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.

Interfacial and Wetting Properties in Shale/Methane/Water and Shale/Methane/Surfactant Systems at Geological Conditions

“…These results quantitatively agree with the limited literature data, which reported that shale samples became more water-wet after treatment with surfactant. Such behavior was linked to the ability of surfactants to manipulate the surface charge distribution in shales. , …”

“…The effect of pressure and temperature on γ water/CH 4 remains similar even after adding CTAB. Some previous studies reported that surfactants reduced γ water/CH 4 at the water/CH 4 interface, ,, although these measurements were conducted either at one pressure point only or at low pressure and temperature conditions. Shales are commonly produced through hydraulic fracturing, where a significant proportion of fracturing fluid gets retained within the shale matrix and hinders the flow of adsorbed CH 4 .…”

“…Cetyltrimethyl­ammonium bromide CTAB (99% purity), a cationic surfactant purchased in powder from Across Organics, was used to treat the shale samples. CTAB was selected as it is a typical surfactant utilized in the oil and gas industry due to its ability to reduce surface tensions and modify wetting behaviors of rocks. , Shale substrates were aged in 0.3 wt % CTAB solution for 24 h at ambient conditions. The effect of surfactant treatment on contact angle and its governing interfacial tensions was evaluated.…”

Surface analysis of liquid adsorption onto shale

“…Before performing fluid any surface analysis, organic matter in both samples were quantified by determine the total organic carbon (TOC). Pervious studies reported that TOC has significant influence on rock wettability [2,[10][11][12][13]. The contact angle measurements were then conducted, and the findings were documented as average contact angles.…”

Application of benchtop humidity and temperature chamber in the measurement of water vapor sorption in US shales from Mancos, Marcellus, Eagle Ford and Wolfcamp formations

“…Usually, an isotherm model describes adsorption equilibrium, providing useful information on pore structure and capacity (Shen et al 2018a, b). Many studies have focused on methane adsorption isotherms in shale in recent years (Abdulelah 2018;Abdulelah et al 2021;Al-Mutarreb et al 2018;Chen et al 2019a, b;Meng et al 2020;Pang et al 2021;Rexer et al 2020). However, little research has been done on water vapor adsorption on shale rocks and the feasibility of isotherm models.…”