Effects Of CO₂ Geological Storage On Pore Structure And Adsorption Property Of Shale

Longmaxi formation of Lower lower Silurian in upper Yangtze platform is one of the most favorable reservoirs for shale gas exploration in C hina,basing on mechanism investigation to analyze the process of Enhance Gas Revovery?EGR?is key to the breakthrough of production.This paper selected shale of Longmaxi formation in Southeast C hongqing as researching object,investigated characteristics of pore structure and adsorption property before and after CO₂ adsorption experiment for further research.The study area is located at outer edge of the southeastern Sichuan Basin,with tectonic activities violent and formations of Silurian widespread.The main kinds of organic matter are type I,organic carbon content of Longmaxi formation is about 3.00% and thermal evolution degree of the organic matter is about 2.47%.The shale is mainly composed of quartz and plagioclase.C lay content is really low,vast majority are illite mixed layer and il ite.Organic pore,interparticle pores,intraparticle pores and microcracks,of which organic pores provide majority of reservoir space and mineral interparticle pores make outstanding contributions to seepage channel.Micropores in Longmaxi Formation mainly contain 3 types: 0.32nm?0.55 nm?0.83 nmand 13.7 to 40.6 nm are the predominant of mesopores.Controlling factors such as TOC?Ro?temperature and pressure must be thought before CO₂ adsorption experiment,through comparison found that the pore volume and surface area were decrease slightly under low pressure,and the former micropore beyond 0.4nm,the latter distributed in the range of 2 to 50 nm and this trend is obvious with higher TOC.Relatively,pore structure parameter is not prominent as Ro changes.According to experiment results,temperature can influence mesopore structure especially adsorption temperature beyond supercritical point,and mesopores parameters between 10 nanometer to 50 nm were increased,furthermore,structure parameters of micropores less than 0.8 nanometer and mesopores between 10 nanometer to 50 nanometer were enlarged differently as the pressure increase.Fractal results show that the average value of D₂ was reduced from 2.792 to 2.763,average value of D₁ was reduced from 2.826 to 2.789,which indicated that adsorption property of pores range from 2 to 4.8 nm and 4.8 to 50 nm was decreased shortly and became serious with the increase of TOC.Oppositely,D₁ increased from2.488 to 2.600,D₂ increased from 2.824 to 2.831 and D₃ increased from 2.899 to 2.911 respectively as experiment pressure changes,the adsorption property enhanced overall,however,the relationship between Ro,temperature and adsorption property were vague.Furthermore,referred to the Maximum Adsorption Effect Model the value of N₁+N₂ show a positive correlation with TOC and pressure,which indicated that these were vital conditions can influence adsorption property of shale before and after experiment.Comparing the TOC,Ro,temperature and pressure control conditions,it was found that the experimental pressure is very important for reconstructing the microscopic pore structure of shale.TOC as an important reservoir parameter can provide a large amount of reservoir space and control the CO₂ squestration.Generally,the effects of CO₂ squestration on the pore structure of shale are two types: the one is that swelling of shale matrix is widespread,which squeezed internal space.The other one is that the time temperature and pressure reached supercritical conditions,the strong solubility and molecular chemical activity of Sc CO₂ fluid can dissolve some of the organic molecules with low polarity in shale,which has positive significance for the reconstruction of reservoir space.