Research On Phase Characteristics Of Multi-period Injection-production Fluid In S6 Gas Cap Reservoir Type Gas Storage

Underground gas storage converted from gas-cap oil reservoiris a kind of underground gas storage,and its operation process is also a multi-cycle injection and production process.Among the existing underground gas storage converted from gas-cap oil reservoir,Liaohe Shuang6 Gas Reservoir has 5.24 million tons of residual oil geological reserves and 105-12.1 million tons of condensate oil,which have low recovery degree and great potential for improving oil recovery.Therefore,it is meaningful to study the improvement of oil recovery and storage capacity of Liaohe Shuang6 gas reservoir during multi-cycle injection and production process.In the process of multi-cycle injection and production of Liaohe Shuang6gas reservoir,there are complex phase change and material exchange between gas-cap and oil-ring,including phase change of gas-cap condensate retrograde condensate and diffusion and retrograde evaporation during injection and production,phase change of dissolution and extraction between injected gas and oil ring,which will affect the design and optimization of gas storage plan and operation.However,at present,there is no reference case for the study of fluid phase characteristics in multi-cycle injection-production process of underground gas storage converted from gas-cap oil reservoir at home and abroad.Aiming at the complex change of fluid phase characteristics during multi-cycle injection and production in Liaohe Shuang6 gas reservoir,the phase characteristics of injected gas,gas cap and remaining oil are studied by using the technology of fluid PVT phase testing and component reservoir numerical simulation.The phase change characteristics of injected gas and gas cap reservoir during injection-production process are revealed,which provides technical countermeasures for improving oil recovery and increasing reservoir capacity.This paper mainly carries out the test and Simulation of the fluid phase characteristics under the original temperature and pressure conditions and the five-cycle injection-production experiment and simulation after depletion production to the present formation pressure,including the gas-cap PVT test under the original formation temperature and pressure conditions(24.76 MPa,89℃),the gas-cap multi-cycle injection-production(10 MPa-24 MPa)test and simulation,the oil-ring oil PVT test under the original formation temperature and pressure conditions,and the simulation of the gas-cap multi-cycle injection-production(10 MPa-24 MPa).Multi-cycle injection-production(10MPa-24MPa)experiment and simulation,gas cap+oil ring multi-cycle injection-production(10MPa-24MPa)experiment and simulation.The main understandings are as follows:(1)The evaporation effect of multi-cycle injection-production process of condensate gas on gas cap is remarkable,and the recovery rate of condensate oil reaches 65%after 5 cycles of exhaustion+injection-production test.The recovery of condensate oil increased by 41.5%in 5 cycles compared with depletion.With the increase of injection-production cycle of gas cap,the saturation of condensate oil decreases,and the change of the saturation of condensate oil is almost zero after multiple injection-production cycles.(2)In the process of multi-cycle injection and production of gas cap,formation fluid composition becomes lighter and lighter,C1 content changes from 86.8%to 91.97%,residual gas composition gradually approaches that of injected gas,and multi-cycle huff and puff has a significant effect on fluid replacement and extraction.After each period of gas injection,the phase envelope of P-T phase diagram moves to the lower left,and the condensation decreases gradually.The P-T phase diagram of 10 MPa is moving to the lower left,indicating that the heavy hydrocarbons are decreasing gradually.(3)After multi-stage degassing to 4MPa remaining black oil is injected into natural gas at the original pressure of 24MPa,the gas-oil ratio decreases,and the dissolved gas capacity of oil ring decreases after multi-cycle injection and production,indicating that the dissolved gas ratio of black oil decreases.(4)After 5 cycles of multi-cycle injection and production in gas cap+oil ring system,the remaining oil saturation is 1.8%lower than the original black oil saturation of 3.8%,which indicates that oil-ring crude oil has also been partially recovered.(5)Multi-cycle injection-production simulation of single well gas cap+oil ring shows that multi-cycle injection-production improves oil recovery by 8.8%(condensate oil+crude oil),single-cycle average oil recovery by 1.8%(condensate oil+oil ring oil),gas cap condensate oil saturation decreases by 28%,oil ring oil saturation increases by 4.6%,multi-cycle injection-production improves gas phase flow ability of near wellbore zone by 53.4%.