Activating Shale Can Create Well Barriers
Authors: Carpenter Chris 
Published: May 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191607, “Activating Shale To Form Well Barriers: Theory and Field Examples,” by Tron Golder Kristiansen, SPE, Torill Dyngeland, SPE, Sigurd Kinn, Roar Flatebø, and Nils-Andre Aarseth, SPE, Aker, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed.

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-abandonment (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.

Creep and Other Shale-Deformation Mechanisms

Elastic Deformation. Elastic deformation is too small generally to close an annulus around a casing. This deformation results from the excavation of the borehole and is proportional to the difference between the horizontal stresses trying to close the wellbore and the wellbore pressure trying to support the wellbore wall and inversely proportional to the shear rigidity of the rock.

Elastoplastic Deformation. This deformation, in rare cases, may be enough to close an annulus. The rock may fail in a ductile manner and reduce its stiffness by a factor of 5 to 10, inducing irrecoverable (plastic) large strains in the rock. Because rocks rarely behave as perfect plastic materials, elastoplastic deformation will seldom be enough to close the annulus.