| Enhancing oil recovery(EOR)is a constant theme in oil and gas field development,no matter at when or at any stage,EOR is the key and core content of oil and gas field development.With the continuous adjustment and deepening of waterflooding in onshore oilfield,most of the oil field has already been in the stage of high water cut and low recovery,invalid water cycle is getting more serious,the producing degree of thin and poor reservoir is low.At present,as a tertiary oil recovery method of enhancing oil recovery,gas injection has been more and more used in various oil fields.Because of the difference between gas characteristics and water characteristics,injected gas can enter the pores and channels which water can not.Gas also can reduce the viscosity of crude oil and reduce the interfacial tension,increase the displacement efficiency of each channel and expand the swept volume.However,in the oil,gas and water three-phase compatible environment,the characteristics of oil-gas contact and seepage is different from the characteristics of simple oil-gas two phases.In the presence of movable water,the mass transfer characteristics of oil and gas,the two-phase mixed characteristics,oil water gas three-phase flow law,as well as the mechanism of gas displacing oil,all need to be further studied.In this paper,on the basis of analyzing the results of previous studies,we use self-developed CO2 miscible visible experiment device to study the static contact and dynamic displacement characteristics of CO2 and crude oil.The change of oil-gas contact form,the formation process and the change stage of mixed phase have been observed,which confirmed the mixed phase is dynamic and gradual changing.The transition zone can be divided into 4 stages.After that,the stepwise sampling experiment in long sand filled tube is implemented under the miscible phase condition,the characteristics of multiphase and multicomponent mass transfer and seepage in porous media are studied.Then,by introducing the theory of relationship between resistivity and water saturation,we use steady-state experimental method to measure the oil,water and gas three-phase relative permeability in the non miscible conditions.The influence of wettability and saturation process on three-phase relative permeability has been studied.The results show that in the three phase flow system,the relative permeability of non-wetting phase is related to the rock wettability,the saturation history and the saturability distribution of the other two phases in the rock.The combination experimental method of core online CT scanning and core simulation micro-model was proposed to qualitatively and quantitatively analyze the the characteristics of water flooding,gas drive and remaining oil distribution from core scale and micro pore scale under different time.The start-up mode and migration process of the remaining oil after water,gas and WAG injection have been revealed.At the last,the suitability evaluation of Q28 reservoir implemented gas injection has been evaluated by the introduction of multi-level fuzzy comprehensive evaluation theory.Eleven evaluation indexes of gas injection have been selected through literature investigation,and membership degree and weight value of each evaluation index has been determined,so that a fuzzy evaluation model can be established to give a comprehensive evaluation on Q28 reservoir for gas injection.On the basis of establishment of geological model and dynamic production model,the reservoir numerical simulation,combined with the theory of orthogonal test analysis,has been carried out to further optimize the parameters of gas injection,and the optimum parameter combination of gas injection also can be obtained,which provide a theoretical basis for the reasonable development of the gas injection. |
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