Field gas huff-n-puff for enhancing oil recovery in Eagle Ford shales – Effect of reservoir rock and crude properties

Abstract

• A unique experimental design for investigating oil extraction from shales by huff-n-puff process. • Utilization of field gas as a solvent for enhancing shale oil production. • Oil extraction efficiency could decrease with gas-oil ratio despite increasing rock porosity. • Oil viscosity strongly influences produced oil composition and recovery factor. Field gas huff-n-puff has a great potential for enhancing oil recovery in shale reservoirs that suffer from dramatic production decline. However, most previous studies have focused on CO 2 huff-n-puff, and a simplified injection hydrocarbon gas composition has been commonly used in nearly all experimental works. This work is a continuation of our previous study on the field gas huff-n-puff process for enhancing oil production from Eagle Ford shales with a focus on the impact of reservoir rock (porosity, permeability, and S 1 content) and crude oil properties (viscosity, density, composition). A unique huff-n-puff process was designed to investigate the characteristics of oil production during soaking and pressure depletion. The efficiency of oil extraction as well as the associated production mechanisms were evaluated based on the ultimate recovery factor and properties of produced oil. Huff-n-puff experiments were conducted on reservoir core plugs with distinct reservoir properties (i.e., porosity, permeability, and S 1 contents). It was found that an increase in Gas Research Institute (GRI) porosity from 4.3% to 12.9% could improve the oil recovery factor from 49.0% to 62.8%. However, such a trend was not observed in the huff-n-puff experiments with a higher OOIP where an increase of GRI porosity from 5.6% to 12% led to a decrease in recovery factor by 24.7%. This remarkable finding could be related to the decrease of gas-oil ratio as OOIP increased at a constant injected mass of gas. It was also found that the average URF decreased from 63.0% to 38.3% with an increase in oil viscosity from 3.56 to 9.59 cp, which could be attributed to the reduced diffusive mass transfer and phase partitioning of components from injection gas and resident oil. This work has advanced our understanding of how rock and oil properties affect oil the efficiency of oil extraction by cyclic gas injection in Eagle Ford shales. It also supports the development of reservoir screening criteria for a preliminary assessment of the suitability of an unconventional reservoir for this EOR method. Finally, the results present a unique high quality data set for further modelling work.