|Measuring Fracture Density and Orientation in Unconventional Reservoirs with Simple-source Vertical Seismic Profiles||Last Reviewed 12/20/2012|
The objective of this project is to develop and demonstrate a technology that uses vertical-force seismic sources combined with vertical seismic profile (VSP) to provide a seismic "log" of natural fracture orientation and density in unconventional reservoirs.
University of Texas, Bureau of Economic Geology, Austin, TX 78713-8924
GEDCO - an integrated geophysical survey design software and services company that is part of Schlumberger?s WesternGeco business unit
A common feature of shale-gas units and tight sandstones is that most of these unconventional reservoirs have embedded fracture systems that need to be understood in order to position exploitation wells. A remote seismic technology that can ?visualize? the internal architecture of unconventional reservoirs and predict fracture orientation and density will be invaluable for characterizing tight sandstones, shale-gas units, and all unconventional resource plays. Current technology has demonstrated that shear (S) waves are more responsive to fractures than compressional (P) waves. Based on this knowledge, operators across unconventional reservoir plays need an effective, low-cost way to illuminate reservoir systems with surface-generated S waves. This project will develop a technology whereby S waves can be produced with simple, low-cost, and widely available seismic sources that apply a vertical force to the Earth. The technology utilizes S modes created directly at the point where a vertical force is applied to the Earth?s surface, which is in contrast to current practices of using sources that apply a horizontal force to the Earth or converted S modes produced at subsurface interfaces by downgoing P wavefields.
In addition, researchers on this project will present field procedures and data-processing strategies whereby S modes produced by vertical-force sources can provide fracture sensitive attributes. These procedures remove current practice limitations because vertical-force sources (vertical vibrators, vertical impacts, shothole explosives) are lower cost, more abundant than horizontal-force sources, and usable over a wider range of terrains. This approach refutes the common assumption that the only way to create a downgoing vertical shear (SV) mode with a vertical-force source is to use a shallow interface to produce a downgoing P-to-SV mode conversion. An important feature of the technology is that fracture properties can be estimated several tens of meters away from a receiver well rather than the one meter distance required for a dipole sonic log.
The outputs of the project will be a demonstration of correct field procedures for acquiring orthogonal SV-wave vectors and software source code that performs data analyses to convert orthogonal SV-wave data into estimates of fracture orientation and fracture density.
The seismic technology developed in this study will allow improved mapping of fracture orientations and densities in unconventional reservoirs, thus addressing the objective of advanced visualization to enhance unconventional production. Technology will be developed using VSP data acquired in shale-gas and tight-sandstone reservoirs, but it can be applied to any fractured reservoir. Providing operators with better knowledge of fracture density and crack orientation should result in increased oil production.
Simplifying S-wave seismic source activity by utilizing S modes created directly at the point where a vertical force is applied to the Earth?s surface will result in less costly data acquisition and fewer environmental issues caused by source deployment. An important impact is that vertical-force seismic sources can be utilized in a wide variety of terrains where horizontal-force sources cannot be deployed.
The experimental methods implemented consisted of analyzing average P and S velocities in a walk-around (WAR) VSP acquired over a Marcellus Shale prospect. Seventy source stations were distributed around the VSP receiver well at azimuth increments of approximately 5 degrees. Data were acquired by a vertical array of 16 receivers positioned across a depth interval of 5820 to 6570 ft. These receivers were approximately centered on the Marcellus Shale. Simple, straight-raypath calculations of average velocity from each source station to each receiver station allow azimuth velocity behavior to be analyzed. Both P and S velocities vary with azimuth, with maximum velocities occurring in a north-northwest direction for both P waves and S waves. BEG tentatively assigned this high-velocity azimuth direction as the direction of maximum horizontal stress. This conjecture is will be verified by an analysis of local stress data.
The data confirmed the following important principles of S-wave physics: (1) SH and SV shear modes propagate with different velocities, and (2) SH velocity is greater than SV velocity when raypaths have a significant horizontal component, as they do for this WAR VSP source-receiver geometry. S waves are produced directly at source stations where a vertical-force source applies its force vector to the Earth.
The project has received two recently acquired VSP data sets that will used to develop fracture delineation methodology. GEDCO, a geophysical company, provided the geophysical data processing software package to be used in the project. Software to be used to analyze the two recently acquired seismic data sets has been developed.
A literature review to determine the relevant seismic data processing software needed to analyze the Vertical Seismic Profile (VSP) data sets has been completed.
Current Status (December 2012)
Two VSPs have been processed and these datasets are currently being analyzed to determine fracture delineation using SH and SV shear modes in shale gas reservoirs. The seismic technology developed in this study will improve mapping of fracture orientations and densities in unconventional reservoirs. The technology will be developed using VSP data acquired in shale-gas and tight-sandstone reservoirs, but can be applied to any fractured reservoir. Providing operators with better knowledge of fracture density and crack orientation should result in increased shale gas production.
Project Start: February 1, 2011
Project End: January 31, 2013
DOE Contribution: $416,688
Performer Contribution: $115,054
NETL ? Chandra Nautiyal (firstname.lastname@example.org or 281-494-2488)
University of Texas at Austin ? Bob Hartage (email@example.com or 512-477-0300)
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