Trapped Gas Saturation Measurement in 2-phase and 3-phase Experiments to Support CCS and CO2 EOR Projects in Carbonate
Authors: Masalmeh Shehadeh , Al-Mesmari Ali , Farzaneh S. Amir , Sohrabi Mehran 
Published: October 2022

Trapped gas saturation (Sgt) is an important parameter for both CO2 storage (CCS) and Enhanced Oil Recovery (EOR) processes. Trapped gas saturation is affected by different parameters such as rock typing, wettability, saturation history, sequence of water and gas injection cycles, etc. This paper presents a comprehensive experimental study that aimed at measuring trapped gas saturation using carbonate rocks to provide measured data in support of CCS and EOR projects in carbonate reservoirs.

The trapped gas experiments were performed using limestone reservoir cores taken from two different reservoirs, B and G. The experiments were performed under different injection strategies to investigate the impact of rock type, wettability and injection sequence. Moreover, the study compares trapped gas under 2-phase and 3-phase flow conditions. The study was performed using different gases, including methane and CO2 and also investigated the impact of the type of displacing fluid (water or oil) on trapped gas saturation. All experiments were performed using vertically oriented reservoir cores and both the gas and liquid phases were injected from the top of the core.

A comprehensive data set has been produced in this study by performing both 2-phase and 3-phase coreflood experiments. The 3-phase experimental data showed that: 1- Trapped gas saturation is strongly dependent on the order of fluid injection sequence, Sgt is much higher during WAG experiments starting with water injection (WAG_W) compared to WAG experiments starting with gas injection (WAG_G), 2- A significant difference was measured between the trapped gas saturation in the core from reservoir G compared to the core from reservoir B, even though reservoir G has a higher permeability than reservoir B, 3- Trapped gas saturation measured using samples initialized at mobile water saturation (to mimic transition zone) increases as the initial water saturation increases and 4- Trapped gas saturation measured in WAG_G experiments starting at mobile water saturation is higher than that measured in WAG_G experiments starting at connate water, even though the initial gas saturation in the second experiments is much higher.

The 2-phase experiments included both oil-gas and water-gas systems in order to investigate whether trapped gas was affected by the type of gas (methane or CO2) or by the displacing phase (water or oil). The results of the 2-phase experiments showed that: 1- Sgt is much higher in 2-phase compared to 3-phase experiments, 2- The water-gas experiments showed that trapped CO2 is higher than trapped methane at high Sgi with the trapped CO2 being as high as 50%, 3- Oil-gas experiments showed that trapped methane is higher than trapped CO2 and 4- Trapped CO2 by water is higher than trapped CO2 by oil, however, trapped CH4 was similar and independent of the displacing phase.

The comprehensive dataset presented in this paper is rarely available in the literature and is vital for validating the predictions of gas injection and storage models available in commercial simulators. The results of this study demonstrated that Sgt obtained in 2-phase flow experiments is not applicable to 3-phase flow conditions. The study also shows that capillary trapped gas in CCS projects is much higher than trapped gas in CO2 EOR projects. Finally, CO2 WAG projects can be designed to optimize CO2 utilization factor when starting with gas injection or optimize CO2 sequestration when starting with water injection.