Multi-scale Study For Seepage Property Of Porous Medium During Hydrate Phase Change Process

Natural gas hydrate is an important new source of energy,having promising application prospect and great exploitation potential with merits of huge reserve,low pollution and high energy density.Hydrate reserve in South China Sea could reach about half of the total amount of onshore petroleum and natural gas in the country.It is of great strategic significance to exploit the natural gas hydrate in China.Practical pilot productions all over the world suggest that stable and efficient exploitation of natural gas hydrate remains a global problem,calling for fundamental researches to understand its productive mechanism and influential factors.The essence of hydrate dissociation and production is to break the pressure-temperature phase equilibrium.The seepage property of hydrate reservoir is a key factor determining the evolution of thermodynamic conditions during the exploitation process.Magnitude and distribution of permeability are important for reservoir evaluating and exploitation strategy making.What is different from the gas and petroleum reservoirs is that the permeability of hydrate reservoir changes with the phase change of hydrate.Essentially,the dynamic change of permeability depends on the evolution of hydrate pore habit.However,quantitative correlating between the hydrate pore habit evolution and the dynamic change of permeability remains a research gap,leading to difficulties in constructing robust permeability predictive model and influencing reliable evaluation for productivity of hydrate reservoirs.This thesis investigates the permeability of hydrate-bearing porous medium during the phase change process in multi-scales.It starts with the numerical model construction for hydrate pore habit evolution at pore scale,proposing a permeability predictive model for hydrate-bearing porous medium at core scale,which is further applied into the evaluation of productivity of hydrate reservoirs in South China Sea at basin scale.The major research contents and results includes:(1)Based on the equilibrium of interphase forces and hydrate phase change kinetics,a numerical model for hydrate pore habit evolution is constructed.Combined with in-situ experimental observations of hydrate pore morphology,the hydro-and thermal-dynamically controlled mechanism is proposed,disclosing the evolution of hydrate growth from grain walls to pore centers.Accordingly,the modifications of hydrate phase change on some critical pore structure properties are quantified,enabling the quantitative correlation between hydrate pore habit evolution and permeability dynamic change.Results indicate that when hydrate saturation exceeds 0.3,permeability would decrease to 10%of the initial value.(2)Permeability of hydrate-bearing core is not only influenced by the porous structure of skeleton but also changes with hydrate phase change.Based on the Magnetic Resonance Imaging for porous structure of various glass bead packs,several versions of Kozeny-Carman model are built to describe their porosity-permeability relationships,which are also used as reference frames to analyze the permeability prediction for various natural hydrate-bearing core skeleton.Based on the variation of the hydrate-bearing core permeability with the hydrate pore habit evolution,a threshold value for hydrate saturation is introduced to quantify the transition point of the influence of hydrate pore habit evolution on permeability and accordingly a permeability predictive model for hydrate-bearing porous medium is constructed.The model weakly depends on porous medium type and hydrate phase change condition and could predict various measurements from experiments and field studies with accuracy as high as 90%,showing a wide applicability.(3)Permeability has the character of anisotropy,which,however,is often neglected in productive potential evaluation for reservoirs.Setting the hydrate reservoir in Shenhu area,South China Sea as an example,influence of different degrees of permeability anisotropy on productivities of reservoirs with various lithologies during the hydrate depressurization productive process.Results show that the permeability anisotropy lowers and delays the peak of gas production rate or even leads to an early termination of production,drawing an adverse impact on effective gas production.When considering its effect,gas productivity of sand reservoir remarkably drops and is no longer superior to that of the siltstone reservoir.(4)Hydrate-filled fractures universally exist in oceanic and permafrost hydrate reservoirs and have high hydrate saturation,porosity and permeability,not only contributing to a higher hydrate reserve but also enhancing the heterogeneity of permeability distribution within reservoir.Based on the numerical simulations for gas production process in fractured hydrate reservoirs located in Shenhu,Ulleung and Mallik areas,it is found that the hydrate-filled fractures can change paths of fluid flow and pressure diffusion during gas production,but degree and scope of their effects strongly depend on infiltration capacity of outer fluids and transport intensity of inner fluids of hydrate reservoirs.In Shenhu area,sufficient outer fluid supplement makes the hydrate within fractures dissociate totally,enhancing the gas productivity at the middle-late period of production.Besides,the difference in efficiency and path of thermal and pressure diffusion in the whole formation caused by diverse fracture distributions is quite remarkable,exerting an unnegligible effect on hydrate dissociation and even making fractures become a potentially "unbeneficial" factor for durable gas production.Much earlier terminations of gas production could possibly occur in the reservoirs located in Mallik and Shenhu areas with permeable overburdens.