Influence Of Hydrate Spatial Distribution In Reservoir On Seepage Characteristics

The essence of social progress is the development and utilization of energy resources.With the increasing demand for energy and environmental awareness,there is a growing call to replace traditional fossil fuels with clean energy.As one of the most potential new energy sources,natural gas hydrate whose combustion products are water and carbon dioxide has attracted researchers' attention.In the process of natural gas hydrate exploitation,the seepage characteristics are crucial for the safe exploitation of natural gas hydrate.And the gas/water phase seepage characteristics directly control the fluid flow and energy transfer in the reservoir.The design of hydrate exploitation scheme also depends on the prediction of seepage characteristics of the reservoir.Therefore,the study on reservoir seepage characteristics is very necessary for the efficient and economic exploitation of gas hydrate.The seepage characteristics of hydrate reservoir not only depend on the composition of sand and gravel,but also are affected by the different spatial distribution of hydrate in reservoir.In order to research the influence of hydrate saturation,heterogeneity distribution and hydrate growth habit on seepage characteristics,this study used CT visualization technology to monitor hydrate sample cores to obtain CT image of hydrate core.And the technology of image enhancement and image segmentation were developed,which solved fuzzy boundary of each component in the core.The hydrate growth habit in reservoir had been identified effectively.Then,a three-dimensional framework model including skeleton structure information of hydrate core was constructed by using the pore network model.And the flow simulation was carried out to obtain a variety of useful information about the seepage characteristics of hydrate core.Finally,the effects of hydrate saturation,heterogeneity and growth habit on seepage characteristics were summarized,and theoretical models available for different growth habit hydrate were established.It was found that hydrate changed the internal structure of the reservoir.Increasing the hydrate saturation could decrease the permeability of the hydrate core.Due to the increase of hydrate content,the radii of pores and throats reduced and the account of pores and throats decreased.Afterwards,the differences in the local seepage characteristics caused by heterogeneity were discussed.Even for the same porosity hydrate cores,samples with higher coordination number and lower pore-throat ratio have better seepage characteristics.If the hydrate accumulated at the pore junction,it would seriously hinder the fluid movement and reduced permeability of the core to 0.It is worth mentioning that in the early stage of hydrate exploitation,the Jamin effect is the main factor restricting the gas flow in the low porosity reservoir.In addition,the hydrate growth habit also affects the seepage characteristics of the reservoir by changing the pore structure.The average pore radius of pore-filling hydrate is much smaller than that of coating hydrate,and the permeability of pore-filling hydrate is also lower than that of coating hydrate.The seepage characteristics of cementing hydrate are the worst.In this study,Kozeny-Carman model was improved by adding parameters to reflect the density of particle distribution.Compared with previous models,the improved model can better predict the variation trend of hydrate reservoir permeability.The results are helpful to the study of hydrate seepage characteristics in future.