When a well is drilled in a hydrocarbon reservoir, the thermodynamic equilibrium of virgin rocks is altered, the natural stresses are redistributed, and a stress concentration occurs around the hole. The alteration of original equilibrium can lead to wellbore yield, evidenced as shear banding and dilatation, circumferential crack, and microfissuring process. Borehole instability represents the main cause for loss of drilling fluids and consequent potential kick problems, and can be so severe to determine the wellbore abandonment. Loss of time associated with stability problems is estimated to account for between 12 and 15% of drilling cost world-wide. Physical and mechanical processes occurring within and around the borehole because of interaction between drilling fluids and rocks can degrade the stability of the wellbore especially in shale intervals, which can make up over 75% of drilled formations. In fact, due to their fine-grained nature and low permeability, in association with high porosity and high fluid saturation, shale minerals are particularly sensitive to time-dependent stability degrade. In order to accurately analyse the complex behaviour of shale minerals, the effects of mechanical deformation, hydraulic diffusion and temperature gradient, as well as their combination must be taken into account. The classic poro-elastic theory, which allows consideration of the coupled phenomena of time-dependent pore fluid diffusion and formation stress variation, fails to capture the effects of temperature gradient and plastic deformation, particularly remarkable in the shale behaviour analysis. So a thermo-poro-mechanical approach, coupling constitutive equations with thermal and hydraulic laws, is necessary to perform accurate time-dependent analyses of stress, pore pressure and temperature distribution around the wellbore in shale formations. This paper presents the analytic formulation of a thermo-poro-mechanical model, in which the basic Desai’s model is used to describe the rock plastic behaviour for a mono-dimensional, axisymmetric problem in plane state of strain. With the aid of an in-house software, the proposed model was first validated, also by comparison with the output of a well-known geomechanical numerical simulator available on the market, and then applied to several real and synthetic cases. Based on the results of one of the examined case history, the paper shows how the borehole stability modelling approach is fundamental to systematically take into consideration the stress variations around the hole and the associated rock deformation and pore pressure changes during drilling. The prediction of natural equilibrium time-dependent alterations allows drilling engineers to improve the design phase, to take decisions on critical effects potentially occurring during drilling as well as to optimize completion (e.g. casing placement and/or cementing) of a well.
Stability modelling applied to wellbore design / ROCCA V; VERGA F.. - In: GEAM. GEOINGEGNERIA AMBIENTALE E MINERARIA. - ISSN 1121-9041. - STAMPA. - XLV:3(2008), pp. 5-12.
|Titolo:||Stability modelling applied to wellbore design|
|Data di pubblicazione:||2008|
|Appare nelle tipologie:||1.1 Articolo in rivista|