Abstract
Hydraulic fracturing is an effective technology for coal reservoir stimulation. After fracturing operation and flowback, a fraction of fracturing fluid will be essentially remained in the formation which ultimately damages the flowability of the formation. In this study, we quantified the gel-based fracturing fluid induced damages on gas sorption for Illinois coal in US. We conducted the high-pressure methane and CO2 sorption experiments to investigate the sorption damage due to the gel residue. The infrared spectroscopy tests were used to analyze the evolution of the functional group of the coal during fracturing fluid treatment. The results show that there is no significant chemical reaction between the fracturing fluid and coal, and the damage of sorption is attributed to the physical blockage and interactions. As the concentration of fracturing fluid increases, the density of residues on the coal surface increases and the adhesion film becomes progressively denser. The adhesion film on coal can apparently reduce the number of adsorption sites for gas and lead to a decrease of gas sorption capacity. In addition, the gel residue can decrease the interconnectivity of pore structure of coal which can also limit the sorption capacity by isolating the gas from the potential sorption sites. For the low concentration of fracturing fluid, the Langmuir volume was reduced to less than one-half of that of raw coal. After the fracturing fluid invades, the desorption hysteresis of methane and CO2 in coal was found to be amplified. The impact on the methane desorption hysteresis is significantly higher than CO2 does. The reason for the increasing of hysteresis may be that the adsorption swelling caused by the residue adhered on the pore edge, or the pore blockage caused by the residue invasion under high gas pressure. The results of this study quantitatively confirm the fracturing fluid induced gas sorption damage on coal and provide a baseline assessment for coal fracturing fluid formulation and technology.
Highlights
Gas production from coal provide clean energy, and improve the safe mining when coal is being mined (Yuan 2016; Cao et al 2017; Zou et al 2021; Tian et al 2020)
The results show that there is no significant chemical reaction between the fracturing fluid and coal, and the damage of sorption is attributed to the physical blockage and interactions
The impact on the methane desorption hysteresis is significantly higher than CO2 does
Summary
Gas production from coal provide clean energy, and improve the safe mining when coal is being mined (Yuan 2016; Cao et al 2017; Zou et al 2021; Tian et al 2020). The raw coal was soaked in 500 g of clean water, and the same length of soaking and drying treatment was carried out with the coal sample after the fracturing fluid treatment Both raw and treated coal samples were tested for gas sorption experiments for both methane and CO2. To quantify the chemical surface property evolution, Fourier Transform Infrared Spectrometry (FTIR) was used to measure the changes in the surface functional groups of the coal sample before and after fracturing fluid treatment. This experiment uses Bruker VERTEX 80v infrared spectrometer, and the probing spectrum ranges from 8000 to 350 cm-1.
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