A Coupled Thermal-Hydraulic-Mechanical Model for Drilling Fluid Invasion into Hydrate-Bearing Sediments

Abstract

Evaluation of interactions of multiple physical fields in natural gas hydrate reservoirs is the basis for risk management during drilling fluid invasion. However, variations and interactions of physical fields, especially stress states and displacements, are still unclear, which limits the stability estimation and risk control during drilling hydrate. Herein, a coupled thermal-hydraulic-mechanical model is establisheded to describe characteristics of geophysical fields . This model has superiority in characterizing coupling mechanisms of multiple fields and elastoplastic deformation induced by ground disturbances. 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 in the downhole zones with massive dissociation. The yield area typically appears in the flushed zone and enlarges with the invasion process, implying that it's prone to damage and failure in this region. Besides, a fail function is introduced into this model to determine the elastoplastic deformation areas, indicating the high-risk regions of instability and even failure. 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.

Keywords: natural gas hydrate, drilling fluid invasion, coupling process, stress state, invasion mechanisms