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The objective of this study is to quantify, by means of numerical simulation, the response of the complex system of gas hydrate accumulations at Site NGHP-02-09, Krishna-Godavari Basin, Indian Ocean, to different production conditions, and to determine the technical feasibility of gas production through depressurization-induced dissociation. The study assesses the suitability of the site for a long-term production test involving a single vertical well, and the long-term potential of the deposit under full-field production using a system of multiple vertical wells. We simulate gas and water flow, estimate the production performance of the accumulation and separately investigate the corresponding geomechanical response of the system. Results indicate that production from Site NGHP-02-09 under the conditions of a long-term field test involving a single vertical well is technically feasible and can yield high gas production rates. However, an inability to fully isolate the water bearing zones results in production that is largely from dissolved gas rather than hydrate dissociation and is thus burdened by excessive water production. Given the estimated physical properties of the reservoir system, Site NGHP-02-09 does not appear to be a promising location for a single-well field test of gas production, but may be a promising production target for full-field production operations using a multiwell system in which exterior wells can mitigate water inflows to allow interior wells to more effectively depressurize the formation and capture methane from gas hydrate dissociation. Geomechanical issues need to be carefully considered as significant displacements are possible, which can be challenging to well construction and stability.

TOUGH+Millstone has been developed for the analysis of coupled flow, thermal and geomechanical processes associated with the formation and/or dissociation of CH4-hydrates in geological media. It is composed of two constituent codes: (a) a significantly enhanced version of the TOUGH+HYDRATE simulator, v2.0, that accounts for all known flow, physical, thermodynamic and chemical processes associated with the behavior of hydrate-bearing systems undergoing changes and includes the most recent advances in the description of the system properties, coupled seamlessly with (b) Millstone v1.0, a new code that addresses the conceptual, computational and mathematical shortcomings of earlier codes used to describe the geomechanical response of these systems. The capabilities of TOUGH+Millstone are demonstrated in the simulation and analysis of the system flow, thermal and geomechanical behavior during gas production from a realistic complex o↵shore hydrate deposit. In the first paper of this series, we discuss the physics underlying the T+H hydrate simulator, the constitutive relationships describing the physical, chemical (equilibrium and kinetic) and thermal processes, the states of the CH 4 +H 2 O system and the sources of critically important data, as well as the mathematical approaches used for the development of the of mass and energy balance equations and their solution. Additionally, we provide verification examples of the hydrate code against numerical results from the simulation of laboratory and field experiments. Keywords Methane hydrates • Reservoir Simulation • Geomechanics • Coupled processes 1 Introduction Gas hydrates are solid crystalline compounds of water and gaseous substances described by the formula G • N H H 2 O, in which the molecules of gas G (guests) occupy voids within the lattices of

Peridynamics is widely used as the theoretical basis for numerical studies of fracture evolution, propagation, and behavior. While the theory has been shown to converge to continuum mechanics in the theoretical limit, its behavior as a discrete numerical approximation with respect to classic problems has not been shown. In this study, we use standard analytical solutions to thoroughly test the numerical accuracy and rate of convergence of the spatial discretization obtained by peridynamics. We analyze the accuracy and rate of convergence of three different peridynamic constitutive responses: of these, two involve a state-based dilation, and the third is based on the estimation of the deformation gradient. Additionally, we study the choice of the peridynamic influence function in each of the constitutive responses. The peridynamic materials are solved in the linear elastic regime by solving a linear system of equations obtained by symbolically differentiating the force states. We test the methods against standard constant-strain solutions for uniaxial compression, isotropic compression, and simple shear. We also apply the methods to a finite material with a pressurized thin crack, using the Westergaard's solution method to obtain an analytical displacement field for comparison. The two dilation-based peridynamic constitutive responses are found to only converge to one of the constant strain solutions, while the deformation gradientbased law converges in all cases with an appropriate choice of the influence functions. We show that a cubic influence function is the best choice of those considered in all methods. Only the deformation gradient-based model converges for all three linear deformation problems, but is less accurate than the dilation-based models for the thin crack problem because of instabilities. We demonstrate an ad hoc smoothing technique based on the influence function that is able to alleviate these instabilities and improve the accuracy of the deformation gradient-based model.

The TOUGH+Millstone simulator has been developed for the analysis of coupled flow, thermal and geomechanical processes associated with the formation and/or dissociation of CH 4hydrates in geological media. It is composed of two constituent codes: (a) a significantly enhanced version of the TOUGH+Hydrate simulator, v2.0, that accounts for all known flow, physical, thermodynamic and chemical processes associated with the evolution of hydrate-bearing systems and includes the most recent physical properties relationships, coupled seamlessly with (b) Millstone v1.0, a new code that addresses the conceptual, computational and mathematical shortcomings of earlier codes used to describe the geomechanical response of these systems. The capabilities of the TOUGH+Millstone code are demonstrated in the simulation and analysis of the system flow, thermal, and geomechanical behavior during gas production from a realistic complex offshore hydrate deposit. In the third paper of this series, we apply the simulators described in Parts 1 and 2 to a problem of gas production from a complex, multilayered system of hydrate-bearing sediments in an oceanic environment. We perform flow simulations of constant-pressure production via a vertical well, and compare those results to a coupled flow-geomechanical simulation of the same process. The results demonstrate the importance of fully coupled geomechanics when modeling the evolution of reservoir properties during production. Keywords Methane hydrates • Reservoir Simulation • Geomechanics • Coupled processes 1 Introduction The TOUGH+HYDRATE (T+H) code (Moridis et al., 2008b) is a simulator developed at the Lawrence Berkeley National Laboratory (LBNL) to model non-isothermal CH 4 release, phase behavior and flow under conditions typical of CH 4-hydrate deposits. T+H is a fully compositional