Suresh Kumar Govindarajan

Barite Sag is an adverse phenomenon observed in horizontal drilling or Extended Reach Drilling (ERD) operations. It has been reported to occur majorly due to settling of weighing material such as barite on the lower side of directional wells. The barite sagging has been reported to cause drilling operational problems such as induced well-bore instability, mud losses and pipe stuck up leading to well control problems. In this context, an attempt has been made to address the barite sagging problem through application of laboratory synthesized nano-particles of barite as they are known to posses' high area to volume ratio, hence required in low quantities in formulating optimum gravity drilling fluid. However, low quantities of barite nano-particles aids in lowering Plastic-Viscosity (PV) and increasing Rate Of Penetration (ROP) due to low solids concentration associated with it. Further, the larger surface area of barite nano-particles also aids in improved heat dissipation from bottom hole, thereby increasing the life of bits. The sagging test results have projected a reduction in sagging phenomenon with the application of nano-barite particles. However, the rheology analysis results have projected a decrease in PV with unchanging filtration-loss properties meeting the API standards. Further, the rheological behaviour at various temperatures better fits with Herschel-Bulkley model establishing its shear thinning characteristics. The present work aids in formulating efficient drilling fluids for economical directional drilling operations.

A variable aperture model, including the random variation of fracture aperture as against the conventional parallel plate model, has been developed to adequately examine the transport of colloids/suspended particles in a single coupled fracturematrix system. Rather than relying on a complex geostatistical method for an accurate representation of fracture aperture, which requires an enormous field data and resource for its validation, a simple statistical method (linear congruential generator) is implemented in the present article. The random variation of fracture aperture is an honest representation of the unpredictable geometry/ morphology of fracture aperture in comparison with widely applied the conventional parallel plate model or the simple mathematical functions based on fractal theory (self-affine structures). A considerable number of parameters are involved in investigating the extent of penetration of colloids into the rock matrix, which creates complexity and ambiguity in the analysis. To overcome this problem, a single parameter “Maximum Penetration Factor” has been introduced for simple and reliable assessment of diffusion of colloids within the rock matrix. Additionally, a non-dimensional parameter ‘Matrix Mitigation Factor’ has been introduced in the present study, which can provide a means of evaluating the diffusion of suspended particles within the rock matrix when it comes to real time applications like microbial enhanced recovery (MEOR) and chemical enhanced recovery (CEOR) in the petroleum industry (nanoparticles and nanofluids). A semi-implicit finite difference model has been adopted for solving the coupled partial differential equations in the present numerical study. Finally, Neumann and Robinson boundary conditions as a function of time have been applied at the fracture inlet to better represent the field scenario as against the conventional constant source condition (Dirichlet). The model results indicate that there is a difference in concentration between the parallel plate model and random fracture model when it comes to colloidal concentration in the fracture and rock matrix. The variance in concentration is due to the inclusion of variation of the aperture in the variable aperture model, which is absent in the parallel plate model. Additionally, the results suggest that the variable source boundary condition has a significant influence on the transport of colloids in fracture-matrix system. Finally, from the evaluation of the extent of diffusion of colloids into rock matrix, it was concluded that that variable aperture model is associated with more mitigation of colloids compared to the parallel plate model, especially in the case of random fracture.

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 of fracture aperture was simulated in this study. Comparison is made between increasing, decreasing and constant fracture profiles to understand their effect on fracture evolution. This study aims to present a methodology to model variable fractures and study its effect on deposition inside the fracture. The model shows that the simple parallel model over predicts the precipitation that occurs in a fracture.

In this work, a mathematical one-dimensional two-phase model (liquid + gas) has been developed to simulate the dynamic flow system in the event of a gas kick during vertical drilling. The flow system is a drift-flux model where the fluid properties are represented by averaged mixture properties rather than by two independent formulations. With this model, different flow scenarios and influx fluid propagation are investigated in vertical wells. The numerical solution is based on finite volume staggered discretization solved implicitly by a first-order upwind scheme. A sensitivity analysis of the influx model parameters, namely, the gas slip velocity, was performed and its impact on the bottom hole pressure and kick propagation is demonstrated. This model is further extended to predict the kick velocity and pressure in the annulus at the bit based on surface flow measurements in real-time drilling. This paper details on the model development of transient two-phase flow along with validation with experimental results. It is foUnd from the study that the developed light-weight simulation model could be employed in real-time drilling to model influx events, and the drift -flux simulation approach is comparable with the experimental and analytical results.

The adaptability of in situ remediation techniques for low soluble fractions in petroleum products such as polycyclic aromatic hydrocarbon (PAH) is generally constrained due to their limited bio-availability owing to hydrophobicity. In the present study, a numerical model is developed to evaluate the effect of unsaturated hydraulic properties, equilibrium chemical partitioning as well as coupled reactions on the fate and transport of a typical PAH (phenanthrene) originating from a surface spill. Simulation of surfactant enhanced remediation using a non-ionic surfactant (Triton N-101) resulted in significant modifications in unsaturated hydraulic properties. The presence of natural organic matter (adsorption partitioning coefficient of 8.97 × 10-4 L/mg) as well as viable bacterial consortium (specific growth rate > 3.06 × 10-7 /sec) in the soil is found to be favouring the biodegradation in order to limit the reach of phenanthrene up to a depth of 200 cm. The results suggest that selection of surfactant type and dosage affected the extent of solubility enhancement of phenanthrene (from 1.27 to 11.5 mg/L); however, ultimately the typical bio-geochemical features of the subsurface seemed to control the success of remediation.

The transport of nitrogen coming from wastewater applied agricultural field is a major problem in assessing the vulnerability of groundwater contamination. In this study, laboratory column experiments are conducted in order to simulate the paddy, groundnut and wheat irrigation with wastewater. The experiments are carried out with high clay content (≈35%) soil from Kancheepuram, Tamilnadu and low clay (≈9%) soil from Ludhiana, Punjab, India. Furthermore, a numerical model and HYDRUS-1D model are developed to simulate the experimental results. The experimental results show that there is no effluent collected at the bottom of the column during groundnut irrigation in Kancheepuram soil and effluent collected except during first irrigation in the case of wheat irrigation in Ludhiana soil. The experimental and numerical results illustrate that when 50 mg/l of ammonium and 20 mg/l of nitrate nitrogen applied during paddy irrigation, the peak nitrate nitrogen concentration of 50 mg/l is arrived after 10 days in Kancheepuram soil due to low permeability and relatively less background soil nitrogen. But in the case of Ludhiana soil with 94 mg/l of total nitrogen applied during paddy irrigation, the peak nitrate nitrogen concentration of 1,620 mg/l is observed at first day due to high permeability and high soil background nitrogen concentration. Additionally, the model results show that the application of high nitrogen content wastewater for irrigation in Ludhiana soil will affect the groundwater quality even when the groundwater table is deep as compared with Kancheepuram soil.

The pesticides applied on the soil surface can be transported vertically downwards through the unsaturated porous system and lead to groundwater contamination. Proper agricultural management practices such as selection of appropriate irrigation techniques, choosing proper irrigation rates, and application of optimum pesticide dosages are necessary to prevent leaching of pesticides to greater depths thus preventing groundwater contamination. A simulation study is conducted based on the one-dimensional numerical model considering Richard’s equation for unsaturated water flow and solute transport which takes into account the effect of both adsorption and biodegradation with inhibitory effect to understand pesticide transport in an unsaturated porous medium. The study addresses the influence of irrigation rate, type of irrigation, and pesticide dosage on soil moisture and pesticide concentration distribution. The numerical results suggest that higher water application rates can carry the pesticides to greater depths. Pulsed irrigation can slightly reduce water losses through the root zone when compared to continuous irrigation. The comparison of the wetting patterns and the pesticide distribution obtained in continuous and pulsed irrigation helps to decide the use of a particular irrigation strategy in order to achieve suitable goals. In addition, the results from this study bring out better understanding of the effect of pesticide concentration and dosage on the resultant pesticide distribution in the unsaturated zone and the pesticide potential to cause groundwater contamination. The better analysis of outputs from this study can help in improving and designing better agricultural management strategies, carrying out risk assessment and bioremediation studies.

A one-dimensional numerical model is developed to predict the benzene concentration in the unsaturated zone with special emphasis on the impact of biological clogging on benzene transport while the bacteria remain in mobile condition. The numerical results suggest that the water saturation during the biological clogging condition reaches its maximum value of approximately 51% after nearly 20 days, whereas the water saturation reaches the maximum value (approximately 54%) within 3 days in the absence of clogging condition while the bacteria remain mobile. A similar trend has been observed at greater depths also. Further, the results indicate that the hydraulic conductivity profile during the presence of biological clogging shows an increasing trend from its initial value, which is observed later in time when the depth increases, whereas the increase in hydraulic conductivity occurs very early for the entire depth during the absence of biological clogging. Similar observations are also experienced for the benzene concentration profile during the scenario of mobile bacteria. Conversely, immobile bacteria play a predominant role on accumulation of more bacteria in the shallow depths as compared with mobile bacteria and eventually increase the microbial saturation. Moreover, the peak value of hydraulic conductivity and benzene concentration are considerably reduced during the presence of biological clogging during the immobile bacterial condition. This situation yields the larger residence time in the shallow depths, which in turn enhances the biodegradation of benzene in the shallow depths; hence, the groundwater contamination by benzene in the deeper zone is prevented.

A one-dimensional numerical analysis on multispecies radionuclide transport in a single-horizontal coupled fracture-matrix system has been performed. Analysis considering linear and nonlinear adsorption cases with linear, Langmuir, and Freundlich adsorption models has been carried out. The results indicate that there is a significant change in the spatial distribution of radionuclides in a coupled fracture-matrix system when a nonlinear adsorption isotherm is considered as compared to the simplified linear sorption isotherm. Sensitivity analysis of Langmuir constant and Freundlich exponent has been performed, and the numerical results indicate that the behavior of radionuclide transport is strongly influenced by these nonlinear sorption isotherm parameters. In addition, an attempt has been made to consider the variation of fracture aperture thickness along its flow direction by introducing logarithmic distribution. A clear distinction in the spatial distribution of radionuclide concentration was observed when variable fracture aperture is considered as opposed to the conventional parallel plate model with a constant fracture aperture thickness. Further, the results suggest that there is an enhanced retardation of radionuclides within the high permeable fracture with varying fracture aperture.

In the present paper an attempt has been made to develop an improved numerical model in order to simulate the oil recovery by microbial flooding under non-isothermal conditions. The proposed model simulates the coupled heat and mass transfer resulting from the mobility of microbes along with its nutrients in a typical petroleum reservoir against the conventional isothermal conditions. The model results have been verified with the existing analytical and experimental results. The present model evaluates the optimum mean fluid velocity in order to achieve the maximum oil displacement under different reservoir temperature conditions. The model also investigates the change in relative permeability of oil and water during microbial flooding within the reservoir under non-isothermal conditions and subsequently estimates the residual oil recovery. The model results suggest that the microscopic oil displacement efficiency increases with the increase in mean fluid velocity and reaches a threshold maximum oil displacement just before the water breakthrough at a relatively lower temperature. In addition, it is observed that the maximum oil displacement efficiency of 82.75% is achieved at a reservoir temperature of 60. °C and at an optimum mean fluid velocity of 1.68. m/day in a sandstone reservoir of length 50. m.