Abstract

Bit optimization played a key role in a recent drilling program by an oil company in its Pilot Production Drilling campaign in the Ruby Field Offshore Vietnam. Advancements in roller-cone and PDC bit technology improved performance significantly. These advancements enabled the operator to save time on the 10 well plus 2-workover project and come in under budget. This paper documents the time savings and the development of the crucial roller cone bit technology.

Introduction

The Ruby Field is located in the southwestern part of Block 01 approximately 155 km east of the port of Vung Tau, Vietnam in approximately 50 meters of water (Figure 1). The initial plans included drilling nine basement granite wells. The scope of the project was later changed to 10 wells plus two workovers that included four basement granite wells and six Miocene wells. All the wells had various directional requirements with proposed TD varying from 2200 meters to 3600 meters measured depth. Drilling fluids to be used included basic gel chemical and seawater for the upper sections in addition to KCL polymers and other systems depending on the individual well. The drilling campaign documented in this paper started on August 18, 1998 and was completed on August 17, 1999.

The RB1P and the RB-2P wells were the first development wells drilled in the Ruby Pilot Production Phase Drilling Program. Both wells were drilled from one slot with separate wellheads using a slim twin wellhead technology. This slim twin well design was new technology implemented as a cost reduction initiative and was the first of its kind to be drilled in Vietnam.

Lithology of Ruby Field is fairly homogeneous with Miocene sediments, Oligocene below the Miocene and basement granite one of the main formations to be drilled. Generally, the upper hole section is characterized by mudstone, siltstone and shale with low unconfined compressive strength (UCS) in the range of 6 to 8000 psi. The Oligocene has some volcanic intrusive and extrusive with UCS as high as 25000 psi but averaging about 15-20000 psi. The granite basement is hard and abrasive with UCS as high as 30,000 psi and a sonic value of 50m/sec ft (Figure 2). Both the Miocene and granite basement well programs were designed using the latest bit technology for optimum performance. Each well posed a different directional configuration and challenge with regards to bit selection. The Miocene well plans were optimized for directional control and maximizing rate of penetration (ROP). To optimize bit selection for drilling the basement wells, the project team focused on bit life, bearing reliability and drilling an in-gauge hole. Previous experience in the field had documented bits utilized in the igneous section were exposed to a very harsh environment. Issues with bits in previous granite drilling campaigns included catastrophic failures in addition to under-gauge hole sections that required reaming adding significantly to overall well cost.

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