Presentation on theme: "Feasibility Study of Fracture Detection Using Seismic Data in Karachaganak Xiang-Yang Li 1, Enru Liu 1 and Tim Pointer 2 1 Edinburgh Anisotropy Project,"— Presentation transcript:
Feasibility Study of Fracture Detection Using Seismic Data in Karachaganak Xiang-Yang Li 1, Enru Liu 1 and Tim Pointer 2 1 Edinburgh Anisotropy Project, BGS 2 Geophysics Skill Centre, BG Group
Contents Geological setting Model building Acquisition parameters P-wave azimuthal analysis Converted-wave splitting Summary
Blocked model Formation tops referred to the cross section Well 25 does not go through Salt Fractured limestone with 10% fracture density
Model building notes: (Tim, please check, here are some questions to you regarding to the model.) 1.The model is consistent with the geological section with a stratigraphic unconformity: the lower Permian sits above the Carboniferous, and hence a velocity inverse. 2.The white zone in the log was interpreted as the velocity inverse. 3.However, this unconformity is not visible on the seismic section recently supply to us (Slide 8 ) 4.The well does not seem to tie with the seismic at well 25 ?
Acquisition parameters (modelling) Four azimuthal directions (0, 30, 60, 90 degrees), angles from fracture normal Offset ranges 0-12km Usable offsets 0-9km 240 channels and receiver interval 50m A Ricker wavelet with 200ms delay time, and dominant frequency 50 Hz is used Gas- vs. Water-filled fractures
Notes on the full-wave field 1.P-wave reflections from all 17 reflectors (R1-R17) can be clearly identified in the vertical components 2.PS-conversions from all 17 reflectors (R1-R17) can be observed clearly in the horizontal radial components 3.In the top half of reflectors (R1-R11), PP- and PS waves are coupled in the vertical and horizontal components 4.PP- and PS events from reflectors R12-R17 are better separated in the vertical and horizontal components. 5.R12 is the top of the fractured zone, and R15 is the bottom of the fractured zone
P-wave azimuthal analysis Azimuthal gathers for offsets: 0-3000m -No observable azimuthal variations Azimuthal gathers for offsets: 0-4500m - Clear observable azimuthal variations More-offset gathers: 0-6000m; 3000-9000m Gathers are NMO-corrected by a single velocity function derived from azimuthal gather 0 (AZI=0, parallel to fracture normal)
Velocity function used for NMO AZI=0 PP wave Gas-filled R12 R13 R14 R15 R12 R13 R14 R15
Notes on azimuthal variation in P-wave moveout 1.P-wave moveout: Event R12 (top of fracture zone) shows no azimuthal variation 2.Events R13-R15 show clear azimuthal variations in P- wave moveout for offsets 3000m or above 3.Gas-filled fractures show stronger azimuthal variations than water-filled fractures 4.An offset-depth ration of 1.5 is preferred to observe P- wave azimuthal variations, or an offset of 6000m is required for a target at depth 4000m.
Notes on azimuthal variation in P-wave stacking velocity and amplitude 1.P-wave stacking velocity at event R12 (top of fracture zone) shows no azimuthal variation 2.Events R13-R15 show clear azimuthal variation in P- wave stacking velocities 3.The picked white curves indicate the stacking velocity from azimuthal gather AZI=0. 4.P-wave amplitude shows similar azimuth variations, but less obvious than the moveout and velocity
Notes on fracture detection using P-wave data 1.Multi-azimuthal coverage is required for using P-wave attributes for fracture analysis 2.Offsets at least up to 6000m is required for a target at depth of 4000m 3.As such, lateral inhomogeneity, such as the salt domes will affect the results of P-wave data. 4.The two salt domes are about 10km apart at the surface, and 4-6km apart at the subsurface. Interferences from the salt domes will degrade the reliability of P-wave fracture detection.
PS converted-wave analysis Azimuthal gathers for offsets: 0-3000m -No observable azimuthal variations Azimuthal gathers for offsets: 0-6000m - Clear observable azimuthal variations Converted-wave splitting is clearly observable on the azimuthal gathers -Fast shear-wave (PS1): Gather AZI=90 -Slow shear-wave (PS2): Gather AZI=0 Gathers are NMO-corrected by a single velocity function from azimuthal gather 90 (AZI=90, parallel to the fracture strike)
Velocity function used for NMO AZI=90 PS wave Gas-filled R12 R13 R14 R15 R12 R13-PS1 R14-PS1 R15 – PS1
Notes on PS converted-wave analysis 1.Converted-wave moveout and stacking velocities for events R13-R15 also show azimuthal variations within the offset ranges of 0-6000m 2.However, this azimuthal variation is complicated by converted-wave splitting and the non-hyperbolic moveout due to the asymmetric raypath. 3.Converted-wave splitting in the fracture zones is observable in all azimuth gathers. 4.The fast and slow converted-waves (PS1 and PS2) are separated in the two azimuthal gathers parallel and perpendicular to the fracture strike (AZI=90 and 0, respectively).
Notes on PS converted-wave analysis - continued 5.Converted-wave splitting can be observable within the conventional offset ranges (0-3000m), and long offset coverage is not necessary. 6.An reliable analysis of converted-wave splitting still requires multi-azimuthal coverage. 7.The salt-free corridor offers a possibility to conduct an experiment using converted-wave splitting to characterize the fractures in this area.
Summary - 1 It is possible to use P-waves to characterize the fracture zones located at depth level from 4200m to 5000km. However, it requires wide azimuthal coverage and long-offsets up to 6000km or more. Consequently, complications will arise due to the presence of two closed spaced salt domes less than 6km apart in the subsurface. The accuracy and reliability of any fracture information derived from the P-wave will be affected by the presence of the salt domes.
Summary - 2 Converted-wave splitting can be observed within the fractured zone and this splitting can be observed within much shorter offsets less than 3km. This offers a better alternative to characterize the fractures along the salt-free corridor using converted-waves. A multi-azimuthal coverage may still be required, although it is not essential.
Acknowledgement We thank BG for providing the data …. (Tim, please add the names of your colleagues, The work is supported through the EAP project and we thank the sponsors of EAP project. ????
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