Drilling Interbedded and Hard Formations with PDC Bits Considering Structural Integrity Limits

Abstract Although PDC bits dominate the footage drilled in the oilfield, transitions at interbedded formations and high lateral vibrations are still problematic for PDC bits and reamers. This paper describes design features, operational parameters, and procedures found to substantially reduce cutter damage, often allowing the section to be drilled in one run, resulting in significant cost savings. It also covers data analysis needed to determine if interfacial severity is the key limiter in a given application. Time and depth based drilling mechanics data are used to determine the sections of the hole that cause the observed cutter damage and optimize parameters. Forensics analysis is used to determine the type of cutter damage seen when drilling these formation intervals. Structural integrity of the cutters is estimated from forensics analysis, lab tests, as well as stress analysis of cutters loaded to replicate the observed damage. Review of the bit and BHA designs often show opportunities for design improvements and cutter selection. Drilling parameters are identified to keep the bit and cutters below the structural integrity limit. Drilling interbedded formations causes the cutters near the nose of the profile to fracture from tangential overload, through the diamond table and carbide support. This is a significantly different failure mode than the cutter spalling fracture on the bit shoulder seen with bit whirl. Review of drilling mechanics data showed significant reamer damage and reductions in ROP at the formation transitions when entering salt with a bit and reamer combination. Drilling parameter and BHA design changes improved drilling performance in these transitions. Finite element studies were requested of bit vendors to estimate the structural integrity of cutters loaded in the tangential direction. Independently, forensic analysis was also used to estimate the range of loads and depth of cut that cause structural overload. Drilling interbedded formations and transitions at a controlled depth of cut, as opposed to weight on bit control, avoids exceeding the structural limit of the cutters while only part of the cutting structure is drilling the harder formation and the rest of the profile is in softer formation. Depth of cut elements are engaged to keep the bit stable while control drilling. Other design changes are considered to eliminate cutter overload using existing designs as a starting point. The structural integrity concept applies to hard, interbedded, conglomerate and/or vuggy formations, as well as formation transitions and shoe drill out and milling operations. Effective drilling of such interfaces can be achieved by knowing the depth of cut that can be sustained in the harder formation, while avoiding damage from vibrations. Cutters must be selected for fracture resistance in the tangential direction, and many bit vendors do not yet track this property.