A Laboratory Investigation of Borehole Stability
Authors: Darley H.C.H. 
Published: December 2007

Before the proper technique can be selected for combatting hole problems, the causes of the problems must be identified. To understand better the nature of borehole instability, a model study was conducted in which reconstituted shale specimens were subjected to triaxial stresses, and various fluids were circulated through an axial hole.

Introduction

The principal causes of unstable boreholes* have been known for many years. For example, in a paper published in 1938, Halbouty and Kaldenbach listed published in 1938, Halbouty and Kaldenbach listed nearly all the classes of troublesome shales that we know today. Much progress has been made since then, but we still have difficulty in identifying the cause of hole problems and consequently in selecting the right technique for combatting them. This subject was discussed at length by Kelly in a recent paper and much valuable information was provided. paper and much valuable information was provided. In the Shell Laboratory it was felt that there was a need for a better understanding of the basic mechanisms underlying the various forms of hole instability, and accordingly, a study was undertaken. This entailed reviewing the physics and chemistry of shales, developing tests for characterizing shales, and constructing a model to study borehole stability. Full simulation of underground conditions was not attempted; for example, we used reconstituted shale specimens, and often simple solutions or suspensions rather than practical drilling muds. Such procedures are necessary in order to establish mechanisms, but caution must be exercised in applying the results to the more complex conditions in the field.

Part I - Compaction Swelling and Part I - Compaction Swelling and Dispersion of Shales

Compaction of Sediments

Shales are formed by the compaction of sediments. Water is expressed as sediments are buried by subsequent layer, and the degree of compaction is proportional to the depth of burial, provided the water proportional to the depth of burial, provided the water is able to escape easily to permeable formations. The younger sediments, when recovered at the surface, soften and disperse when mixed with water; the older shales have undergone diagenesis (alteration of the clay minerals, secondary cementation, etc.), and remain hard and unaffected by water. In this paper we shall apply the term "shale", as the drilling engineer does, to everything from clays, which are highly reactive to water, to completely lithified materials such as claystones and slates, which are completely inert. However, because these materials behave quite differently when encountered in the drilling well, it is desirable to develop a simple means of characterizing them. The feature that distinguishes the various shales is their dispersibility in water - soft clays disperse readily, harder shales disperse slowly when agitated, the lithified materials will not disperse at all unless milled. In the dispersibility test used in our study, a weighed amount of dry shale particles in the 12-to-100-mesh-size range was placed in a tube filled with distilled water and allowed to soak for 16 hours.

JPT

P. 883