Research On The Damage Of Hard And Brittle Shale Based On Wave Velocity

Shale gas development mainly depends on the horizontal drilling technology and segmented fracturing technology,and instability of borehole are complex problems often encountered in the course of drilling.In the process of drilling,with the change of time,shale will occur hydration damage.A method used the acoustic velocity measurement to describe the variation of the shale was proposed in this paper.Velocity of shale,sensitive to stress,makes it possible by elastic wave velocity on stress susceptibility evaluation shale properties,also keeps the experimental samples without damage.Can be carried out on a sample test repeatedly,by this method,also provides experimental study on shale hydration.The specific research contents include the following aspects.Research the properties of propagation of sound waves in the shale.Shale formation has strong anisotropy,and it is need to distinguish between group velocity and phase velocity to improve the accuracy of parameter inversion.Selected a suitable model for elastic parameters inversion of shale.Referred a model based on the effective medium theory which contained the natural properties and micro-structure in shale.Using the change of micro-pores to describe the damage of shale.Designed and conducted shale wave velocity measurements under different confining.Designed shale hydration experiment,found that shale will injury with the effect of hydration.Experimental results showed that the Longmaxi Shale has lower velocity than the Wufeng shale,and is significantly affected by the confining pressure,on the contrary,the Wufeng shale affected by the confining pressure is not obvious.Elastic modulus of the Longmaxi shale calculated by the velocity were smaller than the Wufeng shale,the Longmaxi shale has strong anisotropy.Elastic modulus of shale increased with confining pressure,and anisotropy decreased with confining pressure.Analyzed the acoustic experimental results can found that micro-fracture-pore in the shale sample has distribution direction.Shale in the vertical direction was complete and the crack will develop along the direction of bedding.With the increase of confining pressure,these cracks will gradually shut down,and shale will show the inherent properties of the matrix itself under high confining pressure.The micro-cracks in the Longmaxi shale were more development than the Wufeng shale,and therefore the damage parameters of the Longmaxi shale were far above the Wufeng shale under the same confining pressure.With the result of direct shear experiment,shale will injury because of hydration,and the cohesion and internal friction angle will decline in the macro-performance.The cohesion and internal friction angle of rock both decreased after the rock sample soaked in mud.Shale compressive strength decreased with the soak time,the compressive strength and weakening rate showed anisotropy,water-based mud effects are more obvious,shale strength decreased rapidly after soak,while oil-based mud can be better to keep the strength of the rock sample.The result of linear expansion test found that the characteristics of Longmaxi shale was match with brittle shale.The velocity and elastic modulus of the Longmaxi shale decreased after the rock sample soaked in mud.With the change of time,the micro-cracks increased perpendicular to the bedding plane much than parallel to the bedding plane,but the micro-cracks are most along the direction of parallel to the bedding plane.This study deepened the understanding of brittle shale hydration about the wellbore instability,put forward a method based on wave velocity measurements to analyze the properties of shale after the drilling mud percolated through the wellbore,borehole stability study on shale gas will play a good role in guiding.It will play a very good role in the research of shale gas wellbore stability.