液氮压裂作用下页岩破裂特征试验研究  

Experimental research on shale cracking characteristics due to liquid nitrogen fracturing

作　　者：蔡承政[1,2] 任科达 杨玉贵 胡国忠[3] CAI Chengzheng;REN Keda;YANG Yugui;HU Guozhong(State Key Laboratory for Geomechanics and Deep Underground Engineering,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China;School of Mechanics and Civil Engineering,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China;School of Mines,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China)

机构地区：[1]中国矿业大学深部岩土力学与地下工程国家重点实验室,江苏徐州221116 [2]中国矿业大学力学与土木工程学院,江苏徐州221116 [3]中国矿业大学矿业工程学院,江苏徐州221116

出　　处：《岩石力学与工程学报》2020年第11期2183-2203,共21页Chinese Journal of Rock Mechanics and Engineering

基　　金：国家自然科学基金资助项目(51604263,U1762105);中国博士后科学基金资助项目(2019M650120)。

摘　　要：液氮压裂作为一种新型无水压裂方法,有望为页岩气高效开发提供一条新途径。为揭示液氮压裂作用下页岩破裂特征,利用自主研制的试验装置开展了液氮压裂试验,获得不同初始温度及液氮处理方式下页岩的破裂压力及裂缝形态,在此基础上探讨液氮压裂对页岩的致裂机制,进而提出一种新的液氮压裂方法。研究表明,超低温液氮产生的热冲击效应会对页岩破裂压力及破裂模式造成显著影响,当页岩温度从室温(25℃)升高至80℃和150℃时,破裂压力分别降低22.58%和32.26%,破裂模式从室温状态下的沿初始弱面破坏转变成沿孔眼轴线的拉张破坏。经液氮低温处理页岩的破裂模式以随机分布的局部裂缝为主,局部裂缝的复杂性随页岩初始温度的升高而增大。在液氮低温冷冻状态下,由于页岩的渗透性的增大,在压裂过程中岩样表面出现了严重的漏气现象,破裂模式转变为开度较小的局部裂缝。液氮低温冷冻会导致页岩破裂压力升高,但能够降低流体进入页岩的难度,有利于页岩内压力的传递。液氮压裂可对页岩储层产生热冲击、低温损伤、冷冻开裂和压力致裂多重致裂效应,建议采用氮气压裂–液氮冷冻交替作业的施工方式,以充分发挥液氮的低温冷冻作用。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 L

关 键 词：岩石力学 页岩 液氮压裂 破裂特征 裂缝形态 

分 类 号：TU45[建筑科学—岩土工程]