A constitutive model of natural gas hydrate reservoirs during exploitation by methane-carbon dioxide replacement

Abstract

For the safe exploitation of marine gas hydrate resources, the mechanical stability of the reservoir itself and that of the exploitation equipment system need to be ensured, of which the former is the fundamental factor. The carbon dioxide replacement method uses carbon dioxide to chemically replace methane and then generate carbon dioxide hydrates in the reservoir, enhancing the reservoir stability while also sealing the reservoir with carbon dioxide. In this work, methane hydrate samples containing sediments were artificially prepared. Carbon dioxide replacement for methane hydrates and triaxial compression tests during carbon dioxide replacement were carried out. Based on the experimental results, the parameters of the Duncan–Chang constitutive model were modified according to the replacement ratio and hydrate saturation, and a nonlinear constitutive model for describing natural gas hydrate reservoirs under the effect of carbon dioxide replacement was established. These results indicate that the diffusion of carbon dioxide into a sample may gradually be blocked and inhibited by carbon dioxide hydrate formation during replacement. Overall, the stress‒strain curves of the samples are hyperbolic. The sample undergoes elastic deformation in the initial stage of the triaxial compression test. Subsequently, it undergoes plastic failure without apparent peak strength and shows strain-hardening characteristics. After replacement, the sample strength increases, and the stress‒strain curve is similar in shape to that before replacement, with an upward shift. In the established constitutive model, the replacement ratio significantly affects the initial tangent modulus, cohesion, and initial tangent Poisson's ratio. The calculation results provided by the model fit the experimental data well.