Abstract

A major operator has been drilling deep liquefied natural gas (LNG) wells for several years in central Oman. These wells are typically drilled vertically, featuring a complex geological sequence, that provides a significant challenge to bit performance.

Historically these wells have been drilled in 90 days with Tungsten Carbide Insert (TCI) bits on packed rotary assemblies down to the 9 5/8” casing shoe and Impregnated bits on turbine assemblies with occasional TCI runs in the 8 3/8” section.

A detailed study of the LNG wells focused on drilling performance in each of the three hole sizes in a typical well profile. The findings suggested that the introduction of new PDC technology was viable over much of the intervals and that significant savings in terms of time and cost could be achieved.

This paper reviews the findings of the study subsequent recommendations and the application of advanced PDC technology and stable PDC bit designs. The resulting improved performance is demonstrated by examples shown from two separate fields in central Oman where significant savings were achieved in terms of both rig time and cost per meter.

The collaboration between the Operator, rig contractor, bit vendor and other service companies was necessary to provide the platform for a successful operation.

Introduction

Bit Optimisation can be defined as a continual process, whereas bit selection is defined as a single step. The purpose of a bit optimisation service is to provide a more dedicated engineered and focused approach to bit selection in order to consistently deliver more productive wells with best in class performance at a lower overall cost for the operator.

The operator in Oman had used a bit optimisation service provided by two vendors for a number of years to optimize bit selection. The bit optimisation engineer(s) (BOE) from each vendor would be the focal point for bit selection, working as an integral part of the multi-disciplinary team lead by the senior well engineer, but also including the well engineer, well site drilling supervisor, drilling contractor personnel and representatives from additional service providers.

The operator awarded a third vendor (Vendor C) a rig for a trial period in early 2003 in order to evaluate the vendors bit optimisation service and to gain access to some emerging unique and novel technologies.

The applied methodology focused upon understanding the mechanisms of the entire drilling environment, identifying risk and solving down hole problems with the objective of completing each hole section safely with maximum efficiency. The dedicated optimisation engineer, equipped with the necessary tools and processes together with a comprehensive range of products at his disposal would be expected to deliver quantifiable performance improvements throughout the course of the project.

Background

Campaign drilling has been conducted in a number of fields located in central Oman, most notably in Saih Rawl and the adjacent Saih Nihayda.

The variable geology (Figure 1) encountered provides a significant challenge to optimizing bit selection and performance

The 17½” hole section is drilled into the top Natih formation at around 950m. This section has historically been most economically drilled with TCI and Tooth bits in around four to eight days.