Vol.7 No.4 2015
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Research paper : Applicability of the technologies to the assessment of methane hydrate sediments (N. TENMA)−221−Synthesiology - English edition Vol.7 No.4 (2015) development. It also engages in (2) the evaluation of well stability by looking at the stress distribution in the vicinity of wells during the application of the depressurization method.In the following section, details will be given on the outline and calculations of the geo-mechanical simulator that the Reservoir Simulator Team is currently developing, the research on the contact surface characteristic conducted as part of well integrity, and the evaluation of wide-area deformation conducted through collaboration with private companies and universities within the MH21 framework.3.1 Development of the geo-mechanical simulatorFor the numerical analysis of sediment deformation, development and use are pursued in the fields of civil engineering and architecture. Normally, laboratory experiments are conducted using on-site cores to collect parameters such as the elastic modulus that reveals how much the sediment deforms with pressure. The stress distribution and the amount of strain within the sediment layer are then calculated using the finite element method (FEM) using these parameters.[5] However, in gas production from the MH layer, the MH that is originally present in the layer in solid form dissociates into water and methane gas by the depressurization method, and the stress distribution in the layer changes as the MH that previously existed in solid form disappears. Moreover, the gas and liquid that are produced by dissociation can move through the sedimentary layers. Since MH dissociation is an endothermic reaction, heat exchange takes place within the layer. Therefore, unlike with general sediment deformation, it is not possible to analyze the sediment deformation behavior in MH development until the simulator can handle the mechanical parameters for the elastic modulus and strength of the MH bearing sand sediment, the flow of gas and liquid in the layer, and the changes in temperature due to the dissociation and formation of MH. Therefore, jointly with West Japan Engineering Consultants, Inc. (WJEC) which has experience in numerical simulation and analysis of sediment deformation, a geo-mechanical simulator for MH bearing sand sediment was developed by adding functions for multi-phase flow analysis, heat conduction analysis, and MH dissociation and formation, to the sediment deformation numerical simulator.[6][7] The handling of mechanical MH parameters, which will be explained below, and the basic design of the numerical simulator were conducted mainly by the Reservoir Simulator Team. Currently, it has become a FEM with combined functions for stress, multi-phase flow, heat conduction, MH dissociation/formation, among others. The simulator is called the “Coupled Thermo-Hydro-Mechanical Analysis with Dissociation and Formation of Methane Hydrate in Deformation of Multiphase Porous Media” (COTHMA). Recently, deformation simulators with functions similar to COTHMA have been proposed,[8][9] but as it will be explained later, COTHMA was developed based on the laboratory experiment results from core samples of MH bearing sand sediments. It is a simulator that can most accurately represent the mechanical behavior of MH bearing sand sediments. The following are the characteristic functions of COTHMA that is under development and improvement.1)Analysis of complex processes in multiple phases (vapor, liquid, and solid).Fig. 4 Development of evaluation tool for sedimentary characteristicsTools are developed while integrating the results of the evaluations of well integrity and wide-area deformation that are currently being done.DEM analysisVerification by offshore production test and laboratory experimentTransparent acrylic cell triaxial testing (TACTT) systemSensitivity analysis for the effect of discontinuous surfaceModeling of discontinuous surfaceAcquisition of long-term deformation characteristicEvaluation of stress distribution in the vicinity of the production wellAcquisition of parameters for contact surfaceDevelopment of evaluation toolAcquisition of mechanical parameters by laboratory experimentEvaluation of well integrityEvaluation of wide-area deformation(field-scale employment)Development of a geo-mechanical simulator Technologies are organized to ultimately provide the following methods to plan the project:①Selection of MH development area②Optimum well design method③Facility design on seabed surface④Setting of conditions for safe/secure operation …etc.Reservoir deformationFault characteristicSeabed surface subsidence

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