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| Experimental research on shale cracking characteristics due to liquid nitrogen fracturing |
| CAI Chengzheng1,2,REN Keda1,2,YANG Yugui1,HU Guozhong3 |
| (1. State Key Laboratory for Geomechanics and Deep Underground Engineering,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China;2. School of Mechanics and Civil Engineering,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China;3. School of Mines,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China) |
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Abstract Liquid nitrogen(LN) fracturing,as a novel waterless fracturing technology,is expected to provide a new approach for shale gas development. To reveal the cracking characteristics of shale due to LN fracturing, laboratory simulation experiments were performed on shale samples utilizing a self-designed experimental apparatus. The effects of shale initial temperature and LN treatment on cracking characteristics and fracture pattern of shale were researched,and the shale cracking mechanism induced by LN fracturing was explored. Finally,a novel fracturing technology with LN was proposed based on the experimental results. The investigations show that the thermal shock induced by LN remarkably affects the breakdown pressure and the cracking mode of shale. As the shale temperature increases from room temperature(25 ℃) to 80 ℃ and 150 ℃,the breakdown pressure decreases by 22.58% and 32.26%,respectively. During LN fracturing experiments,the samples at room temperature were fractured along the original weak plane,whilethe cracking mode of 80 ℃ and 150 ℃ samples transforms to tensile rupture along the borehole axis. For LN cool-treated samples,the cracking mode mainly presents in the form of local fractures,whose complexity increases with the sample initial temperature. For the samples under LN cooling condition,the obvious gas leakage phenomenon was observed on the sample surface due to the increase of the permeability of shale. The cracking mode of the samples under LN cooling condition transforms to local fracture with low aperture. Under LN cooling condition,the breakdown pressure increases compared with original samples. However,the re-open of existing cracks induced by LN cooling could make the fluid permeate into the shale more easily,which is beneficial to the pressure transmission. The multiple cracking effect on shale can be generated by LN fracturing,including thermal shock,cryogenic damage,low-temperature cracking and pressure-induced cracking. The alternate treatment of gaseous nitrogen fracturing and LN cooling was proposed to make full use of the low-temperature cooling effect of LN.
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