Measurement of Oil-Gas Diffusivity at Reservoir Conditions for Huff-n-PuffEOR in Shales

摘要

Due to the extremely low permeability and high depletion rate,primary recovery from unconventionalreservoirs is generally low.Huff-n-puff has proved to be a successful EOR technique in tight formations,such as the Eagle Ford.However,the underlying transport mechanism remains to be completely understood.Recent studies show oil-gas diffusion is a key factor for the success of huff-n-puff EOR.Due toconcentration gradient,injected gas molecules diffuse into in-situ oil causing it to swell and consequentlyto be expelled out of the nanopores into the micro-and macro-fractures.Our research group has designed an experiment with a high pressure-high temperature cell havingobservation windows for the measurement of oil swelling and diffusivity in oil-gas mixtures and in thisstudy,we present some preliminary results.The measurements were done on a Meramec oil(API-42.7)with 3 different gas mixtures of methane-ethane,at a temperature of 175°F to evaluate the impact ofinjection pressure(above and below Minimum Miscibility Pressure-MMP)and injectate composition on oil-gas diffusivity.The diffusivity of injectate gas into oil phase as a function of pressure increases to maximumat MMP,beyond which it decreases.Using pure methane(MMP = 5500 psi)as the injectate,the diffusioncoefficient increases by 250% on increasing the pressure from 2500 psi to 5500 psi and then decreases.Based on the data available in the literature,this decrease in diffusivity can be explained by the increasein bulk fluid density and viscosity.For the oil sample used in this study,the diffusion coefficient variesbetween 10~(-10)m~2/s to 10~(-9)m~2/s,regardless of pressure and injectate composition.Tight reservoirs generally have high matrix tortuosity,which impacts the diffusion efficiency in theporous media.Using tortuosity values available in the literature and diffusivities of oil gas systems measuredin this study,we estimate that the injected gas can only travel 0.2-0.75 ft away from the fracture-facesin 1-6 months of injection.This study highlights the importance of stimulated reservoir area(SRA)characterization,nanoporous tortuosity and diffusivity measurements to optimize huff-n-puff recovery inshales.