Summary

Extended-reach drilling (ERD) is playing an important role in the economic development of Alaska's North Slope oil reserves. For some ERD wells, careful well planning and existing drilling practices are sufficient to avoid problems, such as wellbore instability, lost circulation, and stuck pipe. However, recent Alaskan experience has shown that as well step-outs increase, some operational practices developed on conventional wells are inadequate to successfully deliver cost-effective ERD wells. On these wells, careful planning alone cannot always prevent costly drilling problems.

The ability to analyze downhole annular pressures with pressure- while-drilling (PWD) data has proven invaluable to the drilling engineer. The value of this information is not fully realized, however, until it is put in the hands of the rig operator, who can act on it in real time. This paper examines the impact PWD measurements have made in recent Alaska ERD wells and focuses on both planning and operational uses. Field data are presented from problem and nonproblem wells to underscore successful and unsuccessful operational practices. The importance of rig-site training in equivalent-circulating-density (ECD) management and the linkage to drilling, circulating, and tripping practices are developed.

Introduction

The majority of North Slope ERD wells have been drilled to develop offshore reserves from onshore drilling locations. These wells share many design and operational challenges with North Sea and Gulf of Mexico extended-reach wells.1–3 In 1998, two North Slope ERD wells were suspended with significant cost overruns resulting in part from hole-cleaning and wellbore-stability problems. Substantial engineering effort was marshaled in an effort to learn from these two failures and prevent recurrences.4

Findings and Common Themes.

Although the two wells differed significantly in both design and failure mechanism, several common themes emerged from the drilling post-mortems. First, it was apparent that inadequate hole-cleaning practices were being employed. Equally important, the hole-cleaning indicators were ineffective in detecting poor hole cleaning. Second, fragile shales were being repeatedly subjected to unnecessarily high transient-stress levels, possibly weakening them to the point of failure. This was a consequence not only of high ECDs but also of established drilling, tripping, and connection practices.

It was recognized that ECD played a key role as a causal factor in wellbore instability and lost circulation and as an indicator of hole-cleaning efficiency. Measurement and analysis of ECDs with PWD tools were seen as critical to the success of future ERD wells, particularly if the technology was adopted and used by the rig team.

Overview of PWD Tools.

PWD tools are well documented in the literature.1,3 PWD tools, run in the bottomhole assembly (BHA), measure the annular pressure in the mud column. With the mud pumps off and the drillstring stationary, the borehole annulus is at the hydrostatic pressure of the mud column, a function of the temperature and density gradients of the fluid in the annulus. This hydrostatic pressure measurement is typically converted to an equivalent mud weight and referred to as the equivalent static density (ESD).

When circulating the drillstring, the annular pressure consists of the mud column's hydrostatic pressure and a dynamic component resulting from frictional pressure losses as the fluid moves up the annulus. Drillstring movement also dynamically impacts the annulus pressure because of surge and swab effects. ECD is the expression of the hydrodynamic annulus pressure in common oilfield density units.

PWD measurements are taken continuously every few seconds and stored in memory as a time file. These recorded PWD data are downloaded at surface after a trip. Real-time access to the data is also provided through a measurement-while-drilling (MWD) tool, which is programmed to periodically pulse up a pressure measurement with logging and directional data. Both the real-time and recorded data are typically merged with other surface and subsurface measurements and presented with time- or depth-based logs.

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