Abstract

Drilling the 12 ¼” and 8 ½” sections in offshore Abu Dhabi’s Deep Khuff fields is very challenging due to an intricate geological sequence that consists of limestone, dolomite and shales that are highly interbedded with hard stringers. Historically, ADMA-OPCO, the operator, has attempted to efficiently drill the section using roller cone tungsten carbide inserts (TCI), natural diamond and polycrystalline diamond compact (PDC) technology.

PDC bits yielded better performance in terms of rate of penetration (ROP) and bit life over TCI bits. However, it required several PDC bits to drill the sections. Further detailed analysis produced optimum PDC designs that successfully established new benchmarks in the fields.

The objective was to minimize the number of bit trips by enhancing PDC bits performance. This called for new PDC technology coupled with optimized motor drive. To address this challenge, ADMA-OPCO and bit vendor and other service companies worked together to seek optimized solutions to drill the interbedded rocks efficiently.

This paper reviews the findings of the detailed study in drilling the 12 ¼” and 8 ½” sections in typical Deep Khuff wells. The study showed that PDC damage from encountering harder stringers was the primary impediment to achieving better performance. Subsequent to the recommendations, the challenges were overcome by utilizing new PDC cutting structure designs and advanced cutter technology along with optimized downhole motor assemblies. The resultant improvement is also demonstrated by examples from separate fields offshore Abu Dhabi where considerable savings were achieved in terms of footage drilled and cost per foot.

Background

This paper focuses on achieved economic savings in the 12 ¼” and the 8 ½”sections in two different offshore fields where ADMA-OPCO develops deep Khuff wells. These fields are identified as ABK and US and are shown in the study area map in Figure 1. The well plans in the above fields match closely in terms of drilled formations. The 12 ¼” hole section is drilled into the top of Upper Khuff formation and the 8 ½” hole section is drilled in the Khuff formation. The typical interval length for the 12 ¼” hole section ranges from 1,500 ft to 2,500 ft depending on the well profile. The typical interval length for the 8 ½” hole section ranges from 1,500 ft to 2,200 ft.

Until recently, several bit types including TCI and PDC drill bits with different designs and various cutting structures have been utilized to successfully drill the above mentioned sections; however, in most cases these bits did not yield good performances in terms of footage drilled or rate of penetration (ROP), which in turn resulted in using more drill bits that involved several trips and subsequently increased operating costs. The scope of work was conducted initially on the 12 ¼” phase where a comparative analysis for the drilled sections in the US field has shown that PDC drill bits produced better ROP and drilled more footage than TCI drill bits. Ultimately, PDC drill bits replaced TCI drill bits in these applications; however, PDC drill bits still lacked consistent performance in the above mentioned fields.

The challenges presented in drilling the 12 ¼” and 8 ½” sections come from encountering highly interbedded formations with varying confined compressive strengths. A comprehensive study was performed on the 12 ¼” sections drilled in the US and ABK fields in which forty-six drill bits, mostly PDC designs, were evaluated and graded for their dull conditions based on the current IADC dull grading system utilized in the oil and gas drilling sector.1 It was noted that more than 40% of the PDC dull bits had worn cutters (WT) and approximately 50% of PDC drill bits have shown either chipped cutters (CT) or broken cutters (BT) as primary dull conditions Figures 2.