Casing integrity assessment of a wellbore crossing a karst cavity in a carbonate reservoir

摘要

Different measures of porosity can coexist in carbonate reservoirs, ranging from micropores to large-scale cavities. It is well known that the construction of wellbores intersecting karst cavities can be seriously affected with lost circulation and kick. This work focuses on the casing integrity of a wellbore crossing a karst cavity in a carbonate reservoir during production. 3D geomechanical analyses using the finite element method are carried out. The model considers the main steps of the wellbore lifespan: initial stress state in the formation, drilling, casing run, pressure changes and pore pressure variation in the reservoir. The materials are modeled as elasto-plastic - von Mises criterion for the casing and Mohr-Coulomb criterion for the rocks and cement. Casing integrity (Yield Index, YI) is monitored through the evolution of the ratio between the von Mises deviatoric stress and yield stress with the pore pressure variation. The influence of a 1227m~3 cavity on the results is investigated through the development of two models: an open-hole completion with one production tubing, and a regular wellbore with intermediate and production casings. The results point out a maximum increase of 13.5% on the YI due to the cavity presence.