Advances in wellbore stability and shale technology have long been sought because shale instability has consistently produced the highest drilling trouble costs. Most wellbore stability problems occur in shales. Wellbore instability in deep, shale formations is the major cause of stuck pipe, high torque and drag, hole enlargement, poor log quality and poor primary cement jobs. Major improvements in wellbore stability predictions and shale technology are required to significantly impact drilling risks and costs, particularly in expensive wells under high-angle, extended-reach and severe tectonic conditions. This paper presents an overview of recent advances in shale technology and results that show relationships between cuttings index properties and shale mechanical properties determined by special triaxial tests. In addition, it presents an overview of wellbore stability technology with a discussion of practical solutions to wellbore stability problems by qualitative and quantitative applications of theory.
One of the greatest needs in the petroleum industry is a predictive method to prevent wellbore failure during the drilling operation (Cheatham 1984 and Pearson 1988). Well bore collapse causes problems such as stuck pipe, high torque and drag, hole enlargement, poor log quality and poor primary cement jobs. Critical wells are often sidetracked or abandoned before reaching their objectives because of severe well bore failures. Most of the problems occur in weaker formations, predominately shales. These failures lead frequently to excessive drilling and completion costs that cannot be sustained in the current economic climate drilling engineer is often frustrated with the problem of trying to understand and predict the behavior of rock of uncertain composition at great depth under uncertain loading conditions with theoretical models of uncertain validity or viability. In the past, the well bore stability research area was considered high risk with a low probability of successfully finding practical solutions to the problem. Research progress in recent years has reduced some of the uncertainties. There has been increased development of theoretical concepts (Fairhurst 1964, Gnirk 1972, Bradley 1979, Risnes et al. 1983, Zoback et al. 1986 and Vardoulakis et al. 1988), significant advances in understanding and quantifying shale mechanical behavior (Steiger & Leung 1988) and more widespread experience in field applications and tests of concepts that show promise by Exxon and others (Maury & Sauzay 1987 and Fuh et al. 1988). These results provide encouragement and optimism that more practical predictive methods can be developed to prevent wellbore failure. However, major improvements in wellbore modeling and shale mechanics technology are required to implement practical predictive techniques to significantly impact drilling risks and costs, particularly in expensive wells under high-angle, extended-reach and severe tectonic conditions.
Over the past few decades considerable research effort has been expended on both the chemical and mechanical aspects of wellbore stability. Substantial benefits have accrued from the development and widespread use of inhibitive drilling fluids (Clark et al. 1976, Steiger 1982, Bailey et al. 1987, Grinrod et a1. 1988 and Colins & Stone 1989).