An Experimental Study on the Effects of Competitive Adsorption During Huff-N-Puff Enhanced Gas Recovery

Abstract

Gas is stored in tight reservoirs both as a free gas occupying the pores, and as an adsorbed gas on the rock matrix. Adsorbed gas exhibits liquid-like densities resulting in significantly more gas being stored on the rock surface. This research aims to highlight the effects of competitive adsorption during Huff-n-Puff enhanced gas recovery (EGR) on activated carbon to achieve maximum gas recovery. Pure methane was initially adsorbed by the activated carbon sample in four simple pure component adsorption stages. The methane was then produced in a primary production stage, allowing some methane to desorb from the activated carbon. The free and adsorbed methane was then displaced in five subsequent cyclical injection/production stages with a displacing gas, either nitrogen or carbon dioxide. The experiments were conducted at 30 °C, 45 °C, and 80 °C, and the temperature was maintained using a water bath. The purpose of testing a variety of temperatures was to highlight the effect of temperature on competitive adsorption and recovery factors. From the experiments, adsorption capacity was plotted as a function of the isothermal pressure and methane composition. This data was then fitted with the Extended Langmuir model because of its popularity and simplistic approach for multicomponent gas mixtures. It was observed that total adsorption capacity decreased as a function of temperature for both the nitrogen and carbon dioxide displacement experiments. Selectivity ratios were also determined for each experiment. At all temperatures, carbon dioxide had a higher selectivity ratio over methane compared to the selectivity ratio between nitrogen and methane. Selectivity ratios did not correlate with changing temperatures in both sets of experiments. Recovery factors were also determined for each experiment. Incremental recovery factors progressively decreased with each subsequent production stage. Cumulatively, the carbon dioxide experiments exhibited higher recovery at each temperature tested. For these experiments, irreversibilities were not considered due to the authors’ previous experience with single-component adsorption and desorption experiments on activated carbon [1]. To date, there have not been any EGR Huff-n-Puff experiments conducted on highly porous activated carbon samples with a primary focus on the effect of competitive adsorption. This research aims to highlight the effects of temperature and displacement gas type on the competitive adsorption between methane and nitrogen/carbon dioxide and its impact on the recovery factors. By doing so, EGR schemes can be better understood and modeled with improved inputs for competitive adsorption in each injection and production cycle. This will allow for more accurate production forecasting and help minimize the financial risk of costly EGR projects.