Study On Sand Production In Hydrate Decomposed Unsolidated Sediment

The hydrate formation is weakly cemented.Hydrate mining is likely to cause sand production.According to the law of migration and production of silt in the stratum,the sandblasting process of the hydrate reservoir can be divided into two sub-processes: the spalling of the sand on the stratum skeleton and the migration in the hydrated decomposition zone.and the latter is mainly Influenced by the fluid seepage flow in the hydrate decomposition zone.Studying the migration and production law of mud sand in the hydrate decomposition zone under the condition of fluid carrying is the key to rational planning and design of sand control completion measures and production parameters.Therefore,this paper mainly takes the law of sediment production in the loose sediments of natural gas hydrate decomposition zone as the object,and uses theoretical modeling combined with the model of physical model experiment.The loose sediment simulation experiment was carried out.The critical flow rate law of the formation sand movement of the formation sand and the sand grain production law under the condition of no sand retention were obtained,and the experiment of the internal migration law of the formation sand was carried out.Based on the assumption of spherical particle accumulation of hydrated loose sediments,the stress of sand particles in the formation front and decomposition zone of hydrates was analyzed.The calculation of the critical flow velocity of formation sand particles in loose sediments was established based on the moment balance conditions.Models and relevant sensitivity analyses to explore the influencing factors.According to the dynamic process of sand grain initiation and movement,the calculation model of sand mass yield of formation sand unit is established,and the calculation results are compared with the experimental results.(1)There is a critical flow velocity of the sand in the loose sediments,and the critical starting transport velocity increases with the increase of the particle size,the porosity of the reservoir and the cementation strength of the reservoir;(2)unit body The amount of sand increased with the increase of the flow rate,and decreased with the porosity of the unit body,the grain size of the constituent unit and the friction coefficient.The flow rate of the over-flow unit is related to the cumulative sand production of the unit.The coefficient of coefficient is0.85,which is the main factor affecting the amount of sand;(3)there are two forms of migration in the process of sand production,including the migration of small-sized sand inside the sand body and the overall migration of sand body.In the experiment,when the displacement flow rate increased from 10 ml/min to 20 ml/min,the cumulative sand production increased by more than 2 times,and the flow velocity was the key factor affecting the amount of sand;(4)the migration of sand within the reservoir.The pore structure of the reservoir will be changed,and a bridge structure will be formed inside the reservoir to hinder the further migration of the sand.Under the experimental conditions,the migration intensity of the sand is the largest when the flow is 80 ml/min,and the position of the bridge is the most.Close to the liquid flow inlet;(5)during the multi-step depressurization of natural gas hydrate,when the flow increases from a small flow to a larger flow,the original bridge structure is broken to form a new bridge structure,which is smaller under experimental conditions.The flow gradient span of 20 ml/min can effectively slow down the migration of sand.The research in this paper will have certain reference significance for sand production prediction and sand control process design in the future hydrate mining process.