Study On Storage And Microscale Seepage Mechanism Of Shale Gas

The resources of unconventional natural gas in China are quite abundant.The technical recoverable reserve of shale gas is 21.8×10¹² cubic meters,which is close to the reserve of United States,and is a type of promising energy.Different from the conventional reservoirs,shale gas is particular in its tight pore structure and complexity in relationship between gas and formation water.During the development period of some shale gas reservoirs,the productivities of different wells show evident differences,and the production potential is not fully realized.As a result,focusing on the typical pore structure characteristics of shale gas in China,a study on the storage capacity and microscale seepage mechanism is urgent.And it is hope the study can provide theoretical guidance for the high efficient development of shale gas reservoir in China.A combination of experimental characterization techniques and theoretical analysis was applied for the study in the dissertation.To begin with,the organic evolution and hydrocarbon generation processes were clarified.Afterwards,the experimental techniques including X-ray diffraction analysis,scanning electron microscopy,low temperature nitrogen adsorption and mercury injection capillary pressure were adopted to characterize the gas storage affected by formation water and the microscale pore structure.Based on adsorption experiment,molecular simulation and lattice Boltzmann methods,the storage and microscale seepage mechanism with formation water were studied,and suggestions for the development of shale gas were proposed according to the knowledge of the dissertation.The main conclusions of the dissertation are given as follows.The different storage mechanism of gas and water in shale is unveiled,and the microscale pore structure of shale is revealed.The organic pores are mainly occupied by natural gas,while the fluid in inorganic pores includes both water and natural gas.Minerals in shale include quartz+feldspar?averages 58.0%?,carbonate?averages21.75%?and clays?averages 20.28%?.Four types of pores are identified in shale,which are organic pores,inorganic inter-particle pores,inorganic intra-particle pores,and micro fractures.The pore size measured by low temperature nitrogen adsorption is1.79⁶3.61 nm.Meso and macro pores make most contribution to the total pore volume,while micro and meso pores contribute most to the specific surface area.With the increasing residual water saturation,the effective radius of inorganic pores decreases significantly.Based on adsorption experiment,the effects of water cut,maturity and clay content on the adsorption capacity of shale is characterized,and a capacity prediction model with varying depth considering the effect of formation water is proposed.The water absorbing capacity of shale shows positive relationship with both clay content and BET surface area.The methane adsorption capacity of dry shale is controlled by total organic content?TOC?,maturity and specific surface area,while that of the wet shale is related only to the TOC.As a result,the methane storage capacity predicted based on dry samples would exaggerate the actual storage capacity.Based on molecular dynamics,numerical kerogen molecular models including carbonate,hydrogen,oxygen,nitrogen and sulfate elements was built.With the simulation results on the models,the effects of temperature,pressure,maturity,competition adsorption of gas and water,and water cut on the adsorption capacity were analyzed.And the adsorption capacity of organic matter with varying depth was evaluated.The simulation results indicate that the specific surface area and degree of aromatization increase with increasing maturity,resulting in increasing methane adsorption capacity.The water adsorption capacity of organic matter show negative relation with the maturity.In the recoverable depth of shale gas,ranging from 2000 to3000 meter,the methane adsorption capacity is evidently affected by water cut.When the depth is larger than 4000 m,the effect of water cut on the methane adsorption capacity is not clear.A microscale lattice Boltzmann model considering the regularization procedure was proposed.The newly proposed lattice Boltzmann successfully expanded the simulation flow regime from continuous regime to transition regime.The flow of methane in porous media was simulated,and the effects of slippage,adsorption layer,residual water saturation and micro pore structure on the apparent permeability of the porous media were unveiled.The apparent permeability decreases with increasing formation pressure and surface area,while increases with the increasing temperature.Both adsorption layer and residual water decrease the effective pore radius,thus increases the apparent permeability.The dissertation builds a solid foundation for the evaluation of adsorption capacity evaluation and seepage mechanism,which can provide guidance for the successful development of shale gas reservoirs in China.The dissertation has important academic and engineering contribution.