Study On The Influence Of Carbon Dioxide Replacement On Mechanical Properties Of Natural Gas Hydrate Reservoir

As a kind of clean energy,natural gas hydrate is regarded as the most potential alternative energy for oil and gas.However,hydrate reservoirs have the characteristics of weak consolidation and low strength.The hydrate reservoir is featured by weak consolidation and low strength.At the same time,the decomposition of hydrate in the drilling process will lead to the further weakening of the strength of hydrate reservoir,which may lead to a series of geomechanical problems such as reservoir subsidence,submarine landslide and sand production.The CO₂ replacement method can generate CO₂ hydrate in the reservoir while sealing CO₂ gas,enhancing the mechanical properties of the reservoir,and providing guarantee for the stability of the reservoir in the process of hydrate exploitation.In this paper,CH₄ hydrate and CO₂ hydrate sediments were synthesized by in-situ formation method,and triaxial compression experiments with different initial saturations were carried out.On this basis,the CO₂ replacement experiment of CH₄ hydrate was carried out to test the mechanical properties of the samples under different CH₄ hydrate saturation and replacement time.Based on the analysis of the deformation and failure mechanism of hydrate reservoir in the process of CO₂ replacement mining,the constitutive model and strength criterion of gas hydrate reservoir suitable for CO₂ replacement mining are established.Based on the physical characteristics of CO₂-CH₄ hydrate sediments,a thermal-fluid-solid-chemical coupling mathematical model was established to analyze the variation characteristics of hydrate saturation around the wellbore and the sediment sand production rule during the replacement mining process,and the influence of producing conditions on the sand production was analyzed by orthogonal experiments.The results show that the elastoplastic failure occurs in the hydrate bearing sediment samples,and the stress-strain curve is hyperbolic in shape,without obvious peak strength.After the replacement reaction,the stress-strain curves of the sediment samples containing hydrate are basically the same as those before the replacement.The mechanical properties of the hydrate-containing sediment samples after replacement are better than those before replacement.The strength of the sample increases with the increase of initial CH₄ saturation and replacement rate.With the increase of displacement time,the Poisson’s ratio of the samples containing hydrate decreases.A hyperbolic constitutive model and strength criterion considering initial CH₄ hydrate saturation and displacement rate are established on the basis of displacement experiments.By modeling the replacement process,it is found that CO₂ injection will lead to the coexistence of CO₂ and CH₄ hydrate in a small area around the well,and reduce the seepage capacity of the formation during the injection process.The sediment outside the coexisting zone will cause the decomposition of CH₄ hydrate due to the exothermic generation of CO₂ hydrate,forming a single zone of CO₂ hydrate.By modeling the depressed-pressure production process,it is found that the permeability loss caused by the generation of CO₂ hydrate is not conducive to the decomposition of CH₄ hydrate,but it improves the formation strength performance and reduces the risk of sand production.In the process of producing CH₄ hydrate by CO₂replacement method,the amount of sand produced decreases with the increase of CO₂ hydrate generation range and CO₂ hydrate saturation after replacement,and increases with the increase of production pressure difference and production time.The significant degree of influence of each factor on sand production is the formation range of CO₂ hydrate>CO₂ hydrate saturation>Production differential pressure>Production time.