Wellbore Stability Analysis Made Easy and Practical
- F.J. Santarelli (Agip S.p.A.) | S. Zaho (Agip S.p.A.) | G. Burrafato (Agip S.p.A.) | F. Zausa (Agip S.p.A.) | D. Giacca (Agip S.p.A.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1997
- Document Type
- Journal Paper
- 212 - 218
- 1997. Society of Petroleum Engineers
- 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6 Drilling Operations, 4.3.4 Scale, 1.1 Well Planning, 3 Production and Well Operations, 4.1.2 Separation and Treating, 5.5.2 Core Analysis, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11 Drilling Fluids and Materials
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Recent years have seen many scientific and technical progress in the field of wellbore stability understanding. The main causes of instabilities have been identified, models developed and practical remedies proposed and often tried successfully in the field. Yet despite this scientific effort, drilling extra costs associated with wellbore instabilities remain very high. This in turn demonstrates that until now, scientific and technical progress have somehow failed to reach their final technical and economical objectives.
This paper shows that the main reason for such a state of affairs is the complexity of wellbore stability analysis which cannot be performed for every single well on a worldwide basis. Having identified this, Agip undertook a vast effort of field data back analysis in order to identify from historical records the best way to optimize its drilling planning, practice and post-evaluation procedures. The paper presents part of this effort and in particular the back analysis of four clusters drilled in Southern Italy -i.e. 38 wells in total. It then draws conclusions in terms of procedure optimization. Finally, it presents how the approach is used during the appraisal phase of a field in order to speed up the learning curve and hence significantly cut the development drilling costs.
According to many sources in the industry, wellbore stability problems are responsible for an extra 10 to 15% drilling cost. Even more worrying is the fact that Oil Based Muds, which have been recognised as a good mean to reduce instability problems particularly in shales, can no longer be used so easily because of environmental concerns. Were these environmental constraints to become more stringent which is likely, then the percentages indicated above could still rise and become as high as 20 % in some regions. Knowing that drilling costs represent an important percentage of the development costs of a field, one can then understand why the resolution of wellbore stability problems has been given a high priority by many operators. As a consequence of this sustained effort, the field of wellbore stability has seen many improvements over the past decade which will be reviewed briefly below. In particular, it will be shown that many causes of instabilities have been identified, models developed and remedies proposed and tried successfully in practice.
The paper will then address the fact that despite this scientific progress, the problem of extra costs sustained because of wellbore instabilities remains rampant throughout the industry and it will try to identify some of the reasons for such a situation. In particular, the technico-economical background of wellbore stability analysis will be studied in the light of the cost saving policy of all operating companies.
The paper will then present the decision which was made by Agip in order to improve the situation mentioned above - i.e. despite large scientific progress, drilling costs in general are not sufficiently affected. In particular it will describe how the back analysis of the development of fields can be used in order to identify how to optimise the drilling planning, drilling practice and drilling post evaluation procedures applied worldwide and hence affect at a large scale the drilling costs sustained by a company without having to use its experts on every well.
Finally, the case of a difficult area will be presented and it will be shown how the current technology can be adapted and used during the appraisal phase of a field to allow a faster learning curve and hence cheaper development drilling costs.
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