Mechanism of CH4 Sorption onto a Shale Surface in the Presence of Cationic Surfactant

Abstract

A substantial amount of water used for fracking shale formations is trapped by capillary and interfacial forces. Such trapped water is detrimental to gas production because of its potential to obstruct gas’s desorption and, subsequently, its flow path. Surfactants are proposed to alleviate the problem; however, further insight is required to understand the underlying mechanism. In this study, a cationic surfactant, namely, cetyltrimethylammonium bromide (CTAB), and a clay-rich Marcellus shale are used to investigate and explain the mechanism. The study encompasses a series of systematic experiments and molecular simulations. First, laboratory measurements of CH₄–brine interfacial tension, CH₄ surface excess, and zeta potential at different CTAB concentrations were conducted. Then, we evaluated CH₄ adsorption in Marcellus shale before and after treatment with CTAB. Second, a molecular dynamics simulation by GROMACS software was used to explain the phenomenon at the molecular level. Experimental results indicated that CTAB reduced the CH₄–brine interfacial tension by up to 80%. The zeta potential data showed that shale’s dominant surface charge was altered from negative to positive after treatment with CTAB. Furthermore, the presence of CTAB has significantly influenced the distribution of CH₄ in the aqueous phase as indicated by the changes in the CH₄ surface excess concentration. Moreover, the adsorbed CH₄ amount decreased with increasing CTAB concentration when the CTAB concentration was kept below the critical micelle concentration (CMC). The reduction in adsorbed CH4 was explained by the molecular dynamics simulation results, which revealed a 62% shrinkage in vertical distances between CH₄ molecules and clays after introducing CTAB. Simulation findings also unfold that CTAB has reduced the density distribution of CH₄ molecules along with clay layers by 64%. One of the more significant result of this study is that surfactants injected at above CMC values can lessen fracking water trapping by reducing CH₄ brine interfacial tension, changing surface charges, and reducing molecular distances between CH₄ and hydrophilic clays.