The Effect of Casing Eccentricity on the Casing Stability Analysis of a Wellbore Drilled in Gas Hydrate Bearing Sediments

Abstract

Abstract Conductor pipe is a surface casing installed in the first few hundred meters in deepwater drilling operations (tens of meters in onshore operations) and it is used to prevent borehole collapse (and protect unconsolidated surface formations against washing out by drilling mud). Gas hydrate bearing sediments found typically near to the seafloor in deepwater environments are very soft and unconsolidated formations. Hole enlargement and wash-out are problems during the drilling these types of formation. As a result, the casing eccentricity is inevitable in the case of borehole washout (in particular if there are not enough centralizers placed on the casing). The circulation of hot drilling mud during subsequent drilling operations or production of hot hydrocarbon after completion of the wellbore will dissociate gas hydrates in the formation behind the casing. Water and gas resulted from gas hydrate dissociation cannot flow away if the permeability of the formation is very low causing high pore pressure to trap behind the casing. The trapped pore pressure in combination with the casing eccentricity may endanger the stability of the casing depending on the magnitude of trapped pore pressure, degree of casing eccentricity and the mechanical strength of the casing. As a result, the development and drilling operations in deepwater environments can be challenging if there are gas hydrate bearing sediments. In this communication, a numerical model is developed using a finite-element code in order to analysis the casing stability of the wellbores drilled in gas hydrate bearing sediments. The model is used to analysis the casing stability of the wellbore under uniform and non-uniform loadings. The non-uniform loading is introduced in the model by considering the presence of eccentric casing together with the pore pressure increase due to gas hydrate dissociation. The results of the model show that the casing eccentricity is an important issue in the casing stability analysis of the wellbores drilled in gas hydrate bearing sediments in deepwater environments. The higher degree of casing eccentricity, the higher the magnitude of stress generated in the casing during subsequent operations, which may lead to casing collapse