Abstract

Wellbore pressure management is a critical part of normal drilling practices, where static and dynamic fluid pressures are used to contain formation pressures and to assure wellbore stability. Excessive fluid pressure while circulating can create problems including reduced operating margins between fracture and pore pressures and, in the extreme, lost circulation. To address these problems, an Equivalent Circulation Density (ECD) Reduction Tool (RT) has been developed.

The ECD RT is designed to counter the frictional pressure effects that exist while circulating. The tool is expected to have a broad range of drilling applications including the narrow pore/fracture pressure margin deepwater environment, wellbores prone to instability, pressure depleted reservoirs and extended reach wells.

The tool has the potential to:

  • Improve wellbore stability.

  • Extend hole intervals and reduce casing requirements.

  • Improve rate of penetration (ROP).

  • Reduce lost circulation.

  • Reduce differential sticking.

  • Improve hole cleaning in extended-reach drilling (ERD) wells through the use of higher flow rates.

This paper describes a new downhole tool for ECD reduction, which is run as an integral part of the drill string. A prototype tool has been built, to operate inside 10–3/4" to 13–3/8" casing strings, which has undergone testing in a flow loop and in two experimental wells. The design features of this prototype and the test results obtained so far are discussed in this paper.

Introduction

This paper describes the development of a novel system for reducing the Equivalent Circulating Density (ECD) of drilling mud. The need for reducing ECD has become more apparent as the industry is faced with increasingly difficult drilling challenges.

The initial focus in developing technologies for ECD reduction has been directed towards applications in deepwater, where the issue is to overcome the significant hydrostatic pressure in the riser when it is full of weighted mud1. However the concept affords potential benefits in a wide range of drilling applications.

The work reported in this study covers the design and testing of a prototype ECD RT that can have application to many drilling conditions, both onshore and offshore.

Benefits of ECD Reduction

As the industry has strived to recover hydrocarbons in increasingly challenging areas, it has become apparent that one of the major challenges is to maintain downhole pressures within the narrow window between pore pressure and fracture gradient. In practice, the window may become even narrower if the minimum required downhole pressure is governed by wellbore stability issues, rather than just pore pressure2. Since the size of this operating window dictates the maximum ECD that the well can tolerate, there is clearly a need for reducing the magnitude of ECD.

ECD is governed by the hydrostatic head of the mud column and the frictional pressure loss in the annulus and therefore it is influenced by many factors. Conventional well designs often exploit the controlling parameters to minimize ECD. Such optimization methods include:

  • reducing frictional losses through the use of low fluid rheologies.

  • use of casing strings with wider annular clearances.

  • application of expandable tubulars to preserve hole size.

  • use of drilling liners rather than full casing strings.

  • controlled penetration rates to avoid overloading the annulus with cuttings.

In addition, there are more radical methods that can be employed to reduce downhole pressures and hence ECD.

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