Nanostructure vs. Macroscopic Properties of Mancos Shale

Abstract

Abstract Shale gas reservoirs have become financially attractive upon more efficient utilization of technologies such as horizontal drilling and hydraulic fracturing. In order to better identify such reservoirs, estimate their production potential, as well as device optimum drilling and fracturing strategies, a detailed understanding of the connection between gas shale macroscopic mechanical and petrophyscal properties and microstructure is highly valuable. Since a high fraction of both pores and grain structures in gas shales are of nanometer sizes, this requires advanced experimental techniques beyond what has commonly been applied on more conventional coarsely grained reservoir rocks. We present a study of the Mancos outcrop shale where we combine results from state-of-the-art nano tomography with results from standard rock mechanical, petrophysical, and geochemical testing. In addition to providing the first detailed investigation of the Mancos shale on the nanometer pore scale, this multi-scale analysis contributes to bridging the gap between small- and large-scale gas shale descriptions – and to establish the role of nano tomography in relation to more standard rock testing procedures. Our study revealed that the Mancos shale is marginally mature (%Ro ~ 0.66–0.70), gas prone and has a total organic content of 1.0–1.3 wt%. It is observed that even if the porosity of the organic matter itself is very low, organics are found in or around all analyzed fractures and voids in the Mancos sample. Large fractures parallel to bedding are observed to be organic-rich, potentially enabling the soft organic material to serve as a preferential weak plane when the rock is subjected to mechanical stress. This may be reflected in the observed significant strength anisotropy of about 60% deduced from rock mechanical tests on the same material. It is also suggested that nano/microfractures influence the measured acoustic properties, which appear consistent with those of other organic rich shales. Thus, knowledge of the detailed nanostructure, especially the distribution of organic matter, its porosity and its interface towards other solid phases, may be beneficial when attempting to predict shale macroscopic mechanical and petrophysical properties. Knowing these simplifies the process of identification and characterization of shale gas prospects, as well as the subsequent drilling and production.