An advanced elasto-plastic model for borehole stability analysis of horizontal wells in unconsolidated formation

Borehole stability problems can dramatically increase the cost of drilling and completing wells. For example, sand failure and production may lead to costly well shut-in and workover, as well as equipment failure. Wellbore in stability is particularly important when operating in deep unconsolidated f ormations or when horizontal wells are planned. Also, stability requirement s increase when drilling into producing formations. Drilling problems such as stuck drillstring, caving, logging difficulties, well enlargement, inadv ertent side tracking, and stuck casing can be costly to remediate. Traditio nally, analytical methods are used to predict required mud weights. However , they are typically too conservative and are unsuitable for applications t o horizontal wells in unconsolidated formations. In order to analyse horizontal drilling in an unconsolidated formation, a w ellbore stability model has been developed. The model uses a robust finite element elasto-plastic code as a tool to perform effective stress analysis of the near-wellbore tensile and shear failure. The code is capable of hand ling extremely low confining stresses (near tensile regime) in unconsolidat ed formations. It is considered that plastic yielding, occurring at relativ ely low deviatoric stresses under low confining stress conditions, is not a good indicator of wellbore failure in terms of loss of service. Therefore, a more realistic criterion based on the accumulated plastic strain is appl ied. The model has been used successfully to analyse the stability of a hor izontal well with open-hole completion in unconsolidated oil sand. The resu lts show reasonable agreement with field observations.