Research On Mechanical Properties Of Heterogeneous Hydrate-bearing Sediments

Natural gas hydrate(NGH)is a potential new type of clean energy due to its huge reserves and high energy density.The decomposition of hydrate during the production process will destroy the original cement structure of the reservoir,which may cause formation instability and even climate change.Therefore,it is very important to study the mechanical properties of NGH sediments for the efficiency and safety of NGH production.Sample drilling core sample can be found from the spot,the nature of hydrate were showing heterogeneity distribution,the properties of sediments caused by the difference of mechanical properties experimental results,this paper combined with the indoor experiment and numerical simulation,the heterogeneity of the mechanical properties of hydrate sediments were studied,in order to provide analysis basis for improving the stability of hydrate reservoirs.First put forward the three sample preparation methods on the experimental side to simulate different types of hydrate somewhat heterogeneity distribution of three different kinds of samples are obtained.Then the low-field NMR technology was used to detect the distribution of water in the samples,and the one-dimensional saturation curves of water in the three samples were obtained.The distribution of hydrate in the samples was analyzed,which provided a basis for the following experiments and numerical simulation.Later in this paper,the synthesis of three kinds of samples in a series of triaxial shear test.The results show that the samples with uneven hydrate distribution have lower initial elastic modulus and lower strength.In addition,the samples with uniform hydrate distribution show strain softening behavior,while those with uneven hydrate distribution show strain hardening behavior.When the axial strain is less than 15%,the peak strength of the samples with uniform distribution of hydrate is higher than that with non-uniform distribution of hydrate.Finally,the discrete element method was used to reconstruct the sample with the experimental data.After several parameter calibration,the correctness of the parameters was verified.The effects of heterogeneous distribution types of hydrate on the mechanical properties of NGH sediments under a series of triaxial shear experiments were analyzed.The mechanism of stress and volume changes of the samples was described from the particle scale.It is found that the direction and velocity of particles are different inside and outside the shear zone.The axial uniform distribution of hydrate has a more obvious effect on the strength enhancement of the sample than the radial uniform distribution,and the more uniform the radial distribution of hydrate is,the more obvious the inhibition of the bulk expansion behavior of the sample is.