Abstract

Inter-zonal flow of fluids behind cemented casings has plagued the EP industry for years. This has led to the development of many models, but there is still a need for a unified and practical method to design cement sheaths.

Three leakage categories can be distinguished depending on the physical state of the cement when leakage occurs, and this paper focuses on modeling cement sheaths while in solid state (CSSS). It shows first that predictive simulations are not very efficient, even if tremendous improvements have been made the past ten years. Then, a new modeling method (inspired from the Convergence-Confinement Method for designing of tunnel supports) is introduced, the SRC (System Response curve) method, that simplifies CSSS modeling. Using a case of history, where SRC method has been used to help in designing a well, the paper highlights the importance of pore pressure and shrinkage in predictive cement-sheath modeling.

Introduction

Primary cementation of a well aims first at holding the casing in the hole, and second at preventing inter-zonal flow of fluids behind cemented casings. Unfortunately, this second objective is not always fulfilled, and that this failure to prevent cement sheath leakage is a major operating problem for the EP industry. Besides, as early as in the 60s, studies were launched to understand annular gas flow that occurred during cementing of gas storage wells1. Since then, the EP industry has been engaged in many studies that have considerably improved the understanding of how a cement sheath can leak.

However, no unified point of view has been reached yet due to insufficient fundamental knowledge. As a consequence, asking to two different service companies to design a tight cement sheath may lead to two very different solutions. And this situation is becoming dramatic as new environmental policies are issued requesting environmental safe operations, such as those coming from the Kyoto protocol.

Moreover, the number of configurations where gas leakage is concerned is growing:

  • Gas field;

  • Gas storage;

  • Acid gas storage;

  • HT/HP fields;

  • Deep-water fields;

  • Shallow gas formations;

  • Injection/production wells;

  • Geothermal wells.

This is why there is a need to develop a unified method to design cement sheaths based on solid fundamentals. And this method should include a workflow that is compatible with the EP organization in order to become a practical engineering tool and not a new fad.

Three leakage categories can be distinguished depending on the cement physical state when leakage occurs:

  • Cement sheath in a fluid state (CSFS);

  • Cement sheath in a gelled state (CSGS);

  • Cement sheath in a solid state (CSSS).

This paper will focus only on the mechanical simulation of the CSSS, and will put aside other aspects of the cement-sheath leakage. This does not mean that these aspects are assumed to be negligible. On the contrary, it is obvious2 that reliable zonal isolation can only be obtained if effective mud displacement practices are used. But it means that a unified method can only be based on sound components of knowledge, and this paper tries to focus on such a component. More precisely, this paper will focus on introducing a new method to CSSS design. It will be divided into three components:

  • The presentation of prior CSSS studies to show that, even if there have been many improvements in CSSS modeling in the past 10 years, some work needs to be done to reach predictive modeling;

  • The presentation of the SRC (System Response Curve) method which aims at improving CSSS design;

  • The presentation a practical case where SRC method has been used to design cement sheath. This presentation includes a discussion on shrinkage and pore pressure modeling.

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