Study On Mechanism And Optimization Design Of Enhanced Oil Recovery By CO₂ Huff And Puff In Shale Reservoir

There are many nano-scale pores in shale reservoirs,and the clay mineral content in the reservoirs is high,which leads to the poor effect of traditional water injection development.When CO₂ is injected into shale reservoir,it is easy to reach supercritical state,form miscible phase with shale oil under certain conditions,and easily enter shale reservoir to displace crude oil.At present,the mechanism and law of CO₂enhancing oil recovery in shale reservoirs are still unclear.In this paper,the physical properties experiment of crude oil at high temperature and high pressure,the minimum miscible pressure test,CO₂ huff and puff experiment and on-line NMR analysis experiment are planned to be carried out for the target block,so as to clarify the mechanism of shale oil exploitation by CO₂ huff and puff.On this basis,through the numerical simulation technology,the sensitive parameters and influencing rules affecting the development effect of CO₂ huff and puff are analyzed,and the huff and puff scheme is optimized.Through the research,the following understandings have been obtained.Based on the reservoir fluid physical property data of the target shale reservoir block,PVT phase analysis experiments of formation oil were carried out,including flash separation,constant expansion,CO₂ injection expansion and thin tube experiments.The dissolved gasoline ratio of this shale oil is 353.5m³/m³,and the proportion of C₁-C₇ components in the composition reaches 51.74%,so it is judged that this shale oil reservoir is a volatile oil reservoir.The flash experiment shows that the relative volume of shale oil in the target block is 2.05,and the viscosity of degassed oil is 1.61m Pa·s,so it is judged that shale oil is volatile oil and has certain fluidity.In addition,through the expansion experiment of CO₂ injection,it is concluded that after CO₂ injection into shale oil,its volume expansion coefficient reaches1.765 when the molar fraction of CO₂ injection reaches 40%,which shows that CO₂has a good energy-supplying effect on shale oil injection.The minimum miscible pressure between formation oil and CO₂ is 29.5MPa,and the original formation pressure is 37MPa,which shows that miscible flooding can be easily realized when CO₂ flooding is carried out in this shale reservoir.Aiming at four factors（huff and puff rounds,soaking time,reservoir temperature and injection pressure）that have great influence on shale oil development by CO₂ huff and puff,an orthogonal experimental scheme with four factors and three levels is designed and an oil displacement experiment by CO₂ huff and puff is carried out.Based on the grey relational analysis,the sensitive factors affecting oil recovery are sorted in order:huff and puff rounds>injection pressure>soaking time>reservoir temperature;The sensitive parameters affecting the oil change rate are:huff and puff rounds>injection pressure>reservoir temperature>soaking time.The corresponding parameters of the optimal scheme are:injection pressure of30MPa,reservoir temperature of 60℃,three rounds of huff and puff,and soaking time of 2hours.Finally,the pore production characteristics of different schemes for CO₂ huff and puff development of shale oil were studied by on-line NMR displacement,and it was found that the optimal scheme mainly used shale oil in pores above 15nm,and the shale oil in small pores of shale was also passively used by optimizing the scheme.The following understandings are obtained:（1）During huff and puff,the crude oil at the interface between CO₂ and the core edge is used passively first,and then gradually penetrates into the shale core;（2）Increasing the gas injection pressure can reduce the interfacial tension between CO₂and crude oil,greatly reduce the resistance of CO₂ entering the pores,and use the crude oil in the smaller pores in the target layer;（3）The main mass transfer mode of（3）CO₂ entering the pores is diffusion,and the contribution of increasing injection pressure to increasing diffusion rate is very limited,so the overall recovery rate of pores is low.Based on the reservoir physical properties and fluid data of the target block,a200m×200m well group geological model is established and numerically simulated.The CO₂injection pressure,soaking time,reservoir temperature,injection speed and huff and puff rounds are optimized,and the optimal level indexes of each factor are given as follows:injection pressure 30MPa;The soaking time is 20d;The reservoir temperature is 70℃;The injection rate is 20m³/d;The number of throughput rounds is 2.Based on the indoor experiment of CO₂ huff and puff in Chapter 3,the numerical simulation shows that Scheme 1has the highest oil recovery,and Scheme 1 is the best scheme.By analyzing the indicators of block formation pressure,crude oil viscosity and miscibility in Scheme 1,it is found that the optimal scheme has the best energy replenishment effect during CO₂ huff and puff near horizontal wells,the largest macro-swept area of CO₂,and the best miscibility effect.The core recovery rate of the optimal scheme is 45.7%,and the conclusions reached by the numerical simulation scheme and the indoor experimental scheme are in high agreement,indicating that the determined research parameters are reasonable.Through this study,the production mechanism of shale oil and the optimization of CO₂ huff and puff factors are optimized,which has certain reference significance for guiding the efficient development of continental shale oil in China.