To better handle borehole instability problems while drilling through shales, one needs access to relevant rock mechanical data on the formations, as well as an understanding of how the drilling mud interacts with the specific formation. An obstacle may however be insufficient amounts of cored shale material. Therefore, we have developed various methodologies where we utilize smaller samples to produce rock mechanical data on shales. We present various cases where we use these techniques to provide shale characteristics, and illustrate how they may be used to avoid borehole instabilities. These examples demonstrate the basic principle that small samples (mm-cm scale) can be used to characterize in particular shales, producing results that are consistent with those traditionally attained on larger, standard samples tested in triaxial cells. The tests generally provide data both faster and at a lower cost than conventional tests and, last but not least, they are sometimes your only choice when direct rock mechanical data are needed.
1. INTRODUCTION
Borehole instability problems while drilling through shales are known to add substantial costs to the drilling operations. This may be related to incidents like tight hole/stuck pipe, kicks and mud losses. Fundamental control parameters for the well design are generally the mud weight, which should be balanced between the collapse and fracture pressures, and the orientation of the well with respect to stresses and rock mechanical properties (see for instance Fjær et al [1]). Additionally, drilling procedures should be optimized to achieve good hole cleaning and to minimize effects triggered by oscillations in the dynamic mud weight.
In case of shales, another factor is the potential interaction between clay minerals and the drilling fluid. Traditionally, oil based muds (OBM) tend to be preferred due to their sealing capabilities towards the shale surface. However, due to environmental constraints, costs, or due to for instance barite sagging in high pressure/high temperature (HPHT) wells, water based muds (WBM) may be preferred. WBM more easily communicate with the shale pore fluid, potentially leading to loss of pore pressure support and thereby to accompanying time dependent instability problems. On the other hand, by adding various salts to the WBM, one may trigger time dependent interactions between the water phase and the shale that even enhance the stability [2, 3, 4, 5]. Such an interaction may be associated both with activity controlled osmotic processes as well as with ionic exchange effects where ions from the WBM exchange with ions in the clay mineral lattice [4, 5]. Common to such fluid induced phenomena is that their time dependence is controlled by diffusion related processes, which thereby also influence the time dependency of the stable mud weight window. In case of shales, the inherently low permeabilities result in correspondingly small diffusion constants and large time constants.
Thus, a pre-requisite to better handle borehole instability problems is access to sufficient rock mechanical data on the relevant formations, as well as sufficient understanding of how the drilling mud interacts with the formation. This generally includes not only the intrinsic properties of the shale formation as such, but rather the bulk formation when for instance fractures are present since the latter may call for alternative solutions to cure the instability problem [6, 7].
