Study On Multiphase Flow In Marine Natural Gas Hydrate Reservoir During Trial Production By Depressurization

Natural gas hydrate(NGH)is a new type of clean energy attracting much attention in the future,whose resources are abundant in the China sea,but require solving many technical difficulties.Different from conventional oil and gas reservoirs,hydrate-bearing reservoirs in the South China Sea are usually buried in the loose,low permeability and weakly cemented sedimentary beds which are composed of fine silt and even muddy,and reservoir itself has poor stability.Meanwhile,NGH is sensitive to environmental low temperature and high pressure.Its complicated phase transformation between gas,liquid and solid will occur after external disturbance,which may lead to the accidents such as submarine landslide,collapse,sand production and well blocking,affecting production efficiency and even causing huge economic losses.Therefore,it is of great significance to understand the heat and mass transfer mechanism and multiphase seepage law in the hydrate phase transformation process in order to achieve safe and efficient trial production of marine NGH-bearing reservoir.Considering the phase and enthalpy change effects during transformation,and taking NGH as a component of the rock skeleton or multiphase seepage with its own physical and chemical properties,thermal parameters synthesis and phase composition of the sedimentary layer are discussed respectively on the basis of the classical theory of heat and mass transfer of oil and gas reservoirs.Then the roles of NGH and its phase transformation to heat and mass transfer in the sediments are analyzed from the perspective of energy and mass conservation.Theoretical analysis and experimental results show that gas hydrate is sensitive to the disturbance of ambient temperature and pressure field,and its presence and phase transition have an important effect on the temperature and pressure transfer in the sedimentary layer.On the one hand,the change of hydrate content can alter the comprehensive physical property parameters of the sedimentary layer,thus affecting the heat and mass transfer.On the other hand,the phase transition caused by the perturbation of the temperature and pressure field will generate additional energy and mass sources,which will react on the sediment layer.Therefore,the phase change field of hydrate can form a strong coupling relationship with the ambient temperature and pressure field.Taking the hydrate dissociation front and temperature/pressure transfer leading edge as the boundaries,the sediments can be divided into undisturbed areas of the original strata,disturbed areas without phase transformation,disturbed areas with phase transformation and disturbed areas containing no hydrate,so as to clarify the difference and relationship between each area.The results show that gas hydrate can significantly reduce the effective permeability of the sedimentary layer,and its effect is related to its own saturation.When the hydrate saturation is low,its increase can significantly reduce the permeability of the sedimentary layer.However,when the hydrate saturation is high enough,its change has relatively limited effect on the permeability of the sedimentary layer.At the same time,the permeability of NGH-bearing deposits is also influenced by the synergistic effects of confining pressure,initial pore pressure of sediments,particle size and type of substrates.Based on this,an empirical formula of permeability of NGH-bearing sedimentary layers is fitted by experimental results,which improves the accuracy of the prediction results.Stress analysis on the matrix particle with local stress variation during dissociation in pore filling type NGH-bearing sediments is carried out to put forward discriminant method for predicting the critical shedding requirements.It indicates that the original mechanical balance may be broken due to expansion force caused by dissociated compressible products,leading two falling modes of matrix particles in disturbed areas with phase transformation,which are easier to migrate than those in other areas.Under the same conditions,a more intense hydrate dissociation and a smaller size of matrix particles makes it easier to fall off.Once the critical detachment condition is satisfied,the matrix particles can be used as a component of the solid phase to participate in the common migration of multiphase seepage.Applying microfluidic experimental apparatus,the flow law of hydrate phase in multiphase during trial production is studied.The original methane hydrate bulk will not only gradually melt at the edge,but also be broken into smaller pieces,which will migrate together with other components under dragging force,indicating that hydrate can be used as one of the solid components in the multiphase seepage in the sedimentary layer.However,the flow rate of smaller pieces of gas hydrate is 3-6 orders of magnitude lower than that of other phases.What’s more,they continue decompose as they migrate,dissolving completely into gas and water not far from where they break down.Based on the above research results,the interaction between the hydrate phase transition field and the temperature field,pressure field and seepage field of the sedimentary layer is analyzed.Then,a closed multi-physical field coupling system with the hydrate phase equilibrium and the heat and mass transfer as the medias is constructed and the corresponding multiphase flow prediction model for the marine natural gas hydrate trial production is established.Combined with the basic geological characteristics of marine NGH-bearing reservoir and the engineering parameters,the multiphase seepage law is predicted in the process of vertical well depressurization of marine NGH-bearing reservoir.The results show that a lower initial hydrate saturation,a higher pore temperature,a lower outlet pressure leads to a faster dissociation front advance,a shorter overall phase transition period,and a higher gas and liquid flow rates.The flow rates of gas and liquid both increase first and then decrease with the trial production.The closer to the outlet,the faster the flow rate is.When optimizing and designing the construction parameters for depressurization trial production in marine NGHbearing reservoir,the phase equilibrium conditions should be taken as the basis,and the effects of the above sensitivity factors should be comprehensively considered.