The Study On The Reservoir Pressure Control Strategies And Optimization Of Well Pattern For A Co₂-Enhanced Water Recovery Project

Climate change poses the greatest challenge and threat to humanity,and it is a major constraint to our sustainable social and economic development.CO₂ geological storage?CGS?proved to be an effective way to mitigate greenhouse gas emissions,and CO₂-enhanced water recovery?CO₂-EWR?technology may improve the efficiency of CO₂ injection and saline water production with potential economic value as a means of storing CO₂ and supplying cooling water to power plants.Moreover,the continuous injection of CO₂ may cause a sharp increase for pressure in the reservoir system,so it is important to determine reasonable reservoir pressure control strategies to ensure the safety of the CGS project.However,CO₂-EWR is still at the stage of theoretical research,and there is no complete demonstration project.It is necessary to optimize the pattern of wells according to site characteristics before large-scale CO₂ injection and saltwater production,providing scientific guidance for the deployment and construction of injection and production wells and reservoir pressure control strategies in demonstration projects.During the CO₂-EWR,the production of saline water will affect the migration and evolution of CO₂ in the reservoir,changing the intensity of different CO₂ storage mechanisms,the capacity of CO₂ storage and saline water production and storage safety of CGS will be evidently affected.Based upon the typical formation parameters of the China Geological Survey CO₂-EWR test site in the eastern Junggar Basin,laboratoryscale experiments of CO₂-water-rock interaction were carried out to study the effects of CO₂-water-rock interaction and formation water salinity on the transformation of rock micro-physical structure.A series of three-dimensional?3D?injection-extraction models with fully coupled wellbores and reservoirs were established to evaluate the effect of the number of production wells and the well spacing on the enhanced efficiency of CO₂ storage and saline production.The optimal key parameters that control reservoir pressure evolution over time are determined.The main findings are as follows:Through CO₂-water-rock interaction experiments,the dissolution/precipitation rules of minerals in the reservoir system were revealed,as well as the degree of modification to the rock micro-physical and structural characteristics.The pore types of sandstone in Donggou Formation are mainly slit,micropore and mesopore.The interaction of CO₂-water-rock has little effect on mesopore in rocks.Mineral dissolution mainly affects micropore,which is directly reflected in the large change of specific surface area and pore volume of minerals,thus enhancing the capacity of CO₂ adsorption.The transformation of pore structure during CO₂ injection in the early stage of CGS will make CO₂ more firmly sealed in deep saline aquifer.The dissolved minerals are mainly feldspar and calcite,and clay minerals such as illite and montmorillonite are the main precipitation minerals.The precipitation of new carbonate mineral was not observed in a short period of time under the temperature and pressure conditions of this experiment.However,the trend of mineral precipitation is greater than that of mineral dissolution in the later stage of the experiment from the change of rock microstructures.It is speculated that if the reaction time is long enough,stable carbon-fixing minerals should be formed.With the increase of Ca Cl2 concentration,the dissolution trend of feldspar and calcite is more obvious,promoting the precipitation of clay minerals.The influence of pore-permeability heterogeneity on the evolution of pressure and CO₂ is clarified by the long-time scale geochemical reaction simulation during the process of CO₂-EWR,and the dissolution and precipitation mechanism of typical minerals and the transformation rule of CO₂ storage mechanisms are identified.After the injection of CO₂,the zone in the injection well is single supercritical CO₂ phase,the dissolved CO₂ mass fraction is less than the two-phase zone near the injection wells,and the decrease of p H is more obvious in the two-phase zone near the injection wells.Because of the obvious pore-permeability heterogeneity in the vertical direction,the spatial distribution of CO₂ is divided into three different dominant reservoirs vertically,and the CO₂ migration range in the reservoir with larger porosity and permeability is more obvious.CO₂-EWR technology can promote the horizontal migration of CO₂ during the process of CGS,thereby reducing the accumulation of the CO₂ concentration and pressure near the injection wells,which can significantly reduce the risk of CO₂ leakage along the injection wellbore.After 150 years,the main dissolved minerals are oligoclase,k-feldspar,chlorite and iron mica under the condition of mineral constituent of the target reservoir in the study annite,and the precipitated minerals are calcite,siderite,smectite and quartz,among which siderite is the main secondary carbonate mineral.The CO₂-EWR technology can not only enhance the injection capacity of CO₂,but also increase mineral storage and enhance the effect of mineral storage mechanism significantly.Compared with traditional CGS,CO₂-EWR is more conducive to CO₂ permanent storage.The numerical simulation results show that a smaller spacing between injection and production wells and a larger number of production wells can enhance not only the CO₂ injection capacity but also the saline water production capacity.The effect of the number of production wells on the injection capacity and production capacity is more significant than that of well spacing.CO₂-EWR technology can effectively control the evolution of the reservoir pressure,offset the sharp increase in reservoir pressure caused by CO₂ injection and the sharp decrease of reservoir pressure caused by saline production.It can avoid possible reservoir damage during the implementation of a CGS project and ensure the reservoir stability and safety of the project.The main controlling factors of the pressure evolution at a certain spatial point in the reservoir changes with time.The transition time point is affected by the well spacing,the number of production wells and the spatial position in the reservoir.When the monitoring point is in the middle of the injection well and the production well as is the case in our model with one production well and one injection well and a well spacing of 2 km,the pressure evolution is mainly controlled by the water production at the beginning;then,the main controlling factor of the pressure is the process of CO₂ injection.The actual site simulation of the Junggar Basin shows that the simulation scenario with 2 production wells,one injection well and a well spacing of 2 km is most reasonable,and the annual production capacity can meet the water requirements of a 300 MW thermal power plant.Moreover,the optimization strategies of well pattern for the CO₂-EWR project are summarized,which can provide references for the placement of wells in the CO₂-EWR site of continental sedimentary basins in China.The implementation of a CO₂-EWR project plays an active role in the eastern Junggar Basin,which can not only achieve the goal of lowering carbon emissions to mitigate global climate change but also produce enough water resources to meet the local water demand.Overall,the results of analysis provide a guide and reference for the CO₂-EWR site selection,as well as the practical placement of wells.