Development Mechanism Study On Gas Injection In Low Permeability Sandstone Reservoir In High Water Cut Period

In recent years, along With the deepening development degree gradually and neW reserves increasing substantially for loW permeability reservoirs in most onshore oilfields in our country, further research and perfect supporting application technologies for Enhancing Oil Recovery (EOR) effectively in loW permeability reservoir has become a major strategic task for sustainable development in China’s continental petroleum industry. At present, big challenges for EOR in loW permeability reservoirs both at home and abroad, and rare successful cases for oil development on field, Which accounts for the research regarding on the gas injection development technology for loW permeability sandstone reservoir in high Water cut stage is still blank due to lack of mature theory and technical ideas. In this paper, taking Wenxi Block3in Wenmi Oilfield in Tuha Oilfield Company as an example, an indoor simulating physical simulation Was applied integrated, combining the theories and approaches of reservoir engineering and numerical simulation, the development mechanism of gas injection and characteristics of gas flooding had been understood systemically, so that, effective gas injection mobility control and key parameter optimization Were established. Finally, a complete set of technology idea and research method for EOR in loW permeability reservoir by gas injection Were proposed. The main research results are shoWn as belloW:1.This dissertation Was focused on studying the micro and macro displacement mechanism for loW permeability sandstone reservoir by gas injection. The basic mechanism of miscible flooding is that the displacement agent and displaced agent form miscible under the condition of the reservoir, so as to eliminate an interface, the capillary force in porous medium drops off to be zero, consequently, the residual oil saturation of crude oil trapped by capillary retention due to capillary effect is reduced. While the main displacement mechanism of immiscible flooding is of volume expansion, viscosity reduction and gravity differentiation. The heterogeneity of the reservoir and the compatibility of the reservoir fluid may effect on gas displacement efficiency of gas injection.2.The minimum miscible pressure (MMP) of the gas in gas reservoir is determined as33.96Mpa in Wenxi Block3through the slim displacement experiment, and the MMP of front gas is33.00Mpa. Comparing to gas driven by gas of gas reservoir, the MMP of the front gas drive is0.96Mpa lower, but its oil displacement efficiency is3.88%higher. The enrichment degree of injected gas has a great effect on miscible pressure and oil displacement efficiency.3.It is understand through oil and gas multi-contact experiment that the displacement mechanism of gas injection in Wenxi Block3presented a double component mass transfer mechanism of evaporation/condensate Which the evaporation effect Was dominated. The middle composition of produced gas shoWed the trend of decreasing first and then rising; Whether for miscible flooding or immiscible flooding, there Was a mass transfer zone for oil and gas alternating. The oil displacement efficiency of gas injection Was subjected to the area of the mass transfer zone.4.By adopting the simulation coupling of orthogonal experimental design and simulation physical parameter, the approach of Water alternating gas (WAG) of immiscible flooding Was determined for Wenxi Block3, Which could restrain the injected gas to be breakthrough in advance effectively. The parameters featured With an initiate critical temperature corresponding injected gas composition of213.25K, a reasonable injected gas slug size of0.025HCPV, gas-Water ratio of1:1and cumulative gas injection slug reaching to0.25HCPV as Well.5.The program for Enhancing Oil Recovery (EOR) by gas injection in WenXi Block3overall relied on the existing injector-producer Well pattern. The deployment of gas injection Wells Were designed With top gas injection and Water injection from the edge in the Block, and immiscible flooding Was taken as the displacement mode by Water Alternating Gas (WAG) injection Which sWitch period Was six (6) months.The daily gas injection volume for the Block Was about29.49×104m3. It predicted that oil recovery Will be enhanced6.52%by gas injection and cumulative incremental oil production Will be86.64×104t according to numerical simulation.The research fruits have been used in the preliminary study for EOR by gas injection in the secondary development campaign in Tuha Oilfield Company. The application results shoWed that the research methods are characteristic With reliable, practical and operable, and Will get good economic benefits for Oilfield after implementation. Mean While, the research ideas and the approaches described in the dissertation for EOR by gas injection are used especially for loW permeability sandstone reservoir in high Water cut period. It is significant for the reference and theoretical guidance to the development and research of similar reservoirs in other areas in our country.