INTRODUCTION
An efficient approach to the management of wellbore instability in shales is presented. It takes into consideration the factors which determine the degree of complexities required in developing mud weight program to provide the required effective mud support with time. A range of design and analysis tools required for wellbore stability analysis is described systematically. The application of some of the tools is demonstrated through two field case studies to develop strategies to control shale instability in wells drilled in the North West Shelf of Australia. The tools provide a practical means of opfimising the approach in developing the solution, including drilling fluid design (weight, type and chemistry), to manage shale instability efficiently.
Wellbore instability, experienced mainly in shale sections, may be induced by either in-situ stresses that are high relative to the strength of the formations (stress-induced) or physico-cbemical interactions of the drilling fluid with the shale or a combination of both (Tan & Willoughby 1993, Mody & Hale 1993, van Oort et a1.1995, Last et al. 1995, Tan et al. 1998). The dominant stability mechanism(s) is dependent on a wide range of factors including type of shale, in-situ stress environment, thermal gradient and drilling fluid system. Hence, for efficient management of shale instability, the significance of the factors should be assessed, and the most efficient approach adopted on a field by field basis.
This paper describes an efficient approach to the management of shale instability. It takes into consideration the factors which determine the degree of complexity required in developing a mud weight program to provide the required effective mud support with time. A range of design and analysis tools required for the stability analysis is described systematically. The application of some of the tools is demonstrated through two field case studies to develop strategies to control shale instability in wells drilled in the North West Shelf of Australia.
