissolved nitrate concentration for initial skin porosity of 10% in the presence of an unlimited oxygen and primary substrate. When the rate of change of skin porosity remains lower with a minimal variation, the nitrate concentration provided a considerable reduction in the vicinity of the fracture inlet. A similar trend is observed for dissolved oxygen concentration as well. The concentration profile of nitrate showed a higher rate of reduction with an increase in initial skin porosity value from smaller to significantly larger values. The present study clearly indicates the role of skin interface in depicting the solute concentration profile in fracture, especially during the washout of bio-clogged membrane (biofilm) attached to the rock matrix.

Modelling of mineral precipitation in fractures with variable aperture

Abstract: Fractures act as a highly permeable conduit for flow in naturally fractured reservoir. Geo-chemical reactions inside fractures may lead to partial or complete filling of pore spaces inside fractures over time. This reduction in fracture aperture directly affects its capability to transport fluid. The current study presents a 1-D mathematical model to simulate the geochemical filling of natural fractures. The fracture walls have been represented by simple mathematical functions to reflect variable aperture of natural fractures. Mass transfer through convection/diffusion and mineral precipitation due to precipitation/dissolution reaction were solved as a simplified mathematical representation of the actual processes. Precipitation reaction is coupled with mass transport by the fluid to ensure mass conservation of reacting components. For simplification, calcite precipitation in fracture has been modelled. The effect of pressure drop, diffusion constant, type of fracture aperture profile on evolution...

Confronting heterogeneous sorption and hydrodynamic dispersion on solute transport in a fracture-skin-matrix system using spatial moment analysis

Abstract: Understanding the intricacies of inter-dependency of fluid flow and solute transfer at the scale of a single fracture is limited by various simplifying assumptions employed for computational purposes. In the present study, the fracture-rock matrix interface is assumed to be consisting of a skin layer with sufficient mass transfer properties where non-linear adsorption is considered to be the limiting reaction among the various interfaces. A numerical model has been developed using implicit finite difference method with varying grids at the fracture-skin interface to capture the mass transfer during solute transport in the presence of non-linear Sips adsorption. The model was used to identify the critically influencing parameters on the temporal profiles of fluid velocity, macro-dispersion coefficient and dispersivity using the method of spatial moments. The results indicate that the presence of the skin has enhanced the mixing phenomenon as well as the sorptive mass transfer rates. Summary of the sensitivity analysis provides the critical factors to be considered while employing such comprehensive models for elucidating details at a small scale. The presence of fracture-skin evades the sensitive role played by (a) the fracture adsorption coefficient (with a reduced rate constant for adsorption and an increased rate constant for desorption) at early times, while resulting in an enhanced mixing characteristics at later times, and (b) maximum sorption capacity of fracture as a function of solute velocity; and in turn, it provides an improved control over the transportation of solutes through the fracture.

Weighted Spacer Design for Elevated Temperature Conditions to Mitigate Barite Settling by Identifying Suitable Viscosifier

Abstract: Any successful primary cementing operation at elevated temperature condition requires an efficient displacement of fluid surrounding the casing by cement slurry. In such conditions the cement slurry should be designed in such a way that it should be compatible with both cement and drilling mud. To achieve these requirements we designed the cement slurry with weighted spacer. Spacer is a barrier between cement & mud so that they should not mix with each other, also all these fluids should be incompatible inorder to avoid cement aggregation. The displacement efficiency during cementation is directly dependent on discharge rate, but however due to formation fracture pressure constraints, the discharge rate is limited, hence designing spacer becomes very crucial. This phenomenon becomes more pronounced at higher temperature as turbulent flow efficiency reduces due to the presence of weighting agent. The drive of the present work is to identify a suitable viscosifier to avoid settling of weighing agents in spacer and to maintain the stability of rheology admixture at elevated temperature condition. Laboratory tests were performed for compatible deformation and flow of matter with cement slurry-spacer-mud at temperature range (80-140°C) on a rotational viscometer as per the procedure of API RP 10B-2. The volumetric proportions of the cement slurry/spacer and spacer/mud admixtures were prepared with various ratios: 95/5, 75/25, 50/50, 25/75, and 5/95. Rheological compatibility of fluids (cement & spacer and mud & spacer) is evaluated by computing the R-Index Value (R) which is calculated by subtracting highest 100 rpm reading of admixture from highest rpm reading for an individual fluid for the given range of elevated temperature condition. The calculated R-Index Value can then be utilized to comment on fluid compatibility. After finalization of chemical compatibility, rheological hierarchy was achieved by incorporating the friction pressure loss with respect to discharge rate of an individual fluid for the given range of elevated temperature condition. The spacer system used achieved stable compatibility and efficient rheological hierarchy at elevated temperature cementing conditions. In addition, by comparing the results between the two different spacer systems, the role of hydration in attaining rheological compatibility is computed. This study will in turn prove helpful in figuring out the better spacer system which will play a vital role for better displacement and cementation quality.

Experimental and Numerical Investigations on In Situ Chemical Oxidation Model for Groundwater Contaminated with Petroleum Hydrocarbons

Abstract: The subsurface contamination by petroleum hydrocarbons (PHC) from leaking underground storage tanks, pipelines and refilling stations is one of the serious issues directly affecting the quality of groundwater. Application of advanced oxidation process (AOP) has been favoured for the remediation of petroleum contaminated sites due to the spontaneous redox reactions mediated by a strong activating agent. In this study, we propose a methodology for efficient injection of reagents by using two concentric PTFE tubes in a sand box model for simulating the groundwater flow, contaminant transport and in situ chemical oxidation (ISCO) using Fenton’s reagents (hydrogen peroxide and zero-valent iron particles). This injection method has proved to maximize the interaction of chemicals resulting in complete oxidation of petroleum compounds. An attempt has also been made to numerically simulate the mass transfer and transport of petroleum hydrocarbons incorporating the impact of spontaneous mass transfer by means of numerical methods. It is expected to have significant difference in interface mass transfer between free phase (oil) and water leading to increased exposure of residual oil phase, thereby enhancing the complete mass removal. The presence of soil organic matter (SOM) is found to be enhancing the activity of Fenton’s reagents as well as increasing the adsorption of hydrophobic organic compounds.

VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

Abstract: The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

Study on three-dimensional fracture network connectivity path of rock mass and seepage characteristics based on equivalent pipe network

Abstract: Nuclear waste repositories have extremely stringent requirements for geological environment. However, natural fractures in rock mass can be potential channels for nuclide migration, therefore, the influence of fractures on the permeability of rock mass must be assessed. In this paper, a well research was conducted on well-exposed granite outcrops in the Xinchang site (the Chinese high-level radioactive waste repository). The high-precision three-dimensional model of a typical outcrop is built to obtain fracture information combined with field measurement, and then the three-dimensional fracture network model is generated using the relevant parameters by Monte Carlo method. To obtain more comprehensive fracture connectivity while avoiding the traditional method of searching the connectivity path in the complicated 3D fracture model taking up a lot of storage space and costing a lot of time, this paper presents an approach using MATLAB cell array instead of traditional adjacency matrix to search and store fracture network connectivity paths. In DFN model, the fracture disc with certain thickness is equivalent to three-dimensional pipe network model (EPNM) with variable diameter, and the equivalent path permeability coefficient (EPC) is proposed to objectively study the permeability characteristics of the seepage path in fractured rock mass based on that. Especially noteworthy is that some fractures in a certain strike range belong to open type, while those in another range belong to cemented closed fractures, when fresh fractures were exposed by cutting off the surface rock to a certain depth. The calculation of EPC under different conditions shows that the order of magnitude of EPC mean value is 1e−7m/s and 1e−3m/s, respectively, when fractures are cemented and not partly. On this basis, the size of the representative elementary volume (REV) of the fractured rock mass in the study area is determined to be about 25 m. By rotating the matrix in model, the spatial permeability tensor of the region (including permeability principal value and main direction) is obtained, which is within the range of borehole data. The predicted results may provide some reference for the related projects in the future.

Chlorpyrifos in environment and food: a critical review of detection methods and degradation pathways

Abstract: Chlorpyrifos (CP) is a class of organophosphorus (OP) pesticides, which find extensive applications as acaricide, insecticide and termiticide. The use of CP has been indicated in environmental contamination and disturbance in the biogeochemical cycles. CP has been reported to be neurotoxic and has a detrimental effect on immunological and psychological health. Therefore, it is necessary to design and develop effective degradation methods for the removal of CP from the environment. In the past few years, physicochemical (advanced oxidation process) and biological treatment approaches have been widely employed for the pesticide removal. However, the byproducts of this process are more toxic than the parent compound and along with an incomplete degradation of CP. This review focuses on the toxicity of CP, the sources of contamination, degradation pathways, physicochemical, biological, and nano-technology based methods employed for the degradation of CP. In addition, consolidated information on various detection methods and materials used for the detection have been provided in this review.

Effect of Salinity on Silica Nanoparticle Adsorption Kinetics and Mechanisms for Fluid/Rock Interaction with Calcite.

Abstract: This study addresses the kinetics of silica nanoparticle adsorption on calcite from a solution at three salinities: deionized water (DIW), synthetic seawater (SSW), and low salinity water (LSW). The nanoparticle adsorption mechanisms and the effects on calcite dissolution are addressed. It was shown that nanoparticle adsorption was best described with the second-order-kinetic model and that silica nanoparticle adsorption reduced calcite dissolution. This was confirmed by measuring the Ca2+ ion concentration, the pH, and by estimating the amount of calcite dissolved. This is an important conclusion of this work, especially as LSW as an enhanced oil recovery technique is a candidate for use in chalk fields. Less formation damage/dissolution of chalk when silica nanoparticles are combined with LSW can lower the risk of reservoir subsidence. Intraparticle diffusion and the pseudo-second-order models, indicated a reduction in the adsorption rate with increasing nanoparticle concentration in LSW. This is explained by possible repulsive forces among the nanoparticles as they diffuse from the bulk fluid onto the calcite surface. Ion charges reduce the repulsion among the nanoparticles through shielding. However, an increasing nanoparticle concentration reduces the shielding efficiency by the ions. Estimates of the surface forces confirmed that nanoparticle–mineral interaction is less attractive in LSW as compared to SSW and DIW.