Numerical Analysis Of Sand Production In Borehole During Gas Production From Marine Gas Hydrate

Natural gas hydrate(NGH)is an important energy strategic resource.The current research focuses on how to ensure the safe and efficient exploitation of marine NGH.The field trials of NGH in various countries show that sand production is a key obstacle to the industrialization of NGH.Sand production refers to the state that sedimentary particles in the formation are uncontrolllably produced from the borehole.Sand production not only blocks the path of gas migration and wears down the drilling equipment,but also causes formation damage and drilling accidents.To achieve the exploitation safely and efficiently,it is necessary to study the mechanism of sand production in hydrate reservoirs and work out the sand production problem.In this study,the author firstly uses the TOUGH + HYDRATE simulator to solve the hydrate decomposition process,which is based on the mathematical model of gas hydrate decomposition in the sea area.Based on the production pattern analysis of marine hydrates,the temporal and spatial evolution of the parameters that affect the decomposition of hydrates was clarified.The simulation results have a similar trend with the results of Japanese hydrate trial production and it is confirmed that a weak formation is formed in the perforated intervals where hydrates were mined,and hydrate saturation and pore pressure may be the important factors affecting sand production in the marine hydrate reservoirs.Then the perforation formations is focused and highlighted and an elastic mechanics model suitable for the marine natural gas hydrate reservoirs was established,based on the stress situation around the reservoirs concerned during the operation in field trials.By introducing the cohesion model considering hydrate saturation effect into the elastic mechanics model,the influence of pore pressure,Poisson’s ratio and compressibility of rock on sand production was investigated.The results show that the pore pressure has the greatest effect on change in the stress state of the borehole reservoir during the hydrate production,and is an important factor affecting the sand production.The stress state critical function shows that the plasticity deformation area is generated around production well and elastic mechanics model has not been able to describe the change in mechanical state during sand production.In order to clarify the mechanism of sand production in hydrate reservoirs,the reservoir near the borehole is focused to study the evolution path of plastic stress in hydrate exploitation.The results show that inner plastic zone,elasto-plastic transition zone and elastic zone are formed within the distance from production well.The sand production is not caused by the elastic failure.However,it is controlled by the plastic focal strain near the well end and pore pressure in the formation.The parameters influencing sand production are also analyzed in the study.The results show that the decrease in hydrate saturation and over-high in pore pressure are the important factors that control sand production.During the field trial,attentions should be highly paid on the plastic deformation around well and pore pressure changes,and then operating conditions could be adjusted properly to finally prevent the emergence of sand problems.Based on the establishment of the mechanics model of hydrate reservoir,this paper analyzes the sand production mechanism and factors affecting the sand production,and gives some suggestions for the field trial.It is a basis for the establishment of sand production mathematical model using multi-field coupling methods.