Investigating the role of diffusion in hydrocarbon gas huff-n-puff injection-an Eagle Ford study

Fracture complexity, variation in fluid properties, variations in rock types 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 has been conducted to define diffusion coefficients coupled with rock tortuosity resulting in calculation of effective diffusion. The objective of this work is to conduct a comprehensive experimental and simulation study on lower Eagle Ford rock samples to measure diffusion coefficients for different injection cycles in three representative rock samples from the main flow unit. 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 effluent oil and gas compositions were measured. NMR measurement was repeated at the end. Three compositional simulation models were set up for three different samples using tortuosity values from FIB-SEM analysis to simulate experimental diffusion and production. For history matching, relative permeability curves, fracture properties, and tortuosity were tuned to match pressure and production results for three representative samples. Pressure profiles vary significantly between different cycles caused by invasion of the gaseous phase into a new section of the pore network during each cycle. Effective diffusion coefficients were calculated using representative tortuosity values. Diffusion was represented by pressure drop during soaking time. This pressure drop varies across different lithofacies and for different rock samples. For 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 components increased. Gas huff-n-puff injection into shale oil reservoirs were 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, conducted using a constant volume diffusion cell with locally produced hydrocarbon gas and stock-tank oil, show favorable results. Recovery factors for samples A, B, and C are 57.5%, 56.7%, and 51.7%, respectively.