Coal And Rock Damage Mechanism And Methane Migration Law In Thin And Medium Thick Soft Coal Seams By Hydraulic Fracturing

Southwest region of China is major coal production and energy supply area, with totally 386.6 billion tons of coal and 421 million m3 coal bed methane(CBM). However, the geological condition of coal is complex, which main coal layer thickness ranges from 0.8m to 3.5m and most of them are thin and medium thick seams. Besides, most coal seams are soft which f value are 0.2 to 1. What's more, coal's permeability is extreme low, which is about 10-4 mD ~ 10-3mD. Due to the above reasons, it is difficult to develop resources as well as prevent gas disasters, which seriously hamper the development of clean energy efficiently and safe production of coal. Hence, how to effectively increase permeability in thin and medium thick soft seams will decide coal and CBM production efficiency and gas disaster control in southwest region of China.In recent years, Chongqing Songzao coal mine introduced the hydraulic fracturing technology to CBM extraction and preliminary application results showed that: the average drainage volume increased by 3 to 5 times per borehole. But, The mechanism are not clear, such as the law of hydraulic fracture initiation and propagation, the roof damage mechanism by hydraulic fracturing, and methane migration law during hydraulic fracturing in thin and medium thick soft coal seams.. In this paper, theoretical analysis, numerical simulation, laboratory and field experiments methods were used to study the hydraulic fracture initiation and propagation, coal and rock damage and deformation, and gas migration mechanism. The main conclusions are as follows:Establish models to calculate initiation pressure, judge initiation direction and reveal the effects of coal seam occurrences, in-situ stress on initiation pressure and direction. Coal seam occurrence is a major factor to decide drilling arrangement of hydraulic fracturing in the thin and medium thick soft seams. Based on the stress state analysis of borehole in consideration of coal seam occurrence and in-situ stress, and combine with maximum tensile stress theory, hydraulic fracturing initiation pressure and fracture direction calculation model in thin and medium thick coal seams was established. Similar simulation experiments were conducted to verify the calculation model. The results show that the crack initiation pressure will decrease with the increase of horizontal stress difference, angle between coal seam inclination and maximum stress direction, and coal seam dip angle; with the decrease of coal seam inclination, initiation direction will gradually turn from strike direction to inclination direction and the turning speed will increase; with the increase of angle between the maximum stress and coal seam inclination, the initiation direction will turn from inclination direction to strike direction and the turning speed till decrease.Through the construction of thin and medium thick soft coal seams hydraulic fracturing model to describe coal seam roof damage area, hydraulic fracturing damage deformation mechanism of roof area was revealed. Based on theoretical analysis, basic principles of hydraulic fracturing method overcome visco-elastic-plastic deformation, damage of roof and increase permeability of coal was analyzed. Simplify the hydraulic fracturing area as an elastic thin plate, thin and medium thick soft coal seams hydraulic fracturing damage model was established. On this basis, FLAC3 D software was used to analysis of the stress state of roof in soft coal seams during hydraulic fracturing process, which reveals the seam thickness, in-situ stress, hydraulic fracturing parameters' effect on the damage deformation of the roof.The gas migration model in thin and medium thick coal seams was established which reveals the gas migration and enrichment law. Based on theoretical analysis of the basic principle of gas flow for high pressure water in thin and medium thick soft coal seams during hydraulic fracturing process, according to the theory of seepage mechanics analysis of coal which contains gas water displacement dynamics process, two-phase flow seepage model for thin and medium thick soft coal seams during hydraulic fracturing was established. The influences of porosity, permeability, gas pressure and the hydraulic fracturing process parameters of thin and thick soft coal seams on gas migration during hydraulic fracturing were further analyzed.Hydraulic fracturing in thin and medium thick soft coal seams technology process and evaluation methods is developed. According to the thin and medium thick soft coal seams hydraulic fracturing increased permeability principle and occurrence characteristics, suitable process for thin and medium thick soft coal seams hydraulic fracturing was developed; put forward a ?look-detect-measure? method to evaluate hydraulic fracturing performance in thin and medium thick soft coal seams. It is applied to 3121 driving strip in Tonghua coal mine to increase permeability and extract CBM. The results show that when the hydraulic fracturing range is about 50m~60m, the gas enrichment area is 30m~50m; compared with the data from 2127 tunneling which without hydraulic fracturing, the average single hole gas drainage quantity increased from 1.75m3/d to 10.36m3/d, increased by 4.9 times; tunneling efficiency increase by 90.7%, the average wind gas amount decrease from 1.53m3/min to 0.45m3/min, decreased by 70.8%.