Characteristics of stress release during downhole slotting by high-pressure water jet

The high stress of rocks in deep or ultra-deep wells is one of the major reasons for quick bit wear and low rate of penetration.Therefore,high-pressure water jet technology is widely used in the field of well drilling engineering because it can greatly improve the rate of penetration.According to the traditional theory of rock breaking by high-pressure water jet,it is considered that water jet can play a role in rock cleaning and auxiliary rock breaking,but generally little attention has been paid to the impact of the slotting by water jet on the distribution of downhole rock stress state.Further,in combination with the combined rock breaking process of drilling bit and high-pressure water jet,this paper proposes a method for stress release during downhole slotting by high-pressure water jet.Based on Biot's theory of poroelastic mechanics,a physical model of deep formation has been established to analyze the distribution patter of effective rock stress and the release degree of slotting stress at different well depths,in-situ stress states and drilling fluid differential pressures.Based on the mechanical properties of granite at different confining pressures,a user-defined function of compressive rock strength is used to analyze the influence law of downhole slotting on the rock strength distribution characteristics.The research results indicate that the stress release efficiency constantly rises as the well the depth increases,the effective stress release efficiency along the axis of well bottom is above 70%,and that in the peripheral area of the bottom hole is about 50%;the rise of drilling fluid pressure has lowered the stress release efficiency,and the stress release efficient along the axis of well bottom is greatly affected by drilling fluid pressure;the water-jet slotting has significantly reduced the strength of rocks along the axis of well bottom,the release efficiency can reach above 50%,and the strength release efficiency of rocks around the well bottom is around 30%;the existence of downhole slotting has released the effective rock stress and can push the concentrated areas of downhole rock stress away from the cutting surface,resulting in the lower stress value of the drilling zone.This has reduced the rock strength to a certain extent,thus improving the penetration rate. © 2022 Science Press. All rights reserved.