Developmental Research on Cutting a Notch in Deep Borehole for Hydraulic Fracturing with High Speed Waterjets

为了明确高速水射流技术在深井水力压裂中切缝的适用性，研制了试验装置，模拟了2000 m深水中岩石的切缝过程，研究了驱动压力、喷距、喷嘴直径和横向速度等切割参数对切缝深度的影响，在压力为100 MPa、流量为80 l/min的条件下，对白川熔结凝灰岩的缝宽和比能进行了研究。1）虽然切割深度随着周向水压迅速减小，但最终在约10 MPa的水压下变得几乎恒定。白川焊接凝灰岩在20 MPa水压下的最大切割深度为32.2 mm。2）对于小于0.05的较小空化数，其主要切割机理为空化侵蚀。对于较大的空化数，其它机理占主导地位。针对开挖深度与周向水压力有关和与周向水压力无关的两种情况，提出了开挖深度的经验公式结果表明：从能量消耗的角度考虑，存在一个最佳水压和最佳喷距;随着水压的增大，缝宽迅速增大，直至达到一个基本恒定的值;高速水射流可用于深孔劈裂。

In order to clarify the applicability of high speed waterjets technology to making a slit in deep borehole for hydraulic fracturing, the testing apparatus was manufactured first to simulate the slot cutting of rock in deep water up to 2000m and then, the effects of various cutting parameters such as driving pressure, standoff distance, nozzle diameter and traverse rate on the depth of cut, the slot width and the specific energy were investigated for Shirakawa welded tuff using the driving pressure up to 100 MPa and flow rate of 80 1/min.The main results obtained in this study are summarized as follows;1) Although the depth of cut rapidly decreases with circumferential water pressure, if finally becomes almost constant at about 10 MPa of water pressure. Maximum depth of cut was 32.2 mm under water pressure of 20 MPa for Shirakawa welded tuff.2) Main cutting mechanism is cavitation errosion for the smaller cavitation number less than 0.05, and for larger cavitation number other mechamisms predominate.3) Empirical formulae on the depth of cut were proposed for both regions where the depth of cut depends on circumferential water pressure and where it is independent of water pressure.4) It is found that there exists an optimum water pressure and optimum standoff distance from the view point of energy consumption.5) The width of slot rapidly increases with water pressure until it reaches almost constant value.6) High speed waterjets can be applied to making a slit in deep borehole for hydraulic fracturing.