液氮-冰粒新型磨料射流调制机理研究

Liquid nitrogen jet can lead to fluid impact, cryogenic fracturing and gas expansion, etc. when applied to rock-breaking. The addition of abrasives into liquid nitrogen jet can greatly improve the rock-breaking efficiency, which has much potential to be used in petroleum drilling and completion engineering. This project proposed a new idea about liquid nitrogen-ice granule jet, that is, pump some water drops into liquid nitrogen, which congeal as ice granule in a short time and eject with liquid nitrogen. The key issue is the modulation mechanism of liquid nitrogen-ice granule jet based on dynamic heat transfer and parameter control. Focusing on this issue, following works should be done: numerical simulation of the mixture process of liquid nitrogen and water drop, revealing the effects of parameters such as initial temperature, velocity and flow rate ratio on temperature field; based on the simulated temperature filed, conduct visualization experiment of single water drop congealing in liquid nitrogen, illustrating the mechanism of water drop congealing in liquid nitrogen; perform laboratory impacting experiments of liquid nitrogen-ice granule jet, use acoustic emission technique to acquire the effects of water drop, nozzle and liquid nitrogen jet parameters on rock-breaking efficiency, and obtain the optimal hydraulic parameter and structure of jet producer. The results can build the theoretical fundamentals to form a new kind of abrasive jet: liquid nitrogen-ice granule jet.

液氮射流破岩具有流体冲击、低温致裂及气化膨胀等特性，加入磨料可大幅提高纯液氮射流的破岩效率，在油气钻完井工程中具有良好的发展前景。本项目提出了一种液氮-冰粒射流的新思路，即在液氮中泵入适量的水滴，水滴快速凝结成冰粒后跟随液氮喷射形成磨料射流。其关键科学问题是基于动态传热与参数控制的液氮-冰粒射流调制机理。针对这一关键科学问题，拟开展如下研究：数值模拟液氮与水滴的混合过程，揭示初始温度、速度及流量比等参数对温度场的影响规律；基于模拟温度场，开展单水滴在液氮中凝固过程的可视化实验研究，揭示水滴在液氮中凝固机理；开展液氮-冰粒射流冲击特性室内实验，采用声发射监测手段获得水滴参数、液氮射流参数、喷嘴参数等对破岩效率的影响规律，优化出最佳水力参数及发生装置结构参数。研究成果可望为形成一种液氮-冰粒新型磨料射流提供理论依据。

液氮喷射压裂技术是将液氮低温致裂作用与高压射流技术相结合的无水压裂技术，有望在低渗透致密储层中得到推广。为提高液氮射流破岩效率，本项目首次提出液氮-冰粒射流新思路，即在液氮中泵入适量的水滴，水滴快速凝结成冰粒后跟随液氮喷射形成磨料射流。其关键科学问题是基于动态传热与参数控制的液氮-冰粒射流调制机理。围绕关键科学问题，项目开展了液氮与水滴混合动态温度分布规律、单水滴在液氮中凝固可视化实验、液氮-冰粒射流冲击特性实验等研究。数值模拟表明，粒子从磨料入口进入混合腔后，温度迅速降至冰点以下，即可快速凝结成冰粒，且被液氮射流卷吸、掺混、加速，形成液氮-冰粒射流；提高液氮射流压力，可提高液氮-冰粒射流的冲击力；增大冰粒体积，冰粒峰值速度降低；增大冰粒流量，不利于冰粒加速，浓度较小时，影响较小。单水滴凝固实验表明，水滴凝固时，先在外侧形成冰壳，再逐步向内凝结，部分溶解气析出导致冰粒局部破裂；水滴在冷气环境下缓慢凝结存在四个阶段，而在液氮中仅能测得三个阶段；随着水滴体积的增大，水滴凝固时间增大，考虑到打击力，水滴体积不能过小，因此水滴体积存在最优值；环境温度越低，水滴凝固越快，在液氮中4μl的水滴可以在0.5s内凝固。设计研制了液氮-冰粒射流发生装置，并成功调制出液氮-冰粒射流；液氮-冰粒射流的冲击效果显著好于纯液氮射流；且喷射距离越短，喷射压力越大，液氮-冰粒射流打击效果越好。研究成果为形成一种液氮-冰粒新型磨料射流提供理论依据。