Diffusion Process, Diffusion Capacity and Controlling Factors of Shale Gas in the Longmaxi Formation, Southeast of Sichuan Basin, China

Abstract

There are multiple sets of marine organic rich shale in Sichuan basin and its peripheral regions. However, the enrichment degree of shale gas between in-basin and out-basin has an obvious difference, so it is of great importance to study the shale gas’s preservation conditions and diffusion capacity (DC). To make a deep research in the diffusion process, DC and controlling factors of shale gas, we selected the 25 black mud shale core samples of the YQ1 in the Longmaxi formation in the southeast of Sichuan basin, conducted the closed desorption experiments to obtain shale gas content and made the gas components analysis, finally quantitatively calculated the DC of shale gas. Additionally, total organic carbon (TOC), XRD, SEM and mercury penetration tests were also carried out on the 25 core samples.

The DC of shale gas is influenced by many factors. From the shale gas desorption curves, the diffusion rate is very low at 50 centigrade, but the diffusion rate turns very fast first and then decreases lower slightly when at 98 centigrade, indicating that temperature is the main controlling factor of shale gas DC, the higher the temperature is, the faster the DC is. Through calculating the DC of each sample, there are obvious differences in DC value between the upper and lower section of Longmaxi Formation. The DC is lower than 10 in the lower section, but higher than 10 in the upper section. The methane content in the shale gas is higher in the lower section, with DC increasing, the methane content is reducing, but the nitrogen content is increasing, which seems that the stronger the shale’s self-sealing capacity is, the lower the shale gas’s DC is. The DC of shale gas is negatively related to TOC, it indicates that the organic matter (OM) can control shale gas’s DC, which may be associated with the high specific surface area and the strong adsorption ability of OM, additionally OM develops a large amount of organic pores, enhancing the adsorption capacity of methane. Mineral compositions also have a certain effect on the DC of shale gas. The quartz and pyrite content exhibit negative correlations with DC, due to that quartz and pyrite have positive coupling relationships with OM. The content of feldspar is positively related to the DC, mainly because that the feldspar dissolution pores are relatively developed in the shale. In a word, the shale gas’s DC is related to the temperature and pressure, the shale’s self-sealing capacity and shale’s rock properties.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018