| The Tahe fractured-vuggy reservoir has strong heterogeneity which results in complex reservoir fluid flow law.The reservoir mainly relies on natural energy recovery in the early stage.As the extraction progresses,some wells have a tendency of insufficient energy supply,increased water content,and reduced production.The injection of water does not increase the flow channel on the basis of natural water flooding.The water content rises rapidly along the high flow channel which makes it easier to produce flat linear flooding.The gas injection development has been implemented in Tahe Oilfield.The results of laboratory experiments and mines indicate that gas injection can increase the flow channel on the basis of water injection and increase oil recovery effectively.However,the Tahe fracture-vuggy reservoir has a large thickness and poor connectivity between wells which resulting in the enrichment of residual oil in the middle/lower horizon of the well group after gas flooding.Gas-assisted gravity flooding can make full use of these advantages.Stable displacement can be achieved by optimizing injection-production parameters.Oil recovery can be further improved on the basis of gas flooding.But there are still two problems.First,the remaining oil distribution of different karst reservoirs is unclear after gas injection in the existing well pattern.Second,the mechanism and displacement law of nitrogen-assisted gravity flooding are to be studied.For the existing problems of fractured-vuggy reservoir,this paper designed conceptual model and two-dimensional model of different karst system.The karst system includes superficial karst belts,palaeo-subterranean river system and karst-fault system Then the fabricating work was done.this paper used the improved physical simulation experiment method in carrying out experiments.Thus,the type and distribution of remaining oil after gas injection were analyzed.The remaining oil mechanism in nitrogen-assisted gravity experiments was studied.The main conclusions and understandings are as follows:Gas energy is concentrated at bottom of the gas injection well.the bottom water is suppressed to the bottom of the production well.The cross-well gas is laterally displaced.The flooding is synergistic with gas and water on the whole.The remaining oil is mainly enriched in gas injection well,bottom of production well and between them after gas flooding.The remaining oil after gas injection in the palaeo-subterranean river system is mainly focused on blind end caves and outside of well control areas.The middle of dense filling section is rich in remaining oil.The distribution of remaining oil after gas flooding in karst-fault system is controlled by faults,which is similar to distribution of remaining oil in a single cave.It presents a distribution of “upper gas middle oil lower water”.Bottom water is suppressed by injection gas in the early stage of nitrogen assisted gravity flooding.The flooding is synergistic with gas and water on the whole,which makes it more stable.The effect of karst fault system in the three different karst reservoirs is best.The high gas injection is suggested in selection of gas injection location.The gas injection rate shall be controlled reasonably when bottom water is weak.In addition,small/lower fractures are easily shielded for superficial karst belts and palaeo-subterranean river.However,the fracture connection position in karst-fault system determines the degree of gas flooding.The formula for calculating the minimum fracture scale of gas flooding and water flooding is established.It is based on the force analysis of capillary force,gravity and displacement force.Studies have shown that the gas injection is mainly between wells when the fracture-vuggy structure is complex.The gas injection is mainly in bottom of production/injection well when the fracture-vuggy structure is simple. |
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