The assessment of correlation between rock drillability and mechanical properties in the laboratory and in the field under different pressure conditions

• Rock mechanical properties and drillability were separately measured.
• Rock porosity, fluid pressure and pore pressure determined confined pressure.
• The confined pressure affected drillability grade and compressive strength.
• The correlations became better with the increase of confined pressure.

Drillability, used for evaluating penetration rate and in selecting suitable bit types, is greatly influenced by the mechanical properties of rock. To investigate the correlation between drillability and rock mechanical properties, compressive strength, elastic modulus and Poisson's ratio were measured separately under uniaxial, 30 MPa and 60 MPa confined pressure conditions. Drillability numbers were obtained using Rollow's microbit method. Regression analysis results demonstrated the correlation between drillability grade and rock mechanical properties. The correlation improved with increased confining pressure. Additionally, the best correlation was shown between drillability grade (DG) and compressive strength under varying confined pressure conditions, as opposed to the correlation between DG and the elastic modulus or DG and Poisson's ratio under uniaxial conditions. The calculation results of rock compressive strength and DG under varying confined pressure conditions illustrated that the rock effective porosity, wellbore fluid pressure and formation pore pressure played an important role in determining the confined pressure at bottom hole. This further affects DG and compressive strength distribution along the wellbore during field drilling process. These preliminary studies could contribute significantly to understanding the effect of rock mechanical properties on confining pressure and thus on safety and efficiency of drilling in the field.