Fracture Development Characteristics of Coal under Organic Solvent Erosion and Its Nondestructive Testing Method

Abstract

The low permeability of coalbed methane (CBM) reservoirs restricts the efficient extraction of CBM. Organic solvent erosion is a sound method to improve the permeability of coal; however, a nondestructive testing (NDT) method for identifying the effects of erosion on coal is necessary. The ultrasonic characteristics of coal can reflect the physical state of coal; therefore, ultrasonic testing is a suitable method for identifying the erosion effect. First, the characteristics of fracture development in coal were studied through surface observations and mass analysis. Organic solvent treatment is an effective means to promote the development of coal fractures, and the effects of organic solvent erosion on coal can be divided into three grades. The effects of organic solvent erosion on coal are influenced by the extraction rate of the solvent for coal. A high extraction rate can improve the degree of coal fracture development, and coal becomes susceptible to breakage. N-Methylpyrrolidone (NMP) and tetrahydrofuran (THF) have high extraction rates for coal and are suitable for solvent treatment. Subsequently, the effects of solvent erosion on the ultrasonic characteristics were comprehensively investigated. After solvent erosion, the ultrasonic characteristics of coal changed remarkably. The peak amplitude of the time-domain spectrum decreased by 13.5–56.7%, the ultrasonic velocity decreased by 39.1–70.1%, and the ultrasonic attenuation coefficient increased by 12.2–130.7 dB/m. The relationship between the degree of erosion and ultrasonic parameters was revealed, and the physical state of coal was deduced through the ultrasonic parameters. Thus, ultrasonic detection can be used as an NDT method in solvent treatment. These results provide a theoretical basis for the application of solvent treatment in the coal industry.

Experimental Study on Differential Thermal Response and Pore-Fracture Structure Evolution Characteristics of Coals under Microwave Irradiation: A Case Study of Five Different Rank Coals

“…20 The environmental impact of solvent extraction is inevitable. 21 The lowtemperature condition of liquid nitrogen freeze−thaw is difficult to achieve. 22 In recent years, research has shown that thermal stimulation technology can effectively reduce the adsorption capacity of CBM, increase the permeability, and thus promote the production of CBM.…”

“…Gas injection displacement has high energy consumption and low economic benefit, and it is only suitable for coal seams with high initial permeability . The environmental impact of solvent extraction is inevitable . The low-temperature condition of liquid nitrogen freeze–thaw is difficult to achieve …”

Impact of N-Methyl-2-pyrrolidone Erosion on Surface Functional Groups and Pore Evolution in Coals of Different Ranks

“…Ji et al 27 studied changes in the pore structure and adsorption capacity of coal after tetrahydrofuran (THF) extraction and found that the dissolution of small organic molecules reduced active groups, altered the pore space, and decreased the methane adsorption capacity. Lin et al 28 characterized the physical state of coal after organic solvent erosion using ultrasonic method and found that NMP and THF were the most effective in extracting the coal, and the crack width widening effect was obvious. Li et al 29 developed a coal molecular model to study methane adsorption and diffusion, finding that NMP treatment decreased methane adsorption by 41.6% and increased diffusion by 1.72 times.…”