Key Parameters for Liquid-Rich Unconventional Plays: Case Studies from North America

Tom Anderson
Energy & Geoscience Institute, University of Utah, Salt Lake City, UT

There is a considerable interest in understanding the production of liquids from shales with the discovery and exploitation of liquid-producing regions in numerous unconventional plays including the Eagle Ford, Bakken, Niobrara, Utica/Point Pleasant, Mississippi Lime, and others. It is important to understand how liquids are produced from ultra-low-permeability rocks so that production rates and recovery could be optimized.

The geological and engineering considerations in optimizing liquid recoveries from "shales" are complex. A comprehensive study of fluid production from shales should include lithology and mineralogy, natural fractures and faults, petrophysics, micro-imaging, geochemistry including TOC, thermal maturity, and kerogen type, production declines, GOR and other phase changes, recovery factors, fluid properties, relative permeabilities, pressure dependence, and completion practices.

Past assumptions about "shales" have been that they are good seals over conventional reservoirs, as well as source rocks, where the TOC and thermal maturity are conducive. As a seal, the implication is that hydrocarbons are prevented from flowing through them. And yet, as a source rock, we assume that somehow the generated oil is able to escape and migrate into the conventional reservoir. Do source rock systems work only because they have ample geologic time over which to enable oil escape and migration?

Production results in these plays are showing several anomalous characteristics that could overturn previous concepts about what is and is not possible in nano-permeability systems. Previous geologic models for sediment accumulation, water depth, effects of currents, and biologic activity are proving to be over-simplified and in many cases wrong. Mud-rock heterogeneity is a much bigger factor than previously thought.

The Energy & Geoscience Institute at the University of Utah has been studying these liquids-rich systems in depth for the past 2-3 years. We have been using micro-imaging technologies including SEM, QEMScan, and FIB, on core and outcrop samples, ranging upscale to field, regional, and basin-scale characterization and modeling. Several examples of liquids-rich systems will be featured in a case-study examination of controlling parameters, focusing especially on the Bakken, Eagle Ford, and Niobrara.

Also incorporated into this study are existing pore- and pore-throat-size classification schemes, and porosity-type classification systems. Different porosity types may act very differently with respect to effective permeability. Simply visualizing these geometries is a useful step. Hopefully, it will lead to a fundamental improvement in our understanding of fluid flow through nano-pores and pore throats in the matrix, and will help significantly in developing unconventional "shale" reservoirs.

AAPG Search and Discovery Article #90186 © AAPG Geoscience Technology Workshop, Hydrocarbon Charge Considerations in Liquid-Rich Unconventional Petroleum Systems, November 5, 2013, Vancouver, BC, Canada