Solubility And Exsolution Study Of CO₂ And CH₄

In the process of global carbon cycle,CO₂ and hydrocarbon gas are the core carriers,which are the key nodes of organic carbon and inorganic carbon,which are widely distributed in strata,atmosphere and organisms.Under formation conditions,in addition to the formed CO₂ and hydrocarbon gas reservoirs,more CO₂ and hydrocarbon gases exist in the formation water in a water-soluble phase and migrate with the formation water in the formation.In addition,in the study of natural gas reservoir development and geological carbon sequestration,the solubility of CO₂ and hydrocarbon gas in formation water is indispensable for the assessment of natural gas reservoir geological reserves and CO₂ geological storage abundance.Therefore,the dissolution and dissolution of CO₂ and hydrocarbon gas in formation water with temperature and pressure is an important basis for the study of natural gas reservoir migration,development and geological carbon sequestration.Through independent design,this research has developed“high temperature and high pressure multi-phase comprehensive tester”and“high temperature and high pressure fluid solubility comprehensive tester”.The equipment temperature and pressure conditions are 0²00°C,0²00MPa,and the minimum period of data acquisition is 0.5s./time,to meet the formation temperature and temperature conditions test needs.As the main component of hydrocarbon gas is CH₄,in order to explore its basic law,CH₄ is used to replace hydrocarbon gas to carry out relevant experiments.The subsequent work will further explore the influence of different hydrocarbon gas components on its dissolution and dissolution law.Through the"high temperature and high pressure fluid solubility comprehensive tester",the single phase and mixed phase of CO₂ and CH₄ in the temperature range of 40²00°C and 5²00 MPa were dissolved in pure water and formation water.On this basis,in order to better understand the exsolution mechanism of CO₂ and CH₄,the properties and phase behaviors of CO₂ and CH₄ under different temperature and pressure conditions were tested by"high temperature and high pressure multiphase comprehensive tester".Through 994 test tests,6116 experimental data points of phase and fluid properties were obtained,and 1692 experimental data were dissolved and exsolution.The test results showed that the dissolution of CO₂ and CH₄ depended on the change of fluid properties with temperature and pressure.And the fluid properties are controlled by the phase distribution.Finally,taking the natural gas reservoirs in Yinggehai Basin and Tarim Basin as examples,the properties of CO₂ and CH₄ fluids,phase,dissolution and exsolution with temperature and pressure of the formation are discussed in the study of natural gas accumulation,development and the application of CO₂geological storage.The main conclusions and understandings are as follows:?1?The density of CO₂,CH₄ and CO₂+CH₄ increases with the increase of pressure,and decreases with the increase of temperature.The pressure has a greater influence on the density at low pressure,and the temperature has a greater influence on the density under high pressure.The volume deviation factors of CO₂,CH₄ and CO₂+CH₄ decrease first and then increase with the increase of pressure.When the medium and low pressure regions increase,the volume deviation factor increases with the increase of temperature,and the volume deviation factor of high pressure region decreases with the increase of temperature.The volume coefficient and compressibility of CO₂,CH₄ and CO₂+CH₄decrease monotonically with increasing pressure,and increase with temperature.Whether it is single component or CO₂+CH₄ mixed phase,temperature almost no longer affects volume coefficient and compression under high pressure environment.The properties of CO₂+CH₄ mixed-phase fluids are consistent with the single-phase with temperature and pressure,and the values of the mixed-phase fluid properties are always within the interval determined by the single-phase fluid properties.As the molar fraction of CH₄ component in the mixed phase increases,the properties of the CO₂+CH₄ mixed phase fluid move closer to the CH₄ single-phase fluid property,and conversely,move closer to CO₂.?2?Corrected the existing phase diagram distribution of CO₂ and CH₄,and subdivided the original“supercritical phase zone”into critical zone and supercritical state.The gas-critical zone phase change line was corrected by constant temperature line to be subjected to temperature and pressure.Control the curve and increase the critical region-supercritical phase transition line.In this experimental temperature range,the CO₂ phase includes:gaseous,liquid,supercritical and gas-liquid mixed phase and gas-supercritical mixed phase;CH₄ phase distribution is relatively simple,which is gaseous,supercritical and gas-supercritical mixed phase;CO₂+CH₄ phase is the most complex,and its phase states are:gaseous,liquid,supercritical,gas-liquid mixed phase,gas-supercritical mixed phase,gas-liquid-supercritical mixed phase,liquid-supercritical mixed phase.?3?Above the critical temperature,the P-V curve of CO₂ and CH₄ is converted from a piecewise function to a continuous function due to the fact that the supercritical fluid properties are closer to the gaseous state.The upper and lower critical line mathematical models show that the upper and lower critical lines of CO₂ and CH₄ are divergent in the temperature range of 0³00°C,and the critical range is continuously expanded.When it is closed at higher temperature,it is connected with the plasma state.Further experimental demonstration.Under the condition of high temperature and high pressure,the partial pressure theorem is no longer applicable.This study proposes a one-component analysis based on the full pressure theorem for the critical region and supercritical state of CO₂+CH₄.The calculated results have a good fit with the mixed phase measured data?deviation less than 5%,the main deviation is less than 3%?,suitable for mixed phase phase analysis.?4?When CO₂ and CH₄ are dissolved in pure water,the solubility increases with the increase of pressure.As the temperature increases,the solubility decreases first and then increases.Due to chemical equilibrium and molecular polarity,the solubility of CO₂ in pure water is much larger than that of CH₄.In most cases,the solubility of CO₂ is 5-10times that of CH₄ under the same conditions.At low temperature and medium and low pressure,the solubility of CO₂ in pure water is 10²0 times of CH₄.The dissolution of CO₂ and CH₄ in formation water is the same as that in pure water,but its solubility in formation water is 2⁷%lower than that in pure water.Compared with CH₄,H⁺,OH^-,Ca²⁺and Mg²⁺ions in formation water have significant effects on CO₂ solubility.?5?The experimental results of solubility of CH₄+CO₂ in pure water reveal that:?1?When CO₂ enters the critical zone and supercritical zone,due to the extraction of CH₄ by CO₂,the CO₂ mole fraction in the mixed phase is higher than 8%,and CH₄ is no longer dissolved in water.?2?When CH₄ is dissolved and equilibrated,CO₂ is dissolved,and CO₂is also soluble in water,after CO₂ is dissolved,then CH₄ is dissolved,and CH₄ is no longer soluble in water.?3?When CH₄+CO₂ is dissolved in pure water,the solubility of each component increases with the increase of temperature and pressure,and the solubility of CO₂ in the mixed phase is always lower than that of the pure component.However,the solubility of CH₄ component is lower than the pure component with the increase of pressure.The solubility,and then rapidly higher than the solubility in its single phase,the pressure continues to increase,and the solubility of the CH₄ component in the mixed phase is again lower than the solubility of its pure component.?4?The dissolution of CH₄+CO₂ in formation water is consistent with that of pure water,but the solubility in formation water is about 4%lower than that in pure water?90°C,60MPa?.At the same time,the solubility of CO₂ component in mixed phase is more affected by the formation water then that of CH₄.?5?Under high temperature and high pressure conditions,whether it is CO₂ or CH₄,the effect of saturated vapor pressure on solubility is not negligible.?6?Under the condition of temperature gradient of formation,the depth of CO₂dissolution is about 3000m,and the depth of CH₄ dissolution is 2500m.When the depth is less than 2000m,the solubility of CH₄ changes little.After the depth is higher than2500m,the solubility increases rapidly.When the temperature gradient is the same,the greater the pressure gradient,the higher the solubility of CO₂ and CH₄.When the pressure gradient is the same,in the middle and shallow stratum?<2000m?,the solubility of CO₂decreases with the increase of temperature gradient,and the solubility of CH₄ is almost unaffected by the temperature gradient.In the middle and deep strata,the solubility of CO₂ and CH₄ increases with the increase of temperature gradient.?7?Regardless of whether it is exsolution in pure water or formation water,the amount of CO₂ and CH₄ dissolved is determined by the initial temperature and pressure conditions,the difference between the dissolved pressure and the temperature of the dissolved temperature.The initial conditions are the same,the larger the pressure difference is,the larger the amount of dissolved solution is;the larger the temperature difference is,the smaller the CO₂ and the CH₄ are dissolved and then decreased.When the initial temperature and pressure conditions,temperature difference,and pressure difference are the same,the amount of CO₂ dissolved is 2⁵ times that of CH₄.When the pressure is higher than 90MPa,the amount of CO₂ dissolved slowly increases with the increase of pressure difference.When the pressure is lower than 90MPa,the amount of CO₂ dissolved rapidly increases with the pressure difference,and the higher the temperature,the greater the changes of CO₂ dissolution is.For CH₄,the melt-forming transition pressure is around 140 MPa.Compared with CO₂,the temperature difference has little effect on the amount of CH₄ dissolved.The dissolution of CO₂ and CH₄ in formation water is consistent with that in pure water.However,when CO₂ formation water is dissolved,its dissolved amount is also affected by HCO₃^-in the formation water.?8?When the mixed phase is dissolved,CO₂ is dissolved first than CH₄.After the pressure is lower than 60 MPa,CH₄ begins to dissolve.As the pressure decreases,the amount of dissolved solution increases rapidly.When the initial temperature of the miscible phase is lower than 100°C and the pressure is higher than 20 MPa,the temperature and pressure have little effect on the amount of CO₂ component dissolved.After less than 20 MPa,the effect begins to increase.When the initial temperature is higher than 100°C,regardless of high pressure and low pressure.The temperature difference and pressure difference have obvious influence on the amount of dissolved matter.When the pressure is higher than 20MPa,the dissolved amount of CO₂ component increases slowly with the increase of pressure difference and temperature difference.When the pressure is lower than 20MPa,the dissolved amount of CO₂ component increases rapidly.?9?CO₂ and CH₄ phase,fluid properties,dissolution and exsolution laws have wide application prospects in natural gas reservoir exploration,development and CO₂geological storage research.?1?The initial stage of hydrocarbon migration in the DF1-1structure of the Yinggehai Basin is the water-soluble phase+free phase;the water-soluble phase and the free-phase seepage are the most important modes of migration under the pressure difference,and the accumulation mode is free gas+dissolved gas.?2?In the natural gas reservoirs of the Yinggehai Basin,there are three hydrocarbon accumulation modes:CH₄ accumulation alone?with a small amount of organic genesis CO₂?,CO₂charging after CH₄ accumulation,and CO₂+CH₄ mixed phase.The CH₄ is first charged and then CO₂ is charged into the reservoir.The CH₄ is formed separately,and the mixed phase is less.?3?The YKL condensate gas reservoir has a movable natural gas reserve of27.0538×10⁸ m³ in the formation gas,accounting for 32%of the existing reserves.The YKL condensate gas reservoir contains abundant hydrocarbon gas in the bottom water.?4?It is feasible to use CO₂ flooding to improve natural gas recovery in low temperature and high pressure formation environment.?5?In the CO₂ geological storage study,parameters such as CO₂ fluid properties?volume coefficient,etc.?and solubility in formation water are key indicators for assessing the abundance of CO₂ in the formation.