High pressure jet drilling effect in chalk and alteration of local geomechanics properties surrounding the radial hole

This study presents an improved understanding of how high-velocity fluid jet affects the mechanical properties of chalk, building a solid ground for the borehole stability analysis. Our laboratory experiments on water and acid-aided water jet-drilling of chalk samples have shown that high-pressure jet drilling alters the local mechanical properties of the chalk, resulting in a significant weakening of strength properties (tensile, uniaxial/triaxial compressive and pore collapse stress) in close proximity of the jet hole. Acid-aided water has even more impact on chalk matrix as revealed by Scanning Electron Microscopy (SEM) analysis. Numerous micro-perforations are found on the surface of microsparites (calcite). However, no extensive dissolution of calcite cement was observed in the chalk matrix. Results show that mechanical properties degradation is the outcome of both dissolution of the cementation and micro-perforations created on the calcite surface.