Multi-scale Research On Microscale Mechanism And Macroscale Performance Of Water Flooding In Carbonate Reservoirs

Carbonate reservoirs are rich in oil reserves and process high economic value.However,with the quickening development,an increasing number of problems have emerged,such as the unclear distribution of the remained oil and the low oil recovery during water flooding.The development problems limit the production of carbonate reservoirs.It is necessary to study the performance and mechanisms of water flooding in carbonate reservoirs.In this work,the microscale mechanism and macroscale performance of water flooding carbonate reservoirs were investigated by physical experiments,molecular dynamic simulation and numerical simulation.Detachment mechanism of oil molecules from a calcite surface and the effect of salinity on the detachment process were studied by molecular dynamic simulation.Variation in remained oil with water saturation during water flooding was studied by water flooding experiment.The current numerical simulation method was advanced to consider the remained oil variation during water flooding.A scaled 3D water flooding experiment with an interlayer heterogeneous and anisotropic physical model was performed to achieve quantitative characterization of water flooding performance and pattern and the distribution of remaining oil after displacement in a porous carbonate reservoir with bottom water.Finally,the microscale water flooding performance was studied by numerical simulation.On the molecular scale,the detachment mechanism of oil molecules from calcite surface was studied by molecular dynamic simulation.The results show that the weak polar oil molecules can detach from the calcite surface by water molecules while the strong polar oil molecules can’t detach.This phenomenon indicates that the remained oil composition and viscosity may vary with water saturation during the water flooding process in carbonate reservoirs.Salinity and key ions play an important role on the detachment process of oil molecules from the calcite surface.Ca2+ influence the oil detachment level.The detachment level of the oil molecules from calcite surface decreases as the Ca2+ concentration increases.Mg2+ influence the oil detachment time.The detachment time of the oil molecules from calcite surface increases as the Mg2+ and Na+ concentration increases.On the micro-scale,the water flooding experiments results show that the remained oil varies with the water saturation during water flooding.The oil viscosity experiments show that the oil viscosity increases as the water saturation increases.The oil fraction experiment results show that as the water saturation increase,the content of polar oil fraction in remained oil increases while the content of nonpolar oil fraction in remained oil decreases.The gas chromatography results indicate that as the water saturation increase,the content of heavy alkanes in remained oil increases while the content of light alkanes in remained oil decreases.The numerical simulation considering remained oil variation results indicate that remained oil viscosity increasing can weaken the mobility of the remained oil and reduce oil recovery.On the macro-scale,the results of water flooding performance in carbonate reservoirs consist of 2 aspects: 1.A design and manufacture method for heterogeneous and anisotropic physical reservoir model was proposed by combining similarity criterion with space transform method.2.The water flooding performance,named “Smooth spread in high permeability layers and coning type breakthrough in low-permeability layers”,was proposed by physical experiment.The seepage ability and gravity are one of the main controlling factors for water seepage mechanism in multilayer reservoirs.When the bottom water flooded into the high seepage ability area,the driven pressure decreases and the water seepage horizontally.While in the low seepage ability layers,the driven pressure increases and the water floods vertically.The results of study on carbonate reservoir development performances show that permeability range,K_v/K_h and injected water salinity play an important role on the water flooding effect in carbonate reservoirs.When high permeability range of the numerical model is high,the oil recovery should be low and water cut should be high.When the K_v/K_h is high,the water can flow vertically fast resulting fast water breakthrough.Thus,the oil recovery and water flooding level decrease as the K_v/K_h viscosity increases.The effect of the injection water salinity is that the low salinity water should increase the oil recovery,decrease the water cut and decrease the remained oil saturation.The results in this work should provide some reference for the water flooding recovery in carbonate reservoirs.