Geological Characteristics And Physical Properties Of Low Permeability And Weakly Consolidated Hydrate Reservoirs In The South China Sea

Natural gas hydrate is the most potential strategic resource of the 21st century,and has attracted worldwide attention for its multiple advantages including wide distribution,shallow burial,large size of reserves,being clean and pollution-free,and others.China has carried out comprehensive investigation and research on natural gas hydrate resources in Dongsha,Shenhu,Qiongdongnan and Xisha troughs in the northern South China Sea during the past two decades.After unremitting efforts,22 gas hydrate blocks with obvious BSR anomaly have been found,6 hydrate metallogenic prospect areas,19 metallogenic belts,25 favorable blocks and 24 drilling target areas have been delineated,and a series of major prospecting results have been achieved.In particular,two gas hydrate trial productions were performed successfully in Shenhu area of South China Sea in 2017 and 2020,which makes China becoming the first country in the world to realize continuous and stable gas production in the exploitation of hydrate in marine silty reservoir.Nowadays,the natural gas hydrate resource has been officially listed as the 173th new mineral resources in China,and the industrialization process is accelerating.However,the hydrate reservoirs in the South China Sea are mainly composed of argillaceous siltstone and silty mudstone with low-permeability and weakly consolidated,leading to insufficient description of the reservoir,it means the characteristics of hydrate reservoir are still not clear,therefore,the locking of the high value mining target area is quite difficult.In addition,the hydrate reservoirs have the features of poor physical properties,and the understanding of its response behavior and evolution law is lack,which leads to the small decomposition radius of hydrate mining and the difficulty in achieving the production capacity.Meanwhile,the reservoirs are easy to induce the well collapse,sand production,formation subsidence,gas leakage,as well as geological disasters,it makes the industrialization of hydrate in China still subject to the dual constraints of economy and safety.Accordingly,based on the systematically combing and summarizing the previous hydrate drilling plans and hydrate production test in the South China Sea,this paper aims to comprehensively depict the low-permeability and weakly consolidated hydrate reservoirs in the South China Sea in both quantitative and qualitative aspects,the measured and analytical results of hydrate cores obtained from typical low-permeability and weakly consolidated hydrate reservoirs in the South China Sea,as well as combining with the understanding of relevant simulation experiments are presented.The main conclusions are as follows:(1)The hydrate reservoir in Shenhu area is composed of dark gray clayey silt with high water content,rich in foraminiferal shells,calcareous nannofossils and strawberry like pyrite aggregates.The sand particles have sharp edges and good sorting,and the clay minerals are mainly layered illite and the foraminifera are mainly planktonic with high content.The hydrate is mainly the composite distribution mode of pore filling,particle cementation and micro fracture filling,and a large number of hydrate resources occurs in the foraminiferal shell.(2)The depositional environment is characterized by low temperature,low energy,and relatively stable weak reduction in the Shenhu Area.the type of hydrate gas source is controlled by the fault structure.Generally,the gas of hydrate is thermogenic gas or mixed gas in those areas where deep faults are developed,while microbial gas makes an important contribution to hydrate accumulation in the other areas.The formation and distribution of hydrates in the area are controlled by the combination of reservoir and caprock,which are composed of turbidite deposits and fine sediment instability bodies.(3)The enrichment degree of hydrate is mainly controlled by the specific surface area of sediment particles,pore size of reservoir,abundance of foraminifera and other micro factors.Saturation of hydrate reservoir is negatively correlated with specific surface area,while being positively correlated with median pore size and foraminifera.In addition,Microfractures are more developed in the hydrated reservoirs where the sediment particles with relatively small particle size are concentrated,which may be an extraordinarily favorable factor for hydrate accumulation.(4)After hydrate decomposition,the porosity of the reservoir is about 20%,the effective permeability is about 0.5～0.75 mD,the shear strength is about 300 kPa,the friction angle is about 20°and the cohesion is at the level of tens to hundreds of kPa.When the confining pressure increases,the effective permeability decreases sharply,and the mechanical properties weaken obviously.Additionally,under the same shear rate,the greater the confining pressure,the greater the pore water pressure is caused by shear deformation,and there is strain hardening phenomenon,but no obvious peak shear strength.(5)The analyses reveal that the shear modulus increases exponentially and the damping ratio decreases exponentially as the confining stress is increased.The shear modulus decreases exponentially and the damping ratio increases exponentially with a rise in shear strain.Under the same confining pressure,the shear modulus and damping ratio increases as hydrate saturation rises,while the sensitivity index(b)of each incremental value for shear modulus decreases monotonically with an increase in hydrate saturation as the confining pressure changes.The shear modulus is negatively correlated with porosity when the THF hydrate is predominantly pore-filling,and the damping ratio and sensitivity index b are positively correlated with porosity.However,when the hydrate is mainly load-bearing,the shear modulus and damping are both related positively with porosity.This is because the total amount of hydrate increases with high porosity,while the sensitivity index b is negatively related to porosity.It was also found that as the hydrate morphology changes from pore-filling to load-bearing,the shear modulus rise sharply,and an obvious decrease is observed in the sensitivity index b.The hydrate that fills pores can be treated as solid material and therefore leads to an increase in the shear modulus and damping.A film of water that forms between the hydrate and the particles of sediment is most likely the cause of the abnormal increase observed in the damping ratio as the hydrate content rises.Moreover,the fitting formula of normalized permeability Kr:Kr=(1-Sh)3.6/(1+Sh)2.58,which is suitable for predicting the permeability of low permeability and weakly consolidated hydrate reservoirs in the South China Sea.