Experimental study and analysis on physicochemical properties of coal treated with clean fracturing fluid for coal seam water injection

A study on the impact of ultrasonic-stimulated clean fracturing fluid on the pore structure of medium to high rank coal

The pore structure of coal plays a key role in the effectiveness of gas extraction. Conventional hydraulic fracturing techniques have limited success in modifying the pore structure using clean fracturing fluid (CFF), and the stimulating effects of ultrasonic can enhance the effectiveness of CFF in modifying coal pore structures. To research the effects of ultrasonic stimulation on the pore structure of medium to high-rank coal when using CFF, this study employed mercury intrusion porosimetry (MIP) and low-temperature nitrogen adsorption (LT-N2A) methods to analyze the changes in pore structures after cooperative modification. The results indicate that the pore volume and surface area of medium to high rank coal exhibit an increase and followed by a decrease with increasing Ro,max values, while the average pore diameter and permeability demonstrate a decrease and followed by an increase with Ro,max. Although there are some variations in the results of MIP and LT-N2A analysis for different pore size ranges, the overall findings suggest that ultrasonic stimulation in conjunction with CFF effectively alters the coal pore structure. The most significant improvement was observed in coking coal, where pore volume increased by 22%, pore area decreased by 11% and tortuosity decreased by 47%. The improvement of lean coal is the smallest, the pore volume increases by about 7%, and the surface area decreases by about 14%. It is found that the modification of coal pore volume is mainly concentrated in transition pores and macropores. These research outcomes provide valuable insights into the application of ultrasonic technology in coalbed gas extraction.

Physicochemical Effect on Coal Pores of Hydrocarbon Chain Length and Mixing of Viscoelastic Surfactants in Clean Fracturing Fluids

Clean fracturing fluids are environmentally friendly and could have broad applications in permeability enhancement of coal seams. The hydrophobic chain length of the viscoelastic surfactant (VES) and the mixing of VESs with different ionic types have marked effects on the performance of clean fracturing fluids. This paper analyzes the effects of the hydrocarbon chain length of VES and mixing of VESs with different ion types on the pores of coal and discusses the mechanisms controlling the pore changes from a physical and chemical perspective. We found that the coal samples treated with clean fracturing fluid B had the largest porosity change. Adding two methylene groups to the hydrocarbon chain of the cationic VES will increase clay swelling in coal treated with fracturing fluids. Adding 0.1 wt % cocoamidopropyl betaine (zwitterionic VES) to cationic VES fracturing fluids can reduce the extent of clay expansion induced by fracturing fluids. VES with a long hydrocarbon chain has a strong ability to remove kaolinite in hard coal, and the addition of zwitterion VES increases the ability of a clean fracturing fluid to remove kaolinite. These results provide theoretical guidance for the synthesis of new VES molecules and the design of new fracturing fluid formulations.

Evolution Characteristics of the Microstructure with Different Fracturing Fluids: An Experimental Study of Guizhou Bituminous Coal

Fracturing fluid is a key factor affecting the hydraulic fracture morphology and coal microstructure, which plays a key role in the hydraulic fracturing effect. To compare the effect of clean water, clean fracturing fluid, and acid fracturing fluid on the pore structure of coal, this paper used high-pressure mercury injection (MIP), low-temperature N2 adsorption (LT-N2A), and scanning electron microscopy (SEM) to determine the pore structure of Guizhou bituminous coal before and after the action of fracturing fluid. The results show that clean water can cause mineral expansion and reduce pore volume by about 6% and clean fracturing fluid and acid fracturing fluid can increase pore volume by 3 and 12%, respectively, due to different degrees of acidity. The MIP data show that the pore structure of coal samples is more complex after the action of different fracturing fluids, and acidic fracturing fluids can increase the fractal dimension of the pore by about 7%. The LT-N2A data showed that the fractal dimension of micropores and transition pores decreased after the action of different fracturing fluids. In general, acid fracturing fluid has the best effect on the coal microstructure, followed by clean fracturing fluid, and the least effect on clean water.