Study On Seepage Parameters And Dynamic Variation Law Of Formation Damage In Water Flooding Oilfields

Formation damage is a common phenomenon involving the whole process of oil & gas field development,which always lead to water injection shortage and decline rate increasing.Therefore,it is of great significance to profoundly explore physiochemical mechanism,characterization models,seepage theories and relevant dynamic variation laws of formation damage for reducing formation damage degree and improving development effects.Formation damage is deemed to the degradation of macro produciton profile incurred by multiple physiochemical effects.At a micro-scale,chemical compositions,clay mineral compositions and distribution,microscopic physicochemical reaction as well as precipitation processes,etc.should be analyzed in detail.Without a doubt,it is difficult to elaborate all mechanisms on formation damage at the microscopic level.In this paper,not only is mechanism analysis conducted from a macroscopic phenomenological perspective based on continuum mechanics and fractal theory,but corresponding characterization models and seepage law are clarified.To be specific,a pore throat distribution fractal dimension and a tortuosity fractal dimension were both introduced on the basis of the fractal theory.In this way,a theoretical model of relative permeability curves was theoretically derived,establishing a bridge between the micro-pore structure and the macroscopic seepage laws.In addition to summarizing physiochemical mechanisms of formation damage,scaling experiments were performed to define physical property parameters,micro-pore structure related parameters,wettability and relative permeability curve variation rules during formation damage.Based on the capillary model,formation damage model of contaminant settlement and acquisition was respectively derived in theory as well as formation damage model of solid particles.Moreover,three seepage parameters with explicit physical meanings associated with the formation damage were introduced,including a permeability retention rate after bridge blinding,an initial capture rate of contaminant precipitations and the variation index in contaminant precipitation capture rate.Furthermore,the proposed model was proved to be reliable through laboratory tests and specific variation laws of three seepage parameters along with the permeability and the diameter and concentration of solid particles were summarized.On this basis,a dynamically evolving macroscopic constitutive relation was constructed for the formation damage from a comparatively microscopic perspective.Moreover,the characterization model of formation damage was combined with experimental results to investigate how micro-pore structure affects characteristic parameters of relative permeability curves and further establish the characterization model for characteristic parameter variations of relative permeability curves due to formation damage.On this basis,the influencing rules of different formation damage degree on macroscopic water cut variations laws and corresponding decline rate under a static condition were discussed.By introducing key mechanical parameters(depending on the oil/water viscosity and relative permeability curves),it is clarified that injectivity and productivity index before water breakthrough may rise,existing a turning point or decline,even if the formation damage didn’t occure.In addition,mathematical models of the formation damage were further solved for the purpose of exploring dynamic variation laws on one hand,and on the other hand,comparatively analyzing the changing rules with taking either permeability damage or relative permeability damage or both into consideration.By analyzing variation rules of permeability and relative permeability fields,it is found that formation damage characterization merely based on permeability is one-sided.Under the circumstance that permeability declines dramatically,relative permeability curves may change significantly due to variations in the microscopic pore throat structure and the wettability.The formation damage degree could be rather greater in the near wellbore area,because of more highly concentrated contaminants and more serious bridge blinding caused by solid particles.As the cumulative injection increases,a cumulative effect of formation damage is gradually presented.Both influence degree and breadth of formation damage are extended,leading to gradual decline of injectivity and productivity,which exerts a severe impact on relevant development effect.To some extent,the fractal theory is extended in this paper.Based on continuum mechanics,three seepage parameters are introduced to describe formation damage mechanism and mechanical processes.Based on the proposed formation damage characterization models,a formation damage seepage theory is also established,which could provide scientific guidance for prediction,evaluation and prevention of formation damage.