Numerical Simulation On Depressurization Production Of Natural Gas Hydrate

As strategic materials,oil and natural gas play a significant role in the economic development and social progress for all countries.Nowadays,with the rapid economic development,the shortage of oil and gas resources has become an important factor restricting economic development,and it is urgent for all countries to explore new types of energy.Natural gas hydrates are widely distributed and stored,and are considered to be one of the most promising energy sources in the 21 st century.At present,with the efforts of scholars all over the world,the research on hydrate has made rapid progress both in experimental study and numerical simulation.In the numerical simulation,the main effort focuses on the establishment of numerical model and analytical model.The simulated results for the hydrate decomposition process mainly depend on the coordinate system,equations and other conditions used in the model.In the actual depressurization production process,the hydrate reservoir covers a large district of area.Driving by the pressure difference,the hydrate decomposition area spreads along the radial direction.In order to make the numerical simulation results closer to the actual depressurization production process,this research establishes the physical model and mathematical model of hydrate depressurization production in one-dimension cylindrical coordinate system.The finite difference method is used to solve the mass conservation equation,hydrate decomposition kinetics and auxiliary equations in the mathematical model.This modelling is first verified by the test production data of Shenhu area.The variations of pressure,hydrate saturation and permeability in the process of depressurization production are then studied.The movement of decomposition transition zone is analyzed under various boundary conditions.The key parameters controlling the depressurization decomposition process of hydrates are also studied.A new method of hydrate production is proposed by coupling the pressurization and the depressurization in the fracturing process.Modeling results revealed the following facts.(1)After the pressure reduction starts,a pressure drop funnel is rapidly formed around the wellbore,as time goes,the pressure drop funnel spreads farther into the reservoir,the hydrate near the wellbore decomposes first,the hydrate saturation decreases,while the permeability increases.(2)The decomposition transition zone moves outwards along the radial direction,with the outer and inner edge of the transition zone moving at different speed and slowing down in the later stage of the production.The width of the decomposition transition zone increases gradually with the production time and tends to be stable after a certain number of days.(3)The main controlling parameters of the hydrate decompression process include well pressure,initial hydrate saturation,absolute permeability,hydrate decomposition kinetic constants.(4)The selection of the boundary conditions,including boundary type and radius of the model,plays an important role in the simulation of the hydrate decomposition.When the outer boundary type is closed and the radius is relatively small,the modelling result may contradict to the real production process.(5)Fracturing operation can improve reservoir permeability,accelerate the propagation speed of pressure drop,promote the decomposition of hydrate,and increase gas production.