This paper discusses shale creep and other shale-deformation mechanisms and how an understanding of these can be used to activate shale that has not contacted the casing yet to form a well barrier. The authors then explore methods of activating shale for this purpose, concluding that inducing a pressure drop in the annulus is the most-promising such method.

Introduction

Creep is a well-known phenomenon in engineering in many materials. In rocks, it is related typically to grain rotation, grain sliding, cement cracking, and, in some cases, even grain cracking. The rock matrix has a viscous behavior wherein time is required to achieve new stress equilibrium at the grain-contact level as the bulk rock volume is exposed to an altered stress state. The deformation after the new load on the bulk rock volume is in place, and until the bulk rock volume is not significantly deforming in response to the altered load, is referred to as creep. Creep can interact with other physical and chemical processes, so it can be difficult to separate creep from other time-dependent deformation processes around a borehole. Fig. 1 illustrates the three main creep modes:

• Primary (transient)
• Secondary (steady state)
• Tertiary (accelerating)

Transient creep occurs after the wellbore excavation and transforms gradually into secondary creep. Secondary creep then, in some cases, can result in tertiary creep, wherein the deformation accelerates. In most cases, formations will experience steady-state creep. For plug-and-abandoment (P&A) purposes, creep can take place over several decades. Even after that much time, the creep deformation may have been too low to contact the casing, so assisting the shale to form a barrier would be beneficial. In some cases, it is best to establish shale barriers especially quickly, perhaps in a couple of days—in new well construction, for example.