Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Patent
1998-09-29
2000-07-18
Oda, Christine K.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
367 38, 367 68, 367 75, 367 57, 367 52, 367 43, 367 33, G01V 134
Patent
active
060916699
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a seismic method for determining and displaying the relative changes of elastic moduli and that of density of geologic formations and utilizing said relative changes and displaying plots for oil and gas exploration, especially for direct gas and light oil detection.
BACKGROUND OF THE INVENTION
The physical parameters being used most often in oil and gas exploration are velocities and densities of rocks and wave impedance calculated from them. These physical parameters are appropriate for determining underground structures. Nevertheless they result in uncertainty in determining rock property and for direct hydrocarbon detection. Two reasons account for such uncertainty: (1) the variation ranges of velocities and densities for variety of rocks overlap each other to some extent. (2) P-wave velocity, S-wave velocity and density are closely related each other. They have very limited ability for compensating each other to indicate the variation of characteristics of rocks and their pore-filling fluid.
Elastic moduli of rock include Lame constants .DELTA. and shear modulus .mu., the bulk modulus .kappa., Young's modulus E and Poisson's ration .sigma.. Their first introduction into physics was in the field of material science. The way by which they are defined makes them all positive. Those five elastic moduli are not independent from each other. Any one of them can be expressed in terms of two others. Therefore the elastic moduli played an important role in wave equation theory development. It is usually difficult to measure the moduli directly. They are usually calculated using the shear- and compressional-wave velocity and density measured in lab.
Shear-wave velocity V.sub.s and compressional-wave velocity V.sub.p are related to elastic moduli in terms of density .rho.. For example ##EQU1## By original meaning, rock's velocities in seismic exploration indicate only how fast the seismic waves travel through rock. Velocity has indirect relations to characteristics of rock and its pore-filling-fluid. Velocity results from comprehensive effect of moduli and density. Moduli, as compared with velocities, are more closely and more directly related to characteristics of rock and its pore-filling-fluid. The relative changes of elastic moduli and that of densities of geologic formations may have better ability to indicate changes in characteristics of geologic formations and their pore-filling-fluid directly. Therefore determining relative changes of elastic moduli and densities of geologic formations will be very useful for rock property study and direct hydrocarbon detection.
The Common-Depth-Point technique (hereinafter referred to as CDP technique) is one of the techniques which experienced great successes in the field of oil and gas exploration. The CDP technique have been developed and refined over forty years. Assuming a horizontally layered geologic formation, multiple coverage seismic data from each subsurface point can be recorded with CDP technique by using appropriate surface-detector and shot-point spreads. By sorting the seismic data recorded, CDP gathers can be composed. Stacking the traces within one CDP gather will enhance primary reflections from the same sub-surface reflection point and reduce multiple reflections and random noise. Unfortunately, while enhancing primary reflections, CDP stacking destroys valuable seismic information which is of crucial importance for direct hydrocarbon detection.
It has been recognized in theory since at least the end of last century that the amplitudes of shear and compressional reflectivity at a reflection boundary change with the angle of incidence. One of outstanding works is Zoeppritz equations which show that changing of the reflectivity with the angle of incidence is related to the elastic properties at the boundary, (see Zoeppritz, K., 1919, "Uber Erbeduellen", Vol. V II, Gottinger Nachrichten, p.66-84). People have paid their attention in recent years to the use of such changes in oil and gas explorat
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China National Offshore Oil Corp.
Jolly Anthony
Oda Christine K.
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