Drill bit, system, and method for drilling a borehole in an earth formation

A drilling system for drilling a borehole in an earth formation, comprising;

• a drill string provided with a drill bit having a central longitudinal axis and being operable by applying at least a rotary motion about the central longitudinal axis and optionally applying longitudinal reciprocal movement to the drill bit so as to exert a percussive force on the borehole bottom,
• the drill bit comprising a surface provided with a plurality of shear cutters having a rake surface arranged to induce a scraping movement along the borehole bottom upon application of the rotary motion, the rake surface during operation facing the direction of rotation at a rake surface back-rake angle of less than 90°, wherein the rake surface back-rake angle is defined as the angle included between the projection of a line perpendicular to said rake surface on a plane defined by the central longitudinal axis and the direction of the tangential velocity component of the shear cutter and a plane perpendicular to said longitudinal axis, and
• wherein one or more of the shear cutters is provided, in addition to the rake surface, with a pre-cut flat impact surface oriented essentially parallel to the plane perpendicular to the longitudinal axis;
• and
• wherein the rake surface of each shear cutter has a secondary inclination relative to the radial direction of the drill bit, the secondary inclination being such that in operation the rake surface pushes drill cuttings from the rock formation in radially outward or radially inward direction;
• the drilling system further comprising rotary drive means for rotating the drill bit in the borehole about the bit's central longitudinal axis so as to induce a scraping movement of the shear cutters along the borehole bottom.

A method of drilling a bore hole into an earth formation, comprising the steps of

• providing a drilling system comprising
• a drill string provided with a drill bit having a central longitudinal axis and being operable by applying at least a rotary motion about the central longitudinal axis and optionally applying longitudinal reciprocal movement to the drill bit so as to exert a percussive force on the borehole bottom,
• the drill bit comprising a surface provided with a plurality of shear cutters having a rake surface arranged to induce a scraping movement along the borehole bottom upon application of the rotary motion, the rake surface during operation facing the direction of rotation at a rake surface back-rake angle of less than 90°, wherein the rake surface back-rake angle is defined as the angle included between the projection of a line perpendicular to said rake surface on a plane defined by the central longitudinal axis and the direction of the tangential velocity component of the shear cutter and a plane perpendicular to said longitudinal axis, and
• wherein one or more of the shear cutters is provided, in addition to the rake surface, with a pre-cut flat impact surface oriented essentially parallel to the plane perpendicular to the longitudinal axis;
• and
• wherein the rake surface of each shear cutter has a secondary inclination relative to the radial direction of the drill bit, the secondary inclination being such that in operation the rake surface pushes drill cuttings from the rock formation in radially outward or radially inward direction
• placing the drill bit against the subterranean earth formation that is to be drilled;
• exercising a rotary motion about the longitudinal axis while maintaining a force on the drill bit against the earth formation in the axial direction;
• and
• optionally intermittingly providing percussive strikes on the drill bit.