Surfactant Adsorption on Shale Samples: Experiments and an Additive Model

Adsorption of surfactants in shales is not well studied. The goal of this work is to quantify and understand surfactant adsorption in several shale samples. Shale samples were obtained from several formations and are referred to by the formation names; but considering the fact that these formations are highly heterogeneous and huge, these few samples do not represent the shales. Shales are multimineral substrates with pores in the range of 1-300 nm. The adsorption capacity of three surfactants (cationic, nonionic, and anionic) on an Eagle Ford reservoir shale sample was measured. The cationic surfactant, cetyltrimethylammonium bromide (CTAB), showed the highest adsorption capacity in molar units, followed by anionic internal olefin sulfonate (IOS) C15-18 and then nonionic NP-40s. CTAB also had the highest adsorption in mass units, followed by NP-40 and then IOS. Adsorption of the anionic surfactant onto two pure minerals (calcite and quartz) and six shales were investigated, and all of them showed Langmuir-type sorption. The adsorption capacity of calcite and quartz are about the same (similar to 1.1 mg/g of rock). The adsorption capacity of shales depends on the mineral composition. The adsorption in Mancos outcrop and Green Shale samples is dominated by clay, whereas that in Wolfcamp and Eagle Ford outcrop shale samples is dominated by calcite. The adsorption in Eagle Ford (reservoir) and Marcellus shale samples is dominated by total organic carbon (TOC). An additive model was built to estimate the adsorption capacity, given the mineral composition and TOC. The model shows that organic matter and clay have the most significant impact on adsorption per unit mass; the contribution of each shale component on adsorption depends on its corresponding mass fraction.