Occurrence And Transport Of Gas And Water In Nanopores Of Tight Medium

Unconventional oil and gas resources which have attracted more and more attention are becoming important supplements of conventional oil and gas resources.Most of these unconventional resources are in tight media that have amounts of nanopores.Both during the process of reservoir formation and hydraulic fracturing,water always exist.In addition,water and gas existing in nanopores may show different characteristics comparing with the bulk phase.Even though many experimental breakthroughs have been achieved currently in the aspects of reservoir formation and fluid transport in tight media,detailed descriptions on micro scale cannot be provided because of the limitations of experiments.Nevertheless,with the rapid development of computer technology,all-atom molecular dynamics simulation method has become a powerful tool to study micro mechanisms on the molecular scale.In the present work,all-atom molecular dynamic method has been employed to study the occurrence and transport of gas and water in nanopores of tight media.The research work is carried out from three main aspects: the adsorption,desorption and occurrence of gas in nanopores in an aqueous environment;the phase change and transport of water in nanopores of tight media;the manipulation of water flow in nanopores.Firstly,the interactions of gas molecules and the pore walls in the process of hydraulic fracturing are studied.Through simulating the gas accumulation on the pore walls and the gas occurrence state in the nanopores,the underlying mechanisms are discussed.It is found that the gas molecules can break away from the water molecules spontaneously,accumulating on the interface of water and the wall and forming a gas layer finally.This accumulation process has nothing to do with the initial state.For smaller slits,the gas molecules can expel the water molecules from the slit and occupy all the slit space.What's more,single,double and triple gas layers are observed respectively with different slit width.Considering multi-field coupling in the practice of development,an electric field is implemented for the purpose of desorbing accumulated gas and enhancing gas recovery.Once the static electric field is implemented,desorption of accumulated gas on the interface of water and wall can be seen clearly.Interestingly,the complete desorption which has a close relation with the direction of the electric field can only occur when the vertical component is larger than the horizontal component of the electric field relative to the wall.Secondly,simplified nanopores and nanochannels with charges are constructed to mimic the compounds with different charge distribution in the real shale rocks.This section focuses on the influence of charge on the phase change and transport of water in nanopores without gas molecules.For the nano-slits which can only accommodate two layers of water molecules,the two dimensional ordered hexagonal ice(Type 1)in it can melt and form a new two dimensional ordered hexagonal ice(Type 2)with increasing charge.For larger nano-slits,the diffusivity of the first layer of water molecules near the wall decreases obviously with increasing charge.However,for the second layer and inner layers of water molecules,when the charge is less than 1.0e,the change of their diffusion coefficients are similar to that of the first layer with the increasing charge.Once the charge is larger than 1.0e,their diffusion coefficients will increase with the increasing charge.Finally,in order to speeding up the water flow in nanopores,a vibrational charge is put outside the nanopore and its influence on the flow is studied.When the vibrational frequency is low or high,the net flux of water in the nanopore is small.However,the water net flux reaches a maximum which is about nine times larger than the minimum net flux when the vibrational frequency matches the inherent frequency of hydrogen bond inside the nanochannel.This phenomenon is called electrical resonance.Besides,the effects of both vibrational electric field and static electric field on the water flow in nanochannels are studied and an opposite relation has been found.This work has further expanded the scope of manipulating water flow in nanopores.In summary,both the occurrence state and transport of gas and water in nanopores of tight media are complex dynamic processes.The present work,based on the simplified nanopores,has studied the micro mechanisms of the occurrence and transport of gas and water.Nevertheless,the physical property of the real tight media will never limit the molecular dynamics simulation study to the research work mentioned above.Instead,the applications of the computer simulation technology bring new opportunities,new researches and new developments to this field.