Stress release mechanism of deep bottom hole rock by ultra-high-pressure water jet slotting

To solve the problems of rock strength increase caused by high in-situ stress,the stress release method with rock slot in the bottom hole by an ultra-high-pressure water jet is proposed.The stress conditions of bottom hole rock,before and after slotting are analyzed and the stress release mechanism of slotting is clarified.The results show that the stress release by slotting is due to the coupling of three factors:the relief of horizontal stress,the stress concentration zone distancing away from the cutting face,and the increase of pore pressure caused by rock mass expansion;The stress concentration increases the effective stress of rock along the radial distance from O.6R to 1R(R is the radius of the well),and the presence of groove completely releases the stress,it also allows the stress concentration zone to be pushed away from the cutting face,while significantly lowering the value of stresses in the area the drilling bit acting,the maximum stress release efficiency can reach 80%.The effect of slotting characteristics on release efficiency is obvious when the groove location is near the borehole wall.With the increase of groove depth,the stress release efficiency is significantly increased,and the release range of effective stress is enlarged along the axial direction.Therefore,the stress release method and results of simulations in this paper have a guiding significance for best-improving rock-breaking efficiency and further understanding the technique.

Flow structure and rock-breaking feature of the self-rotating nozzle for radial jet drilling

For improving the hole-enlarging capability,roundness and rock-breaking efficiency of the nozzle in radial jet drilling,a new structure of self-rotating nozzle was put forward.The flow structure and rock-breaking features of the self-rotating nozzle were investigated with sliding mesh model and labortary tests and also compared with the straight and the swirling integrated nozzle and multi-orifice nozzle which have been applied in radial jet drilling.The results show that the self-rotating jet is energy concentrated,has longer effective distance,better hole-enlarging capability and roundness and impacts larger circular area at the bottom of the drilling hole,compared with the other two nozzles.Forward jet flow generated from the nozzle is peak shaped,and the jet velocity attenuates slowly at the outer edge.Due to periodic rotary percussion,the pressure fluctuates periodically on rock surface,improving shear and tensile failures on the rock matrix and thereby enhancing rock-breaking efficiency.The numerical simulation results of the flow structure of the nozzle are consistent with the experiments.This study provides an innovative approach for radial jet drilling technology in the petroleum industry.

Erosion characteristics and mechanism of the self-resonating cavitating jet impacting aluminum specimens under the confining pressure conditions

In order to study the effects of the confining pressure on the erosion characteristics of the self-resonating cavitating jet under wellbore and deep-water conditions,experiments are conducted on aluminum specimens impinged by the organ pipe cavitation nozzle and the conical nozzle with the confining pressure in the range 0 MPa–10.0 MPa.Meanwhile,through the numerical simulation of the collapsing process of the cavitation bubble and the noise test,the cavitation erosion mechanism is analyzed.The experimental results show that the optimal standoff distance and the confining pressure can be obtained for the maximum erosion quantities,and the optimal standoff distance is 5 to 7 times greater than the equivalent nozzle outlet diameter and the confining pressure is about 2.0 MPa.Under the same conditions,the erosion caused by the cavitation nozzle is up to 2 times larger than that caused by the conical nozzle.According to the numerical simulation and the noise test,the cavitation erosion on the aluminum specimens is mostly caused by mechanical forces due to the high-frequency pressure pulse generated during the collapse of cavitation bubbles,while just a small part is caused by micro-jets.