Numerical Modeling of Cased-hole Instability in High Pressure and High Temperature Wells



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Down-hole damages such as borehole collapse, circulation loss and rock tensile/compressive cracking in the open-hole system are well understood at drilling and well completion stages. However, less effort has been made to understand the instability of cemented sections in High Pressure High Temperature (HPHT) wells. The existing analysis shows that, in the perforation zones, casing/cement is subject to instability, particularly in the presence of cavities. This dissertation focuses on the instability mechanism of casing/cement in the non-perforated zones.

We investigate the transient thermal behavior in the casing-cement-formation system resulting from the movement of wellbore fluid using finite element method. The critical value of down-hole stresses is identified in both wellbore heating and cooling effects. Differently with the heating effect, the strong cooling effect in a cased hole can produce significant tension inside casing/cement.

The confining formation has an obvious influence on the stability of casing/cement. The proposed results reveal that the casing/cement system in the non-homogeneous formation behaves differently from that in homogeneous formation. With this in mind, a three-dimensional layered finite element model is developed to illustrate the casing/cement mechanical behavior in the non-homogeneous formation. The radial stress of cement sheath is found to be highly variable and affected by the contrast in Young?s moduli in the different formation layers. The maximum stress is predicted to concentrate in the casing-cement system confined by the sandstone.

Casing wear in the cased-hole system causes significant casing strength reduction, possibly resulting in the casing-cement tangential collapse. In this study, an approach for calculating the stress concentration in the worn casing with considering temperature change is developed, based on boundary superposition. The numerical results indicate that the casing-cement system after casing wear will suffer from severe tangential instability due to the elevated compressive hoop stress.

Gas migration during the cementing process results from the fluid cement?s inability to balance formation pore pressure. Past experience emphasized the application of chemical additives to reduce or control gas migration during the cementing process. This report presents the thermal and mechanical behaviors in a cased hole caused by created gas channels after gas migration. In conclusion, the size and the number of gas channels are two important factors in determining mechanical instability in a casing-cement system.