The instability of clay walls is one the crucial problems during the drilling process. It is mainly caused by the reactions between clay and mud. These reactions are the results of the interactions of clay and polar composites, due to their large specific areas and also their structure. In order to minimise this problem, an experimental method is reported. This method consists of the: (1) characterisation of well clay formation, (2) formulation and optimisation of a drilling fluid system, and (3) then the characterisation of the influence of mud upon clay. (C) 2003 Elsevier Science Ltd. All rights reserved.