Abstract

In an oil field, drilling engineers had encountered numerous drilling troubles such as sloughing and stuck pipe problems across a shale interval overlaying the reservoir. In a preliminary study the authors carried out, the instability phenomena contradicted to the simple theory of homogeneous material. For example, hole condition worsened in the highly deviated well although the stress regime is strike-slip fault type.

The study aims to find out the root cause of such complex instability phenomena, to develop numerical method that can be used for mud weight optimization, and to establish the drilling guideline to mitigate the troubles

Using the 3D poroelastic and laminated formation model, we found that wellbore condition can be worsened as hole inclination increase even in the strike-slip stress condition. In such case, higher mud weight cause wider failure area due to the fluid pressure penetration into weak planes of the laminated shale formation.

The paper shows that the appropriate numerical method with three -dimensional poroelastic model with heterogeneous the strength and permeability can predict such unconventional wellbore instability phenomena.

Introduction

In the recent years, the oil industry has acknowledged the importance of mechanical wellbore stability problems. Some numerical tools are now commercially available. Such tools are basically based on the simple elastic theory in homogeneous and isotropic media. On the other hand, most of the wellbore instability problems have happened in shale formations. In shale, heterogeneity and anisotropy are common nature. The low permeable nature makes fluid-solid coupled approach important, because the effective stress around borehole changes dynamically in such rocks (Detournay and Cheng, 1988). Also, micro fissures can add discontinuous characters.

In such formations, conventional analytical solutions and numerical methods based on the linear elastic theory do not work. By such reasons, many studies to improve the models have been done. To solve the problem in heterogeneous media, discrete element model (DEM) are applied to the failure of fractured and discontinuous rocks (Santalleri, 1992; Yamamoto et al., 2002). Aoki (1994, 1996) developed the modified analytical solution for laminated media. For the fluid-solid coupling, the analytical solution of inclined wells in poroelastic media have been developed (Ekbote et al., 2004; Abousleiman and Ekbote, 2005).

In this paper, an example of a finite difference model (FDM) code application for a poroelastic problem in a real field to explain the unconventional behavior of a shale formation of the field.

Problem to be Solved

The target formation is a cap rock of the reservoir of an oil field in the Middle East. Numerous drilling troubles such as tight hole and stuck pipe problems happened across a shale interval overlaying the carbonate reservoir.

To solve the problem and reduce the drilling cost, Japan National Oil Corporation (former name of Japan Oil, Gas and Metals National Corporation, JOGMEC) conducted collaborative study with operating companies of the field. This study was a comprehensive one that includes statistic analysis of past drilling troubles, data acquisition in shale formation using wire line and LWD logging tools, laboratory testing using core and cuttings samples, analyses of chemical and mineralogical characters of the shale and formation fluid, and modelling studies using numerical and analytical solutions (Tantawi and Fadaq, 2000, Yamamoto et al., 2002, Yamamoto et al., 2004).

In the preliminary studies, the authors found that the instability phenomena in the shale formation have some interesting features, and such features sometime contradict the conventional theory; such as:

  • The stress condition of the format ion is evaluated as the strike-slip fault type. Under this type of stress condition, a inclined well is more stable than the vertical well. However, most of the drilling troubles due to shale instability happened in the highly deviated well.

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