Study On Geological Controlling Factors Of CO₂ Burial In Coal And Effect On Reservoir

In the paper,the main controlling factors and influencing mechanism of CO₂ adsorption in coal seam are clarified by adsorption experiments with high temperature and high pressure,different coal rank,different water content and different CH₄ content.The accuracy of nine adsorption kinetic equations including Langmuir adsorption model for CO₂ adsorption of different temperature,different coal rank,different water content and different CH₄ content is verified.The nine commonly used adsorption kinetics models are Langmuir model（L type）,Freundlich model（F type）,Toth model（T type）,Langmuir-Freundlich model（LF type）,extended Langmuir model（EL type）,DR model DR type),quadratic equation model（quadratic type）,cubic equation model（cubic type）and logarithmic equation type（logarithmic type）.The parameters of adsorption kinetics model under different temperature,different coal rank,different water content and different CH₄ content were fitted by SPSS software.The effects of temperature,coal rank,water content and CH₄ content on the adsorption model parameters is clarified;The chemical reaction of CO₂ and coal was used to analyze the variation law and influence mechanism of CO₂ chemical burial,and the change rule and influence mechanism of mineral content,water quality and permeability of coal reservoir caused by CO₂ chemical burial were clarified.The simulation of CO₂ buried seepage test reveals the change law and influence mechanism of coal strain and permeability caused by CO₂ adsorption and burial.CO₂ adsorption experiment was carried out under high temperature and high pressure.The highest experimental temperature was 55 ℃ and the experimental pressure was 16 MPa.The experimental sample was medium coal rank coal in Yuwu coal mine.With the increase of pressure,the higher the CO₂ adsorption capacity,the higher the temperature,the CO₂ adsorption capacity decreases.When the experimental temperature exceeds 31.1 ℃,the experimental pressure exceeds 7.3MPa,CO₂ enters the supercritical state,and the supercritical state is the same as the CO₂ adsorption capacity under the low temperature and low pressure condition.The effect of temperature on the adsorption amount is negative.From the CO₂ buried consideration,high pressure,low temperature coal should be chosen as a target buried reservoir.The maximum adsorption capacity of the parameters in the model decreases with the increase of the temperature,the fitting error of the cubic equation is the smallest,the fitting effect is the best,and the F model has the largest fitting error.The adsorption experiment of CO₂ was carried out in high coal rank,medium coal rank and low coal rank coal.The high coal rank anthracite was from the Sihe mine,and the Ro was 2.7.The medium coal rank lean coal was from Yuwu mine,the Ro was 1.56;low coal rank long flame coal was from Ningdong Yangchangwan 1#,the Ro was 0.55.Under the same temperature and pressure conditions,the higher the coal rank,the higher the adsorption capacity,the adsorption capacity of the anthracite coal in Sihe mine is 1.2 1.4 times the adsorption capacity of the lean coal in Yuwu mine.The adsorption capacity of the medium lean Coal in Yuwu is 1.6 to 2 times of the adsorption capacity of low coal rank long flame coal in Yangchangwan mine.With the increase of Ro,the maximum CO₂ adsorption capacity in coal increases linearly.The effect of coal rank on the CO₂ adsorption capacity was analyzed.The effect of the adsorption of vitrinite content and fixed carbon content was positive.The effect of ash and volatiles was negative.The adsorption model simulates the adsorption capacity of coal in different coal rank,LF model has the least error and the fitting effect is the best.The experimental results showed that CH₄ concentration was 0,20%,40% and 50%,respectively,under different CH₄ content.Under the same temperature and pressure conditions,the adsorption capacity of CO₂ in Sihe coal is 1.34 2.84 times of the adsorption capacity of CH₄,and the adsorption capacity of Yuwu coal is 1.61 2.35 times of the adsorption capacity of CH₄.With the increase of CH₄ concentration,the CO₂ adsorption capacity decreased.Compared with F model and L model,it is considered that the F model has better effect on CO₂ adsorption capacity under different CH₄ gas content conditions.The experiment of CO₂ adsorption was carried out under different water content conditions.The experimental water content was 0/5%/10% and 15%.With the increase of water content,CO₂ adsorbed capacity on coal decreases.The adsorption capacity of the dry anthracite coal is 1.48 times of the adsorption capacity of 5% water,the adsorption capacity of 5% water is 1.24 times of that of 10% water and the adsorption capacity of 10% water is 1.08 times of the 15% water.The adsorption capacity of dry Yuwu coal is 1.44 times of that of 5% water content,the adsorption capacity of 5% water content coal is 1.2 times of the adsorption capacity of 10% water,and the adsorption capacity of 10% water is 1.1 times of the adsorption capacity of 15% water.The adsorption capacity of dry coal in Yangchangwan is 1.37 times of that of 5% water,5% water coal is 1.25 times of adsorption capacity of 10% water coal,10% water coal adsorption capacity is 1.12 times of the adsorption capacity of 15% water.Ettinger model simulates the CO₂ adsorption capacity in coal with different water content,which is better for medium and high coal rank coal rock and is not suiTab.for low coal rank coal.The adsorption kinetic model simulates the maximum CO₂ adsorbed capacity in coal with different water content.With the increase of water content,the maximum adsorption capacity decreases,the third equation has the smallest fitting error,and the fitting effect is the best.The chemical reaction experiment of CO₂ and coal in the CO₂ solution with pH value of 4.3 clarified the change law of CO₂ chemical burial in coal and the main influencing factors.CO₂ capacity of chemical burial fluctuates in coal,the maximum is 101.5mg / L,the total content increases initial content.The main reason is that CO₂ solution is very unsTab.,CO₂ capacity of chemical burial in acidic solution increases,and decreases in alkaline environment.The main factors influencing CO₂ capacity of chemical burial are calcite,dolomite,potassium feldspar and chlorite in coal and Coal water Ph value;The effect of CO₂ adsorption on the strain and permeability of coal is revealed by the seepage experiment of CO₂ injection into coal seam.According to the experimental results,CO₂ burial increases porosity pressure,and the coal strain increases.In Yuwu coal radial strain is 2.5 times of the chamber axial strain,in Sihe coal radial strain is 2.3 times of the chamber axial strain,volume strain is mainly the result of radial strain.The strain decreases linearly with the increase of effective stress,and the axial strain decreases by 70%,and the radial strain and volume strain are reduced by 80% on average.Adsorption expansion has little effect on coal strain.Changes of coal strain is mainly due to the change of effective stress,the effective stress is increased to 13 MPa,the axial strain is reduced by 63%,the radial strain is reduced by 70%,and the volume strain is reduced by 65%.The permeability decreased first and increased after.Yuwu coal permeability decreases by an average of 0.05 mD,and Sihe coal permeability decreases by an average of 0.01 mD.The effective stress of Yuwu coal increases to 23 MPa,the permeability decreases by 44.7%.The permeability decreases 0.011 mD per unit effective stress.The permeability of Sihe coal decreased by 41% compared with the initial permeability with the increase of effective stress,and permeability decreased by 0.02 mD per unit effective stress.The paper reveals the effect of CO₂ chemical burial on mineral content,water quality,porosity and permeability of coal.Sihe coal in the Ph value of 6.3 solution soak for 8 months,the largest relative reduction in the amount of mineral content is calcite,the relative reduction of 88%,potassium feldspar content decreases by 71%,dolomite content decreases by 68 %.The content of bicarbonate ion increased by 150 mg / L and the content of sodium and potassium increased by 293% after soaking for 8 months.Yuwu coal in the Ph value of 4.3 solution soak for 8 months,The content of bicarbonate ion and the content of calcium and magnesium increased first and then decreased,and the contents of potassium and potassium increased gradually.The change of porosity and permeability is due to the change of acid environment with water and calcite,dolomite and potassium feldspar Dissolution,alkaline solution dissolution reaction,calcium and magnesium ion precipitation reaction;six Yuwu porcelain sample porosity have increased by 2% to 31% range compared to the initial porosity,the average increase in coal Porosity of 13%,the permeability increases by 0.15 mD,increased by 28.6%;for eight months the permeability in Sihe1 sample increases of 11.51 mD,compared to the original permeability increases by 25 times,Sihe2 sample permeability increases by 11.18 mD,Compared to the original permeability increases by 48 times;Based on the dual pore structure model of coal,the match box model and the pore volume distribution function,the mathematical model of the change of permeability caused by the physical and chemical reaction of CO₂ and coal seam is established,and the model is verified.CO₂ burial project by injection to coal seam in North Shizhuang is introduced as an example.North Shizhuang is evaluated as a good area for CO₂ buried in coal.The injection in SX006-1 well,SX-006 well and SX006-2 well has been done,and the total injection amount is 4491 tons of liquid CO₂ by the end of 2015.In the process of CO₂ injection,the temperature dropped rapidly,the minimum temperature was 25.7 degrees Celsius,after 25 days’ injection,the temperature basically no longer changed.The temperature returned to the original level,between 29 degrees and 30 degrees;injection rate has a greater effect on temperature.The minimum temperature was 26 degrees,27.75 degrees,28.5 degrees respectively with 10 tons’ CO₂ injection per day,8 tons’ CO₂ injection per day,6 tons’ CO₂ injection per day.After the CO₂ injection,the bottom well pressure increased.The faster the injection rate is,the faster the pressure rises.The contents of potassium ion and sodium ion were increased gradually,and the contents were increased by 900 mg / L,20 mg / L 10 mg / L,respectively.The permeability fluctuates and is mainly affected by the negative effect of effective stress.The larger the injection volume is,the faster the permeability increases,the maximum is 0.1mD.After the injection,the permeability began to decline.The potential amount of CO₂ burial in well SX-006,SX006-1 and SX006-2 is 9.87 × 107m³ 1.24 × 108 m³.