Experimental evaluation of hardness models by drillability tests for carbonate rocks

• Drilling tests are performed for carbonate rocks.
• Different hardness indices are studied and the best one for carbonate rocks is presented.
• The relationship between hardness indices and Drilling Rate Index (DRI) is studied.
• Rate of Penetration has been correlated as a function DRI.

Drillability, specific energy, resistance to drilling alteration index and several other indices have been proposed to estimate the rock capability to fail (inversely proportional to hardness). Specific energy, resistance to drilling and alteration index are the indices that apply Rate of Penetration (ROP) and operational variables (weight on bit and rotary speed) to quantify the rock hardness. Using a laboratory drilling setup, values of these indices were calculated for six different carbonate rocks in more than 20 different operational conditions. Experiments showed that specific energy has the highest consistency with ROP measurements and is the best parameter to quantify hardness of carbonate rocks. In addition, Drilling Rate index (DRI) was measured for the available samples as an indirect indicator of drilling rate. Specific energy and resistance to drilling reveal a linear correlation with DRI, while alteration index has a polynomial correlation with DRI. Specific energy, resistance to drilling and alteration index can be calculated for carbonate rocks using these correlations with DRI measurements (without performing ROP tests). In addition, ROP shows a polynomial correlation with DRI when weight on bit and rotary speed are constant.