牙轮钻头的岩屑破碎机理及可钻性的分形法

Breaking cuttings mechanism of cone bits and fractal method for rock drillability

为满足优质高速钻井的要求,需要对岩石可钻性做出快捷、精确的判断。利用分形几何理论,研究了钻井过程中牙轮钻头的破岩机理以及岩屑的等破碎概率问题;依据Bond裂纹学说,提出了一种以井底返出的岩屑为对象的岩石可钻性测试方法,并建立了岩石可钻性极值和岩屑分形维数的关系模型。室内实验表明:微钻头岩石可钻性级值与破碎岩屑分形维数存在明显的相关性,由此制定了一个基于岩屑粒度分布分形维数表示的岩石可钻性预测指标。经现场检验,分形法确定岩石可钻性级值具有较高的精度。这表明钻井过程中上返岩屑的分形维数能反映岩石可钻性程度,可以用分形维数衡量岩石可钻性大小。

To fulfill the need of high drilling quality and high drilling speed, more rapid and precise observations need to be done on rock drillability. This paper analyzes the rock breaking mechanism of cone bits during drilling and the equiprobable breaking cuttings by using the fractal geometry theory. According to Bond crack theory, this paper sets forth a new rock drillability testing method which takes the detritus from the bottom hole as the research subject. A correlation model of drillability grades and cuttings fractal dimensions is established. The laboratory test shows that there is high correlation between micro-bit drillability grades and cuttings fractal dimensions. Based on this, a predicting index for rock drillability is established, which is shown by the fractal dimension and distribution of the cuttings size. Practically it proves that the fractal method has higher precision to ascertain rock drillability grade. The rock drillability can be reflected from the fractal dimension of upward cuttings during drilling. So fractal dimensions can be used to measure rock drillability.