Pore structure, wettability and tracer migration in four leading shale formations in the Middle Yangtze Platform, China

Abstract

Deposited in different sedimentary settings, four leading shale formations (Late Ordovician Wufeng, Early Silurian Longmaxi, Late Permian Dalong, and Early Jurassic Dongyuemiao Shales) are currently the most promising zones for shale gas development in the Middle Yangtze Platform of South China. Based on complementary tests [low pressure gas physisorption, mercury injection capillary pressure (MICP), contact angle measurement, fluid imbibition into initially dry shale, and tracer diffusion into initially fluid-saturated shale followed by tracer mapping with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)], this work investigates their pore structure (both geometry and connectivity) and wettability characteristics, as well as the coupled effects of these characteristics on fluid flow and tracer migration.Pores <50 nm account for the majority of the pore volume in these organic shales. The shapes of hysteresis loop from gas physisorption show that the pores are mostly inkbottle-shaped in Wufeng, Longmaxi and Dongyuemiao Shales, while they are mostly narrow plate- or slit-like shaped pores in Dalong Shale. These four organic shales are strongly oil-wetting and moderately water-wetting. According to the imbibition behaviors toward aqueous (deionized water) and oleic (n-decane) phases, hydrophobic pores are better connected than hydrophilic pore networks, which is consistent with the measured contact angles. Diffusion of nano-sized nonsorbing (perrhenate [ReO4−]) and sorbing (cesium [Cs+]) tracers into brine-saturated shales indicates a high spatial variability and limited pore connectivity in these organic shales. The effective diffusion coefficient values are on the order of 10−13 m2/s with an associated geometric tortuosity ranging from 11.1 to 41.4. Due to the limited edge-connected pore spaces and low diffusion coefficients, the migration of hydrocarbons will be very slow from the shale matrix to hydraulically created fractures, with the initially high production of hydrocarbons from connected pore spaces near the sample edge (e.g., fracture face).