Study On The Variation Of Pore Scale And Injectability Of CO₂ Saltwater Sequestered Sandstone Reservoir

In recent years,climate change becomes a common problem affecting human survival and development since extreme weather frequently occurred universally.There is a universal consensus to reduce greenhouse gas emissions in response to climate change.China has the energy endowment characteristics of more coal,less oil,and a lack of gas.At the same time,it is facing the great development task of the common prosperity of 1.4 billion people.Therefore,CCUS(carbon capture,utilization,and storage)are regarded as the only technology that can promote low-carbon and carbon neutralization without disruptive changes to the existing fossil energy and industrial structure.CCS(Carbon capture and geological storage)is one of the technologies most likely to achieve large-scale emissions reductions.CO₂ geological sequestration has been tested and demonstrated in many countries and is in the preparation period of industrialization.Nevertheless,"Injection feasibility"and"Storage security"are still two core problems that have not been totally solved.Based on the research basis of geological storage at home and abroad,this study fully understands and analyzes the previous research results,and combs the common scientific problems of geological storage of CO₂.On this basis,taking Ordos Shenhua CCS demonstration project as the research object,we make full use of the powerful conditions of the field test,especially the actual cores of sandstone reservoirs before and after injection are obtained in this study.The injectability of the CO₂ injection process and reservoir transformation in the process of reservoir diffusion are systematically studied to provide a reference for CO₂ geological storage site selection and reservoir selection.The main research contents and results of this thesis are as follows:(1)Study on injection feasibility at engineering scaleThe injection feasibility at the engineering scale is studied based on the actual injection conditions.The actual temperature,pressure,flow,and other parameters in the wellbore are obtained through logging analysis,unstable well test analysis,and injection performance analysis.The results show that the seepage capacity of the reservoir near the well is greatly improved.The inspiratory starting pressure decreased by 22%,and the inspiratory index increased by 10 times.The permeability has increased by 80 times.The wellbore has a typical variable skin effect.With the increase of injection volume,the CO₂ gas-water two-phase mixing area near the well zone dissolves or evaporates the residual water in the pores due to high-purity CO₂.This results in the reduction of water saturation and increases gas-phase permeability,which is conducive to improving reservoir injectability.(2)Study on injection feasibility at pore scaleTaking the sandstone reservoir cores from the Shenhua CCS demonstration Project in the Ordos as the research object,the pore-scale injectivity of the cores before and after injection was studied,including obtaining porosity,permeability under formation temperature and pressure,mineral composition test,whole-rock chemical analysis,high-pressure mercury injection,scanning electron microscopy and other basic parameters.The results show that the clay content decreases,the feldspar content increases,and the quartz ratio decreases slightly after CO₂ injection,indicating that the water-rock reaction obviously changes the composition of bedrock after CO₂ injection.After CO₂ injection,dissolution and precipitation in the reservoir have little effect on the porosity of the sandstone reservoir.The pore size distribution characteristics of microscopic pores change greatly before and after CO₂ injection.Before CO₂ injection,pores with a pore size of 1-100um will react and dissolve after CO₂ injection and expand into pores with a larger pore size of 100um and above,thus greatly improving the permeability of the reservoir.(3)Simulation of thermal effect on injection feasibilityWe further study the influencing factors of CO₂ injection under isothermal and non-isothermal conditions,which is based on a completely implicit reservoir simulation framework proposed by predecessors.Results show that the injection temperature directly affects the density and viscosity distribution of the fluid,thus affecting the injectivity.Cold(liquid)CO₂ tends to distribute in the lower part of the reservoir,while supercritical CO₂ accumulates in the upper part of the reservoir.The temperature evolution is dominated by fluid convection during CO₂ injection,followed by conductivity.However,the conductivity effect has a significant impact once the injection is stopped.(4)Simulation of microscopic pore structure variationBased on the CO₂ injection conditions and actual monitoring data of Liujiagou formation,the main reservoir of Shenhua CCS demonstration project in Ordos Basin,combined with numerical simulation,the CO₂ geological storage reactivity migration model of sandstone reservoir is constructed.The simulation shows that the dominant reaction is the dissolution of calcite,chlorite,albite,and potassium feldspar and the precipitation with illite for the injection area.The overall reaction result is to increase the porosity and permeability of the injection well,which is conducive to CO₂ injection.The diffusion area,affected by the components in groundwater such as Ca²⁺and dissolved CO₂,is mainly manifested in the dissolution of chlorite and potassium feldspar and the precipitation of calcite,illite,and calcium montmorillonite.The overall reaction result is to reduce the porosity and permeability of the reservoir,inhibit the migration and diffusion of CO₂,improve the mineralization rate of CO₂,and improve the security of CO₂ geological storage.