Innovative Steerable System, Combined With Block Set Impregnated Diamond Bit Design, Dramatically Improves Economics of Dutch Horizontal Well
Authors: Stapel G. , Lucas S. , Illerhaus R. , Doogan P. 
Published: December 2007

This paper describes the design and application of a new bit/steerable system combination. The design of the new bit, featuring a block set type of segment arrangement, will be discussed. The performance of the bit/steerable system combination will be presented and analyzed.

Traditionally, impregnated diamond bits were reserved for the harder and more abrasive formations, which normally cause severe damage to conventional PDC bits. In the North Sea, instantaneous penetration rates as high as 150 ft/hr have been recorded with this bit design on turbine drive.

Generally, the shallower and softer formations encountered in directional work have been drilled quite satisfactorily with soft formation roller cone and PDC bits used in conjunction with bent housing positive displacement motors. However, this technology has exhibited serious limitations in the harder and/or abrasive sequences, particularly when poor homogeneity is present. Addressing these problems focused on creating full-system compatibility of all the components involved.

A new IADC Class M84 1 impregnated diamond bit and a 6 5/8" turbodrill dressed with a 0.75 bent housing successfully completed the build-up phase from 45 to 90 and the 521-meter horizontal section in a well drilled offshore The Netherlands. The section, which historically required as many as 12 bits, was completed with only two IADC Type M84 1 impregnated diamond bits at an average penetration rate of 5 m/hr, as opposed to an average penetration rate of 1.7 m/hr for the best available offset. The latest well was drilled 20 days faster than the best available offset, resulting in savings of approximately $2 million US.

The new system not only resulted in faster drilling and fewer trips, but also provided more tangible benefits associated with a higher quality wellbore.

Introduction

Two wells were drilled horizontally offshore The Netherlands with completion in the Lower Graben sandstones that are normally encountered around 3000 m (TVD). Fig. 1 illustrates the lithological characteristics of a representative well in the block. The Lower Graben is preceded by soft clays that exhibit sonic log values from 100 sec/ft to as high as 135 sec/ft. These clays are interrupted by hard and abrasive sandstones (around 60 sec/ft). As detailed in Fig. 2, the calculated unconfined compressive strength ranges between 9000 psi to 22000 psi.

The first well drilled (designated as Well A) represents the offset. On that well, the operator used a combination of roller cone, TSP and PDC bits on positive displacement motors (PDM) to drill the 8 1/2" production interval. On Well A, a total of 12 bits were used to drill the section. In nine of those runs, the bits had to be pulled prematurely, because of problems in meeting the wellpath directional requirements. Besides the directional control problems, the performance of the PDC bits, in terms of life and rate of penetration (ROP), was equally discouraging.

Heretofore, it was widely accepted that steerability was primarily a function of the bit. However, an optimum steerable system must be considered as a single unit with the bit as an integral component.

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