Research On Mechanism And Scheme Adjustment Of Assisted Gravity Flooding By Hydrocarbon Injection Gas In DH Oilfield

DH oilfield is a typical anticlinal structural reservoir with deep burial depth,large dip angle and thick reservoir.The development status shows that water injection causes high water cut in low part of reservoir and high remaining oil saturation in high position.The horizontal displacement of upper sandstone formation is obvious,while the oil increase effect of lower sandstone formation is weak.So hydrocarbon gas-assisted gravity drainage(GAGD)is proposed according to geological characteristics.However,this technology has not been well applied in China,so it is quite necessary to study GAGD's displacement mechanism,so as to provide a certain theoretical basis and reference significance for formulating a reasonable development plan.Based on the phase behavior experiment between hydrocarbon-injected gas and formation fluid,this paper establishes a mechanism model by combining laboratory experiment and numerical simulation.The phase state characteristics,miscibility mechanism of hydrocarbon-injected gas drive and gravity displacement mechanism in target reservoir are analyzed.At the same time,the main factors to affect gas-assisted gravity drainage are studied.Finally,the development plan is designed and the effect is evaluated.The main research results are as follows:(1)The reservoir fluid is characterized by low viscosity and low gas-oil ratio.With the increase of injected hydrocarbon gas content,the saturation pressure of the fluid system increases greatly,while the density and viscosity decrease linearly.When the molar content of injected gas is 20.79%,the oil viscosity decreases from 12.74mPa·s to 1.21mPa·s,and the viscosity-reducing range reaches 90.5%.This shows that Gas injection can effectively improve the physical properties of crude oil.(2)The minimum miscibility pressure of hydrocarbon gas injection is about 44MPa,lower than the current formation pressure.Therefore,miscibility between hydrocarbon gas injection and current formation fluid system can be achieved,which can greatly improve oil recovery.(3)The miscible phase mechanism research indicates:the higher injection pressure,the shorter transition zone under the same injection volume;the larger of the injection volume,the larger swept region would be under the same injection pressure.Then higher oil recovery can be achieved.(4)The mechanism of gas-assisted gravity drainage mainly includes gravity differentiation,stable displacement interface,volume expansion of dissolved gas in oil,reduction of interfacial tension between two phases and improvement of fluid flow.(5)The main factors affecting the effect of gas-assisted gravity flooding include reservoir parameters and process parameters.The greater permeability,the better vertical permeability,the thicker oil layer thickness and the bigger dip angle can lead to the more stable gas-drive interface and the larger swept volume,then the higher oil recovery is achieved.There are optimum gas injection rate and oil recovery rate in hydrocarbon gas-assisted gravity drainage.(6)Aiming at the contradiction of gas injection in DH reservoir,some means can be used,like perforations adding,acidizing and drilling new wells to expand the scope of gas drive;higher gas injection intensity and perfect injection-production patterns nets to highlight the function of gravity drive.Numerical simulation shows that the recovery degree of crude oil at the end of the prediction period is 47.88%,which is 8.37%higher than that of the basic scheme.