Enhanced Oil Recovery Mechanism By CO₂Injection And Storage Study In Near-Critical Condensate Gas Reservoir

The present research was carried out under the CNPC Key Technologies R&D Program during the Twelfth Five-year Plan Period which entitled "Research on new technologies for gas condensate reservoir development (2011B-1507)"According to oil and gas reservoir technology development trend, enhanced oil recovery through CO2injection combined with CO2storage is one of the most effective and economical techniques to reduce the greenhouse gases emission and deal with the worsening environmental problems. Presently, a number of low permeability oil reservoir in the middle and later development (Jilin and Jiangsu oil fields) have been carried out by CO2injection and has achieved preliminary success.But enhancing oil recovery by CO2injection technology mostly applied in low permeability oil reservoirs, while the research on the problem of enhancing condensate oil recovery by CO2injection technology in the condensate gas reservoir is less. Especially, the research on the problem of enhancing condensate oil recovery by CO2injection at the same time considering on the storage of CO2in the condensate gas reservoir is more less. And the research on the problem of enhancing condensate oil recovery by CO2injection at the same time considering on the storage of CO2in the near critical condensate gas reservoir is few. Thus, based on a real near critical condensate gas reservoir, the indoor experiments and combined with numerical simulation method was run to study the EOR mechanism and CO2storage potential, migration mechanism and its affecting factors in the middle and later development of near critical condensate gas reservoir, which helps provide experimental and theoretical basis for the projects designing and potentiality evaluation of CO2-EOR and CO2storage in the middle and later development of near critical condensate gas reservoir.The present paper mainly describes the mechanism of EOR by injecting CO2, its feasibility as well as the potential and stability of CO2storage in the near critical condensate gas reservoir. The priority goals of the research are:special phase behavior of original reservoir fluids in initial development phase; special phase behavior of residual oil and gas system or residual oil and gas system by CO2injection; the MMP of residual oil system by CO2injection; percolation characteristics during the strong phased CO2displacement process in the near critical condensate gas reservoir. On the basis of these studies, the one dimensional linear, two dimensional section plane and three dimensional numerical stimulation models were used to evaluate the feasibility of CO2injection for enhanceing the recovery of condensate oil. Furthermore, the effect of condensate oil recovery by CO2injection and the CO2storage potential were carried out. Finally, an optimal scheme was obtained for CO2injection and CO2storage.(1) Based on the special phase behavior of near critical gas condensate reservoir original fluid, the special phase behavior of injecting CO2into the remaining condensate oil and gas system/condensate oil system under present reservoir conditions, and the percolation characteristics of strong CO2displacement process in the s in the near critical condensate gas reservoir, the experimental research method of enhancing condensate oil recovery mechanism by CO2injection and CO2storage capacity evaluation in the near critical gas condensate reservoir has been further developed.(2) During the PVT experiments of original reservoir fluid in early stage of development, in the adjacent area of the critical point, the fluid behaves from black brown in the upper and reddish brown in the lower to black brown in the whole cell with misty phase transition observed, this is the famous "critical opalescence phenomenon". Moreover, the phenomenon observed proves that the fluid of near critical gas condensate reservoir is a fluid with high density, liquid drop highly dispersed and no interfacial tension near the critical point.(3)The fully visually experimental result of injecting CO2into remaining condensate oil and gas system/condensate oil system has shown that after injecting a certain proportion of CO2, critical opalescence phenomenon can be observed in the remaining condensate oil and gas system/remaining condensate oil system. Critical opalescence phenomenon means that there is no obvious interfacial tension between CO2and the remaining condensate oil and gas system/remaining condensate oil system and the fluid has been mixed to one phase. Therefore, it is feasible to achieve miscibility by injecting CO2into the remaining condensate oil and gas system/the remaining condensate oil system. So the injected volume of CO2achieved miscibility should be considered.(4)The experimental results of homogeneous one-dimension long slim tube filled with sand have indicated that the tested MMP is less than original reservoir pressure, which means that CO2and the remaining condensate oil system is prone to achieving miscibility. However, to enhance condensate oil recovery the injection pressure of CO2must be larger than MMP, which indicates that the injection pressure should be considered when injecting CO2for enhancing condensate oil recovery.(5)Due to the limitations (narrow range for application) and large error of the existing empirical CO2-oil system MMP models,through the collection of domestic and overseas data in CO2-oil MMP experiments, the MMP models of CO2-oil system are presented by using optimization algorithm in this paper. And the MMP model is tested by using the minimum miscible pressure data tested in this paper and the other data unused to establish the model.The result from the test shows that the model is of higher accuracy for CO2-oil MMP prediction than the other models and the scope of prediction is wider than the other models. Thus, the purpose of the improvements is achieved.(6)Combined long core flooding experimental results have indicated that condensate oil recovery is related to injection pressure of CO2(maintaining level of reservoir pressure), injection scenario (considering the well shuted in for pressure hold and convective diffusion and without considering the well shuted in for pressure hold and convective diffusion). Thus, injection pressure and injection scenario should be comprehensively considered while conducting CO2injection for enhancing condensate oil recovery. (7) By utilizing compositional model of commercialized numerical simulation software CMQ numerical simulation model of CO2flooding for enhancing condensate oil recovery and CO2sequestration has been established. In this model, some sensitive factors are taken into consideration, such as injection-production well pattern, injection rate, injection pressure (maintaining level of reservoir pressure) obtained from the experiments and injection volume. The result has shown that these several factors play an important role in enhancing condensate oil recovery. Furthermore, under the same conditions, the effect of only one factor has been simulated and it can be found that the higher injection pressure, the higher condensate oil recovery and the greater the CO2storage potential; the more injected CO2, the higher condensate oil recovery and the greater the CO2storage potential; the higher injection rate, the higher condensate oil recovery and the greater the CO2storage potential. However, with the oil replacement rate and the case of gas channeling considered, the reasonable injection rate, injected volume and injection pressure are optimized to be60000m3/day,0.35HCPVå'Œ28MPa, respectively.(8) Based on the optimum numerical simulation scheme of injecting CO2for flooding condensate oil and CO2storage, stability evaluation of CO2storage for50and100years has been carried out. The result shows that after storing CO2for50to100years, reservoir pressure barely changes and CO2has a little diffusion. But the diffusion coefficient decreases by the storage time,with a minimum of only1.43×10-5cm2/s, which has proved that CO2storage can be reached stably.