Abstract

The rising need for hydrocarbon forces operators to drill more aggressively even in harsh formations and to drill well profiles with ever-increasing directional complexity. As a result, drillstrings are increasingly exposed to tough drilling conditions causing mechanical fatigue due to vibrations and bending stress cycles in tortuous wellbores. Additional challenges are posed by high torque and drag situations, corrosive drilling mud environments, abrasive formations, high temperature, high pressure and other unconventional drilling practices. On the economical side the situation is worsened by limited supply capacity and high prices for new drillstrings, forcing operators to rely on older drill pipes in critical applications.

Mechanical fatigue, corrosion or abrasion makes a drillstring vulnerable to wash-out, allowing mud to flow from the inside of a drillpipe through an initially small leak in the pipe wall into the annulus. If not detected in time, the leak will wash out rapidly and eventually cause a drill string twist-off which can be catastrophic. The result will be significant non-productive time for fishing operations or, in worst case, loss of an expensive BHA or even an entire well section. Presence of radioactive sources in such a situation can compound the problems many fold.

This paper presents a method to detect wash-outs at a very early stage where it is not possible yet to identify the wash-out from classic surface measurements such as stand pipe pressure. This is based on real-time transmission and close monitoring of the rotary speed of a downhole turbine positioned in the MWD BHA close to the bit. The effectiveness of the method was demonstrated recently on a year long period of a drilling campaign in a difficult drilling environment in India. A total of 9 washouts were detected, but not one twist-off occurred on these 9 runs. Not a single washout was found at the surface that was not indicated earlier by a reduction in MWD turbine RPM.

The paper starts with an overview of the washout detection concept and presents several data examples to illustrate the method in detail. The case study from India describes the implementation and the effectiveness of the method that has resulted in a 100% success ratio in washout detection in this application. After this services were introduced to the operator, the operator has made the requirement of washout detection services a standard part of its job planning.

Introduction

Drilling industry has been using Standpipe pressure measurement and flow rate correlation as a washout detection means. The occurrence of a drillstring washout near surface can normally be detected through vigilant monitoring of surface pressure. If a washout occurs at a deeper depth in the drillstring, this can be more difficult to detect from surface pressure loss measurements alone. To assist in early detection of a deep drillstring washout, we need to monitor the flow rate of drilling fluid through the BHA. We will also discuss the need to understand and identify all available measurements that can be used to identify a possible wash out.

Many MWD/LWD downhole systems are powered from an alternator driven by a mud turbine located in the BHA. By monitoring the turbine speed (Turbine RPM) in real time and integrating those data with flow rate and surface pressure data, washouts deeper in the drillstring can be detected quickly—allowing timely remedial action before catastrophic drillstring failure occurs.

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