Increased efficiency of finding and developing hydrocarbon resources while reducing the environmental impact through geophysical technology is critical to the future of the petroleum industry. This technology must remotely determine rock-fluid property characteristics, their geometry, and changes over time. In reviewing advancements in exploration seismology, three component 3-D seismology stands out as the next wave and lends promise to the continued role that geophysics will play in the future of the petroleum industry.
Exploration seismology began with dynamite recording on single vertical component geophones from which compressional (P) wave refractions and reflections were identified and interpreted on paper records. Further advancements came with the development of reproducible seismograms and digital processing which enabled P-wave reflections to be stacked (CDP) to enhance the 2-D seismic expression of the subsurface. With advancements in digital acquisition and processing, 2-D has given way to 3-D which has lowered costs and increased the efficiency of finding oil and gas reserves. The full potential of 3-D has not and cannot be achieved without three-component recording.
Three-component 3-D seismology involves the acquisition of seismic data in three orientations at each receiver location: two orthogonal and one vertical. The horizontal components of source and receiver displacements enable the recording of shear (S) waves. which are a powerful complement to P-waves. When three components of source are used, nine times the data of a conventional (P) wave 3-D can be recorded at approximately one-third more cost thanks to advancements in today's acquisition and processing systems. The cost effectiveness and power of three-component 3-D will increase as new systems are developed.
Acquisition and processing of three-component 3-D will increase the fidelity of seismic data to determine lithology, including subtle diagenetic changes, fractured reservoirs, and rock-fluid property changes in the subsurface. Seismic wave propagation characteristics determined from three-component 3-D, including traveltime, anisotropy and attenuation, are critical to the seismic characterization of the subsurface. Through 3-D 3-C, much has been learned about seismic waves in fluid saturated and fractured rocks.
The orientation of in-situ stresses, the relative magnitude of these stresses, and estimates of permeability and fluid conductivity can be derived from seismic anisotropy. Rock properties, including lithology and porosity, can be obtained from comparative traveltimes of velocities of P and S waves. Fluid properties may be determined from comparative P and S traveltime and attenuation measurements.
In sight is three-component 4-D seismology, introducing time as the "fourth dimension". Stress and compressibility changes in a reservoir with production over time generally precludes the use of conventional P-waves in monitoring the flow and movement of hydrocarbons in the reservoir. By recording P and S waves as they pass through the reservoir, we can probe the reservoir dynamically and equate propagation characteristics to reservoir parameters including monitoring the fluid/rock interaction over time as the reservoir is produced and developed. Unless we use 3-D 3-C geophysical technology, we run the risk of leaving reserves behind at unprecedented rates.
With world population growth and economic stability, energy consumption will continue to grow well into the next millennium. As a result, there is a greater need for using geophysical technology for discovering new hydrocarbon reserves and for producing more from existing fields. Through three-component 3-D and 4-D seismology, these new reserves will be found, enhanced recovery will be achieved, and better resources management with reduced environmental impact will occur.
About the Author(s)
Dr. Tom Davis is currently Professor of Geophysics at the Colorado School of Mines (CSM) and Director of the Reservoir Characterization Project. He began his career in geophysics at the University of Saskatchewan in the late 1960's as a singular student in the geophysics option of the geological engineering program. Following graduation with a BE in 1969 he went on to complete a MS in geophysics at the University of Calgary (1971) and a PhD at Colorado School of Mines (1974). While at CSM he held an SEG Foundation Fellowship. His career in the oil patch began when he worked as a summer hire for Amoco (1968) and Chevron (1969-1970) and then as a permanent employee of Amoco (1971-72).
Except for a 3 1/2 year sojourn at the University of Calgary (1977-1979), Tom has been a faculty member of the Department of Geophysics of the Colorado School of Mines since 1974. He is dedicated to his students and currently teaches courses on seismic interpretation and integrated exploration and development. Since 1980 he has team-taught integrated, multidisciplinary courses at Mines. Having dedicated his career to education and research, he still takes time to consult and lecture worldwide and to watch the wheat grow in Saskatchewan.
Tom joined SEG in 1971 and has taught extensively in SEG's continuing education program from 1980-1994. He served as Second Vice-President during 1989-90. His main focus in addition to serving as SEG's Spring 1995 Distinguished Lecturer is co-editing a 3-D Seismic Atlas to be published jointly by SEG, AAPG, SPE and SPWLA. He is a member of the Denver Geophysical Society, the Canadian Society of Exploration Geophysicists, the European Association of Exploration Geophysicists and a host of other professional societies.