Study On Phase Characteristics And Reasonable Development Technology Of Volatile Oil Reservoir In Bozi Block

Tarim Basin is rich in condensate gas and volatile oil,which has great exploration potential.However,its oil and gas reservoirs are mainly composed of deep high temperature and high pressure reservoirs,which lead to complex phase characteristics and unclear development technologies.Taking Bozi-7 volatile oil reservoir in Tarim Basin as the research object,the phase state characteristics of volatile oil reservoir were clarified by PVT experiment of formation fluid at high temperature and high pressure,consistency test and phase state simulation of hydrocarbon fluid.Further fine tube experiments and simulations were carried out to explore miscibility and near-miscibility of different injected gases（CO₂,N₂,associated gas）.On the basis of phase fitting,the influence of injected gas on the phase characteristics of volatile oil was studied,and the feasibility of developing volatile oil by injecting gas was explored in combination with the miscibility of injected gas,so as to provide reasonable technical countermeasures and suggestions for field implementation.The hydrocarbon fluid composition of Bozi-7 formation fluid measured by HIGH temperature and high pressure PVT experiment is 20.62%C₂-C₆ and 22.66%C₇₊,with high intermediate components,light oil and heavy gas,30.95MPa saturation pressure and 108.98MPa saturated pressure difference.The comprehensive gas-oil ratio is judged as volatile reservoir fluid.The consistency test of PVT experiment shows that the experimental results of Bozi-7 PVT are reliable.Through calculation and comparison,the PR equation of state with Peneloux volume conversion is optimized for phase state simulation of hydrocarbon fluids.The modified equation has high accuracy for phase state prediction under high temperature and high pressure,and the PVT-sim phase state simulation software has good convergence.The binary interaction coefficients between C₁/C₂ and heavy components and their critical parameters such as eccentricity factor and critical pressure should be considered in parameter tuning.Tubule experimental results show that compared to the rest of the two gases（associated gas/N₂）,note the minimum miscibility pressure（MMP）of CO₂as low as 31 Mpa,embodies the CO₂ miscible ability,good comprehensive judgement combining interface applanation tonometry method and the method of oil displacement efficiency near miscible pressure interval,makes the minimum miscible pressure prediction is more reasonable and forecast results show that in the design of gas injection pressure,CO₂ injection and associated gas injection can achieve miscible flooding,while N₂ injection shows immiscible flooding.The results show that CO₂ injection and associated gas can slightly increase the saturated pressure and decrease the viscosity of crude oil,and the light component of crude oil can be extracted.N₂ injection can sharply increase the saturated pressure,and the density of CO₂ increases to nearly 1000 kg/m³ under reservoir temperature and pressure.This characteristic indicates that CO₂ can effectively reduce the wellhead gas injection pressure.Numerical simulation was carried out to optimize the pressure drop rate of 8 MPa/year and the oil recovery rate of 2%during the depletion development of Bozi-7.After 9 years of depletion development,CO₂ injection and associated gas injection at the injection rate of 15000 m³/d could increase the recovery degree by 11.48%and 11.88%,respectively.The study on the phase state of gas injection and the prediction of numerical simulation show that timely gas injection is feasible after the depletion development of Bozi-7 volatile oil reservoir,and it is suggested to use associated gas or CO₂ as injection medium.