Mechanism Study Of CO₂-Water-Rock Interactions During CO₂ Flooding Process

CO₂ flooding is an effective method to enhance recovery,which is suitable for different types of reservoirs as well as different periods of development.However,the interaction mechanism of injected CO₂ with formation water and rock,and its influence on recovery rate need to be studied and evaluated.Takes the CO₂ flooding demonstration project in Xinjiang oilfield as the target block,this paper studies the CO₂-water-rock interaction through formation water scaling experiment,geochemical reaction experiment and flow reaction simulation in the reservoir,and evaluates the reservoir adaptability of the water-alternating-CO₂ injection and the pure CO₂ injection process.The target block belongs to low-permeability conglomerate light oil reservoir,which was developed by water injection at the early stage.Currently,due to the shortage of water source and difficulties in water injection,CO₂ injection is planned to enhance recovery.Based on the composition of rock and formation water in the target block,this simulation of underground environment using high-temperature and high-pressure reactor for water scale experiment and CO₂-water-rock geochemical reaction experiment,so as to evaluate the scale formation risk in the reservoir under different injection modes.According to the experimental results,the geochemical reaction model was established,and the flow reaction simulation method in reservoir mathematical model was used to study the influence of chemical reactions on reservoir physical properties in the CO₂ flooding process,reveal the interaction mechanism between CO₂,and formation water and reservoir minerals.Experimental results of water scale between injected water and formation water show that the currently available injected water contains relatively high CO₃^2-and HCO₃^-ions,which will produce Ca CO₃ scale precipitation with Ca²⁺in formation water,leading to formation scale.The incompatibility between injection water and formation water is obvious.Mixing CO₂ with injected water can alleviate the scaling phenomenon caused by incompatibility of injected water.High temperature and high pressure reactor experimental results show that when the temperature is 100?,the total system pressure is 10 MPa,CO₂partial pressure increases from 1 MPa to 5.5 MPa,the average scale amout of calcium carbonate decreased by 84.4%.Experiments of CO₂-water-rock geochemical reaction show that CO₂ injection can cause calcite dissolution and dolomite precipitation,but it has no obvious influence on the concentration change of scaling ions(Ca²⁺and Mg²⁺)in water,and temperature and pressure have certain influence on the geochemical reaction rate.The simulation results of geochemical reaction show that CO₂ dissolving in water will acidify formation water and lead to the dissolution of calcite in rocks.However,due to the high concentration of Ca²⁺in formation water,the dissolution of calcite is not large.At the same time,a small amount of dolomite precipitation.Compared with the CO₂-water injection method,the water-gas injection will increase the water saturation of the reservoir and increase the influence of the formation internalization near the injection well,resulting in a certain degree of scaling and porosity reduction.However,the pure CO₂ injection process has a good adaptability with the target formation,which can slow down the deposition of scaling ions,and the reservoirs porosity is less affected by the CO₂-water-rock interaction.Therefore,it is feasible to inject CO₂ into the target block to enhance recovery.