Dynamic Capillarity Characteristics And Application During The Waterflooding Process In Low Permeability Oil Reservoirs

Waterflooding is often adopted to supply the energy for low permeability reservoirs.Waterflooding in low permeability reservoirs faces many challenges,such as the difficulty to inject enough water,the requirement of high injection pressure,the low oil recovery and the much faster fluid flow in fractures than in matrix.Mechanism behind these challenges including dynamic capillarity,thick boundary layer,etc.,which cause the special fluid flow characteristics(e.g.non-darcy flow)in low permeability reservoirs.This paper aims at the characteristics,mechanism,model and application of dynamic capillarity during the waterflooding in low permeability sandstone reservoirs.A series of experiments were conducted to clarify special properties of the rock and fluid in a typical low permeability sandstone reservoir.Lithology,geochemistry,pore structures porosity-permeability and basic fluid flow properties of the sandstone samples were deterimined by X-Ray diffraction,casting thin sections,scanning electron microscope,pressuring mercury,nitrogen adsorption-desorption analysis,nuclear magnetic resonance,contact angle measurement,interfactial tension measurement and sensitivity evaluation of the reservoir to fluid.Results demonstrate that facters such as dynamic capillarity,boundary layer have strong effect on the waterflooding process in low permeability reservoirs.A set of dynamic capillarity measurement system and the dynamic capillarity evaluation method were determined,which evaluate the dynamic capillary pressure,dynamic relative permeability,steady capillary pressure,steady relative permeability and the dynamic coefficient More over,the static capillary measurement system and evaluation method were developed,which was calibrated by the comparision with pressuring mercury.Taking medium to ultra-low permeability sandstone rock for instance,we determined the dynamic capillarity of these rock.To reveal the dynamic capillarity characteristics in low permeability sandstone reservoirs,factors which influence the dynamic capillarity were examined.These facters are permeability,pore structure,fractures,interfacial tension,wettability,fluid viscosity and density,and pressure difference.In addition,the forces on flowing fluid and the flow characteristics in low permeability reservoirs were analyzed to explore the mechanism of the dynamic capillarity.Results indicate that the dynamic capillarity is more pronunced in a lower permeability porous media.The dynamic capillarity becomes stronger when the fluid viscosity is higher,or the pressure difference is higher.The dynamic capillarity can be weaker by fracures and surfactant.The dynamic capillrity model,including macro and micro parts,was derived based on the dynamic capillarity characterics and mechanism.The displacement pressure,action length,wettability,permeability,porosity,fluid viscosity and density etc.are included in the macro dynamic capillarity model,to reveal the reservoir physical property,homogeneity and fluid flow ability.The wettability hysteresis coefficient was defined to reflect contact angle hysteresis and impact of dynamic capillary pressure.The GML and GBL models were developed to calculate the relative permeability.The production model,taking dynamic capillarity,water boundary layer effect and boundary slip effect into consideration,was developed.Sensitivity analysis shows that the boundary layer effect and the boundary slip effect can be neglected,while the dynamic capillarity should be included when performing production prediction.Numerical simulation softwore was coupled with dynamic capillarity to predict single well production.This method was verified by comparing its results with the developed production model,so it can be applied for the production prediction of low permeability reservoirs.The achievements of this paper can attribute to the fluid flow theory in low permeability reservoirs.This paper helps to improve the waterflooding production prediction accuracy,and provides theory for the increase of injection and the reduction of the injection pressure during waterflooding in low permeability reservoiors.