Study On Sand Production Mechanism Of Natural Gas Hydrate Exploitation

Natural gas hydrate is a potential clean energy source in the future.It has huge reserves in the permafrost and marine environment.At present,the on-site mining conditions all show that the strata are easily destabilized and destroyed during the hydrate exploitation and decomposition,and a large amount of sand are entered into the wellbore by the fluid forced to stop mining which seriously reduces the development efficiency of hydrates.Hydrate formation and decomposition process will cause damage to the formation,resulting in reduced stratum strength and easier sand production.Aimed at this problem,this paper conducts a preliminary uniaxial compressive and Brazilian splitting test on rock samples under different conditions before hydrate formation,after hydrate formation,and after hydrate decomposition.The comparative analysis found that for strong cementation sandstone skeleton and weakly cemented sandstone skeletons,the strength of the specimens was significantly reduced after hydrate through formation and decomposition,and the weakly cemented sandstone specimens showed a greater reduction;the formation of sandstone hydrates with strong cemented skeletons significantly improved the compressive strength.But for weakly cemented sandstone,the compressive strength of the hydrate did not change significantly after its formation,and the elastic modulus even decreased,and cracks appeared on the surface of the sample.After the hydrate was decomposed,the sample was fragmented.The calculation results of the theoretical model of particle-incorporated composites show that when the pure hydrate elastic modulus is low,the elastic modulus of the hydrate sediment sample is approximately unchanged or slightly lower as the hydrate saturation increases.Based on the statistical damage constitutive model calculations,the damage variable curve of the original sandstone sample after the hydrate formation and decomposition is significantly different.When in the uniaxial test the stress of the sample before the decomposition reaches the peak,the damage variable reaches about 40%;In the triaxial test,the initial axial stress damage variable of the sample as high as 98%,and the hydrate produced significant damage to the sample during the formation and decomposition process,resulting in serious sand production from hydrate production.From the perspective of the change of the formation water saturation during the process of hydrate production,the relationship between the inter-particle liquid bridge force and the water saturation angle after the cementation was lost after the hydrate decomposition was studied in combination with a particle-scale microscopic model.Based on Bishop effective stress theory,a formula considering the relationship between the change of water saturation and the formation pore pressure was established.Combined with the failure criterion of rock and soil,the critical bottom hole pressure difference and sand production radius calculation method for hydrate reservoir was established.As the cohesion of the reservoir decreases,the pressure difference at the bottom of the critical sand production decreases;when the values of the cohesion force and the internal friction angle are low,a slight change causes a sharp increase in the sand production radius,and the sand production becomes more serious.