Abstract

The process of drilling the 12 ¼-in. section in the deep Khuff field offshore Abu Dhabi is challenging because of an intricate geological sequence. This sequence consists of carbonates (limestone and dolomite) and anhydrite that are interbedded with shale and sandstone.

In the past, the operator, ADMA, has attempted to efficiently drill the section by using roller cone tungsten carbide (TCI) and polycrystalline compact (PDC) technology. PDC bits yielded a better performance in terms of penetration rate and durability in comparison to the TCI bits. However, because several PDC bits were required to drill the section, the amount of time required for drilling and tripping at a relatively deep interval increased. The increased time exposes the well to many risks that could occur as a result of tripping in a long open hole interval. To reduce the time and risk factors, additional analyses were performed that led to the creation of an optimum PDC design and established new benchmarks in the field at the first trial.

The objective of the study was to minimize the number of bit trips by enhancing the PDC bit performance and bit durability to drill longer intervals with a higher rate of penetration (ROP). Meeting this objective required a new PDC technology in conjunction with optimized motor drive. To address the challenge, ADMA, the bit vendor, and other service companies worked together to seek an optimized solution to drill this section efficiently.

This paper reviews the findings of the detailed study in drilling the 12 ¼-in. section in typical deep Khuff wells. The study shows that PDC damage from encountering harder stringers was the primary impediment to achieving better performance. The challenge was overcome by implementing a new cutter technology and drilling simulation software to optimize the cutting structure design.

The improvement that occurred in Umm Al-Shaif field is demonstrated by comparing the performance of the most recent well, in which the new technologies have been implemented, to the performance of earlier wells to illustrate the significant savings in time and eventual drilling cost.

Background

This paper focuses on achieved performance and economic savings in the 12 ¼-in. section in Umm Al-Shaif offshore field in which ADMA develops deep Khuff gas wells in United Arab Emirates.

The 12 ¼-in. section is drilled from the Hith formation, which is encountered at 9,000 ft depth average, down into the Upper Khuff formation. This drilling interval crosses ten different formations, and the section length ranges between 5,500 and 6,400 ft. The section is drilled tangentially, with an inclination of 30 to 35 degrees, with medium speed positive displacement motors (PDM), but other driver mechanics, such as rotary steerable systems (RSS) and turbines, were used in previous wells. The turbine performance in this field as driver mechanisms was studied by Salman and El Raggal (1999).

Until recently, the bit types used to drill this section consisted primarily of PDC drill bits with varying design and cutting structures; however, the performance of most of the bits was less than planned in terms of footage drilled and ROP. Several bits failed prematurely, which resulted unplanned trips and the use of additional bits (Salman and El Raggal 1999). As shown in Table 1, Well X, drilled several years ago, used eleven bits to drill the interval, with an average of 360 ft drilled per bit. Most of the bits were pulled out of the hole because of low ROP, and most of the PDC bits showed severe cutter damage.

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