Abstract
Abstract
Wellbore instability is a major cause of stuck-pipe incidents. Wellbore enlargements induced by instability work to produce enlarged rock fragments known as cavings. These fragments can overwhelm the rig hydraulics system, which can potentially lead to tight spots, overfill, stuck-pipe, and even loss of well. Therefore, it is essential to manage, prevent, or anticipate the production of cavings through reliable modeling techniques. Conventional geomechanics models can predict yielding and rock failure through a fitted failure criterion. This failure assessment does not offer a prediction of size or quantity of cavings that can be introduced to the wellbore. This is because failed rock, as predicted by a conventional geomechanics model, will not necessary dis-integrate and fall off into the wellbore. To overcome this blind spot, a numerical geomechanics model is coupled with spalling rate analysis.
The analysis is carried out using a 3D poro-elasto-plastic finite element model (FEM). The FEM uses offset wells data along with the best fitting failure criterion to estimate the wellbore instability limit. The considered offset wells data used consist of wireline logs, drilling reports, and mechanical testing lab results belonging to the formation of interest. The model uses this data to estimate the stress distribution and define the best fitting failure criterion. The model is then coupled with spalling rate analysis. This relies on hollow cylinder tests, which are conducted on cores extracted from the formation of interest, to determine the spalling criterion. The end result is prediction of quantities and sizes of cavings that will be introduced into the wellbore under the specified conditions.
Coupling spalling rate analysis with the 3D FEM model makes way for de-risking cavings production. It is shown that by applying different controls, such as the mud weight and its rheological properties, instability related challenges can be overcome within the means of the rig hydraulics system. It is also shown that at different values of mud weight, the quantities and sizes of cavings can be constrained in a manner that would allow the well hydraulics to manage them. Finally, the results make a clear distinction between yielded or failed rock, as decided by the failure criterion, and spalled or damaged rock, which will be introduced into the wellbore as caving fragments.
The coupled modeling approach presented in this work amplifies the impact of geomechanics analysis on drilling operations. This is done by not limiting the geomechanics modeling recommendations and output to mud weight only, but other controls within the rig hydraulics system.