Pore Network Modeling Of Oil/Water Two-Phase Flow In Shale

With the great advancement of exploitation technology,unconventional oil and gas are playing an increasingly crucial role in the global energy mix.Shale oil and gas are the main components of unconventional resources,which are stored in shale reservoirs.Because of poor permeability in shale,large-scale hydraulic fracturing technology is generally used to enhance the oil production during the process of developing shale oil.Many field tests show that the majority of fracturing fluid(mainly water)doesn't return to the ground duiring hydraulic fracturing,but flows into shale matrix.As a result,oil/water two-phase flow happens.However,the fluid transportation in nano-pores is different from that of the conventional sandstone reservoirs.Therefore,it's particularly important to understand oil/water two-phase flow in shale.First of all,based on the pore-size bimodal distribution,this paper established a Gaussian mixture model for organic and inorganic pore distribution functions,which could be solved by expectation-maximization algorithm.A multiscale pore network model for shale is presented through the modification of traditional random pore-scale modeling technology.On account of the nano-effect in shale reservoirs,there is obvious underestimation when the traditional Hagen-Poiseuille equation is applied.Therefore,for precisely characterizing the fluid slip flow in nano pores,new hydraulic conductivity models for single phase and two phases are then built through the computational fluid dynamics method.Particularly,in order to further quantify hydraulic conductivity accurately,a hybrid technique of support vector machine and genetic algorithm is proposed for predictions of the corner conductivity coefficient.Statistical analysis results show that this intelligient model has excellent prediction accuracy and robustness.Finally,via the quasi-static simulation,oil and water two-phase flow can be simulated in the multiscale pore network model of shale for the acquisition of capillary pressure curves and relative permeability curves.Results show that slip effect could not affect the capillary pressure,but would exert a significant influence on the relative permeability curve.When oil and water coexist in a pore or a throat,slip effect could enhance the more transportation capability of wetting phase,i.e.,water.Besides,development of organic pores is the primary factor of pore structure affecting two-phase flow properties,rather than pore-throat ratio and coordination number.