Downhole rebound hardness measurement while drilling or wireline logging

A method of drilling a wellbore, comprising:

• conveying a tool having a testing surface, an orientation sensor and a rock density sensor into a wellbore in the formation;
• determining an orientation of the testing surface using the orientation sensor;
• propelling the testing surface in a selected direction corresponding to the determined orientation to impact the formation, wherein an energy of impact of the testing surface against the formation is affected by a component of gravitational force along the selected direction;
• determining the rock density of the formation using the rock density sensor;
• using a processor to:
• select a hardness scale for the determined orientation,
• measure a rock density of the formation using a downhole sensor,
• select a linear relationship between the selected rock hardness scale and rock strength for the measured rock density, and
• determine a rock strength of the formation indicated by the measurement of hardness using the selected hardness scale and the relationship between the selected rock hardness scale and rock strength, and
• affect a parameter for drilling the wellbore using the estimated rock strength.

An apparatus for estimating a rock strength of a formation, comprising:

• a tool configured to be conveyed into the wellbore;
• a testing surface disposed on the tool configured to propel against the formation in a selected direction and rebound from the formation, wherein an energy of impact of the testing surface against the formation is affected by a component of gravitational force along the selected direction;
• and
• an orientation sensor disposed on the tool configured to determine an orientation of the tool in the wellbore;
• a rock density sensor disposed on the tool;
• a processor configured to:
• obtain a measurement of hardness of the formation under in-situ stress conditions from a rebound of the testing surface from the formation,
• obtain the orientation of the tool from the orientation sensor,
• obtain the rock density of the formation from the rock density sensor,
• select a hardness scale for the determined orientation,
• measure a rock density of the formation using a downhole sensor,
• select a linear relationship between the selected rock hardness scale and rock strength for the measured rock density, and
• determine a rock strength of the formation indicated by the measurement of hardness using the selected hardness scale and the relationship between the selected rock hardness scale and rock strength.