|Secondary Natural Gas Recovery in the Appalachian Basin
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.
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.
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
- Analyzed data from drilled sidewall cores and borehole image logs from three study wells in western Pennsylvania (Henderson Dome area, Mercer and Butler Counties) to determine the orientations (azimuth and facing direction) of the 27 cores,
- Ranked each core as to the reliability of the orientation determination,
- Identified large-aperture fractures and faults on Formation Microimager® (FMI) logs from two study wells,
- Prepared and photographed oriented thin sections from the sidewall cores with the highest orientation rankings, using standard petrographic techniques,
- Classified each microfracture observed in the thin sections and manually measured fracture attributes,
- Measured grain cement compositions and cement volumes from thin sections as well as from a suite of outcrop samples for diagenetic analysis,
- Identified the existence of multiple fracture sets in the cored intervals, with the single most prominent set striking N85W, the next most dominant N20E, and the third distinct set striking N30W,
- Performed power law scaling analyses on the fracture data,
- Determined that the location of zones of high fracture quality (intense fracture development with little or no mineral filling to occlude fracture conductivity) can be assessed from sidewall cores in the study area.
- Developed and tested a new horizontal-vector seismic explosive package that creates a downgoing wavefield that has a stronger S-wave component than does a conventional seismic explosive, and successfully tested the explosive concept in a western Pennsylvania setting,
- Formed a technical alliance between the Bureau of Economic Geology and a major supplier of explosive products (Austin Powder) to develop and test horizontal-vector explosive technology,
- Developed a second approach the relies P-to-S mode conversion to generate downgoing S-waves that will illuminate fractured reservoirs using a standard seismic source, such as a conventional shot hole explosive or a vertical vibrator, and
- Demonstrated, using VSP data, that robust S-waves generated by P-to-S mode conversions over Henderson Dome is a viable S-wave imaging technique.
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.
Current Status and Remaining Tasks: This project is complete.
Project Start: August 18, 1997
Project End: December 31, 2000
DOE Contribution: $550,551
Performer Contribution: $444,046
NETL – Thomas Mroz (firstname.lastname@example.org or 304-285-4071)
WVU – Douglas Patchen (304-293-2867)
Final Report [PDF]