Case Study of Performance Improvements Offshore Drilling Carbonates Using Specially Designed PDC Bits

Abstract

In the 12 ¼-in. section of an Abu Dhabi offshore company's wells, complex directional well profiles and varied formation characteristics create a challenging drilling environment. Outcomes of the drilling efforts in this environment include non-productive time (NPT) as a result of tripping, as well as lower rates of penetration (ROP) as a result of vibration. Conventional positive displacement motors (PDM) were typically used in this offshore application. Polycrystalline diamond compact (PDC) bits were run with the PDMs because of their longer life and increased ROP through the varied formations; the particular designs used had provided good steerability in previous applications. The conventional PDM-PDC system, however, often creates high vibrations that reduce overall performance. In addition, the section has always required two runs so that the bent housing could be changed in the second run, which resulted in lost time for tripping. To address these issues, a new system was designed that provides significant advantages over conventional steerable assemblies. This system includes a specially designed long-gauge PDC bit run on a point-the-bit rotary steerable tool, both of which were designed and modeled together. The advantages observed using this system in the field includes improved hole quality and increased ROP, as well as reduced hole spiraling and tortuosity, reduced vibration, better steerability and hole cleaning, increased bit life, and reduced tripping time. This paper provides three case studies of the use of this system in which one of Abu Dhabi offshore operators achieved the objective of drilling the 12 ¼-in. interval, which averaged more than 2,500 ft, in a single run, and optimized the ROP without compromising hole quality or cleaning efficiency. A summary of results indicates the effectiveness of design improvements, with the new bits achieving the highest single run ROP in history for this particular operator and longest footage achieved in a single run. These benefits are expected to have considerable implications on future drilling operations. Introduction For our offshore client, saving drilling time and improving ROP without compromising the hole quality was always important. For this reason, after drilling the first two wells in their field, the client's drilling team conducted a workshop and invited all service providers to call for an overall improvement. Increased ROP was only part of the company's goal because increasing only the ROP can lead to issues related to wellbore stability and hole cleaning. In addition, the combination of increased ROP with formation characteristics can create severe vibrations, which increase the probability of malfunction of drilling tools. Concentrating on hole quality only without allowing optimum drilling parameters was not the ultimate goal; therefore, the balance of improving the ROP, drilling efficiency, and hole quality was set as the first goal to achieve. Two different drilling bottomhole assemblies (BHAs) were used to accomplish the drilling of the 12 ¼-in hole section, as requested. The first run was usually almost vertical and a new or used PDC bit was normally used. A bent housing was usually added for a slight build-up and to continue the tangent section for the remainder of the hole. As a result of using a normal PDC bit and conventional motors, the following issues were always encountered while completing the course of this section:Lower ROP resulting from the directional work and, in some intervals, from severe vibrationsFaster bit wear as a result of hole condition and spiralingDifficulties and extra NPT while running the casing as a result of hole quality