Characteristics of Non-linear Flow in Porous Media of Ultra-low Permeability Reservoirs Considering Boundary Layer

Abstract

AbstractUltra-low permeability reservoirs are the commercial replaceable resource. The rocks are tight and micro-nano pores and throats are widely distributed in ultra-low permeability reservoirs. The environment and mechanism of flow in porous media are more complex than conventional reservoirs. As a result, existing theories of flow dynamics are not applicable. In micro-nano pores and throats of ultra-low permeability reservoirs, molecule motion greatly affects boundary resulted from the reaction of fluid and rock. Conventional continual mechanics cannot explain this phenomenon exactly. Traditional permeability parameter and Darcy Equation cannot describe flow behavior of ultra-low permeability reservoirs. In this paper, flow in micro-nano pores and throats is studied using dissipative particle dynamics (DPD). DPD can reflect the mechanism of reaction of different fluid and rock. The characteristics of boundary layer in micro-nano pores and throats are quantitative characterization. A new key parameter to characterize the flow behavior in ultra-low permeability reservoirs is came up with. The non-linear flow characteristics in microscale and mesoscale are exactly described. Non-linear motion equation of ultra-low permeability reservoirs and tight oil reservoirs is built and the factors of non-linear flow characteristics are studied. It is found that molecule motion affects velocity distributed greatly in molecule scale and pressure gradient is dominant in macroscopic scale. Non-linear flow characteristics are obvious in nano throat and porous media of ultra-low permeability reservoirs. Pressure gradient, throat radius and fluid viscosity affect non-linear characteristics. When pressure gradient decreases, throat radius decreases and fluid viscosity increases, non-linear flow characteristics in ultra-low permeability reservoirs are more obvious. Effective permeability is the immediate cause of the non-linear flow characteristics in ultra-low permeability reservoirs. The reduction of effective flow space resulting from boundary layer is the essential cause of the non-linear flow characteristics.KeywordsNon-linearBoundary layerEffective throat radiusEffective permeabilityDPD