APPLICATION OF LITHOFACIES MODELS TO CHARACTERIZE UNCONVENTIONAL SHALE GAS RESERVOIRS AND IDENTIFY OPTIMAL COMPLETION INTERVALS

Unconventional shale gas reservoirs are economically viable hydrocarbon prospects and their development has rapidly increased in North America. These reservoirs must be routinely drilled horizontally and hydraulically fracture stimulated to maximize production rates. Identification of the different chemostratigraphic units or lithofacies that make up these reservoirs is crucial for devising completion strategies because some lithofacies are more favorable to gas recovery in terms of their organic content and geomechanical characteristics. Lithofacies are indicative of eustatic changes during deposition and are typical geo‑markers related to the preservation and amount of accumulated TOC for a given basin. Gas content is related to TOC and varies according to lithofacies. Based on the mineralogical and TOC content, some lithofacies are favorable for gas production (e.g., siliceous lithofacies) and the geomechanical properties of these lithofacies often possess low fracture gradients that are conducive to forming extensive fracture fairways for recovery of gas. Other lithofacies can be fracture barriers and zones of fracture propagation attenuation (e.g., carbonate lithofacies). A shale gas facies expert system was developed with the goal of chemostratigraphically characterizing different shale plays and utilizing an integrated petrophysical reservoir evaluation approach to identify optimal completion intervals. This system can aid operators design selective completion strategies, which can potentially reduce fracturing expenses and optimize well productivity. The expert system incorporates a combination of density, neutron, acoustic, nuclear magnetic resonance and geochemical logging measurements. This system first characterizes the lithofacies based on their geochemical makeup and then the most favorable and unfavorable zones are flagged using a simple "stop‑light" approach based on the petrophysical and geomechanical properties. This paper presents facies models for the Barnett Shale, the Haynesville Shale, the Marcellus Shale, the Woodford Shale, and the Eagle Ford Shale applied to a case study well in each of these plays.