Techniques To Determine Natural and Induced Fracture Relationships in Devonian Shale

Abstract

Summary Extensive study of natural fractures in oriented core, well logs, induced fracture orientation, lineaments, and Seisviewer TM surveys resulted in a log-core correlation of subsurface fracture systems and established a relationship between surface and subsurface directional aspects of Devonian shale. Core and well log analysis techniques discussed have the potential to identify the major producing intervals in Devonian shale. Introduction The primary objectives of the U.S. DOE's Eastern Gas Shales Project (EGSP) are to characterize the nature of the reservoir, determine how gas can be extracted in a cost-effective manner, and reduce the uncertainty of the resource base. The unconventional reservoirs in Devonian shale are considered potentially the most productive underdeveloped source of natural gas in the northeastern U.S.. Field data indicate that gas is produced from Devonian shales mainly as a function of the presence and density of natural fractures. Although the detection of a fracture system does not guarantee commercial production, it increases that probability. In support of this effort, a series of joint industry/U.S. DOE research projects was designed to investigate and study the nature, orientation, and density of natural fractures in Devonian shales. Within several project areas in the Appalachian basin. holes were drilled, cored, and logged (Table 1). This paper deals with the detailed correlation of the natural fractures determined from these cores and geophysical well logs to determine the validity and reliability of logging tools to locate fractured internals in Devonian shales. In the latter part of the paper, correlation of natural and hydraulically induced fracture orientations, along with surface photolineaments traces, are discussed for a specific project area. Background The Morgantown Energy Technology Center (METC) is responsible for the management and technical direction of the U.S. DOE research and development activities related to the unconventional gas recovery (UGR) program. As part of the UGR program, the EGSP was formulated in 1975 to determine the resource potential of Devonian shale that underlies the Appalachian, Illinois, and Michigan basins. Within the EGSP. resource characterization activities including drilling, coring. and logging of Devonian shale wells are complete. This paper addresses primarily the coring and logging activities related to the identification of fractures, Correlation of natural fracture density and orientation as derived from well logs and oriented formation cores may aid in defining and identifying gas reservoirs and improving prospects of commercial gas production from shales. The objective of coring various Devonian shale intervals is to provide chemical and physical characterization of the cored formations. Formation characteristics studied include mechanical rock properties, geochemistry, and natural fracture incidence and orientation. Two types of fractures are detected in Devonian shales: existing natural fractures and fractures that occur as a result of stresses produced by the core extraction process. A classification scheme for identifying and defining the coring-induced and natural fractures has been developed. Kulander et al. defined such terms as hackle, petal, and torsion to classify coring-induced fractures. JPT P. 1371^