Numerical Simulation on Production Trials by Using Depressurization for Typical Marine Hydrate Reservoirs:Well Type and Formation Dip

Natural gas hydrate has huge reserves and is widely distributed in marine environment.Its commercial development is of great significance for alleviating the contradiction between energy supply and demand.As an efficient research method,numerical simulation can provide valuable insights for the design and optimization of hydrate development.However,most of the current production models simplify the reservoir as a two-dimensional(2D)horizontal layered model,often ignoring the impact of formation dip angle.To improve the accuracy of production prediction and provide theoretical support for the optimization of production well design,two three-dimensional(3D)geological models with different dip angles based on the geological data from two typical sites are constructed.The vertical well,horizontal well and multilateral wells are deployed in these reservoirs with different permeabilities to perform production trial,and the sensitivity analysis of dip angles is also carried out.The short-term production behaviors in high and low permeability reservoirs with different dip angles are exhibited.The simulation results show that 1)the gas and water production behaviors for different well types in the two typical reservoirs show obviously different variation laws when the short-term depressurization is conducted in the inclined formation;2)the inclined formation will reduce the gas production and increase the water extraction,and the phenomena becomes pronounced as the dip angle increases,particularly in the low-permeability reservoirs;3)and the impact of formation dip on hydrate recovery does not change significantly with the variation of well type.

Numerical Investigation on Dynamic Characteristics of Casing Deformation During Marine Hydrate Production

Formation subsidence is inevitable during marine hydrate decomposition,and the consequent casing deformation seriously threatens the security of sustainable hydrate production.Owing to insufficient observed data of formation subsidence in field,displacement boundary condition of casing is undetermined.Thus the conventional static methods are inapplicable for the calculation of casing deformation in hydrate production well.The present work aims at proposing an approach to investigate dynamic deformation of the casing during hydrate production.In the proposed methodology,based on the movement theory of hydrate decomposition front,hydrate decomposition process can be simulated,in which hydrate reservoir strength formation subsidence showed time-dependent characteristics.By considering the actual interactions among casing,cement and formation,three models of hydrate production well are developed to reveal the static and dynamic deformation mechanisms of the casing.The application of the proposed methodology is demonstrated through a case study.Results show that buckling deformation and bending deformation of casing reduce the passing ability of downhole tools in deformed casing by 4.2%and 7.5%,respectively.With the progress of hydrate production,buckling deformation will increase obviously,while a little increase of bending deformation will occur,as the formation slippage induced by formation inclination is much larger than that caused by hydrate decomposition.The proposed approach can provide theoretical reference for improving casing integrity of marine hydrate production.