Mechanisms for kerogen wettability transition from water-wet to CO2-wet: Implications for CO2 sequestration

Juan Zhou,Jiajun Zhang,Jiapei Yang,Zhehui Jin,Kai H Luo

doi:10.1016/j.cej.2021.132020

Juan Zhou, Jiajun Zhang + Show 3 more

Open Access

https://doi.org/10.1016/j.cej.2021.132020

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Abstract

Geological CO2 sequestration (GCS) is an essential building block of the global strategy to alleviate greenhouse gas emissions and mitigate the climate change. Injecting CO2 into the shale formations can not only reduce carbon emissions but also enhance oil recovery (EOR). Rock wettability is of great importance to CO2 storage as it determines the efficiency of structural and residual trapping of CO2 and plays a crucial role in CO2-EOR. In this work, molecular dynamics (MD) simulations are adopted to investigate the CO2-H2O-kerogen systems under various CO2 pressures. In a vacuum or under low CO2 pressures, kerogen surface is weakly water-wet thanks to the hydrogen bonding between H2O and kerogen. As CO2 pressure increases, kerogen wettability shifts from water-wet to CO2-wet, because more CO2 molecules accumulate at the H2O-kerogen interface and a distinct CO2 thin film emerges. Density functional theory (DFT) calculations reveal that the O-containing functional groups preferably adsorb H2O molecules over CO2 through hydrogen bonding, which is responsible for the weakly water-wet tendency at low CO2 pressures. In contrast, the carbon skeleton of kerogen exhibits a stronger affinity to CO2, leading to the formation of CO2 thin film on the kerogen surface. The CO2 crowding close to the kerogen surface at high CO2 pressures gives rise to the CO2-wet state. This study provides, for the first time, the fundamental mechanism for the kerogen wettability transition from water-wet to CO2-wet. The work also indicates that wettability of the mature kerogen is more likely to be CO2-wet during GCS, which is unfavorable for capillary trapping of CO2, but is favorable for CO2-EOR.

Highlights

Since the Industrial Revolution, the atmospheric CO2 concentration has been climbing, reaching the level of ~420 ppm as of May 2021, which is ~50% above the pre-industrial level found in 1850 [1]
In a vacuum or at a low CO2 pressure, the CO2-H2O-kerogen contact angle is mainly determined by H2O-kerogen and H2O-H2O interactions which can be revealed from Density functional theory (DFT) as well as molecular dynamics (MD) simulations, while it is greatly affected by CO2 crowding driven by the hydrophobic interactions at a high CO2 pressure which can only be captured by MD simulations
The effect of CO2 pressure on the CO2-H2O-kerogen contact angle is investigated through MD and DFT simulations

Summary

Introduction

Since the Industrial Revolution, the atmospheric CO2 concentration has been climbing, reaching the level of ~420 ppm as of May 2021, which is ~50% above the pre-industrial level found in 1850 [1]. There are four main geological CO2 storage mechanisms: structural trapping, where a caprock acts as a seal barrier preventing CO2 from migrating to shallower zones [7]; residual or capillary trapping, where CO2 is immobilized by capillary forces in pores and narrow throat in formation rocks [8]; solubility or dissolution trapping, where CO2 dis solves in the formation water [9]; mineral trapping, where CO2 reacts with rocks and formation water, forming carbonate minerals [10] Among these mechanisms, the structural and residual trapping mecha nisms are dependent on rock wettability, in which the CO2-H2O-rock contact angle plays a crucial role [11]. A large amount of free CO2 gas is trapped by a high capillary force which is strongly dependent on the CO2-water-rock contact angle, while a waterwet rock is generally favored [12,13,14,15]

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