The most critical real-time data required for drilling directional wells is the angle of inclination. The advantages of measuring inclination within a few feet of the bit are enormous as it allows the driller to determine the directional behaviour of the bottom hole assembly, reduce the number of sidetracks due to directional steering problems, and more precisely place a wellbore in a reservoir.

This paper discusses operational procedures, best practices, and lessons learned during the drilling of Bonga 10ST 3(sidetrack # 3) horizontal well in Deepwater Offshore Nigeria. In the course of the well execution, achieving the well objective necessitated the introduction of at-bit-inclination sensor and agitator tools in optimally placing the Bonga 10ST 3 drain hole in the reservoir.

The application of these particular horizontal drilling technologies was a first for the operator.


The Bonga field lies 120 kilometers off the coast of the Niger delta in water depth between 3100 ft and 3600 ft. Quite a number of challenging horizontal wells have been drilled since the inception of operations in late 2000 and one of them was the Bonga 10 horizontal sidetrack well drilled and completed in early 2004. The key drivers for the operator, Shell Nigeria Exploration and Production Company, were completion reliability and sand control which could be achieved if the wellbore is less tortuous.

In the 80's and early 90's, drillers have had to contend with the lack of at-bit sensors. Hole problems such as well tortuosity, high doglegs, spiral hole, poor weight transfer to the bit and poor rate of penetration have been common experiences, especially in extended reach horizontal wells. In this case report, the introduction of ABITM1 (at-bit inclination sensor) and the Andergauge Agitator helped to minimize all of these problems while also providing a comparable cost effective alternative to rotary steerable systems. The at-bit inclination sensor was developed to specifically measure inclination at the bit as opposed to the practice of measuring it about 60 feet from the bit thereby eliminating guess work and keeping the bit within the pay zone. Drillers can also react quickly to changes in the build or drop tendencies of the Bottom Hole assembly (BHA).

The Agitator used in conjunction with a mud motor was selected because of its proven ability to aid weight transfer to the bit, provide excellent toolface control and improve rate of penetration in rotary and oriented drilling modes.

The Bonga 10 horizontal producer well was designed to be drilled and completed as a high rate producer in a major reservoir complex in the Bonga field. The well's target is to complete a minimum of 1000 ft of net sand along hole, with the completed section passing from the reservoir top through the reservoir base.

The well was successfully drilled into the reservoir, placing the 12 1/4" hole at 82degrees inclination and setting the 9–5/8" casing just above the top of the reservoir. Two sidetracks were required.

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