| Natural gas hydrate,also known as "combustible ice",is an ice-like crystalline compound with cage structure formed by methane gas molecules and water molecules under suitable conditions of low temperature and high pressure.It mainly occurs in the permafrost and in marine sediments.Due to the special nature of hydrate reservoir,its exploitation is a coupling problem involving many physical and chemical processes,such as phase change,multiphase flow,heat transfer,and rock-soil deformation.Hydrate production should not only seek for the optimal scheme to obtain the efficient gas production,but also pay attention to the possible mechanical problems,such as wall damage,sand production,submarine landslide,and seafloor subsidence.The practical operations of field production tests are very difficult with high investment and unpredictability,while numerical simulation is a very effective and economical method for the early evaluation before production.This paper focused on the multi-field coupling problems relate to methane gas and water production,temperature-pressure-stress changes,and formation stability during hydrate production.The research was carried out by using the combined methods of theoretical analysis,code development,numerical simulation,and field application.The behaviors of multiphase flow and formation deformation for hydrate production by depressurization were investigated in detail,which has important theoretical and practical significance for the safe and efficient exploiation of hydrate in China.Firstly,the thermal-hydrologic-mechanical(THM)coupling mathematical model and numerical model were established closely considering the special characteristics of hydrate production.Then,a THM coupling simulator,Hydrate Biot,was developed by incorporating geomechanics into TOUGH+Hydrate based on modular design.Through cross comparison with analytical and numerical solutions,the reliability of Hydrate Biot in mechanical coupling function was verified,which provides an improved tool and method for productivity evaluation and formation mechanical stability analysis of marine hydrate exploitation in China.Based on the first offshore production test site in the Nankai Trough of Japan,a history matching model was estabilished by incorporating the available geological data,which considers the heterogeneity of porosity,permeability,hydrate saturation,and lithology simultaneously.The effective multiphase flow and mechanical parameters of hydrate bearing sediments were obtained by fitting the field measured data,including gas and water flow in the production well and seafloor subsidence,which lays a data foundation for the subsequent long-term prediction of gas production potential and formation mechanical stability.The long-term production behaviors(multiphase flow and geomechanics)through vertical well by depressurization were predicted based on the verified heterogeneous reservoir model in the Nankai Trough.The range and trend of seafloor subsidence were predicted,and the effects of homogeneous and heterogeneous reservoir model on hydrate production performance were analyzed systematically.For the muddy silt hydrate reservoir of the South China Sea,the scheme of horizontal well through depressurization was proposed for hydrate production from the low permeability reservoir.The key parameters including horizontal well placement,well length,and depressurization scheme were optimized and evaluated systematically.On this basis,Hydrate Biot was used to predict the formation mechanical stability under the optimal scheme of horizontal well.The geomechanical responses were revealed,including the degree and range of formation deformation,the location of sand producing,and the trend of seafloor subsidence.In addition,the engineering measures for consolidating the formation skeleton and preventing sand production were put forward.According to the above research,the following conclusions can be drawn:(1)Numerical model with the actual formation conditions can accurately reproduce the field hydrate production processes,including the gas-water multiphase flow of production well and the temperature evolution data of monitoring well.Therefore,numerical model should be based on more perfect judgment of reservoir conditions,so as to provide reliable production scheme and productivity prediction.(2)Based on the field hydrate trial production simulation,it was found that the layered heterogeneous hydrate reservoir might be the better candidate for methane gas extraction than the massive hydrate reservoirs by depressurization.This provides a theoretical basis for the selection of marine hydrate trial production target and the design of perforation scheme.(3)Depressurization through vertical well results in funnel-shaped seafloor subsidence,and the maximum seafloor subsidence reaches 0.28 m after 1 year depressurization.The evolution of seafloor subsidence indicates that the initial stage of depressurization is the key period to prevent formation damage,thus the slower depressurization is recommended firstly.(4)The placement of horizontal well can significantly affect methane gas recovery volume and hydrate production efficiency.Consequently,the advanced technology,such as hydrate reservoir precise exploration and offshore directional drilling,should be developed in the future.The excessive length of horizontal well will result in serious wellbore water inrush problem.Considering the water inrush problem and the production efficiency,the length of horizontal well should not exceed 400 meters.(5)The subsidence mainly comes from the compression of pore volume in hydrate reservoir during depressurization,which leads to rigid body subsidence in the overburden and results in seafloor subsidence.Depressurization through horizontal well results in more concentrated region of formation compression,when compared with vertical well.The modeling results indicate that maximum seafloor subsidence reaches 0.5 m after 1 year depressurization through horizontal well.(6)Depressurization leads to stress concentration and shear stress in the formation around the production well,but not results in shear failure according to the Mole-Coulomb failure criterion under the specified model conditions both for the vertical well and the horizontal well. |
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