Gas adsorption capacity calculation limitation due to methane adsorption in low thermal maturity shale: A case study from the Yanchang Formation, Ordos Basin

A series of integrated and repeatable methods were used to demonstrate the methane adsorption limitations associated with calculating low thermal maturity shale gas adsorption capacities. These methods include X-ray diffraction of bulk and clay minerals, Soxhlet extraction, Ar-ion polishing, focused ion beam scanning electron microscopy (FIB-SEM), low pressure N-2 adsorption and high pressure CH4 adsorption. The laboratory data suggest that the Yanchang Formation shale primarily consists of clay minerals ( approximate to 47.4%) and detrital minerals (approximate to 51.14 An illite smectite mixed layer is the principal clay mineral constituent, while quartz is the major detrital mineral component. The thermal maturity is relatively low with Ro ranging from 0.84%Ro to 1.1%Ro, suggesting the absence of organic matter pores and the presence of soxhlet-extractable bitumen and oil, which clog pores with diameters around 4 nm and 50 nm. The specific surface areas and quantities of adsorbed N-2 significantly higher for samples extracted using a mixed solution (CH2Cl2 and CH3OH) compared to non-extracted samples. However, the methane adsorption volumes, including volume that were measured and calculated using Langmuir fitting parameters, minimally varied during high pressure CH4 adsorption experiments. CH4 dissolution in bitumen and oil causes the difference exhibited by these two gas adsorption analyses. Non-extracted samples produce inaccurate results using methane adsorption procedures and do not effectively represent practical shale reservoirs. Extracted samples are limited by CH4 adsorption when calculating gas adsorption capacities for low thermal maturity shales. (C) 2016 Elsevier B.V. All rights reserved.