Experimental Study On Effect Of CO₂ On Mechanical Properties Of Tight Sandstone

In China,the low permeability sandstone reservoir has a large amount of resources,but it has such problems as low permeability and insufficient natural energy supply.Therefore,CO₂ has been studied as an oil displacement agent to improve oil recovery.However,after CO₂ is injected into the wellbore as an oil displacement agent,it reacts with sandstone to change the pore structure of the microscopic components of the rock,and affects the mechanical properties of sandstone,thus affecting the oil displacement effect of CO₂ and the long-term stability of the formation.Therefore,it is of great significance to develop the effect of carbon dioxide on the mechanical properties of tight sandstone.This paper takes north China Xiliu with dense sandstone mouth area of low permeability reservoir as the research object,through the X-ray diffraction experiment,study the CO₂ effects under different conditions of sandstone microscopic mineral composition variation characteristics,using nuclear magnetic resonance analyzer,obtain the CO₂ before and after processing of sandstone porosity,pore structure and pore size distribution,combining with triaxial stress test,soaking time and temperature on the mechanical parameters of sandstone over CO₂ evolution law of sandstone micro mineral composition and pore structure characteristics is analyzed on the relationship with the macro mechanical properties.The main research results obtained in this paper are as follows:?1?According to X-ray diffraction experiment,the law of the effect of carbon dioxide on the microscopic composition of sandstone was revealed.After soaking in CO₂,the mineral components of sandstone changed to different degrees with the soaking time.During the period of 0-72 h of CO₂ soaking,the content of quartz in sandstone increased significantly,and the content of feldspar minerals and calcite minerals decreased gradually.During the soaking period of 72¹68h,the content of quartz began to decrease gradually,and the content of feldspar minerals and calcite minerals increased.This shows that in the early stage of the CO₂ soak,sandstone with CO₂ in a certain chemical reaction,makes the feldspar,calcite mineral dissolution change the main mineral composition of sandstone structure,results in the decrease of the total mass of the sandstone and in the late CO₂ soak,solution began,in the material in the form of secondary minerals solidifying in sandstone,again changed the main mineral composition of sandstone.?2?The experiment of CO₂ soaking sandstone was carried out under different temperature conditions.With the increase of CO₂ soaking temperature,the quartz content increased significantly,with a range of 35.16%.The mineral content of potassium feldspar,plagioclase and calcite continued to decrease,with a range of 14.5%,8.33%and 16.29%,respectively.This indicates that temperature has a certain influence on the chemical reaction between carbon dioxide and sandstone.With the increase of temperature,the dissolution rate of mineral components is accelerated and the total mass of tight sandstone decreases.?3?According to the NMR experiment,the effect of carbon dioxide on the pore structure of sandstone was revealed.With the increase of CO₂ soaking time,the pore volume of sandstone decreased significantly.The volume of the mesopore increased slightly,and the volume of the macropore decreased slightly after significantly increasing.Meanwhile,the porosity also showed a trend of increasing first and then decreasing,and the porosity increased to the maximum after soaking for 50h.Therefore,the increase and decrease of the porosity was mainly caused by the change in the ratio of the macropore to the micropore.According to the results of CO₂ immersion at different temperatures,temperature has a greater influence on the volume change of micro and macropore,and a smaller influence on the volume change of mesopore.With the increase of CO₂ immersion temperature,the faster the micropore decreases,the faster the macropore increases,and the greater the change in the porosity of sandstone.?4?Based on the triaxial stress experiment,the effect of carbon dioxide on the mechanical properties of sandstone is revealed.With the increase of CO₂ soaking time,the young's modulus and poisson's ratio of sandstone gradually decreased,and the compressive strength first decreased and then increased.Among them,young's modulus and poisson's ratio had the best correlation with CO₂ soaking temperature,which were 0.9997 and0.9231,respectively.With the increase of CO₂ soaking temperature,the young's modulus and compressive strength of sandstone decrease,and the poisson's ratio of rock samples changes little.Among them,the correlation between compressive strength and CO₂ soaking temperature is the best,which is 0.9894.