It has long been recognized that mud plays a role in avoiding or triggering wellbore instabilities.
The key factors governing such instabilities in shale are supposed to be: 1- the mud pressure penetration through the formation, acting on the stress state of the rock (controlled by rock permeability); 2 - the morphological/mineralogical transformations occurring when mud filtrate invades the pores of the shale, that may change shale strength.
The former is a quite well accepted mechanism and some strategies have been developed to control such a pressure propagation (plugging additives, mud developing an "osmotic" pressure to balance the formation pressure).
The latter has been only recently addressed investigating the effect on shale strength of different cations: matrix shrinking due to potassium ions in smectitic-rich shale has been pointed out with consequences on wellbore stability, and a few speculations have been done on matrix weakening in kaolinitic-rich shales.
In this paper, results of a project addressing the consequences on wellbore stability of the drilling fluid interactions with shaly formations have been reported to rationalize field evidences.
High pressure-high temperature experiments have been run with original set-up to analyze the combined effect on wellbore stability of two commercial glycols with different potassium and sodium salts.
Contrary to expectations, viscosity effects and pore blocking to prevent the pressure trasmission through the formation have been found to be sligthly effective.
More relevant turned out to be the salts effect on pressure unbalance (related to osmotic phenomena) beween formation and drilling fluid.
Parallel to these experiments, ageing on cuttings prepared from the same outcrop shale has been run in the same solutions of the pressure transmission test (2 weeks at 80°C).
Indentation tests show a different range of hardness depending on glycol and salts used. According to these results, considerations on the successful wellbore stabilization in smectitic-rich shale with glycols and potassium salts have been drawn; in particular, the most effective combination giving a relevant synergistic effect turned out to be with glycols and potassium carbonate.
S. Carminati , L. Del Gaudio (EniTecnologie), M. Brignoli, F. Zausa and D. Giacca (ENI-AGIP Division)
Source:E.N.I.COM
© 2001 Mena Report (www.menareport.com)
