The Migration Transformation And Rock Mechanical Response Law Of CO₂ Geological Storage In Low-permeable Oil Deposits

In recent years,with the rapid development of society and economy,people’s demand for energy mainly from traditional fossil fuels keeps increasing,resulting in increasing greenhouse gases produced by fossil fuel combustion.As a populous country,China will continue to use fossil fuels as its main energy source now and for a long time thereafter.Therefore,it is particularly important to implement energy conservation and emission reduction measures to protect the atmospheric environment.According to the research,CO₂geological storage is one of the effective ways to achieve large-scale CO₂ emission reduction at present.However,current research on CO₂ geological sequestration has focused on conventional reservoirs,researches on CO₂ migration and distribution,storage mechanism and evolution characteristics during carbon sequestration in low porosity and low permeability reservoirs are relatively lacking.Nowadays,the research on CO₂ geological storage in Ordos Basin is mainly focused on the two aspects of deep salt water layer carbon sequestration and injection CO₂ improving crude oil recovery.However,many low-hole low-seepage oil and gas reservoirs in the basin are used to bury CO₂ after depletion,especially the study of the geological storage process by numerical simulation method is more scarce.Therefore,the relevant research on this aspect becomes particularly urgent.Based on the above background,this paper takes the Chang 4+5 and Chang 6 layers of Yanchang Formation in Huaziping oil area of Xingzichuan Oilfield,Ordos Basin as the research object.Systematically collect and sort out the relevant literature and basic data,based on the preliminary analysis of the CO₂ geological storage conditions in the study area and the establishment of the corresponding three-dimensional geological model.The CO₂ geological storage process of Yanchang Formation in the study area was studied by numerical simulation from the CO₂ migration and transformation law,storage mechanism and rock mechanical response characteristics.The effects of physical parameters such as porosity,horizontal permeability,temperature,formation stress,vertical and horizontal permeability ratio,capillary pressure and residual gas saturation on the numerical simulation process of CO₂sequestration were discussed,and the safety of CO₂ geological sequestration in the study area was verified.Finally,through comprehensive monitoring,the migration law after CO₂injection into the reservoir is found out,so as to provide a successful example for the establishment of long-term and safe CO₂ geological storage technology in low permeability reservoir.The results show that the Chang 6 reservoir of Yanchang Formation in Huaziping study area has good conditions for CO₂ geological storage.During CO₂ perfusion,the original pressure balance of the formation is broken,and the formation pressure around the injection well rises rapidly.Supercritical CO₂ forms a funnel-like diffusion halo in the reservoir.The formation pressure will gradually stabilize after CO₂ stops.The state of the CO₂"tongue-like"aggregation band is gathered at the top of the reservoir and the lateral distribution range is expanding.Influence CO₂ gas capture according to the standard sensitivity from high to low are formation stress,temperature,residual gas saturation,horizontal permeability and porosity.Influence CO₂ dissolution capture according to the standard sensitivity from high to low are formation stress,residual gas saturation,temperature,horizontal permeability and porosity.The Chang 4+5 layer of Yanchang Formation is used as the cap layer,which has a very good tightness.By simulating the formation stress change with the maximum injection rate of a single well,it is found that it is far from reaching the degree of fracture pressure of Chang 6Formation.It also proves from the side that choosing Huaziping study area as a CO₂geological storage site has low risk and high economy for long-term operation.Through field practice,compared with the average CO₂ migration rate of 0.0157m/d in the simulated gas injection period and the average CO₂ migration rate of 7.35m/d in the field dynamic monitoring experiment after hydraulic fracturing.It is found that early hydraulic fracturing of low porosity and low permeability reservoirs in Ordos Basin can greatly accelerate the migration rate of CO₂ and improve the injection capacity.