Fracture complexity, phase behavior, lithological variations and diffusion of gas from the fracture into the oil-saturated nano-pores are the main contributing factors in oil recovery using gas huff-n-puff injection. Limited research was conducted to define diffusion coefficients coupled with the rock tortuosity. The objective of this work is to conduct a comprehensive experimental and simulation study on Lower Eagle Ford rock samples to measure the diffusion coefficients for different injection cycles in three representative litho-facies.

Three representative rock samples were selected based on their differences in petrophysical properties. Saturated volumes were measured using a low-field nuclear magnetic resonance (NMR) measurement and confirmed with material balance for cores saturated at reservoir conditions. Pressure was recorded during a one-day diffusion process before it was dropped linearly at the end of each cycle for production, and the effluent oil and gas composition were measured. NMR measurement was repeated at the end. A compositional simulation model was set up using tortuosity values from FIB-SEM analysis to simulate the experimental diffusion and production. History matching on pressure and production results was conducted and diffusion coefficients were estimated for one representative sample.

Pressure profiles vary significantly between different cycles due to different effective diffusion coefficients. This may be caused by invasion of the gaseous phase into a new section of the pore network during each cycle. Diffusion coefficients, represented by pressure drop during the soaking time, vary across different litho-facies and for different cycles. For the produced oil, the concentration of lighter oil components declined from the first to the last cycle of gas injection while the concentration of the intermediate and heavier components increased.

Gas huff-n-puff injection into shale oil reservoirs is being investigated from the point of view of diffusion and variations in rock properties for the first time and measurements were validated using numerical simulation. The huff-n-puff experiments show favorable results, using constant volume diffusion cell with locally produced hydrocarbon gas and stock-tank oil, the recovery factors for samples A, B, and C are 57.5%, 56.7%, and 51.7%, respectively. The history matched oil diffusion coefficients are in the range of ten to the power of negative seven, and are in close relation with the remaining oil composition.

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