Study On Gas Injection Techniques And Mechanisms For Water Control And Oil Increment In Edge-Bottom-Water Flock- Block Oil Reserivoirs

Edge-bottom water encroachments usually occur during the development of complex flock-block oil reservoirs with infinite or finite aquifers.The quick and sever water invasions can lead to low individual well producing rate,and finially cause low oil recovery of the reservoirs.Water control and oil increment?WC&OI?can be achieved by injecting gas into reservoirs;however,the mechanisms of gas-WC&OI still remain unclear.Based on typical edge-bottom-water flock-block oil reservoirs in Jidong Oil Field,China,gas injection eperiments and simulations including CO₂ injection,N₂ injection,mixed gas?CO₂ + N₂?injection,and chemical assisted gas injection?foams + gas,gel + gas?are studied to reveal the mechanisms and technical adaptability for gas-WC&OI in this paper.The research methods include PVT analysis,one dimensional?1D?laboratory experiments,three dimensional?3D?laboratory experiments,and numerical simulations using base reservoir models.PVT analysis show that the influence rules for volume factor and oil viscosity are CO₂ > mixed gas > N₂,and for saturation pressure is N₂ > mixed gas > CO₂.Compared with N₂,CO₂ shows better capacities for oil swelling and viscosity reduction,while mixed gas are between CO₂ and N₂.Dissolving capacity of different gas in liquids is CO₂ > mixed gas > N₂,while for the same gas in different liquid is oil > mixture > water.Gas sobubility formulas in oil,water and oil-water mixture are then derivated according to liquid and gas components,which can be used to determine sobubility for different kinds of gas into mixture under any oil-water propotion.The interaction mechanism between injecting gas and formation fluids can be used to reveal the mechanisms for gas-WC&OI.1D experimental results for gas-WC&OI show that three types of gas are formed after gas injected into the formation: oil-phase solution gas,water-phase solution gas and free gas.Water control is attributed to the dissolving capacity by water-phase solution gas and energy supplement by free gas,while oil increment is attributed to the capacity of oil swelling and viscosity reduction by oil-phase solution gas.During the later period development with water cut of 98%,40⁵0% of free gas can be formed with CO₂ injection,and its effect on water control cannot be ignored.However,little gas is dissolved in oil and water with N₂ injection,and the effect of WC&OI by N₂ is mainly due to energy supplement by free gas.WC&OI mechanisms for mixed gas are the comprehensive effects of CO₂ and N₂.With strictly design of CO₂/N₂ ratio and injecting mode,significant effect of WC&OI can be achieved by combining the advantage of CO₂ and N₂ during composite gas injection.CO₂ or composite gas with the ratio of 9:1 can be used for WC&OI in middle-high permeability layers,while N₂ or composite gas with the ratio of 1:1 can be used for WC&OI in low permeability layers.3D gas injection results for WC&OI in horizontal well group show that synergetic effect can be observed between interwell after gas injecting into the well group,then Horizontal-well Synergetic Gas Injection?HSGI?,and Horizontal-well Mixed Gas Injection?HMGI?are proposed for WC&OI.For HSGI and HMGI,CO₂,N₂ or mixed gas can be injected into formation through a couple of horizontal wells with careful design of gas type,injection mode,and injection allocation.After gas is injected into the formation,interwell residual oil can be effectively displaced by the gravity segregation,natural water flooding,and secondary gas flooding.Simulation results of base reservoir models reveal that major effect factors for HSGI and HMGI include gas injection volume and its allocation,injection timing,distance between well group and acquifers,and well spacing.HSGI technique is preferred to be applied in middle-high permeability oil reservoir,high inclination oil reservoir,thin oil reservoir,etc.While,HMGI can be used in low,middle,and high permeability oil reservoir,and more appropriate in low permeability oil reservoir for WC&OI.For the purpose of gas-WC&OI for fractured oil reservoirs,Horizontal-well Chemical Assisted Gas Injection?HCAGI?is also proposed in this paper.After water blocking using foam or high-stregth gel,the edge-bottom-water encroachment is effectively delayed along the channels and fractures,and the following gas can be then injected into the rock-matrix by enlarging gas sweeping volume.HCAGI can further be subdivided into Horizontial-well Foam assisted Gas Injection?HFAGI?and Horizontialwell Gel assisted Gas Injection?HGAGI?according to the level of water channels.HFAGI technique is more preferred to be applied to the reservoir with permeability rush coeffiicient ? 30,and HGAGI is more effective for WC&OI in the reservoir with permeability rush coeffiicient > 30.