Numerical Simulation Study Of Thermal-Hydrologic-Mechanical Coupling And Sand Production During Gas Hydrate Exploitation

The stabilization of the reservoir is the foundation for achieving long-term,safe,and efficient development of gas hydrate resources,which in China generally have characteristics such as shallow burial depth,poor compactness,and non-diagenesis.Fluid extraction and migration as well as hydrate phase transformation during natural gas hydrate exploitation will lead to changes in the mechanical properties of the reservoir,which may cause a series of geological disasters such as reservoir subsidence,wellbore damage,and sand production.Moreover,a serious formation sand production can cause the formation deficit,which can induce large-scale collapse around the well.Therefore,the behaviors of reservoir deformation and sand production during gas hydrate exploitation are key factors in evaluating reservoir stability.Based on the views above,this work focuses on establishing the thermal-hydrologic-mechanical coupling and sand production numerical simulation programs and conducting a series of numerical studies on the exploitation of gas hydrate reservoirs in the South China Sea by depressurization.The main research contents are as follows:(1)Based on the thermal-hydrologic coupling model in TOUGH+Hydrate and the geomechanics model in FLAC3 D,the two-way coupling simulator of T+H-FLAC3 D was developed by introducing a porosity correction model based on the variation of formation volumetric strain and writing an interface program between the above two software.Compared to the previous simulator of T+F,this simulator adds a correction term about the change of heat flow field caused by the mechanical deformations such as formation subsidence and expansion,which can more truly describe the dynamic seepage characteristics within the reservoir during gas hydrate exploitation.The reliability of the simulator was confirmed by comparing and analyzing the prediction results of numerical simulation with previous research results.(2)The exploitation of a heterogeneous hydrate reservoir with irregular layers by depressurization in two horizontal wells was studied,which revealed the impacts of physical parameters of each hydrate layer on the production behaviors of gas and water,spatial distribution characteristics of physical parameters,and geomechanical response characteristics of the reservoir.Compared to the low-permeability hydrate layer,the high-permeability hydrate layer has characteristics such as a larger propagation velocity of pressure drop,more serious stress concentration,and greater sensitivity of the production rates of gas and water as well as formation deformation to the depressurization amplitude of the production well;The hydrate with a high degree of saturation is easier to strengthen the formation skeleton and reduce formation deformation caused by the decrease of pore pressure.(3)Based on the mechanism of sand production by erosion,the numerical simulation module of solid particle shedding and migration was developed by introducing the corresponding mathematical model.And the sand production simulation program based on the two-way coupling between TOUGH+Hydrate and FLAC3 D was built by embedding the numerical simulation module of solid particle shedding and migration into the framework of the two-way coupling simulator of T+H-FLAC3 D.Compared to the two-way coupling simulator of T+H-FLAC3 D,the sand production simulation program adds a correction term about the porosity change caused by sand production to the porosity change model,which more comprehensively reflects the impact of geomechanical response such as formation subsidence,formation expansion,and sand production on the heat flow field.The reliability of the simulator was confirmed by comparing and analyzing the prediction results of numerical simulation with previous research results.(4)The sand production process during the exploitation of Class 2 hydrate reservoir by depressurization was investigated by using the self-developed sand production simulation program,and the predictive analysis of the gas-water-sand production behavior,temporal and spatial distribution characteristics of physical parameters,and geomechanical response characteristics of the reservoir under the thermal-hydrologic-mechanical coupling effect were also conducted.The simulation results showed that the decrease of pore pressure and the decomposition of hydrates around the production well led to the formation skeleton compression and sand production during depressurization production.Especially in the early stage of production,due to the relatively large pressure gradient,the problem of formation sand production was even more serious,and the concentration of solid particles in the produced fluid was higher,which could easily cause wear and damage to the production equipment.In addition,this part also revealed the impacts of depressurization amplitude,depressurization mode,reservoir permeability,and initial hydrate saturation on the sand production behavior.(5)The sand production process during the exploitation of Class 2 hydrate reservoir by depressurization combined with warm water injection was further investigated,and a comparative analysis was conducted with pure depressurization in terms of gas production,sand production,formation mechanical stability,and energy efficiency ratio.The simulation results showed that compared to pure depressurization,the combined method could promote the decomposition of gas hydrates and improve the gas-water ratio,but the overall energy efficiency ratio of which was reduced.In terms of the reservoir stability,the combined method could reduce formation deformation by regulating the pressure of the injection-production well.In addition,the combined method could significantly reduce the sand production rate of the reservoir in the medium term of production,and maintain the concentration of solid particles in the produced fluid within a moderate sand production range.