Problems related to wellbore instabilities are a well known source of additional costs. According to published data they may amount up to 15% of the total drilling budget of a well. The development of an efficient industrial methodology to solve these problems is therefore compulsory for any operator.
To be successful, this methodology has to include three fundamental steps:
diagnosis of the mechanism at the origin of the problems;
determination of the main parameters governing the mechanism;
modeling of the problem in order to properly size its cure.
To be efficient, all three steps should be performed not only during the planning phase but also in real time, directly at the rig-site during drilling.
The paper will present the wellbore stability package developed by Agip and will put special emphasis on its real time dimension at the rig site. Several field cases in a critical area of Southern Italy will be used to illustrate it. The methodology is based upon three main moments which will be described in detail:
careful monitoring of the instabilities signals - e.g. tripping conditions, cavings formation, physico-mechanical characterization of the cavings, detection of ageing in mud, etc…:
formation evaluation by means of measurements on cuttings to determine their main properties - e.g. strength, stiffness, permeability, etc.;
quantification of the problem with a series of desktop based computer models - e.g. linear elastic, elasto plastic, coupled thermo-poro-mechanic, etc.
The classical ways to tackle wellbore stability problems can be grouped into two main categories:
empirical approaches, mainly based on monitoring and adjusting drilling parameters;
theoretical approaches, where more or less sophisticated models (both analytical and numerical) can be applied in order to investigate wellbore behaviour.
The former accounts for instabilities considering, as critical, only those parameters related to drilling operations. Possible instabilities could then be caused, as an example, by too high or too low mud density, inappropriate mud system rheology, swab and surge while tripping, unsuitable BHA, etc. This method does not give too much importance to in situ conditions and formation features.
The other approach, viceversa, usually takes into account in situ conditions, such as stress magnitude and direction, together with formation characteristics (petrophysical and mechanical parameters) but less attention is devoted to drilling operation itself. Very sophisticated models exist today. Moreover, their applicability to treat real situations is often extremely poor as essential parameters such as in situ stresses or rock strength have to be guessed or parametric studies run.
The above considerations are of primary importance, when defining the critical features to be improved, in order to define a suitable approach to solve wellbore instabilities. First of all drilling parameters and drilling conditions have to be carefully analysed and considered so that to recover the best picture of the field situation. Yet, suitable models to simulate formation behaviour and its response, when interacting with defined in situ stress condition and drilling operations, are also extremely important.