First Successful Drilling and Underreaming of Hard, Chert-Rich Formations

Abstract

Historically hole-opening and underreaming operations have been restricted to softer formations. This paper describes what is believed to be the first successful drilling and underreaming of a very hard, chert-rich formation.Many producing wells have been drilled into the deep Cretaceous formations of south Mexico. These consist primarily of hard limestones containing dolomite stringers, large chert nodules, with very erratic hardness and some natural fractures; UCS estimates range from 17,000 to 30,000 psi. On average it takes three TCI bits and two or three PDC bits to drill the Cretaceous interval, which is typically from 4,800 m to 5,400 m TVD.After several unsuccessful attempts to drill exploratory wells into Jurassic formations below the Cretaceous, the operator redesigned these wells with a contingency casing string. Rig hoisting limitations required each hole interval between conductor and the Jurassic to be underreamed -larger diameter casing strings could not be handled safely. This meant drilling the Cretaceous formation in 8 ½-in. hole and underreaming to 12-in. diameter in order to accommodate the desired 9 5/8-in. casing.A new eccentric fixed-blade reamer and pilot PDC bit were designed for this application, using latest technology cutters, and profiles, cutter layouts and backrakes, which provided the highest possible durability and thereby gave the best chance of withstanding the high, erratic rock hardness and chert nodules.The entire Cretaceous interval was successfully drilled and underreamed in a total of five runs. In each case, the assembly was tripped for logging, coring or section TD, with the pilot bit and reamer in good condition. Logging confirmed the desired 12-in. diameter hole. This case history confirms the ability to simultaneously drill and underream hard, chert-rich formations, in this instance greatly reducing the operator's risk of failing to reach their target and giving a wider range of completion options.