Microscopic Kinetics Characteristics Of Methane Under Pulsating Hydraulic Fracturing And Mechanism Of Liquid Retention Effect

About 70 percent of China’s coal occurrence has micro pores, low permeability, high adsorption characteristics, resulting difficult extraction and more serious gas disaster in most coal. Hydraulic fracturing is an effective technique to improve the efficiency of gas extraction from coal seam, but the existing hydraulic fracturing technology has unstable technical effect, and the presence of the relief is not sufficient. Pulsating hydraulic fracturing is a new technology on the basis of conventional hydraulic fracturing aiming at improving the effect of relieving pressure and increasing permeability. In this paper, we deeply study microscopic kinetics characteristics of methane under pulsating hydraulic fracturing and mechanism of liquid retention effect, and optimize the key technical parameters and process through theoretical analysis, laboratory experiments and numerical fitting methods.Using the mercury experiment and liquid nitrogen adsorption method combined with environmental scanning electron microscopy and energy dispersive spectroscopy testing, we comprehensively make quantitative and qualitative analysis about the changes characteristics of porosity, pore volume with the pore size distribution after pulsating water intrusion different coal. Pulsating water intrusion coal porosity is greater than hydrostatic water intrusion porosity of coal samples. The micropore volume reduced as the increasing pulse frequency and pulsating pressure, while the mesopore and macropore volume increased. Pulsating water intrusion have occurred mainly two types of coal pore:(a) reaming role in the formation of the original pore expansion;(b) the erosion of the role for the formation of a new pore space of mineral crystal.Under the self-test system of pulsating hydraulic fracturing influencing gas desorption, we analyze replacement-flooding gas characteristics gas properties, gas post-desorption characteristics, and comprehensive analysis of the impact of hydraulic fracturing gas desorption effect under pulsating parameters cooperative control. Replacement-flooding gas amount of pulsating hydraulic fracturing process is significantly larger than the static pressure fracturing, and which increases with increasing pulsating frequency according linearly increasing trend first and increased logarithmically later, with the pulsation peak pressure was increased logarithmically. Late natural gas desorption and desorption rate is less than the amount of dry coal gas desorption, the attenuation ratio is generally between 20% and 60%. The static pressure and low-pressure low-frequency fracturing inhibits synthesis gas desorption, with the increase of frequency and pressure pulsation fracturing of the integrated gas desorption play a catalytic role. Description water fracturing in any case not inhibit gas desorption, through the implementation of pulsating hydraulic fracturing technology, and rational allocation of parameters, can greatly improve the comprehensive gas desorption.Using gas diffusion kinetics model of third boundary conditions can scientifically analyze gas diffusion dynamics characteristics under pulsating parameters cooperative control. Internal pore water into the coal, gas diffusion channel will be block and hinder the diffusion of gas in coal. With the increasing pulse frequency and peak pressure, the gas diffusion resistance of pulsating hydraulic fracturing is decreased. Meanwhile, due to the combined effects of coal gas concentration gradient and gas diffusion resistance, the gas diffusion ability in coal increases with the pulse frequency and the peak pressure increases, and there should be a set threshold, the gas diffusion capacity after pulsating fracturing will be greater than original coal.Capillary force and the fracturing fluid retention effect is the two reasons produced water lock effect. The surface tension and contact angle of the fracturing fluid in the five factors affecting coal seam water lock effect is the deciding factor whether to lift the water lock. Using the DSA-type optical droplet morphology analysis system, research clean fracturing fluid critical tension and contact angle changes the surface properties of the surfactant-based. And using magnetic resonance equipment to analyze the unlocking evaluates effect of water blocks, the anionic surfactant active agent SDS is the most optimum fracturing fluid additives, and to determine a reasonable concentration of 0.022%.Finally, we manufactured the integration equipment of pulsating hydraulic fracturing, proposed drill- fracture-drainage trinity technology for pressure relief and permeability increase in low permeability coal seam. Application of research results in the field showed that gas drainage effect is obvious after the implementation of optimization pulsating hydraulic fracturing. The stabilized gas concentrations of fracturing hole and pilot hole were around 4 and 5 times of ordinary drainage holes, gas flow were 4~5 times contrast ordinary drainage holes, seam permeability coefficient increased by 48 to 217 times.In addition, 6 papers about research results were published, in which 1 has been indexed by SCI, 3 have been indexed by EI. There are 78 figures, 38 tables and 206 references in this thesis.