围压条件下电脉冲破碎干热岩机理

Breaking mechanisms of hot dry rock by electric pulse under confining pressure

干热岩储集层具有埋深大、温度高、岩石硬度高、研磨性强、可钻性差、地层环境复杂多变且施工难度大等特点,现有钻井技术难以满足地热资源的有效开发。利用高压电脉冲破岩技术开采干热岩热能有利于避免机械钻井过程中钻头磨损的问题,但其破岩机理尚不清楚。为此,基于离散元颗粒流GBM和等离子体通道路径生成的概率发展模型建立了电脉冲破岩热—力耦合模型,并就围压—液柱压力范围介于5~25 MPa,围压—液柱压力比K介于0.05~20时的花岗岩进行电脉冲破碎后产生的裂纹类型及数量、破碎体积分数分析。研究结果表明:①电脉冲破碎花岗岩后,花岗岩内部产生了晶内拉伸、晶内剪切、晶间拉伸与晶间剪切4种微观裂纹,并以前3种裂纹为主;②当K≥1,且围压大于15 MPa时会促进花岗岩的电脉冲破碎,随围压的增大,晶间拉伸裂纹的萌生和扩展得到加强;③K<1时,对于不同的K值,各个类型的裂纹数量曲线呈现出不同的形态,并且产生的裂纹总数少;④在围压处于很低水平时,无论液柱压力如何变化,相对于围压较大的情况电脉冲破岩效率会更高,破碎体积分数更大。结论认为,利用高压电脉冲对岩石进行破碎,是一种新型的破岩方式,围压条件下电脉冲破碎干热岩机理的揭示,为干热岩的高效钻井开发提供了理论支撑和技术参考。

Hot dry rock reservoirs are characterized by great burial depth,high temperature,large rock hardness,strong abrasivity,poor drillability,complex reservoir environment and high construction difficulty,so existing drilling technologies can hardly achieve the efficient development of geothermal resources.The use of high-voltage electric pulse rock breaking technology to exploit hot dry rocks is beneficial to avoid the problem of bit wear in the process of mechanical drilling,but its rock breaking mechanisms have not been understood clearly.In this paper,the thermal-mechanical coupling model of electric pulse rock breaking is established based on the GBM of discrete element particle flow and the probabilistic development model of plasma channel path generation.In addition,the type and number of cracks generated in granite and the breaking volume fraction after electric pulse rock breaking under the confining pressure-liquid column pressure ranging from 5 MPa to 25 MPa and the confining pressure-liquid column pressure ratio K ranging from 0.05 to 20 are analyzed.And the following research results are obtained.First,after granite breaking by electric pulse,four types of microscopic cracks are generated in the granite,including intracrystalline tensile crack,intracrystalline shear crack,intercrystalline tensile crack and intercrystalline shear crack,among which,the first three types are dominant.Second,when K≥1 and the confining pressure is higher than 15 MPa,the granite breaking by electric pulse is promoted.And with the increase of the confining pressure,the initiation and propagation of intercrystalline tensile crack is enhanced.Third,when K<1,the number curves of different types of cracks morphologically vary with K value,and the total number of cracks is small.Fourth,when the confining pressure is at a very low level,no matter how the liquid column pressur e changes,the rock breaking efficiency of electric pulse will be higher and the breaking volume fraction will be larger than those in the case with higher confining pressure.In conclusion,high-pressure electric pulse is new type of rock breaking mode,and the studies on the mechanisms of hot dry rock breaking by electric pulse under confining pressure provide theoretical support and technical reference for the efficient drilling and development of hot dry rocks.