Development Mechanism And Application Of Gas-assisted Gravity Drainage In Porous Carbonate Reservoir

Carbonate reservoirs are an important part of global oil and gas resources,water flooding and gas flooding are the main development methods,especially gas assisted gravity drainage(GAGD).In this paper,the mechanism and application of GAGD for porous carbonate reservoir in Brazil was investigated through theoretical derivation,numerical simulation,and physical experiments.We also established a method for predicting the oil recovery factor of GAGD,providing the theory support for porous carbonate reservoir in Brazil of GAGD technology.The pore throat was divided into four types based on the fractal characteristics of high-pressure mercury intrusion curves.According to the volume ratio of four type pore throat and average pore radius,the reservoir was divided into four categories.Hence,we calculated the main and minimum flow pore radius of the four types reservoirs based on the theory of minimum limit pore radius of oil seepage.The conditions for the occurrence of free gravity drainage were determined from microscopic force analysis.A new model for calculating the critical interface transport velocity of GAGD was proposed.The model was based on the minimum oil movable pore radius in the GAGD process and the theory of minimum limit pore radius of oil seepage.Therefore,we presented the critical interface transport velocity of the four types of porous carbonate reservoir in Brazil of GAGD technology.The pore-scale gas flooding experiment was conducted.The results show that the injected gas can break the force balance of the microscopic residual oil and displacement the residual oil.It was also found that the three-phase seepage mechanisms of water,gas,and oil were mainly dominated by double-displacement and film flow.The characteristics and mechanism of GAGD were clarified.The oil gas interface migrated in the form of a transition zone.The continuous oil film under gravity has a hydraulic connection to help overcome the capillary force of the small pore throat.Before the gas breakthrough,crude oil flow pattern at the low altitude of the un-swept zone was bulk flow,in contrast,it at the high altitude of the swept zone is film flow under gravity.After the gas breakthrough,crude oil flow pattern in the large pore is thin film flow driven by gravity,in contrast,it in the small pore is thickness film flow by gravity assistant hydraulic connection effect to overcome the capillary force.A numerical simulation method for pore-scale immiscible GAGD was established.The pore-scale dimension number-Npore was presented to quantitative analysis the effect of various factors for immiscible GAGD.Various factors comprehensively control the microscopic seepage process of immiscible GAGD by affecting the relative magnitudes of gravity,capillary force,viscous force,and oil-gas viscosity ratio.When the gravity number is greater than 500,the oil displacement process is greatly affected by gravity.When the gravity is less than 500,the oil recovery factor is not affected by gravity number,when it is greater than 500,the greater the gravity number,the higher the oil recovery factor.According to the heterogeneity of the reservoir,a 3D similar heterogeneous model was made,and the GAGD under high pressure was conducted.The effects of gas injection rate,gas type and injection direction on the GAGD process were investigated.The results show that the effect of gravity,viscous force,and capillary force should be considered when implementing GAGD,the effects of gas and oil interaction should also be considered.A model of oil recovery factor was established to accurately predict the oil recovery factor of GAGD in heterogeneous reservoirs.The paper clarified the development mechanism of GAGD,quantifies the influence of various factors on the development effect and provides an important theoretical basis and technical support for the application of GAGD in porous carbonate reservoirs.