Separating Bit and Lithology Effects from Drilling Mechanics Data

Abstract

Abstract Most real time drilling interpretation is performed using some sort of normalized rate of penetration to infer changes in lithology and diagnose the condition of the bit. This frequently leads to ambiguous or confusing results because it is impossible to separate all bit effects from lithology effects with only one measurement. This paper shows how a second measurement, downhole torque, can be used with rate of penetration to confidently separate the bit effects from the lithology effects when drilling with milled tooth or PDC bits. Simple drilling equations for rate of penetration and bit torque are derived and compared with well known formulae. These lead to simple "quicklook" interpretations which can be made at the wellsite with a hand calculator. A detailed analysis can be performed on a wellsite computer. Many field case studies were made to examine the effects of lithology changes on the drilling response of milled tooth and PDC bits. A few of the studies are presented. The results indicate that: changes in bit torque can be used to broadly classify the lithology into three categories, namely: porous, argillaceous (shaly), and tight, corresponding to high, medium and low torque respectively, trends in bit torque and rate of penetration (ROP) in shale type formations can be used to separate the wear of milled tooth and PDC bits from changes in shale strength, it was not possible to interpret bit wear in non-shaly type formations, surface drilling measurements can be insensitive to major formation changes (eg. sand/shale boundaries), particularly in deviated wells.