| The condensate gas reservoir have vast economic values for the dual characteristics of oil and gas reservoirs it possesses.However,due to the override of injected dry gas,the development process witness a poor gas injection efficiency and a less efficient exploitation.At present,the understanding of overlapping mechanism is insufficient,and there is a lack of mathematical models and evaluation systems that can better reflect the overriding of injected dry gas.Besides,the main influencing factors as well as the mechanism involved within are not yet clear.To address these issues,we have firstly conducted an in-depth investigation on the study of the gravitational differentiation in reservoirs,and secondly,the development of numerical simulation for the diffusion and migration of injected gas.On the basis of literature research,a series of studies have been carried out on the overlapping mechanism of injected dry gas,relative mathematical models as well as evaluation and mitigation of overriding phenomena.1.It is revealed that gravitational differentiation plays a vital role in overlapping after we analyzed the flow behavior of supercritical CO2 in natural gas reservoirs and that of injected dry gas in condensate gas reservoirs.Further studies demonstrated respectively,the effects of convective diffusion between dry gas and condensate gas,phase behavior of condensate gas,reservoir properties,and relative production parameters on the overlapping of injected dry gas.2.Based on the previous study,we introduced the overlapping characteristic equation and established the mathematical model depicting the override of dry gas under cyclic injection development mode.The models not only take into account of the convection and diffusion of each component,but also the phase properties between two phases and the gas compressibility.Besides,the Maxwell-Stefan equation is used to describe the component diffusion behavior.All of the above improvements help to yield an efficient and precise mathematical description for the flow behavior of injected gas during the overlapping process.3.We developed a computer program and adopted the finite difference method and the modified Netwon-Raphson iteration method to solve the mathematical model.Using the ideal model,we simulated the distribution of each component and the properties of dry gas-condensate gas transition zone after injecting dry gas into a condensate gas reservoir.The results revealed the overlapping mechanism of different injected gases and demonstrated the effects of density differentiation and phase transformation on the overlapping of injected dry gas.4.This thesis presents an evaluation system to assess the overriding of injected agent under gas injection development mode.This method realizes the qualitative and quantitative evaluation of overriding by comparing and analyzing the dynamics of injection and production wells from five aspects.5.We established a set of adjusting technologies to mitigate the influence of gas overriding and selected eight main factors affecting gas overriding.Furthermore,their influence on gas overlapping is demonstrated through single-factor indicator analysis and multi-factor variance analysis.The optimized adjusting measures can mitigate the influence of gas overlapping,thereby improving the scientific and accurate development plans for gas injection.6.Results of this thesis were applied on the on-site development of Yaha 2 block in Yaha condensate gas reservoir,the process combined the understanding of overriding mechanism,the improvement of numerical simulation methods,as well as the establishment of evaluation methods and adjusting techniques.It is verified that the mathematical model established in this paper can provide reliable and effective results.After the assessment of gas overlapping in target block,further adjusting measures and optimized injection plans have been put forward to reduce the disadvantages of gas overlapping.The results of this thesis provide new understanding and technical support for the rational development of condensate gas reservoirs under cyclic gas injection mode. |
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