Microscale Flow Mechanisms Of Oil In Shale

Shale oil has been a new highlight in the development of worldwide unconventional resources.However,owing to its widespread nanopores,conventional approaches and technologies cannot be used to characterize the physics of oil flow through tight formations,which has impeded the process of industrial exploitation in China.Studying the microscale flow behaviors of oil in shale,exploring the nature of liquid slip,understanding the transport mechanisms of shale oil,not only shed lights on the rapid breakthrough of exploration and development technologies,but also be beneficial to addressing the fundamental issues in microfluidic from the perspective of applications.In the present work,the flow behaviors of oil in shale are studied on the molecular scale and pore scale.First,molecular dynamics(MD)was employed to prode the occurrence state of liquid alkanes upon shale organic surface.Results show that the density profiles of confined alkanes shows intense fluctuations acrosss the slit.“Solid-like” adsobed layer is present in the near-wall region,and its density approximates to 1.5~3 times that of the bulk phase.There are always multiple adsorbed layers of liquid hydrocarbons in the organic slits,and its total number is largely dependent on slit aperture and oil composition.The thickness of each layer is close to the width of linear alkanes and the adsorption propensity of heavier hydrocarbons is more propounced.Based on the thickness and average density of the adsorbed layers obtaind from MD,a new mathematical model is proposed to estimate the recoverable oil-in-place by dividing the adsorbed phase and bulk fluid.Nonequilibirum molecular dynamics(NEMD)model was built to study liquid hydrocarbons flow through shale nanopores fabricated by diffferent minerals,e.g.,quartz,graphene,and calcite,and the microscale flow mechanisms of shale oil were probed at the molecular scale.Their behaviors were observed to be completely different: under the same conditions,the flow velocities decrease in the order graphene,quartz,and calcite.The reason for this phenomenon should be attributed to the distinct fluid-solid interaction potentials and friction coefficients.Wettability also affects the nanoscale flow behaviors,but it is not the only factor.By introducing slip length and apparent viscosity,two mathematical models were developed to describe oil flow in different mineral nanopores,which lay a solid foundation for the pore scale flow modeling of shale oil.Then the correlation of contact angle with the pore size in shale nanopores were analyzed.As the pore becomes smaller,the surface becomes more hydrophilic but the contact angle of mercury tends to be greater.This correlation was coupled with a model describing the variation of surface tension agianst the droplet size to correct the common interprtation method of mercury intrudion capillary pressure(MICP).By this way,the variations of contant angle and surface tension with pore size were taken into account.The comparison with low-temperature gas adsorption validates the effectiveness of the improved method.The realtive error of the esimated pore radius produced in ignoring these microscale effects could be as high as 44%--samples that contain smaller pores deviate more.Moreover,the prediction models for the capillary pressure of oil,water,and gas were proposed,which may provide a important guidance for the accurate characterization of the pore structure and multiphase flow modeling in tight formations.Finally,effective slip length was defined to describe the flow behaviors of oil in rough nanopores.If the roughness degree increases to some extent,“negative slip” occurs in different minerals pores.Considering that the size of pores within organic matter is always one order of magnitude smaller than that of the inorganic matrix and the flow behaviors of oil in different minerals are completely different,a pore network model and the corresponding flow simulation method were developed for shale.The results reveal that the relationships of flow rate and pressure gradient for oil flow through shale is a nonlinear curve passing thourgh the origin;the complex pore structure,surface roughness,and fluid property are the key reasons for the nonlinear flow.