RESUMO
Nanoparticles are added to clean fracturing fluids to formulate nanoparticle-modified clean fracturing fluids, compared with ordinary clean fracturing fluid, it has the advantages of good temperature resistance, low loss of filtration, and so forth, and has good application prospects in coal-bed methane. However, the current research on nanoparticle-modified clean fracturing fluids is mostly focused on the study of their rheological properties. The mechanism of nano-fracking fluid influence on methane adsorption-desorption characteristics is not clear. Therefore, this study chooses Jiulishan anthracite coal (high-rank coal), Pingdingshan coal (medium-rank coal), and Geng village mine long bituminous coal (low-rank coal) of the three rank coal samples. Using indoor experiments and molecular simulation methods, a study on the influence of methane adsorption and desorption capacity and diffusion ability of coal samples provides a modified fracturing fluid formulation of 0.8% CATB + 0.2% NaSal + 1% KCl + SiO2. The experimental results show that nanofracturing fluid-treated coal samples compared to clean fracturing fluid treated coal samples, both methane adsorption and desorption capacities, were increased to some extent. Construction of methane adsorption systems with different apertures and calculation of isosteric heat of adsorption, indicating that the interaction force between methane and coal molecules is smaller after nanofracturing fluid treatment, which facilitated methane desorption. A simulation study of methane diffusion in coal samples treated with two systems of fracturing fluids at different aperture was carried out using molecular dynamics methods, indicating that nanoparticle-modified clean fracturing fluids can reduce the damage of clean fracturing fluids to the desorption-diffusion ability of coal reservoirs. Comparison of 6 MPa as the most suitable pressure for nanofracturing fluids to function provides a basis for the future development of nanofracturing fluids and their popularization.
RESUMO
In the hydraulic fracturing process, fracturing fluid contacts coal rock and physical and chemical reactions occur, which inevitably damage the pore structure of the coal rock and affect the adsorption and desorption capacity of the coal rock. In this paper, a low-temperature N2 adsorption method and scanning electron microscopy (SEM) were used to characterize coal samples. Using gas adsorption/desorption tests, high-, medium-, and low-rank coal samples before and after the clean fracturing fluid treatment were systematically studied. According to the relationship between coal pore structure parameters and gas adsorption/desorption characteristics, a correlation between the microscopic pore structure and the macroscopic gas adsorption/desorption characteristics of coal was obtained. The results show that the number of closed pores in high-, medium-, and low-rank coal samples increased after the clean fracturing fluid treatment. The micropore volume increased by 0.0009, 0.00143, and 0.0035 mL/g, respectively, and the specific surface area increased by 4.87, 9.06, and 57.60%. The fractal dimension also increased compared with that of raw coal. SEM analysis indicated that the influence degree of clean fracturing fluid treatment on the pore structure of different-rank coal samples was Gengcun low-rank coal > Pingba middle-rank coal > Jiulishan high-rank coal. The experimental results of methane adsorption and desorption showed that the adsorption capacity of the coal samples after clean fracturing fluid treatment was enhanced, which is related to increases in the micropore proportion, micropore volume, and specific surface area of the coal. The desorption capacity of the coal samples was also enhanced. The desorption rate of medium- and high-rank coal samples increased after the clean fracturing fluid treatment but that of low-rank coal samples decreased. The main reason is the increase in the number of micropores in low-rank coal, which enhances the gas adsorption ability and makes gas desorption difficult. Therefore, clean fracturing fluid is suitable for medium- and high-grade metamorphic coalbed methane mines. These research results provide a theoretical basis for the application of clean fracturing fluid in different coalbed methane wells.
RESUMO
The prediction exactness of coalbed methane (CBM) content and productivity correlates closely with the gas adsorption rules of coal, but there is a noticeable difference in the gas adsorption rules between deformed and undeformed coal. One of the main factors affecting the gas adsorption capacity of coal is pore structure, which is affected by the particle size, and it is also one of the essential differences between deformed and undeformed coal. In this work, we experimentally study the law of the pore structure and gas adsorption capacity with the particle size. Results show that the specific surface area and the pore volume of undeformed coal increase significantly as the particle size decreases, while the variation trend of those of deformed coal is insignificant. The fractal dimension D 2 and the particle size show a U-shaped correlation. The fractal dimension D 2 reaches the minimum value at a coal particle size of 1-3 mm and 0.2-0.25 mm for deformed and undeformed coal, respectively. The D 2 values of deformed and undeformed coal are closest in the case of particle sizes smaller than 0.1 mm. The difference in the adsorption capacity between deformed and undeformed coal diminishes with the decreasing particle size as the pore structure characteristics of undeformed coal gradually approach those of deformed coal. The obtained conclusions provide a theoretical foundation for the selection of the particle size of coal samples so as to predict coal and gas outburst disasters and CBM productivity accurately.