Experimental Study On The Behavior And Micro Mechanism Of Sand Production During Hydrate Exploitation

Natural gas hydrate is an unconventional natural gas resource,which is widely distributed in permafrost and deep-sea sediments.Due to its abundant reserves,high energy density and environmental friendliness,it is considered as a traditional fossil fuel alternative energy with great potential in the future.In the process of natural gas hydrate exploitation,due to the large production pressure difference and the weakening of reservoir cementation,fine sand will migrate to the wellbore,resulting in sand production.Serious sand production will affect the reservoir stability and even damage the exploitation equipment which will bring huge economic losses.In the actual exploitation project,sand control is carried out by a variety of technical means such as gravel packing and filter.However,the existing means mostly follow the traditional oil and gas production engineering experience,lack of theoretical basis.Therefore,it is of great significance for the safe development of hydrate in the south China sea to conduct in-depth theoretical research on the sand production characteristics,clarify the sand production mechanism,and then evaluate the risk of sand production.In this work,sandbox experiments are carried out to observe sand migration and blocking phenomenon in simulated hydrate reservoir,and quantitatively studied the influence of fluid flow rate Q,median diameter ratio of gravel to sand(D50/d50),and initial hydrate saturation Si on sand production.It is found that sand production can be divided into three stages: rapid sand production,unstable sand production and stable sand production.With the increase of the fluid flow rate,the cumulative sand production and the median particle size of produced sand increase.Moreover,when the fluid flow rate reaches 5 L/min,cumulative sand production shows a sudden increase and sand production rate begins to fluctuate violently.When the median diameter ratio of gravel to sand is about 5-8,the median particle size of the produced sand is very close to that of the silty sand,which conforms to the principle of proper sand production.The higher the initial hydrate saturation is,the greater the cementation strength of the hydrate reservoir is,which reduces the possibility of sand production to a certain extent,but prolongs the time required for the complete decomposition of the hydrate,that is to say,it delays the time for the reservoir to reach the stable stage of sand production,which increases the risk and uncertainty of sand production.In addition,the experiment can observe the migration and plugging process of fine sand,and the phenomenon of sand layer collapse and settlement will occur with the increase of sand production.Meanwhile,this paper also uses the Micro-CT device to obtain the migration and plugging mechanism of fine sand in hydrate sediments from the perspective of microscopic visualization.The results show that the mechanism of gravel layer plugging mainly includes inlet plugging,sand bridge plugging,pore packing plugging and non-formation sand plugging,and the range of the optimal median diameter ratio of gravel to sand obtained from the quantitative sand production experiment is verified.By analyzing CT images of mixed sand samples,it can be found that in the initial stage of the experiment,the driving force provided by the fluid is almost used to destroy the cementation state and dredge the internal flow channel,and the fine sand barely moves.In the middle and late stage of decomposition,large amounts of free sand form and migrate,which accelerates the decomposition of hydrate.The existence of closed gas and its interaction with fine sand migration are also found.In addition,particle image velocimetry(PIV)is used to describe the fluid's sand-carrying capacity.The study found that the migration direction of sand is somewhat chaotic,and the migration velocity magnitude of sand is not uniform.Due to the change of the internal mechanical equilibrium state,the skeleton sand with larger particle size will also move slightly under the action of confining pressure.