Design Approach to Sealant Selection for the Life of the Well

Abstract Recent experience in the field has demonstrated that the mechanical properties of the annular sealant are a critical factor in the success of a well. A demanding operational regime such as High Pressure/High Temperature (HP/HT) and well interventions, e.g. pressure testing stimulation, place high demands on the mechanical design of a candidate sealant. The following steps should be followed to determine the required mechanical properties of such well sealant candidates: The stability of the bore hole should be evaluated prior to placing the sealant in the annulus. The stability is affected by the far field stresses, the weight (s.g.) of the drilling fluid and the mechanical properties of the formation. The integrity of the sealant should be investigated for the operational envelope of the well. Finite Element models were used to determine the influence of wellbore stability aspects and the effects of loading on the integrity of the sealant. The mechanical properties of the sealant considered were elasticity, tensile strength, shear strength and bonding strength. In this way it was possible to examine aspects of non-linear material behavior of the sealant. The following factors were investigated in this study: What are the well parameters and its operating envelope? What are the total stresses on the formation and on the sealant under various HP/HT conditions? Which mechanical properties does the sealant have to possess in order to be able to withstand different types of stress? The study indicated that there is a strong interaction between on the one hand the behavior of the sealant and on the other hand the formation in-situ stresses, the formation properties, the well completion parameters and its operating envelope. Depending on these input conditions, the sealant was observed to behave in one of the following manners. The sealant remained intact without any damage. A micro annulus occurred between sealant and casing/formation. The sealant failed because of either tensile or shear deformation. Calculations were performed for various wellbore stability scenarios, well completion characteristics, well loading envelopes and properties of the sealant. Typical oil well cement slurries, latex slurries, foam cements and cement hybrids were investigated. Typical results from the study are discussed and procedures for field application are given.