Abstract

By eliminating the need to under-ream, the combination of PDC bit technology and a newly developed tool that enables the operator to simultaneously ream and drill ahead reduced by four days and approximately US$465,000 the time and costs of an offshore Australian well.

This paper describes the development of the reaming while drilling system and its application in a highly cost-intensive well offshore Australia. A detailed operational summary, emphasizing the application/performance of the system will be presented and analyzed.

Using previous under-reaming applications as a benchmark, the operator sought a risk free and cost-effective alternative to conventional hole opening. Various designs were reviewed with the aim of developing a device that would best suit the targeted application. Every facet of the drilling process was analyzed to develop a plan of best practice. For example, the use of pseudo oil-based mud was considered critical to not only stabilize the wellbore, but also to enhance the life of the reaming/drilling system.

The system successfully drilled the targeted section, with caliper logs showing 98% of the hole being drilled to the desired size. Although partially control-drilled to control potential pack-off problems, the well recorded penetration rates similar to those expected for a 17 1/2-in. PDC bit.

From this experience, several operational recommendations arose that will permit similar intervals to be drilled even more cost effectively. The authors will discuss those recommendations.

Introduction

Conventional bi-center PDC bits have long proven to be cost-effective alternatives to traditional under-reamers and their associated mechanical risks. Primarily, this technology has been employed to drill a larger hole size than the inner diameter of the casing through which it passes. Furthermore, bi-center bits have effectively reduced bit and bottomhole assembly sticking in fast-moving salt formations, and eliminate the need for a second under-reaming trip in well deepening operations, thus significantly lowering costs.

As operators move toward deeper and high-pressure, high-temperature (HPHT) wells with extra casing strings and longer intervals drilled through unstable or encroaching formations, the ability to simultaneously ream and drill becomes even more economically attractive.

Operator Western Mining Corporation Ltd. considered the emerging technology in reaming while drilling systems when planning its 3800-m TVD Bay-1 exploration well offshore Australia. The well was located off the North West Shelf on block WA 215-P some 25 kilometers southwest of Barrow Island, Fig. 1 shows the location of Bay-1, highlighting the primary offset fields used in developing the bit program.

As shown in Fig. 2, the lithology of the Bay-1 upper sections comprised interlaminated hard limestone and friable fine sandstone, overlying sticky clays, and unstable water-sensitive shales. The lower intervals encountered the Malouet and Dingo/Dupuy formations characterized by a long siltstone/shale interval and, at the base, extremely hard (60 - 70 Sec/ft) and highly abrasive sand/shale sequences.

On the basis of the proposed well plan, which utilized an additional string of 16-in liner, a drill bit program was developed with the objective of eliminating under-reaming in suitable intervals.

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