| The deep oil and gas resources in our country have great potential for exploitation.With the number of deep wells and ultra-deep wells increasing gradually year by year,the contradiction between the exploitation of deep oil and gas resources and the problem of low rate of penetration(ROP),long drilling cycles and high costs in deep hard and strong abrasive formations becomes more and more poignant,restricting the efficiency and benefit of deep oil and gas resources exploitation.The high hardness and strong abrasive rigid particles in particle water jet could break the rock with high velocity and frequency.With these particles,particle impact drilling technology could reduce the rock breaking threshold pressure and the specific energy consumption,improving the rock breaking efficiency and ROP in deep hard and strong abrasive formations.Based on the wave equation of sphere stress in rock medium,stress components of the rock mass point was analyzed to build the mathematical model of stress wave of particle impacting rock.And to study the dynamic process,damage evolvements,and failure mechanisms of rock under the impact of particle water jet,the smoothed particle hydrodynamics method(SPH)was coupled with the finite element method(FEM)to combine their advantages.Based on a stochastic algorithm,rigid particles built by FEM were dispersed among water built by SPH to construct a new mechanical model of a particle water jet that can actually reflect the large-diameter and low-concentration features of the solid phase in a particle water jet.The conformity of simulation results and indoor experiments showed that rock fragmentation was achieved due to the impact dynamic load and rock softening caused by the stress wave: initial cracks were formed under the impact pressure;rock fragments were formed by the propagation and penetration of cracks under the comprehensive effect of tension stress and shear stress;unloading resulted in the tensile failure of rock;the convergence and connection of the cracks caused by the synergistic effect of water jet;propagation of the stress wave increased the range of the rock damage field.The differential equation of particle movement was built with liquid bridge force model and discrete element method.The dynamic movement process and the distribution state of the particles in high pressure vessel were analysised by the differential equation of particle movement.The mechanism of the instability and blocking of the particles injection process was analysised then and a criterion to evaluate the stability of particles injection was set.The modulation method of pre-mixed particle water jet was built.The study of control mechanism of the uniformity and stability of particles in pre-mixed particle water jet showed that the funnel flow is major reason causing the instability and blocking of the paritlces injection process.The special designed modulation part could change flow condition of the partilces in high pressure vessel from funnel flow into bulk flow,solving the instability and blocking problem effectively.The pre-mixed particle water jet modulation system based on that mechanism worked well.The particles injection process was stable and particles blocking accidents had never arised,indicating that the pre-mixed particle water jet modulation method in this paper was applicable and reliable.The indoor experiments of particle water jet impacting rock under confining pressure were conducted with the pre-mixed particle water jet modulaton system.The effect of generation method of particle water jet,flow channel structure of jet nozzle,confining pressure,jet angle,standoff,jet time,particle volume fraction and paritlce diameter on rock breaking efficiency were studied.The results showed that with a higher compressive strength of the rock,the reduction of the rock compressive strength caused by particle water jet impacting became larger but the dimensionless depth and dimensionless volume of the rock breaking holes decreased.The dimensionless depth and dimensionless volume of rock breaking holes increased greatly at first and the increasing rate reduced sharply with the increase of jet time.The dimensionless depth and dimensionless volume of rock breaking hole decreased nonlinearly with the increase of confining pressure.The jet angle,standoff,particle volume fraction and particle diameter had significant effect on rock breaking efficiency and the optimal ranges existed.The optimized range of dimensionless jet angle was 0.62~1.31,the optimized dimensionless standoff was 7.5,the optimized range of dimensionless particle diameter was 0.35~0.5 and the recommended paricle volume fraction was 2.5%~3.5%.In addition,it showed that pre-mixed particle water jet had higher energy transfer efficiency than post-mixed particle water jet.Based on similarity theory,using spatial vector and solid geometry method,a particle impacting rock breaking bit whose jet angle could be adjusted continuously was designed and the structure of the bit was optimized then.The rock breaking experiments were conducted with the experimental bit.The results showed that the request that forming a rock breaking hole covering the whole bottomhole under non-contact,comprehensive drilling condition could be satisfied.The bit could form a continuous and integral rock breaking hole totally relying on the particle water jet impacting at a relative high efficiency.The rotating speed had little effect on rock breaking efficiency and the feeding speed of the bit had significant effect on the rock breaking efficiency and the shape of the rock breaking holes.The feeding speed of the bit should be controlled according to the lithology of the formation to aquire a relative high ROP.Theoretical analysis,numerical simulation and experiments were combined in the studies of this paper,including the dynamic process,damage evolvements,and failure mechanisms of rock under the impact of particle water jet and the mechanism of pre-mixed particle water jet modulation.The corresponding pre-mixed particle water jet modulation system was developed,and the particle impacting drilling bit was designed.The technical route that the comprehensive rock breaking drilling was achieved totally by the impact of particle water jet was verified by rock breaking experiments with the designed bit.The conclusions in this paper laid the foundation of the development of paricle impacting dirlling and its application in petroleum engineering. |
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