A coupled thermal-hydraulic-mechanical model for drilling fluid invasion into hydrate-bearing sediments

Evaluation of interactions among multiple physical fields in natural gas hydrate reservoirs is the basis for risk management during drilling fluid invasion. However, variations of physical fields, especially stress states and failure behaviors of invaded formation, are still unclear, which limits the stability estimation and risk control during drilling hydrate. Herein, a coupled thermal-hydraulic-mechanical model is established to describe characteristics of geophysical fields and wellbore failure. This model has superiority in characterizing stress states and invasion-induced deformation of the near-wellbore formation, which can reflect the effect of borehole shapes and drilling operations. Results reveal that drilling fluid invasion causes both stress and strain concentrations occurring around the wellbore. Stress states depend on the borehole shapes and invasion degree, especially the downhole zones with massive dissociation of hydrate. The yield area typically appears in the flushed zone and enlarges with invasion time. Besides, a fail function Ff is introduced into this model to determine the elastoplastic deformation areas, indicating the high-risk regions of wellbore instability. Consequently, coupling effects of invasion and phase transition under various stresses can lead to unavoidable deformation and stress changes. Thus, coupled analysis of geomechanical behaviors is an indispensable part of risk control in drilling hydrate reservoirs.