Study On Hydraulic Fracturing In Mining-Coal Bed And Permeability Improved Range

Gas disaster is a serious problem in the world and it has been most serious in China across the world, gas disasters will be more serious when the depth of the mining deeply increases. Hydraulic fracturing technology, as one of the main methods of gas extraction, can effectively increase the air permeability of coal seam, increase the rate of gas extraction, fundamentally resolve the problem that is caused by gas. This study concentrated on hydraulic fracturing of coal seam and the influence of permeability improved range for the study mainline, utilized numerical simulation and experiment as he research tools based on the theoretical analysis. Analysed the initial initiation position and expansion direction of hydraulic fracturing crack, whether matching with the lumerical simulation result. Analysed the effect of fracturing fluid expelled on gas, and fracturing fluid that has impact on gas adsorption desorption and seepage. According to the coal matrix pore, fracture structure and the differences of physical and mechanical properties of coal and rock, analysed the permeability improved mechanism of hydraulic fracturing of coal, and examined horizontal permeability improved range and area establish a simulation model. Through the above research and analyses, the progress in the following several aspects was obtaied:① Obtained the stress distribution formula around borehole by building a theoretical mode of hydraulic fracturing. It was found that, the crack direction is vertical, the initial pressure and the expansion pressure is presenting, as gradually growth trend satisfied the following conditions:when λ<1 and the crack direction is horizontal and the initial pressure and the expansion pressure are presenting as gradually decrease trend; when λ>1 by using first strength theory; When the λ= 1, crack direction is randomness, he minimum pore water pressure of crack initiation is by deducing of first strength theory; when introducing the Mohr-Coulomb strength criterion, the minimum water pressure is when introducing the Drucher-Prager criterion, the smallest pore water pressure is② This paper simulated hydraulic fracturing process of the single-hole coal models and coal samples containing prefabricated crack models by numerical simulation software analyzed crack initiation position and expansion direction, and verified the theoretical lanalyses. Through calculation of prefabricated crack model, it was that, the main crack extension direction does not change, and principal stress is the dominant factor to control the crack when λ< 0.95; the macroscopic crack propagation direction is changed, and the dominant factor gets weakened of principal stress, prefabricated crack can guide and control the propagation of cracks when λ> 0.9625; When λ∈ [0.95,0.9625], the fracture crack initiation at the same time along the maximum principal stress and pre-existing crack tip with the help of the tensile stress. Through the analysis of the influence of the angle of prefabricated crack in fracturing, the prefabricated crack angle has small effect on the direction of crack propagation, but can change the crack point.③ Through the analysis of the permeability increase mechanism of fracturing fluid, we discussed the adverse effect that was brought from high pressure water injection. The injection water drives dissociative gas, and gas enrichment zone is formed on the edge of fracturing effect region, through the gas content distribution of field; through adsorption desorption experiment of different moisture content shows that the increase of coal moisture not only inhibits the process of gas adsorption, reduces the gas adsorption amount, but also suppresses the gas desorption process, and it is not conducive to the gas extraction to a certain extent, however, it can be ignored compared with permeability improved effect of overall coal seam; through seepage test of different moisture content, it was indicated that the increase of coal moisture can adsorpt on coal pore surface or occupy the channel for the gas flow, then augment flow resistance and reduced the permeability of the gas in coal, so it is not conducive to gas extraction to some extent.④ Followed by the analyses of the coal pore-crack structure, the coal rock mechanics properties, the seepage characteristics and permeability improved mechanism of fracturing, models were subsequently established. After numerical simulation, models could be divided into macro-crack zone, macro-crack crossing zone, micro-crack generation zone, and native crack disturbed zone, through stress, flow, water pressure and crack development trend; and obtained range of permeability improved for 54m of λ= 1.2; the growth trend of permeability increasing area which was obtained by digital image processing can be described by quadratic function; through the locale experimental, it was found that the effect area is 40m when water injection time of 120min and injection flux of 25m3 of borehole w17; the effect of gas extraction increased 3-5 times after implementation hydrofracturing of N2706 working face in ming-coal bed.