Search

To investigate the interaction between coal and fracturing fluid and its influence on the pore structures of the coal, three commonly used water-based fracturing fluids were selected and used to treat Illinois coal. A series of low-temperature nitrogen ad-/desorption experiments and nuclear magnetic resonance measurements were conducted to comparatively analyse the evolution of the coal pore structure after treatment. The results show that the intrusion of the water-based fracturing fluid reduces the pore volume and pore area of the coal. In addition, the residue of the VES fracturing fluid may change the shape of the ink-bottled pores of coal. The evolution of the fractal dimension of the coal pore system after fracturing-fluid treatment illustrated that the residue of the fracturing fluid may adhere primarily to the relatively rough region of the pore wall. There is no significant chemical reaction between the water-based fracturing fluids and the Illinois coal. • Intrusion of residue significantly reduces the pore volume of the coal. • The shape of coal pore can be changed after treated by the VES fracturing fluid. • The residue of fracturing fluids mainly adhere to the rough region of the pore wall. • No significant chemical reaction occurs between fracturing fluids and coal. [ABSTRACT FROM AUTHOR]

• Evaluation of influence of various water-based fracturing fluids on gas sorption and diffusion. • Microscope observation of coating and residue behavior of fracturing fluids on coal surface. • Evaluation of water production effect on removal of fracturing fluid residue. • A preliminary data set for fracturing fluid selection for Illinois coal in terms of sorption and diffusion. The water-based fracturing stimulation is commonly used for low permeability coalbed methane (CBM) reservoirs. The influence of various fracturing fluids on the gas sorption and diffusion behaviors were experimentally studied in the work. The Illinois basin coal was selected for the experimental measurements. We used slickwater, guar gel and viscoelastic surfactant (VES) fracturing fluids to treat the coal. The sorption and diffusion behaviors were measured and quantified by the volumetric sorption experiments. The microscope was employed for the coal sample surface observation to qualitatively characterize the coating and residue behavior of fracturing fluids. It was found that all types of fracturing fluids can significantly influence the gas sorption capacity of coal. Apart from the low concentration of polyacrylamide (0.1% and 0.3%) slickwater, all other fracking fluids damage the gas sorption capacity. For most fracturing fluids, the sorption capacity can be recovered closely back to its virgin coal condition with extensive water washing. For the coal samples treated by the VES fracturing fluid, both the Langmuir volume and Langmuir pressure were dramatically increased. The adverse effect of fracturing fluids on coal is apparent and all diffusion coefficients decrease with the treatment. Even with extensive washing, the diffusion coefficient cannot recover for slickwater and VES fluids. But guar gel fluid has the best diffusion recovery because of the easy removal of the guar gel. This study provides a preliminary data set for fracturing fluid selection for Illinois coal in terms of sorption and diffusion. [ABSTRACT FROM AUTHOR]

Raman spectroscopy was used to evaluate rank for a series of coals collected adjacent to an igneous intrusion. For these intruded coals, Raman parameters for collotelinite and its coked equivalent show complex changes during the transformation from low to very high levels of maturity (with vitrinite reflectance increasing from 0.55% to 5.0%). With increasing coal rank up to anthracite-level reflectance, the Raman spectra show a decrease in both G band full width at half maximum (FWHM) and D band intensity. The D band shape becomes increasingly asymmetric during the bituminization and de-bituminization stages, whereas in the anthracitization stage, the width of the D and G bands continues to decrease while D band intensity increases. In meta-anthracites (as seen in the ancillary Hunan samples), the G band has a low intensity and undergoes an apparent broadening due to the presence of the D2 band that occurs as a shoulder on the G band. Differences in Raman spectral parameters for vitrinite and inertinite macerals suggest differences in molecular structure between the macerals; as such, the heterogeneous nature of coal may affect Raman spectral results significantly if different maceral types are not considered. Within individual samples, Raman spectra indicate a high level of structural homogeneity between and within vitrinite particles, allowing assessment of differences between samples of different ranks. For vitrinite, the Raman spectral parameters G FWHM and DAs/GA (Area (1100-s) /Area (s-1650) , s is the saddle point between 1100 cm−1 and 1650 cm−1) are highly correlated (R2 > 0.9) with vitrinite reflectance (R r) and maximum alteration temperature (T peak), and allow establishment of geothermometers over the temperature range of ~70–300 °C that can be used to evaluate temperatures attained in intruded coals. • R r increases from 0.55%–5.0% for a coal transect adjacent to an intrusion. • With increased R r , Raman spectra parameters G FWHM and D band intensity decrease. • G FWHM and DAs/GA are correlated (R2 > 0.9) with R r and T peak. • Two equations were developed to estimate maximum temperature from Raman data. [ABSTRACT FROM AUTHOR]