Multi Field Coupling Analysis Of Geo-Thermo-Mechanics In Hydrate Bearing Sediments For Drilling Engineering

The subsurface natural gas hydrates across the world are considered as potential source of energy.However,at the same time natural gas hydrates lying under sea are also considered as challenge for drilling a well through hydrate bearing sediments because of hydrate dissociation.Therefore this thesis presents an approach of learning from the past practice by revisiting the geographical and historical activities that were related with development of natural gas hydrates.Likewise,unconventional gas production from marine or permafrost hydrates can change the future of global energy interest.The amount of energy trapped in natural gas hydrates across the world is in fair abundance.Further the main objective of this study is to couple the thermomechanics of hydrate bearing sediments for which experimental assembly is developed to achieve a study of hydrate dissociation behavior for natural gas hydrates and control the consequences of drilling through hydrate bearing sediments.When drilling is being performed specially at offshore sites the presence of natural gas hydrates expand the level of challenge by escalating the risk of wellbore instability of well failure.However,if thermo-mechanics of hydrate bearing sediment can be controlled by changing well temperature or mud weight the level of challenge can be reduced.Therefore,this study provides an integrated methodology to simulate and develop an investigational model for thermo-mechanical behavior analysis of natural gas hydrates accumulated in hydrate bearing sediments during drilling process.Additionally,this study also provides a unique methodology to investigate the percolation mechanics of hydrate bearing sediments.Density,hydrate saturation and thermal regime are the base parameters used to evaluate the effect of percolation in hydrates.The methodology adopted in this research includes development of unique experimental assembly to simulate the hydrate dissociation process and percolation mechanics.Visual details were obtained by using digital microscope and infrared thermal camera to observe the physical and thermal effects.A pressure gauge was fixed in assembly to observe pressure changes and geo-mechanical stresses were evaluated empirically.Further,numerical simulation is adopted to simulate the near wellbore parameters that affect the stability of wellbore during drilling process.The condition for simulation include pressure-displacement module under axisymmetric model with quad-structured mesh.Furthermore,for the percolation mechanics the density is measured by stereotypical method,saturation by Ghiassian method and thermal regime by infrared thermal camera.Indeed,the use of infrared thermal camera is novel approach itself to research the thermal behavior of hydrate bearing sediments.Moreover,certain mathematical models are developed to estimate the subsurface hydrate behaviors based on investigations of this study.The mathematical replication of original sample is said to be virtual sample which is developed on matrix approximation technique.The findings of this study suggested that,hydrate dissociation is a gradual process that does not produce a rapid kick and the minimum time taken by natural gas hydrates accumulated around the wellbore walls takes around 8 minutes to dissociate absolutely.Moreover,the thermal,pressure and geo-mechanical profiling during hydrate dissociation has similar pattern of reduction under geo-dynamic and geo-static condition just time interval at dynamic condition is lower.Consequently,the geo-mechanical analysis based on evaluation of thermal induced stress revealed that when temperature is being changed the independent hydrate sample is getting weaker at all the test conditions.Thus,Percolation of particles at geo-static condition is very low however at geo-dynamic condition the percolation mechanics change the entire structure of hydrates accumulated in subsurface.Further the results of near wellbore analysis proved that hydrate saturation has direct effects on wellbore plasticity thus the chances of well failure can be predicted from hydrate saturation around the wellbore.The axisymmetric modeling approach provided more precise results for thermo-mechanics of hydrate bearing sediments.Moreover,the thermo-mechanical analysis is based on thermal induced stress which was found to be 6 MPa thus the well can sustain stability during drilling process.The model is designed to avoid borehole instability by maintaining the pressure and temperature condition,which can be done by injecting the drilling mud while penetration through hydrate bearing sediment with pressure above 6 MPa and temperature below 5 °C.Additionally the results of percolation analysis provided that the rate of error between the outcomes of simulated and physical investigations is very low which can be considered as negligible.By comparing physical and simulated investigation visuals for percolation mechanics the rock model and computer model both showed similar physical shape.Finally,the existence of percolation mechanics was indicated in hydrate bearing sediments.