Petrologic Evaluation of Significance of Natural Fractures in Low-Porosity Shale Gas Reservoirs--Results of Investigation in Upper Devonian of Virginia and West Virginia: ABSTRACT

Abstract

Economic production of gas from the Devonian shales of the Appalachian basin is dependent on the presence of natural or induced fracturing. Investigation of natural fractures in five cored wells of varying productivity, located in areas of regional fracturing and not crestally located on folds, indicates that fracture frequency alone is not the sole control of well productivity. A one-to-one relation between natural fractures seen in the core and gas shows indicated by temperature and sibilation logs is not present. This is attributed to degrees of permeability enhancement by different fracture types and the presence of favorable shale lithotypes for recharging the fracture system. No system of abundant microfractures was documented after petrographic study of nearly 400 hale samples by radiography, thin section, and SEM. Thus, only macroscopic fractures are of importance. Horizontal to subvertical slickensided fractures, even with frequencies of 2 to 3 per ft, are not associated with gas shows in organic-rich, laminated shales. Examination of their surfaces by SEM at 30,000 × shows complete obliteration of grain-to-grain boundaries and a uniform, glassy surface of low permeability. Stimulation by hydraulic fracturing of a well dominated by slickensided fractures resulted in production equal to that of a well (same formation thickness and porosity) that possessed only one fracture (slickensided). Presence of slickensided fractures does not greatly influence open flow or final flow after fracturing of a shale reservoir. High-angle vertical fractures, associated with higher gas productivity, retain some openness and permeability in the subsurface due to mineralization and slight movement between fracture surfaces characterized by coarse twist hackles. Mineral-filled fractures, commonly 1 to 2 mm in width, were seen in thin section to be tightly mineralized by dolomite with little intercrystalline porosity. SEM observation reveals that many seemingly unmineralized fracture surfaces have minor mineralization. Tightly and partially mineralized vertical fractures in nonproductive portions of the shale sequence were associated with organic-poor, nonlaminated shales which have less potential for recharging the meager fracture porosity present. The most productive well, final open flow of 1,007 MCFGD, possessed high-angle vertical fractures in its pay zone of organic-rich, laminated shale. These fractures have coarse twist hackles which show evidence of later vertical movement. Slight offsetting along these hackles opened widths up to 28 mm. This well is not located near photolineaments. Vertical fractures with no mineralization or indication of offsetting along hackles are interpreted as being closed at depth and do not contribute to shale productivity. End_of_Article - Last_Page 1590------------