Study On The Influence Of Clay Minerals On Hydrate Formation And Reformation

In 2021,China's dependence on foreign crude oil exceeded 70%,and China's energy security is facing severe challenges.Natural gas hydrate has the characteristics of large reserves,shallow burial and wide distribution.It is a substitute resource for clean oil and gas in the future.The lithology of natural gas hydrate reservoir in Shenhu sea area of the South China Sea is clayey silty sand type,with clay content of about 26%? 30%.Clay minerals have diffuse electric double-layer structure and strong binding force to water molecules.Different clay minerals can expand in varying degrees when absorbing water.Therefore,clay minerals have a significant impact on the formation conditions of natural gas hydrate and the conductivity of argillaceous silt sediments.In this paper,theoretical and experimental studies are carried out on the phase equilibrium properties of natural gas hydrate,the formation characteristics of bound water hydrate and the law of hydrate reformation.Different from silt media,the surface of clay minerals has an electric double layer structure,and water molecules are bound by the electric double layer.According to the different binding force,it can be divided into strong bound water and weak bound water.The conditions for the formation of hydrate by bound water are different from those of free hydrate.In this paper,the phase equilibrium conditions of clay mineral bound hydrate in the range of 1?-10? and 3 MPa-10 MPa are measured by the step-by-step temperature rise method.By increasing the synthesis pressure and prolonging the synthesis time,this method promotes the full transformation of weak bound water in clay minerals into hydrate,and then gradually increases the temperature to promote the gradual decomposition of bound hydrate from the inner layer to the outer layer,the phase equilibrium conditions of weakly bound hydrate at different positions from the clay surface were determined.The phase equilibrium temperature and pressure of weakly bound hydrate in the innermost layer on the surface of different clay minerals are measured experimentally.The phase equilibrium temperature and pressure of sodium and calcium montmorillonite are 1.0?,4.30 MPa and illite are 1.0?,4.06 MPa,indicating that the formation condition of clay mineral bound hydrate is much higher than that of free hydrate(1.0?,2.82 MPa,calculated by Chen Guo hydrate thermodynamic model).The results show that the closer to the surface of clay minerals,the higher the formation conditions of weakly bonded hydrate,which is mainly due to the lower activity of bound water and the greater difficulty of constructing hydrate cage structure by bound water molecules;The farther away from the surface of clay minerals,the closer the activity of bound water is to free water,and the formation condition of weak bound hydrate tends to free hydrate.In addition,the formation pressure of weakly bound hydrate in the same layer increases significantly with the increase of temperature,which is similar to the relationship between temperature and pressure of free hydrate.The phase equilibrium property of hydrate is the theoretical basis for the exploration and development of natural gas hydrate resources.The research results of this part provide an important theoretical basis for the evaluation of natural gas hydrate resources,the design of production technology and the determination of production process.The study of clay mineral bound water phase equilibrium provides a guiding direction for better understanding the formation dynamic characteristics of hydrate in clay minerals.In this paper,the process of forming carbon dioxide hydrate from bound water in wet montmorillonite and illite was studied experimentally at 2?.The type and content of water were determined by low field NMR.The effects of clay mineral types,water content and formation pressure on gas storage capacity are analyzed.Clay has higher water absorption capacity than sand.With the increase of water content,the storage of gas changes from the adsorption on the surface of montmorillonite to the formation of hydrate.The inflection point is about 15% water content.When the water content increases from 0% to 15%,the decrease of gas storage may be due to the competition between water molecules and gas molecules and occupying the adsorption site.When the water content is higher than 15%,due to the reduction of the binding force of weak binding water,carbon dioxide hydrate begins to form and the gas storage capacity increases.Different from montmorillonite,in illite with low content of strong bound water,with the increase of water content from 5% to 20%,the gas continues to transform into hydrate in weak bound water.During the formation of hydrate,the peak position of T2 distribution spectrum of wet clay moves to the left,indicating that the weakly bound water is orderly transformed into hydrate from the outer layer to the inner layer of the diffusion electric double layer.Residual water is considered to be strong binding water with very low fluidity,which can not be completely transformed into hydrate.Due to the change of binding water activity in the diffusion electric double layer,the final amount of hydrate produced by weak binding water increases with the increase of pressure.These results will not only broaden the understanding of the formation of natural gas hydrate in clay bound water system,but also promote the new assessment of the safety and potential of carbon dioxide hydrate storage in clay sediments.Hydrate reformation is a widespread phenomenon in the process of gas hydrate depressurization and exploitation,and it is also an important obstacle affecting gas production rate and production safety.Clay is a common mineral in marine sediments,and its water swelling and binding properties are likely to change the local permeability of porous media.In this paper,a one-dimensional long tube hydrate simulation platform is independently developed to study the temperature field of gas throttling expansion effect under anhydrous condition and the law of hydrate reformation and evolution under aqueous condition in the decomposed zone of hydrate in pure sand and clay containing quartz sand,and summarize the role of clay.Experiments show that with the increase of the inlet pressure and the decrease of the outlet pressure,or with the increase of the flow rate under the same pressure difference condition,the throttling expansion effect is enhanced in the two porous media,and the temperature drop and the drop rate increase;the hydrate reformation starts.However,the starting position of hydrate reformation is almost unchanged,that is,50 cm away from the inlet end,and it mainly expands in the direction of airflow.Under the condition of water content,due to the decrease of permeability,the gas throttling expansion effect is weakened,and the degree of temperature change is weakened,the formation time of hydrate is delayed and the expansion range is reduced.After a large number of hydrates are formed,the local permeability decreases rapidly,and the pressure rise leads to the further expansion of the hydrate reformation area.Due to the formation of aggregate structure by clay,the pore diameter becomes larger and the permeability increases,the throttling expansion effect at the inlet end is greatly weakened,and the temperature drop is reduced.There is no phenomenon of a large number of hydrates in the middle of pure sand medium.In addition,the migration of clay changes the local permeability,increases the decrease range of local temperature,and leads to the change of the starting time and position of hydrate formation.After the formation of hydrate,the gas in the pipe is calculated in sections,and the hydrate saturation distribution at the end of the experiment is obtained.In the pure sand medium,a large number of hydrates are formed near 50 cm from the inlet,while the clay containing porous medium is at the gas outlet.In the process of mining,the formation of secondary hydrate can be restrained by means of pressure regulation and peripheral heating.In this paper,the phase equilibrium properties and formation mechanism of bound water natural gas hydrate in clay minerals are experimentally studied,and the reformation characteristics and evolution law of natural gas hydrate in pure sand and clay containing quartz sand sediments are revealed.The research results can provide a theoretical basis for the exploration and development of argillaceous silt natural gas hydrate.