EOR Technology And Related Mechanism Research Of Gas Injection To Fractured-cavity Carbonate Reservoir

Tahe Oilfield is a special type of reservoir in China,and the fractured-cavity carbonate reservoir is buried deep,developed under reservoir conditions with high temperature and high pressure.With various fracture networks and cavities as reservoir spaces,complicate structures with strong heterogeneity and sophisticated flow relationship between oil and water,such reservoirs generally rely on natural depletion at early stage,and water flooding during middle process demonstrates transient water-free production and inefficient water utility in recovery.Gas injection after water flooding process achieved great results in oil increasing production in the oilfield experiments.Yet the distribution characteristics of residual oil after water flooding,the types of residual oil recovered by gas,the types of injected gas and the related mechanism of enhanced oil recovery remains poorly understood.So it is necessary to establish a systematical physical simulation technology of the fractured-cavity reservoirs,and then to propound specific EOR methods and theories in fractured-cavity reservoirs by gas injection,thus providing theoretical support for oilfield application.Firstly,for the uncertainty in distribution of the residual oil after water flooding,both 2-dimensional and 3-dimensional visualization models are designed along with the corresponding simulation systems of physical experiments.By conducting bottom water flooding in the above models,seven main types of residual oil are summarized and analyzed:residual oil from water coning,residual oil within the blind hole,residual oil in packed sediments,attic oil,lateral oil,wrapped oil and oil film.Based on the types and distributing characteristics of residual oil,particular displacing medium are recommended to start the residual oil:water flooding,polymer flooding,surfactant water,N₂ flooding,CO₂ flooding,and composite gas?N₂+CO₂?flooding.Corresponding studies on laws of mobilizing residual oil are also carried out.Experimental results show that increased recovery by gas injection is much higher than that of polymer flooding and surfactant water,and that gas can supply reservoir energy and enlarge the swept volume,which effectively mobilizes residual oil to some extent.Secondly,the common pattern of developing fractured-cavity carbonate reservoir is natural depletion at first,water injection to supply natural energy at second stage,and gas injection for enhanced oil recovery?EOR?after the failure in water flooding.In order to minimize the marginal effect in panel model experiments,and to restore real fluid flow patterns under reservoir conditions,a pressure-endured 3-dimensional model is devised.And method optimization experiments of reservoir energy supplied by water flooding post bottom water flooding as well as EOR experiments of gas injection?N₂,CO₂,and composite gas?under different pressures are carried out.Results in water flooding optimization shows that the EOR of periodic water flooding and pulse water flooding are higher than that of constant rate water flooding.Due to the total water injection of periodic water flooding is less than that of pulse water flooding,the optimized energy supplement method is periodic water flooding.Results in gas injection experiments demonstrates:?1?higher injection pressure achieves larger volume of injected gas;?2?increased recovery factor by N₂ flooding decreases with the rise of pressure,and results in the other two gases?CO₂ and composite gas?are opposite;?3?the final recovery of composite gas flooding is higher than that of single gas flooding under 8 MPa gas flooding.The composite gas flooding takes the advantage of both gas since high pressure facilitates the CO₂ EOR effect and inhibits N₂ EOR effect,demonstrating synergistic effects.In gas flooding experiments,single well works as production unit by taking full consideration of the effect of well interference on flooding results,and the production performance changes as the alteration of injecting pressure and types of gas.Theoretical researches in combination with PVT experiments shows that gas flooding by N₂ and composite gas is non-miscible flooding while CO₂ flooding is miscible flooding under reservoir condition in Tahe Oilfield.Based on the results of visualized physical simulation experiments,the mechanisms of mobilizing residual oil by gas flooding are as follows:N₂ flooding,?1?formation energy supplement,replacing residual oil in high portion by gravity segregation and?2?displacing residual oil deposited in narrow fractures which water is not able to enter by the lower interfacial tension between oil and gas than that between oil and water.CO₂ flooding,?1?viscosity reduction,?2?expanding by dissolved gas,?3?dissolved gas flooding.The CO₂ displacing efficiency is directly related to gas solubility in the crude oil,and higher pressure results in higher gas solubility,leading to higher gas utility.Composite gas flooding integrates EOR mechanism in both CO₂ and N₂,not only mobilizing residual oil in the upper portion by gravity segregation from N₂,but also reducing oil viscosity by dissolved gas under high pressure from CO₂,as a result,achieving a favorable oil recovery result.