SHALE GAS EVOLUTION IN CANISTER: EVOLUTION CHARACTERISTICS, CONTROLLING FACTORS AND IMPLICATIONS FOR EVOLVED GAS OCCURRENCE STATE

Shale gas evolution in canister can significantly enrich our understanding in gas-in-place characteristics. However, studies on shale gas evolution characteristics and its controlling factors are rare, and its usefulness in understanding gas occurrence state has not been fully evaluated and understood. In this study, gas evolution curves of 52 shale samples and 3 coal samples were measured using wellsite canister testing technique. According to characteristic shape, three curve patterns including L-shaped, S-shaped and M-shaped are identified, and the difference among three curve patterns mainly lies in the fractional gas volume evolved in surface temperature stage. To evaluate the dependence of evolved gas content on shale properties, the variation in evolved gas content with organic matter, minerals, porosity, permeability, specified surface area and pore volume are analyzed, and found that the evolved gas content shows a strong dependence on the properties that control or could increase gas adsorption and diffusion capacity, such as TOC content, specified surface area and permeability, while shows no dependence on the properties that control free gas storage capacity, such as minerals, porosity and pore volume. Additionally, correlations of evolved gas content with adsorbed/free gas show that the evolved gas during canister testing is the gas that was in the adsorbed state in reservoir, and the free gas has been lost during coring, as well as a fraction of adsorbed gas. Above findings provide insightful information not only on gas-in-place evaluations but also on the dynamic behavior of adsorbed/free gas from producing well.