The goal of this project was to extend development of existing Appalachian Basin reservoirs by improving the likelihood of drilling higher-productivity wells through the identification of high-conductivity sweet spots in fractured reservoirs. The objectives were to develop, test and evaluate advanced technologies designed to: (1) identify large-scale, production-controlling fractures from small-scale microfractures observed in matrix grains of sidewall cores, and (2) verify methods whereby robust seismic shear (S) waves can be generated to detect and map fractured reservoir facies in Appalachian fields.
West Virginia University Research Corporation – Project management
Bureau of Economic Geology – Testing, analysis and research products
Atlas Resources, Inc. – Sidewall cores, logs, and base map
To test and evaluate technologies that would result in improved characterization of fractured natural-gas reservoirs in the Appalachian Basin, the Bureau of Economic Geology (Bureau) worked jointly with industry partner Atlas Resources, Inc. to design, execute, and evaluate several experimental tests. The tests were of two types: (1) tests leading to a low-cost methodology whereby small-scale microfractures observed in matrix grains of sidewall cores can be used to deduce critical properties of large-scale fractures that control natural-gas production, and (2) tests that verify methods whereby robust seismic shear (S) waves can be generated to detect and map fractured reservoir facies.
The grain-scale microfracture approach to characterizing rock facies was developed in an ongoing Bureau research program that started before this Appalachian Basin study began. However, the method had not been tested in a wide variety of fracture systems, and the tectonic setting of rocks in the Appalachian Basin composed an ideal laboratory for perfecting the methodology.
As a result of this Appalachian study, a low-cost commercial procedure now exists that will allow Appalachian operators to use scanning electron microscope (SEM) images of thin sections extracted from oriented sidewall cores to infer the spatial orientation, relative geologic timing, and population density of large-scale fracture systems in reservoir sandstones. These attributes are difficult to assess using conventional techniques.
An innovative method was also developed for obtaining the stratigraphic and geographic tops of sidewall cores. With currently deployed sidewall coring devices, no markings from which top orientation can be obtained are made on the sidewall core itself during drilling. The method developed in this study involves analysis of the surface morphology of the broken end of the core as a top indicator. Together with information on the working of the tool (rotation direction), fracture-surface features, such as arrest lines and plume structures, not only give a top direction for the cores but also indicate the direction of fracture propagation. The study also determined that microresistivity logs or other image logs can be used to obtain accurate sidewall core azimuths and to determine the precise depths of the sidewall cores.
Two seismic S-wave technologies were developed in this study. The first was a special explosive package that, when detonated in a conventional seismic shot hole, produces more robust S-waves than do standard seismic explosives. The importance of this source development is that it allows S-wave seismic data to be generated across all of the Appalachian Basin. Previously, Appalachian operators have not been able to use S-wave seismic technology to detect fractured reservoirs because the industry-standard S-wave energy source, the horizontal vibrator, is not a practical source option in the heavy timber cover that extends across most of the basin.
The second S-wave seismic technology that was investigated was used to verify that standard P-wave seismic sources can create robust downgoing S-waves by P-to-S mode conversion in the shallow stratigraphic layering in the Appalachian Basin. This verification was done by recording and analyzing a 3-component vertical seismic profile (VSP) in the Atlas Montgomery No. 4 well at Henderson Dome, Mercer County, Pennsylvania. The VSP data confirmed that robust S-waves are generated by P-to-S mode conversion at the basinwide Onondaga stratigraphic level. Appalachian operators can thus use converted-mode seismic technology to create S-wave images of fractured and unfractured rock systems throughout the basin.
This project is complete.
NETL – Thomas Mroz (firstname.lastname@example.org or 304-285-4071)
WVU – Douglas Patchen (304-293-2867)
Final Report [PDF]