--> Relationship Between Canister Desorption Behavior and Composition of Upper Paleozoic Shales in the Ordos Basin, NW China

AAPG ACE 2018

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Relationship Between Canister Desorption Behavior and Composition of Upper Paleozoic Shales in the Ordos Basin, NW China

Abstract

A series of canister desorption tests were carried out on 31 over-mature Lower Permian-Upper Carboniferous fresh shale cores (>3000 m) at reservoir temperature, 75 or 80 °C, and a higher temperature, 95°C under atmosphere pressure. In order to investigate the effect of composition on canister desorption behavior, organic chemistry and X-ray diffraction were combined. High-pressure methane sorption experiments at reservoir temperature and pressure up to 20 MPa were conducted to compare sorption behavior and better understand the emission mechanisms of shale gas. Geochemical measurements show the total organic carbon (TOC) contents range from 0.488 wt % to 4.310 wt %. The depositional setting is lagoon and delta. The type of organic matter is mainly Type III. The dominant minerals of the shale samples are clay (25.4-97.0 wt %, average 58.8 wt %) and quartz (1-62.1 wt %, average 33.3 wt %). The content of clay minerals shows a significant negatively correlation with that of quartz with a coefficient of determination, R2, of 0.726. The results show that an increase in temperature from the reservoir temperature to a higher temperature results in an average 31% increase in desorbed gas volume. The desorbed gas volumes at both temperatures are linearly correlated with the total organic carbon (TOC) content, which support the positive relationship with TOC found by high-pressure methane sorption isotherms. The coefficients of determination, R2, at reservoir temperature and 95°C, are 0.658 and 0.644, respectively. The desorbed gas volumes at both temperatures show a slight correlation with clay minerals, indicating that adsorption sites on clay minerals may be significantly impacted during canister desorption tests. In addition, a small negative correlation between quartz and desorbed gas volume was observed. The shale samples with a lower content of quartz and higher desorbed gas volume generally have a higher content of clay minerals and TOC, which may indicate that both clay minerals and TOC are the dominant contributors of sorbed gas capacity even after spontaneous imbibition.