Research on borehole stability of shale based on seepage-stress-damage coupling model

摘要

In oil drilling, one of the most complicated problems is borehole stability of shale. Based on thetheory of continuum damage mechanics, a modified Mohr-Coulomb failure criterion according to plasticdamage evolution and the seepage-stress coupling is established. Meanwhile, the damage evolution equationwhich is based on equivalent plastic strain and the permeability evolution equation of shale are proposed in thispaper. The physical model of borehole rock for a well in China western oilfield is set up to analyze thedistribution of damage, permeability, stress, plastic strain and displacement. In the calculation process, theinfluence of rock damage to elastic modulus, cohesion and permeability is involved by writing a subroutine forABAQUS. The results show that the rock damage evolution has a significant effect to the plastic strain andstress in plastic zone. Different drilling fluid density will produce different damage in its value, range and type.This study improves the theory of mechanical mechanism of borehole collapse and fracture, and provides areference for the further research of seepage-stress-chemical-damage coupling of wall rock.