Experimental Study On Sand Production Law Of Hydrate-bearing Sediments By Depressurization Under Different Reservoir Factors

Natural gas hydrate(hereinafter referred to as hydrate)resources are abundant.It is estimated that the total amount of natural gas in hydrate is about 2×10¹⁶ m³,which is equivalent to twice the carbon content of global conventional fossil energy(coal,oil and natural gas).Because of its wide distribution,abundant resources and little environmental pollution,it is regarded as a potential alternative new clean energy in the future.In nature,hydrate is mainly accumulated in terrestrial permafrost and deep-sea sediments.The hydrate reservoirs in the sea area are shallowly buried,and mainly composed of weakly consolidated or unconsolidated sandy and clayey-silt sediments.The hydrate phase transformation and fluid migration during hydrate exploitation will lead to the redistribution of reservoir stress and the change of mechanical properties,which lead to a series of geological problems,such as reservoir subsidence,sand production and so on.According to the hydrate test production projects in the world,sand production is an inevitable phenomenon in hydrate mining,which has become one of the bottlenecks restricting the safe and efficient exploitation of hydrate.It is the premise of scientific and reasonable sand control to reveal the sand production behavior of hydrate during depressurization under different reservoir factors,and it is also the key to solve the above bottleneck.In this paper,we took the sandy and clayey-silt hydrate reservoirs as the research objects to simulated the depressurization exploitation under the conditions of different hydrate saturation,overlying stress,median particle size of the reservoir and initial reservoir temperature by laboratory experiments simulation using self-developed hydrate reservoir sand production and sand control experimental equipment.The sand producing behavior characteristics of sediment samples in different dissociation stages(before,during and after hydrate dissociation)during the process of depressurization exploitation and the accompanying behavior laws of gas production,water production and subsidence response are studied.The sand production law during the process of depressurization exploitation under the above four reservoir factors is revealed,and the possible reasons are discussed.Then,the sand production behavior of two kinds of hydrate reservoirs is compared,and established the sand production prediction models for depressurization exploitation of sandy and clayey-silt hydrate reservoirs based on the laboratory experimental data.The main conclusions and understanding are as follows:(1)In the process of depressurization exploitation of sandy hydrate reservoir at the experimental scale,the amount of sand production before hydrate dissociation is the least.The amount of sand production during hydrate dissociation increases greatly,and the amount of sand production is higher in the whole dissociation stage.After hydrate dissociation,sand particles are still produced continuously,but the amount of sand production is decreased compared with that during dissociation.There is basically no gas production before hydrate dissociation,and water production and reservoir subsidence are relatively small.During the whole process of hydrate dissociation,the rate of gas production,water production and reservoir subsidence are high,and after hydrate dissociation,the rate of gas production,water production and reservoir subsidence decrease.(2)During depressurization exploitation of clayey-silt hydrate reservoir,the sand production before hydrate dissociation is the least,and the gas production,water production and reservoir subsidence are also lower than those of sandy.When the hydrate begins to dissociation,the sand production,gas production and water production increase sharply,and the subsidence rate is faster.The amount of sand production during this period is mainly concentrated in the stage of high-speed gas and water production in the early stage of dissociation,and then with the gradual dissociation of hydrate,the amount of sand production,gas production and water production gradually decrease.After hydrate dissociation,the amount of sand production is very small,which is significantly lower than that during dissociation.The gas and water production rates are low,and the subsidence rate gradually tends to 0.The difference between sand production and reservoir response behavior during and after hydrate dissociation of two types of hydrate reservoirs is the difference of the characteristics of sandy and clayey-silt hydrate reservoirs and the different characteristics of sand retaining media formed at sand control screens during depressurization.(3)The total sand production increases with the increase of hydrate saturation,and when the hydrate saturation exceeds 40%,the total sand production increases obviously.There is a critical value of overlying stress on sand production behavior.When the overlying stress is less than 12MPa,the total sand production increases with the increase of overlying stress,and when the overlying stress is greater than 12MPa,the total sand production decreases with the increase of overlying stress.When other conditions remain unchanged,the smaller the median particle size of the reservoir is,the more the total sand production is.The initial reservoir temperature has little effect on the total sand production of sandy hydrate reservoir.(4)The total sand production decreases with the increase of overlying stress during depressurization exploitation of clayey-silt hydrate reservoir.There is a critical value for the effect of hydrate saturation on total sand production.When the hydrate saturation is less than 40%,the total sand production increases with the increase of hydrate saturation,and when the hydrate saturation is more than 40%,the total sand production decreases with the increase of hydrate saturation.When other conditions remain unchanged,the total sand production increases with the decrease of the median particle size of the reservoir.The lower the initial reservoir temperature is,the more the total sand production is.(5)Combined with the influence of hydrate saturation,overlying stress,reservoir median particle size and initial reservoir temperature on sand production of two kinds of hydrate reservoirs,a sand production prediction model of sandy hydrate reservoir by depressurization exploitation based on"hydrate saturation-overlying stress-reservoir median particle size"and a sand production prediction model of clayey-silt hydrate reservoir by depressurization exploitation based on"hydrate saturation-overlying stress-reservoir median particle size-initial reservoir temperature"are established respectively according to laboratory sand production data.It provides a theoretical reference for sand production prediction of two kinds of hydrate bearing sediments under different reservoir factors,and has a guiding significance for the priority of target area and reservoir and the design of sand-control methods.