Study On Reservoir Fluid Variation During Gas Injection In Carbonate Reservoir

Fracture-vuggy carbonate reservoir is one of the more complex reservoirs,which is more difficult to develop because of the diversity of internal reservoirs and the complexity of reservoir fluid flow relationship.Taking S2 reservoir unit of Tahe Oilfield as the research object,based on experimental method and numerical simulation application,this thesis carries out a series of studies on the variation of reservoir fluid in the process of gas injection.Firstly,a typical fracture-vuggy unit geological model is established according to the development characteristics of S2 reservoir unit and field production data.Then,the production performance of each well was analyzed,typical well crude oil was selected as experimental samples,and conventional PVT experiments such as single degassing and constant mass expansion of reservoir fluids were conducted successively.Corresponding numerical simulation software was used to complete PVT data fitting,so as to obtain a series of oil phase characteristics data.Three kinds of gas（CO₂,N₂,natural gas）were selected to carry out gas injection expansion experiment to analyze the change of gas injection on the fluid status of crude oil and the influence degree of gas injection on each component of crude oil.The mechanism of gas injection was clarified,and the law that gas injection would reduce the relative content of light components and increase the relative content of heavy components of crude oil system was obtained.At the same time,the minimum miscibility pressure of the three gases and crude oil was calculated by using the thin tube experiment.Under certain conditions,the formation fluid can miscibility of CO₂ and natural gas,but N₂ cannot.In addition,a lot of the experimental results showed that the asphalt content is higher in the S2 unit of heavy oil,heavy component relative content due to gas injection increase,may cause asphaltene deposition,so a asphaltene deposition model is established to predict the conditions of different pressure injection,asphalt deposition amount of asphaltene deposition and the damage degree of reservoir physical properties,The results show that if there is asphaltene deposit in the formation,the oil recovery will be reduced.Based on this,some suggestions and asphaltene control schemes are proposed.Finally,under the condition of the influence of gas injection on fluid,the development parameters of the well group in S2 unit are optimized,and the gas injection development scheme with CO₂ injection gas,6000m³/day best injection gas and 15MPa best production pressure is selected.The results of this thesis have clarified the change law of reservoir fluid after gas injection in fracture-vuggy carbonate reservoir,which provides theoretical support for oilfield development and has certain experience value.