Variation of Wettability of Organic-Rich Shales With Thermal Maturity and the Implications for Oil and Gas Distributions

Abstract

Wettability of pore surfaces in organic-rich shales can significantly affect the relative distribution, storage, and productivity of gas and oil in these lithologies. Changes in organic matter structure during thermal maturation will affect its surface chemistry, therefore changing the wettability, as indicated by our recent experiments on methane and water vapor sorption. Consequently, the relative distribution of gas, oil, and water in shale systems is closely related to the thermal maturation of organic matter. An immature sample of the Woodford Shale was prepared by hydrous pyrolysis at five different time-temperature conditions to generate a suite of samples having different thermal maturities. Methane (CH₄) adsorption isotherms were measured on the moisture-equilibrated and dry samples at 35°C and pressures up to 14MPa. Nitrogen and water vapor adsorption experiments were also conducted on the dry samples to characterize the pore-size distribution, surface area, and surface wettability. Water distribution and morphology were observed for these samples under various vapor pressure conditions with environmental SEM (ESEM). Comparison of water-wet and dry samples shows that moisture added to the system does not substantially reduce CH₄ adsorption. This is consistent with observations from water vapor sorption isotherms that indicate the dominant hydrophobic nature of all the samples. However, the reduction of CH₄ adsorption capacity is larger for the samples in the late oil-generation (367°C/72 hr, 40% reduction) and early oil-cracking (400 °C/72 hr, 53% reduction) stages compared to the early bitumen (300 °C/72 hr, 35% reduction) and maximum bitumen (333 °C/72 hr, 34% reduction) generation stages. These findings suggest that although organic matter is generally hydrophobic, there is a wettability trend for the organic matter of the studied samples that correlates with higher thermal maturity stages. These stages of petroleum formation are notably associated with the formation of insoluble hydrocarbon residue or pyrobitumen. In addition, the distribution and morphology of water observed by ESEM illustrates the increase in wettability of the high-maturity samples. This provides further evidence that pyrobitumen may play an important role in controlling the wettability of organic matter. These results also suggest that whole-rock wettability may be a useful proxy for determining the proportion of kerogen, bitumen, and pyrobitumen in shales.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015