Multi-physics Coupled Numerical Simulation Of Natural Gas Hydrate Exploitation

Natural gas hydrate,which is mainly distributed in deep oceanic sediments and permafrost regions,is regarded as an alternative to the traditional fossil fuels in the future due to its huge reserves and cleanliness.During hydrate exploitation,hydrate dissociation and gas production are controlled by the coupled hydro-thermo-mechanical-chemical processes.Establishing the reliable multi-field coupled model to numerically simulate gas production performance and sediment deformation is the important basis for assessing efficiency and security of different hydrate exploitation methods.Numerical simulation of hydrate dissociation in hydrate-bearing porous media based on laboratory scale are widely used to analyze the hydrate dissociation behavior and explore the factors that affect hydrate dissociation characteristics.Intrinsic permeability and effective permeability,governing multiphase flow,and thus influencing hydrate dynamics and gas production performance,are the key parameters of core-scale hydrate dissociation models.Currently,there are several models proposed to describe the effect of hydrate formed in pore space on the seepage ability of hydrate-bearing porous media.Selection of these models when establishing the hydrate dissociation model lacks of physics-related criteria,which is one of the main sources of uncertainties of the numerical model.Based on the numerical sensitivity analysis of model outputs(i.e.,gas production rate,cumulative gas production volume,hydrate dissociation front and evolutions of temperature and pressure)to the difference in effective permeability models,the equivalency and replaceability between different effective permeability models are demonstrated.Accordingly,these models are correlated to the most widely used Masada model,and the parameter assignment method of the Masada model under different intrinsic permeability values is illustrated.This largely simplifies the effective permeability model selection and parameter assignment when establishing the hydrate dissociation model.At field scale,this study builds a hydro-thermo-mechanical-chemical coupled model to simulate the first hydrate exploitation test in the Shenhu area,South China Sea.The simulated gas production rate and cumulative gas production volume match well with the measured data,which discloses and demonstrates the coupled control mechanism of multiple physical fields.On this basis,this study further analyzes the effect of various physical fields on the gas production performance.The results illustrate that evolution of stress in the sediment is highly related to the change of pore pressure.The sediment deformation mainly occurs near the well at the initial stage of hydrate exploitation.The quick drop of pressure at the initial stage of hydrate exploitation leads to a fast climb of the effective stress,whereas both the pressure and effective stress remain stable at the later exploitation stage.The gas production rate when ignoring the mechanical behavior is almost twice that when considering the mechanical deformation.The different permeability magnitudes can also lead to notable predictive deviation in gas production performance.The uncertainties in both the mechanical behavior and permeability magnitude can largely increase the possibility of multiplicity of solutions for the numerical analysis.Therefore,a short-term detection of the mechanical behavior near the well is suggested for the future exploitation tests in order to provide a strong constraint and effective verification for the mechanical module and the numerical simulation.