| As China’s economic development, the demand for energy is increasing, and our state is in good geological conditions of shale gas formation and enrichment, so the exploitation of shale gas has widespread attention. Shale gas reservoir has typical characteristics of low porosity and permeability, and with the depth of reservoir increasing, physical properties are going to be deterioration, resulting in exploitation difficulty and low recovery ratio. But changing the permeability of the reservoir through artificial increasing permeability methods can effectively improve the recovery ratio. Hydraulic fracturing is one of the important artificial increasing permeability methods, but some deficiencies of hydraulic fracturing are found in the actual implementation. Large friction of fracturing fluid leads itself to be detained in the cracks easily and brings water lock and Jamin effect, so that the reservoir permeability decrease. After fracturing, fracturing fluid reclaim difficulties will cause secondary damage to the reservoir, which reduces fracturing effect. Furthermore, mineral composition of the shale contains more clay minerals such as illite, kaolinite, illite smectite mixed layer and so on, which expands upon imbibing water resulting in closure of cracks and decrease of recovery ratio. The most serious thing is mostly shale gas rich areas where they dry, so non-aqueous fracturing techniques get widespread attention, and it is expected that the reservoir is remolded with pneumatic fracturing.Through laboratory experiments, theoretical analysis and numerical simulation methods to systematically study on the difference of crack initiation between hydraulic and pneumatic fracturing. The followings are mainly achievements.(1) Through fracture mechanics, fluid mechanics in porous medium and seepage-stress coupling theory, a breakdown pressure criteria was established based on the different characters of seepage of water and gas. A new equation to calculate seepage field of gas was deduced, considering the effect caused by the changes of gas density in space on mass conservation equation.(2) The numerical calculation method of breakdown pressure was designed, and seepage-stress coupling computation programs which can achieve boundary expansion of seepage field were programed by Comsol with Matlab.(3) Red sandstone hydraulic and pneumatic fracturing experiments under different conditions of loading rate of water pressure and gas pressure have been implemented successfully. The experiments show that loading rate of water pressure and gas pressure has little effect on the breakdown pressure and increments of volumetric strain of specimens for both hydraulic and pneumatic fracturing in a certain range, and breakdown pressure of pneumatic fracturing is less about 40 percent than it by hydraulic fracturing. After pneumatic fracturing the specimens failure more seriously and have two or three main cracks run through the whole specimens. The width of main cracks is large and there are many small cracks around main cracks. However, after hydraulic fracturing the specimens failure slightly and just have two small symmetrical cracks which doesn’t run through the specimens. The crack width also is small. Comparing the numerical and experimental results of breakdown pressure and time when specimens failure, we found that the calculated model can be used to predict breakdown pressure and fracturing time appropriately.(4) Through numerical methods to simulate and analyze the effect of the loading rate of water and gas pressure, the reservoir permeability, the crack length around the well and the crustal stress of different depths on hydraulic and pneumatic fracturing, we found that the pneumatic fracturing has a distinct advantage under different conditions because the viscosity of gas is small and it is easy for gas to penetrate in the reservoirs. |
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