The Seepage-stress-damage Coupling Mechanism Of CO₂ Storage In Depleted Oil Field And The Effect On The Security Of The Storage

At present, the natural disasters caused by climate change have brought about great casualties and economic losses. Emission of greenhouse gases such as CO2 is considered as the leading cause of climate change, therefore, how to reduce CO2 emissions has become the focus of attention. Sealing CO2 into deep formations (CCS) is one of the effective ways to reducing greenhouse gas emissions, and it has been widely used. For CCS projects, whether CO2 leakage occurs is the main evaluation index to judge the success of the projects.Depleted oil field is considered to be one of the suitable sites for CO2 geological sequestration. Increasing pore pressure due to the injection of CO2 can lead to stress and strain changes of the reservoir, and the macro characteristics are rock deformation and reservoir uplift. The rock get damage when the volume deformation exceed a certain limit, which can lead to irreversible mechanical damages of the reservoir and impact the integrity of caprock which could potentially provide pathways for CO2 leakage through previously sealing structures. In this paper, a mathematical model of rock mass damage was established by using laboratory test and theoretical analysis, geological mechanics responses of the reservoir during the process of CO2 storage and the main factors that affect the storage security were studied through numerical simulation. The research results have both academic sense and applied value on achieving the safety storage of CO2.The main work and results of this paper are as follows:(1) Get the physical parameters and mechanical parameters of the target reservoir through the rock mechanics experiments; through loading-reloading experiments measured how that volume strain changes as a function of axial force; get porosity and permeability data of the target reservoir through displacement experiments and studied the effect of the carbon dioxide phase on the porosity of reservoir rocks.(2) Established a mathematical model which can describe the characteristics of percolation mechanism and mechanical mechanism during the process of storage CO2 into a depleted oil field. In this model, using non-Darcy’s law describes the fluid flow in the reservoir; using the hyperbolic model describes the stress-strain characteristics of the reservoir rock, while the effect of effective stress on porosity was also considered, those two parts were coupled by using the poroelastic theory.(3) A homogeneous model was established to analyzed the change law of porosity and permeability and geological mechanical response characteristics under the condition of signal well gas injection, in addition, the effect of interlayer and well pattern on the security of the storage were studied.(4) A numerical calculation model was established and simulation studies on CO2 storage in chang 4+51 block of Yougou oil field were carried out. Geological mechanics response under different gas injection scheme and different injection parameters were studied. Failure mechanism of target reservoir caused by rock damage were analyzed. Finally, we evaluated the efficacy and safety of storage CO2 in chang 4+51 block and established the safety evaluation method.(5) A model that describe the chemical reaction between CO2 and formation water was established. Numerical simulation was carried out to research the effect of CO2 leakage on pH and mineral dissolution/precipitation f the formation water.