Modulation of plume interaction and diameter prediction of boreholesinduced by novel flash boiling jet drilling
LIU Wenchuan，TANG Jiren，ZHANG Huidong，LI Yijing
射流钻进多层分支孔构造裂缝网络是进行煤层气储层改造增产的有效手段，高质量高效钻 进成孔是延长气井寿命、提高煤层气采收率的关键。 为实现均衡冲击，进一步提升成孔质量与孔壁 稳定性，提出了过热液体闪沸多孔喷射钻进技术，利用闪沸相变形成气液射流协同热应力共同作用 于煤岩钻进成孔。 为了调控羽流相互作用以进一步利用闪沸现象的优点，必须获得羽流相互作用 的调控准则及其对射流宏微观特性的影响。 通过高速摄影(DBI)和相位多普勒测试(PDA)获取了 不同羽流相互作用阶段闪沸喷射流场宏微观特性。 引入喷嘴喉部压力-焓关系计算得到新无量纲 参数，分析射流形态和宏观特征。 基于等熵-绝热两阶段模型以及喷嘴喉部流体状态，获取了闪沸 喷射钻头几何结构设计指标 CP 和工况参数调控指标 OP 。 提出了综合考虑液体性质、喷孔布置以 及环境压力的羽流相互作用调控新准则，并通过不同类型液体进行了验证，为羽流动态调控匹配冲 击力与冲击区域提供了支撑。 根据新准则，研究了羽流相互作用各个阶段的射流特性，发现不同类 型过热液体喷射锥角均在喷射坍缩临界点附近达到最小值，索特平均直径随闪沸的剧烈程度增加 而减小，其时空分布在羽流剧烈相互作用阶段更加均衡。 研究了导致羽流横向膨胀的主导因素，以 及在高过热度条件下不同因素对羽流膨胀的贡献。 高饱和压力有利于气泡的成核破裂以及随后的 羽流膨胀，并且由于能垒对成核的依赖性，表面张力也会影响羽流的膨胀。 喷嘴喉部的流动条件对 于确定以闪沸主导和羽流相互作用主导的阶段的射流特性都至关重要，据此提出了综合考虑成核 能量势垒及成核速率的冲击区域预测模型，能够有效预测不同类型过热液体、不同喷射参数、不同 靶距下的闪沸喷射冲击区域无量纲直径，准确度高达 95%以上。 羽流调控准则和冲击直径预测模 型的建立为闪沸喷射钻头结构设计准则及工艺参数选取提供了基础，研究成果还可进一步扩展至 可压缩流体、超临界流体等射流冲击作用区域预测及调控，服务于我国能源开采。
The formation of multilayer subboreholes using jet drilling is an efficient technology to improve coal seam permeability for CBM recovery,and its quality and efficiency are crucial in enhancing the CBM recovery rate and the life span of the wells. The flash boiling jet drilling was proposed to achieve an equivalent impaction and the subsequent formation of highquality and stable subboreholes. In this method,the multiphase jets are formed through the phase transition,accompanied by the thermal stress to efficiently break coal. To modulate plume interaction for further utilizing the merits of flashboiling phenomena,it is essential to obtain the criteria of plume interaction and their effects on jet characteristics. In this study,Diffused Back Illumination (DBI) imaging and Phase Doppler Anemometry (PDA) were performed to capture the macroscopic and microscopic characteristics of the superheated jet,respectively. The new nominal parameter calculated by the pressure enthalpy graph was introduced to analyze the jet morphologies and macroscopic characteristics. Based on the isentropeadiabatic model,the nominal geometrical CP and nominal operating condition parameter OP were obtained on the basis of flow condition at the nozzle throat. The new criteria considering fluid properties,nozzle geometries,and ambient pressure were obtained,and validated by various types of fluids,providing the basis for the adjustment of the impacting force and area. Based on the new criteria,the jet characteristics in various stages of plume interaction were investigated,and the smallest jet angles were achieved at the onset of the severe plume interaction. Values of Sauter mean diameter decreased with the intensified superheat degree and achieved a more uniform distribution when severe interaction occurred. The dominant factors accounting for the lateral expansion of the plume were examined,and thermal factors contributed to the plume expansion under high superheat degree conditions. High vapor pressure facilitates the bubble nucleation and burst and subsequent plume expansion,and the surface tension also affects the plume expansion due to the dependence of the energy barrier to nucleation. The flow conditions at the throat are crucial in determining the jet characteristics in the region of both flash boilingdominant and plume interaction dominant. The new nominal parameter calculated by the energy barrier and nucleation rate was introduced to predict the dimensionless diameter impacted by the flash boiling jet,showing good agreement under various operating conditions,with a confidence of 95%. The proposed criterion and prediction model supplements the guidance on the nozzle design and injection strategy,and are also suitable for the jet with considerable compressibility and supercritical jet,providing a basis for energy exploration.
CBM recovery;superheated liquid;flash boiling jet drilling;plume interaction;drilling diameter