Application of New Generation Large Roller-Cone Bits Reduces Drilling Costs In Eastern Venezuela

Abstract

Abstract An analysis of recent run data from two fields in Eastern Venezuela documents large metal seal bearing roller cone bits (16" & 17–1/2"), with a new generation steel tooth cutting structure, can be used for a much greater number of total bit revolutions than prior generation bits. The new generation metal seal has proven its ability to significantly extend bearing life and reliability and thereby provide significant reductions in drilling costs. Penetration rates have more than doubled with total footage up 89% in some instances, with less overall cutting structure wear. Cost savings of up to $121,000 US per well have been documented. The authors will describe the application of these new bits and present case studies that document significant performance improvement in the Santa Barbara and Mulata fields of Eastern Venezuela. They will also describe the development of the latest generation metal sealed bearing package and the metallurgical and design improvements of the new steel tooth cutting structures. Application Background Since 1990, over 400 wells have been drilled in the oilfields surrounding Punta de Mata in Monagas County of Eastern Venezuela. Since 1994, a turnkey operator has drilled over 55 wells, under contract with Petroleos de Venezuela S.A. (PDVSA). These wells were mainly in the Santa Barbara, Mulata and Pirital Fields (Figure 1). The hole sizes are generally 26", 17–1/2", 12–1/4" and 8–3/8". The general casing program is depicted in Figure 2. The 17–1/2" hole section is typically vertical and generally drilled from 1,000 ft to 7,000 ft, measured depth (MD). However, several 17–1/2" sections were drilled directionally when high vertical displacement was required. By the end of the operator's turnkey contract with PDVSA, a much higher number of 17–1/2" directional sections were required. The 12–1/4" interval is where the majority of the directional work takes place in these "S" type wells. The subject wells are typically drilled to the Naricual (Triassic) or KP (Jurassic) sandstones. Formation penetrated during the drilling of the 17–1/2" hole section includes Mesa-Las Piedras, La Pica and Carapita Shale formations. In some instances the Sigmolina and Textularia formations are penetrated. All of these formations are Pleistocene to Miocene in age (Figure 3). The Mesa-Las Piedras formation is comprised of soluble claystones with sandstone stringers and coal seams. Additionally, some limestone stringers and siltstones can be encountered. The La Pica formation is comprised mainly of claystones and hydrateable shales. The Sigmolina and Textularia are also shales, with high amounts of claystone and siltstone. Rate of penetration generally slows when these formations are encountered. The 17–1/2" hole section usually bottoms in the Carapita Shale where 13–3/8" casing is set. Open hole sonic logs are not run in the 17–1/2" section, thus unconfined compressive strengths are not available. Attempts to drill the entire section with a single PDC bit have resulted in inconsistent performance. This is due mainly to durability problems with light set PDC bits while drilling the hard sandstone stringers, and balling problems with heavy set PDC bits while drilling the reactive claystones.