Method for testing accumulated damage effects of liquid nitrogen on fracture structures of water-containing coal samples

Abstract

The invention relates to a method for testing accumulated damage effects of liquid nitrogen on fracture structures of water-containing coal samples. The method includes manufacturing coal samples; soaking the coal samples with different saturation degrees in the liquid nitrogen for single cycles and circularly soaking the coal samples in the liquid nitrogen for multiple cycles; weighing the saturated coal samples after each procedure to determine the increased weights of the coal samples and acquiring volume variation of fracture inside the coal samples by means of computing; observing fracture extension conditions of the surfaces of the coal samples before and after liquid nitrogen soaking tests for the coal samples by the aid of microscopes and measuring the increase quantities of the fracture; comparing propagation speeds of sound waves in the coal samples before and after the liquid nitrogen soaking tests and acquiring propagation attenuation speeds of the sound waves by means of computing so as to represent integral fracture conditions of the coal samples; measuring the uniaxial compressive strength of the coal samples, analyzing liquid nitrogen soaking effects of the coal samples with different percentages of saturated water contents and periodic circular liquid nitrogen soaking test results of the coal samples and drawing a conclusion. The method has the advantages that operation processes are simple, the method is high in applicability, fracture extension laws of liquid nitrogen soaking on the water-containing coal samples can be effectively reflected, and the method has an important reference significance on increasing permeability of coal beds, exploiting coal bed gas and increasing coal bed gas extraction rates.

Description

technical field [0001] The invention relates to the technical field of coal seam anti-permeability, in particular to a test method for the cumulative damage effect of liquid nitrogen on the fissure structure of water-containing coal samples. Background technique [0002] Coalbed methane is a new type of unconventional natural gas. Exploitation of coalbed methane is of great significance for optimizing energy structure, protecting the environment, and promoting safe production in coal mines. Coalbed methane reserves account for more than 30% of the world's total natural gas reserves. According to IEA estimates, the global onshore coalfields with coalbed methane resources buried at a depth of 2000m are about 260 trillion m 3 . my country is rich in coalbed methane resources, more than 500 billion m 3 There are 14 coalbed methane-bearing basins in China, and the geological resources of coalbed methane before the burial depth of 2000m are about 36 trillion m 3 . The storage ...

AI Technical Summary

Problems solved by technology

The storage method of coalbed methane is mainly adsorption. Due to low reservoir pressure, poor coal seam permeability, and low gas saturation, the existing mining technology has a high degree of pollution to the surrounding environment of the mining area, resulting in an average coalbed methane extraction rate of less than 20% in my country. Compared with Russia, the United States, Canada, Australia, Germany and other countries, there is a big gap

Second: The vertical well technology used abroad is not applicable in my country

However, this technology requires the coal seam to have high permeability, low in-situ stress, stable roof and floor of the coal seam, and high strength of the coal reservoir. Otherwise, it is easy to cause the cave to collapse and the migration channel of coalbed methane to be blocked. Therefore, the risk is very high, and the later maintenance High cost; the risk of casing completion is small, which can ensure stable production, but the completion cost is high, the completion process is relatively complicated, and improper operation may easily cause mud to invade the coal seam and cause pollution

Third: Limitations of hydraulic fracturing technology

Hydraulic fracturing is a commonly used technology at present, but because the coal itself is soft, the proppant is easily embedded in the coal, resulting in pore closure; the permeability of some coal seams that expand with water will decrease after hydraulic fracturing; choose other fracturing methods Howe

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