Connectivity, Wettability, and Tortuosity of Leading Chinese Shales
With estimated shale gas resources greater than that of US and Canada combined, China has embarked on an ambitious shale resource development program. However, nearly 30 years of American experience in shale hydrocarbon development indicate a steep initial production decline and low overall recovery of hydrocarbons; this can be implicated in the complexity of nanopore structure (geometry and connectivity) and “dalmatian” wettability of shale formations. We have collected a variety of leading hydrocarbon-producing shale formations in China, including Wufeng-Longmaxi, Niutitang, and Shahejie. These formations have different ages and geologic characteristics (e.g., mineralogy, total organic content, thermal maturation, porosity, permeability,). We studied their “dalmatian” wettability (droplet contact angle measurement), pore structure (MICP, low-pressure gas phisisorption), as well as tracer imbibition and saturated diffusion tests in both hydrophilic (API brine) and hydrophobic (n-decane) fluids with a suite of wettability tracers of different sizes and reactivities developed in our laboratory; the 2-D/3-D presence of tracers in shales are mapped with laser ablation-ICP-MS technique. A shale can have a good pore connectivity for a wetting fluid, but poor connection for a non-wetting fluid. Furthermore, pore spaces of different wettability are very likely associated with the compositions of shale, which also have different pore sizes. In other words, oil-wetting pores are smaller (~10 nm) and yet well-connected, while water-wetting pores are larger (>50–100 nm) but sparsely-connected. The low connectivity also creates extensive dead-end pore complexes that are connected to the transport “backbones”, and this steep decline of edge-accessible connected pore spaces leads to very limited tracer imbibition and diffusion for brine fluid. On the other hand, though shale is oil-wetting to exhibit classical behavior of imbibition and diffusion, yet its connected pore spaces are predominantly limited to the sub-mm range to experience additional implications of molecular size effects, as shown from our organic-I and organic-Re tracers of different sizes in n-decane fluid. Our work illustrates the intertwined relationship of connectivity, wettability and fluid migration in influencing hydrocarbon movement from shale matrix to stimulated fracture networks to lead to a steep initial decline and low overall recovery.
AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018