Effect of Anionic Surfactant on Wettability of Shale and Its Implication on Gas Adsorption/Desorption Behavior

Abstract

During the fracking process in shale, an interaction occurs between shale and fracking fluid that contains a cocktail of chemicals. One of the chemicals used in fracking fluid is often surfactant, which is generally used as a viscofier. However, surfactants also have the potential of significantly influencing the wettability and thus gas desorption−key factors affecting ultimate gas recovery from shale reservoirs. Even though a few studies discussed the ability of surfactants to alter wettability in shale, the implication of that change in adsorption/desorption behavior has never been experimentally investigated beyond hypothetical inferences. In this study, the influence of the wettability change by anionic surfactant on gas adsorption/desorption behavior in shale was investigated through a series of experiments. Baseline wettability readings of two shale samples were established by measuring the contact angles (BG-1 = 22.7°, KH-1 = 35°) between a drop of pure water placed on their polished surfaces, indicating that the affinity of pure water for the BG-1 surface was greater than that for KH-1. This difference can be attributed to the higher clay content and lower total organic carbon found in BG-1 as compared to KH-1. To investigate the impact of the interaction between shale and surfactants on wettability during the fracking process, we measured the contact angles again, this time with 1 wt % solution of internal olefin sulfate surfactant. The surfactant-induced wettability changes of the two shale samples were investigated by measuring the contact angles again (BG-1 = 3.5°, KH-1 = 19.2°) between a drop of surfactant solution and their polished surfaces. The effect of wettability changes on gas adsorption/desorption was then evaluated utilizing the United States bureau of mines' modified method. Experiments were conducted on the two shale samples in two ways: after pure water treatment, and after surfactant treatment. The results suggest that due to the wettability alteration of the two shale samples by IOS surfactant toward more water-wet during the treatment, the methane adsorption/desorption characteristics were influenced. In BG-1 sample, IOS solution dramatically changed its wettability to become completely waterwet. Therefore, the volume of desorbed methane dropped by nearly 54%. A similar but less pronounced influence was found in the KH-1 sample, where its desorbed methane dropped by 10% because of wettability alteration toward more water-wet. These reductions in the amount of desorbed gas suggest that prior to selecting a surfactant for addition to fracking fluid, its effect on wettability and gas desorption should be investigated to optimize shale gas recovery potential.

Influence of Water Immersion on Pore System and Methane Desorption of Shales: A Case Study of Batu Gajah and Kroh Shale Formations in Malaysia

“…The objective of this test was to evaluate the influence of water immersion on shale's desorption characteristics and compare it with the moist conditions. The results of the water immersed samples have been reported in our previous work, and utilized in this work for the sake of analysis [1]. Based on Figure 9a, the desorption volume in the BG sample was 0.94 cc/g and 0.60 cc/g for the immersed and moist samples, respectively.…”

“…Such differences in the water uptake of the samples were related to the hydrophilic and hydrophobic nature of the clay minerals and organic matter, respectively [16,43,44]. In addition to that, in our previously published work on the same samples, the BG sample was more water wet than the KH sample [1]. Therefore, we can correlate the high moisture and immersed water uptake of BG when compared to KH due to the large amount of clay in the BG and of organic carbon in the KH samples.…”

“…The second sample was collected from the Batu Gaja formation from the Devonian age (ID-BG) [24]. Some characteristics of the samples were reported in our work published previously [1]. The shale samples were collected from outcrops located at the latitude and longitude shown in Table 1.…”

“…In this study, the experimental set-up used is shown in Figure 1, which was also used in our previously published work [1]. The basic principle of desorption measurement is based on mass balance's law, and we used an adsorption-desorption unit to perform the measurement.…”

“…The growing energy demand and the recent advances in stimulation techniques including horizontal well drilling and hydraulic fracturing have made shale gas a significant source of energy around the world [1,2]. Shale gas has been in the spotlight as a new unconventional fossil-fuel energy source.…”