CO2快速吞吐提高页岩油采收率现场试验OA北大核心CSTPCDEI

The technology of CO2 huff-n-puff for enhancing shale oil recovery is still in an exploratory stage.There are some technical challenges,such as unclear mechanism of interaction between CO2 and shale reservoirs/fluids,immature numerical simulation techniques,and absence of large-scale injection-production and low-cost recovery processes.A shale oil well in the second section of the Funing Formation of the Paleogene in the Qintong Depression of the Subei Basin was taken as a case of study.To figure out the dominant mechanism of CO2 huff-n-puff for enhancing shale oil recovery,this paper conducts an experiment on water-rock reaction with supercritical CO2 to investigate the dissolution of shale minerals at high temperature and pressure and its impacts on porosity and permeability.Then,the constant-mass expansion(CME)and minimum miscibility pressure(MMP)tests with CO2 injection are carried out to evaluate the changes in physical properties of crude oil at high pressure after CO2 injection into overpressured reservoirs.On this basis,multi-factor numerical simulation is conducted to optimize the injection parameters.Finally,a CO2 thermodynamics and fluid dynamics data template is established to optimize the surface process.The proposed technology is verified through field tests.The results show that the water-rock reaction with CO2 is dominated by the dissolution of carbonate minerals,and partial dissolution of felsic minerals,forming medium and large pores.By injecting appropriate volume of CO2 into formation oil,the intermediate hydrocarbon components of the crude oil are extracted significantly,allowing the viscosity of crude oil to decrease from 5.151 mPa·s to 1.250 mPa·s.Moreover,CO2 first extracts the light hydrocarbon components,and then the heavy hydrocarbons with the production time.Based on the compositional numerical model of mixed media(artificial fractures and natural fractures),the parameters of huff and puff in a single well are optimized with the injection rate being 1.7×104t,the injection velocity being 500-600 t/d,and the soaking time being 50 days.The surface process of side heating at low pressure for CO2 huff and puff achieves an accurate temperature control with the support of the multi-parameter model of surface process for CO2 injection,so that the problems such as freezing/blocking of injection pipes and brittle failure of donwhole tubing and casing at low temperature can be avoided.When the concentration of CO2 in produced gas is greater than 80％,the gas phase recovery and direct injection process is adopted to reduce the cost of produced gas recovery to CNY104/t.It is concluded that the technology of CO2 rapid huff and puff effectively enhances the recovery of shale oil.The relevant understanding on mechanism and techniques can provide reference and guidance for shale oil development by huff-n-puff.