Study On The Mechanism Of Enhancing Shale Oil Recovery By CO₂ Huff-n-Puff

As a very important unconventional oil and gas resource,shale oil reserves account for 10%of recoverable crude oil reserves worldwide,it has huge mining potential.At present,horizontal well hydraulic fracturing and other technologies are used for development,but the economic benefits are poor,and only a small amount of proven reserves can be produced.After fracturing,the initial recovery is high but decreases rapidly.Recently,with the global response to greenhouse gas storage measures and further exploration of shale development,CO₂ huff-n-puff has become a new method to improve reservoir recovery after fracturing.It can interact with crude oil and effectively increase the flow capacity of crude oil.The research on the mechanism of CO₂ huff-n-puff to enhance oil recovery also has important theoretical significance.Based on the full investigation of the existing development technology of shale oil and the application of CO₂ injection technology,this paper tests and analyzes the CO₂ properties,and performs simulated oil compounding in the laboratory according to Eagle Ford formation crude oil.The PVT parameters and fluid components of the reservoir are divided,which effectively reduces the simulation time of the component model.Through the interaction experiment(crude oil expansion experiment,diffusion experiment,minimum miscibility pressure test),the swelling phenomenon and mechanism of shale oil injection of CO₂ were investigated.We also proposed a set of experimental procedures for indoor CO₂-shale oil injection,explored the effects of different operating conditions to the recovery degree.On the basis of the actual experiment,the combination of experimental and reservoir numerical simulation is used to demonstrate the relevant parameters of the actual CO₂ stimulation to improve the shale oil recovery process.The main research results and understandings of the thesis are as follows:(1)During the same operating time,the shorter well time can maintain a higher pressure gradient in the core,thus achieving higher recovery,but when the soaking time is too long,the oil displacement capacity will gradually lose its advantage,resulting in economic and material waste,so in actual production,there is an optimal well drainage time.(2)The mechanism of enhanced oil recovery includes:the mixed phase pressure of CO₂and crude oil is lower than the minimum,the main mechanism for oil phase are expansion,viscosity reduction and gas oil displacement,higher than the minimum miscible pressure,resulting in zero barrier capillary pressure of carbon dioxide entering the oil-filled pore space,thereby improving local displacement efficiency,and the miscible condition is the best conditions for CO₂ stimulation to improve shale oil recovery.The results of the physical experiment of CO₂ huff-n-puff indicate that in a fixed operating period,a shorter soaking period will repressurize core more frequently and maintain higher pressure gradients to achieve a higher oil recovery.However,when the soaking period is further extended,the oil extraction capacity of CO₂ will be significantly reduced.Therefore,in actual production,there is an optimal soaking period.