Study On The Nagetive Phenomenon Of Shale Sample Isothermal Adsorption Test And Its Correction

In the process of isothermal adsorption determination of shale reservoirs,the phenomenon of"negative adsorption"often appears in isothermal curves,that is,after reaching a certain experimental pressure,the adsorption amount does not increase but shows a declining trend.In view of the anomaly of isothermal adsorption curve in isothermal adsorption experiments,the authors selected six typical samples and tested them for physical properties including adsorption.Based on this analysis,the negative adsorption characteristics of samples were analyzed,various factors causing negative adsorption and its influence degree and mechanism were studied,and the theoretical and technical ways to weaken or eliminate negative adsorption were discussed.The study found that the"negative adsorption"of shale isothermal adsorption curve is related to the pore characteristics of the sample.The maximum adsorption range of isothermal adsorption test original values is 0.78 cm³·g^-12.88 cm³·g^-1,with an average value of 1.72 cm³·g^-1;Negative adsorption range 0.086⁰.661,mean0.314.For samples with high shale organic carbon content,microporous development has higher adsorption capacity and lower"negative adsorption degree".The improper measurement of free space volume is an important reason for negative adsorption.The blank test shows the comprehensive error of the instrument,which is accidental,and has a great influence on the sample with weak adsorption capacity of methane.The adsorption amount measured by the experiment is excess adsorption,ignoring the methane in the shale adsorption phase,and the excess adsorption amount must be converted into absolute adsorption amount.The correction of the isothermal adsorption curve must first undergo phase conversion,the parameters are optimized,and the selection of the smaller adsorbent phase density?_a=0.345 g/cm³ is better.Secondly,adjust the change of the appropriate free space volume and correct the equivalent temperature curve.Finally,we find the weak link of error from the fundamental reasons such as optimization of test process,improvement of technology and improvement of equipment,reduce the possibility of negative adsorption,and obtain more realistic data as far as possible.