Abstract

Since the 1980's, oil field developments in West Africa have grown tremendously, with the major emphasis in the last few years concentrating on offshore projects in ever-deeper water. This increase in development is of course associated with and continues to be associated with exploration drilling. Offshore activity in West Africa covers a huge area ranging from Mauritania in the North to South Africa, a coastline of approximately 4,000 miles and water depths of up to 10,000 feet. A large part of the challenges associated with servicing these requirements is the need to service mobile exploration rigs, which may move considerable distances between wells, frequently drilling consecutive wells in different territorial waters.

These drilling programs by their nature often require last minute design changes and associated laboratory testing. This means that in whatever offshore location the present well is being drilled, it will require a complete technical back up in the form of a cement laboratory, operational support and engineering back up. Additionally a sufficient inventory of cement and additives to handle any final changes in design must be available. Not least of these requirements will be the ability to deal with unexpected shallow water flows. One of the main characteristics of these deep-water offshore wells is the need for lightweight slurries in the top-hole sections, which can develop high early compressive strength relatively quickly. This will reduce expensive waiting on cement time and ensure mechanical integrity of the conductor casing and BOP support.

Finally, commercial tenders for these cementing services can be difficult to evaluate and compare proposals on an equitable commercial basis. The requirement to design a slurry that meets the requirements and is as competitively priced as possible, can make comparison of actual overall cementing costs difficult to compare.

This paper discusses the engineering and logistical issues associated with these operations.

Introduction.

Exploration drilling in West Africa has been at the fore-front of deepwater drilling and covers an ever larger geographical area. Many of the problems associated with deepwater cementing, such as high early compressive strength development at low sea floor temperatures, potential shallow water or gas flows and the requirement for low specific gravity cement slurries have been well documented1,2. These specific requirements associated with deep-water operations have to be considered in addition to the more usual requirements of servicing exploration rigs. The needs of deep-water operations usually impose an additional logistics and operational challenge in that the need for changes in well design and material supply mean operational support must be flexible and responsive. Most cementing services are tendered based on providing a slurry design to meet a dictated set of parameters, such as setting time at temperature, compressive strength development and specific gravity. The associated cost of these surface casing cementing programs can be a significant cost of the overall program and careful evaluation and testing of slurries submitted with commercial tenders is required to evaluate one proposed system against another

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