Abstract

High frequency of wash-outs and cracks in drillstring has been experienced in a specific formation in oil fields located in the Southern part of Iran. The rig time consumed on this challenge was high due to time spent on replacing damaged pipes. In a few cases fishing operations were necessary to recover twisted off drillstrings. The majority of drill string failures happened during top hole drilling (17–1/2" hole section). So far several solutions have been tried out to overcome the drillstring failure, which to some extent has decreased the washout frequency.

In this work we tried to define the challenge as precisely as possible;where, how and why the problem occurred. The state-of-the-art with respect tocause/solution to pipe washouts is included in the paper. Possible reasons and practical solutions to this challenge are evaluated.

Analysis of relevant factors that affect drillstring failure revealed that drillstring vibration in torsional mode is the most probable factor to induce drillstring failure in the hazard formation. This factor is therefore discussed in detail. Inasmuch as the failure occurred just in one particular formation, the lithology that induced vibration is another factor that was investigated and analyzed in this study. The complete understanding of the cause of the problem has however still not been reached.

Introduction

Drillstring failures (DSF) increase drilling cost dramatically especially in high drilling cost environment. The types of failure experienced and reported in this study were drillstring wash-outs, cracks and twist offs. As often isthe case in drilling, the consequence of one drilling problem induced another. The consequence of DSF can be; loss of bottom hole assembly (BHA), fishing, bit balling and hole instability. One of the main consequence of DSF was hole instability in term of hole wash-out due to time consuming operations leading to interaction between mud and formation.

There are two main processes that can lead to DSF; fatigue and corrosion. The drillstring is subjected to different types of forces. One of the harmful forces is vibration. Improper BHA design and wrongly selected drilling parameters in hard rock or sticky formation can lead to intensive vibrations.In Iran, as also in several parts of the Middle East, the rate of DSF is reported to be high mainly due to hard rock and sticky formation.1In this paper we will report a case study of extraordinarily many DSF while drilling through a specific formation which we will refer to as either the hazardous formation or simply M1. In the M1 formation the rate of failure in some distinctive parts of two fields, called field A and B, out of about 30neighboring fields, were extremely high; from 10 to 70 DSF per well. In others field the rate of DSF was relatively low, typically 1 or 2 DSF per well. This relatively low failure rate was also observed in field A and B outside three distinctive areas within field A and B. These distinctive areas cover only approximately 20 % of the total area of field A and B. The M1 formation exists in most fields in Southern Iran placed below the surface formation and normally penetrated with the 17–1/2" BHA. The relevant potential causes of DSF are discussed and we will suggest some solutions which are easy to implement. In the mean time, several methods have been tested out in order to decrease washout frequency. These methods were;

  • Use of downhole mud motor

  • Changed BHA (increased stiffness)

  • Use of corrosion inhibitors

  • Increased bit lubricity (up to 2 % oil)

  • Changed bit type

  • Changed to brand new drill pipe

  • Decreased axial tension by reduction weight of BHA

Some of the tested methods have been partly successful, especially the applying of down hole motors, while we saw a failure rate decrease of approximately 25 - 30 % was observed. However, the problem can be characterized as unsolved.

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