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Fractured Reservoirs (fractured + reservoir)

Distribution by Scientific Domains
Earth and Environmental Science83%

Selected Abstracts

The Geysers geothermal field: results from shear-wave splitting analysis in a fractured reservoir

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2005
Maya Elkibbi
SUMMARY Clear shear-wave splitting (SWS) is observed in 1757 high signal-to-noise ratio microearthquake seismograms recorded by two high density seismic arrays in the NW and the SE Geysers geothermal fields in California. The Geysers reservoir rocks within the study area are largely composed of lithic, low-grade metamorphism, well-fractured metagraywackes which commonly lack schistosity, warranting the general assumption that shear-wave splitting here is induced solely by stress-aligned fracturing in an otherwise isotropic medium. The high quality of observed shear-wave splitting parameters (fast shear-wave polarization directions and time delays) and the generally good data spatial coverage provide an unprecedented opportunity to demonstrate the applicability and limitations of the shear-wave splitting approach to successfully detect fracture systems in the shallow crust based on SWS field observations from a geothermal reservoir. Results from borehole stations in the NW Geysers indicate that polarization orientations range between N and N60E; while in the SE Geysers, ground surface stations show polarization directions that are generally N5E, N35E-to-N60E, N75E-to-N85E, and N20W-to-N55W. Crack orientations obtained from observed polarization orientations are in good agreement with independent field evidence, such as cracks in geological core data, tracer tests, locally mapped fractures, and the regional tectonic setting. Time delays range typically between 8 and 40 ms km,1, indicating crack densities well within the norm of fractured reservoirs. The sizeable collection of high resolution shear-wave splitting parameters shows evidence of prevalent vertical to nearly vertical fracture patterns in The Geysers field. At some locations, however, strong variations of SWS parameters with ray azimuth and incident angle within the shear-wave window of seismic stations indicate the presence of more complex fracture patterns in the subsurface. [source]

Processing, modelling and predicting time-lapse effects of overpressured fluid-injection in a fractured reservoir

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2002
Erika Angerer
Summary Time-lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs. We analyse two 4-D, 3C onshore surveys from Vacuum Field, New Mexico, USA, where the reservoir of interest is a fractured dolomite. In Phase VI, a time-lapse survey was acquired before and after a pilot tertiary-recovery programme of overpressured CO2 injection, which altered the fluid composition and the pore-fluid pressure. Phase VII was a similar time-lapse survey in the same location but with a different lower-pressure injection regime. Applying a processing sequence to the Phase VI data preserving normal-incidence shear-wave anisotropy (time-delays and polarization) and maximizing repeatability, interval-time analysis of the reservoir interval shows a significant 10 per cent change in shear-wave velocity anisotropy and 3 per cent decrease in the P -wave interval velocities. A 1-D model incorporating both saturation and pressure changes is matched to the data. The saturation changes have little effect on the seismic velocities. There are two main causes of the time-lapse changes. Any change in pore-fluid pressures modifies crack aspect ratios. Additionally, when there are overpressures, as there are in Phase VI, there is a 90° change in maximum impedance directions, and the leading faster split shear wave, instead of being parallel to the crack face as it is for low pore-fluid pressures, becomes orthogonal to the crack face. The anisotropic poro-elasticity (APE) model of the evolution of microcracked rock, calculates the evolution of cracked rock to changing conditions. APE modelling shows that at high overburden pressures only nearly vertical cracks, to which normal incidence P waves are less sensitive than S waves, remain open as the pore-fluid pressure increases. APE modelling matches the observed time-lapse effects almost exactly demonstrating that shear-wave anisotropy is a highly sensitive diagnostic of pore-fluid pressure changes in fractured reservoirs. In this comparatively limited analysis, APE modelling of fluid-injection at known pressure correctly predicted the changes in seismic response, particularly the shear-wave splitting, induced by the high-pressure CO2 injection. In the Phase VII survey, APE modelling also successfully predicted the response to the lower-pressure injection using the same Phase VI model of the cracked reservoir. The underlying reason for this remarkable predictability of fluid-saturated reservoir rocks is the critical nature and high crack density of the fluid-saturated cracks and microcracks in the reservoir rock, which makes cracked reservoirs critical systems. [source]

The Geysers geothermal field: results from shear-wave splitting analysis in a fractured reservoir

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2005
Maya Elkibbi
SUMMARY Clear shear-wave splitting (SWS) is observed in 1757 high signal-to-noise ratio microearthquake seismograms recorded by two high density seismic arrays in the NW and the SE Geysers geothermal fields in California. The Geysers reservoir rocks within the study area are largely composed of lithic, low-grade metamorphism, well-fractured metagraywackes which commonly lack schistosity, warranting the general assumption that shear-wave splitting here is induced solely by stress-aligned fracturing in an otherwise isotropic medium. The high quality of observed shear-wave splitting parameters (fast shear-wave polarization directions and time delays) and the generally good data spatial coverage provide an unprecedented opportunity to demonstrate the applicability and limitations of the shear-wave splitting approach to successfully detect fracture systems in the shallow crust based on SWS field observations from a geothermal reservoir. Results from borehole stations in the NW Geysers indicate that polarization orientations range between N and N60E; while in the SE Geysers, ground surface stations show polarization directions that are generally N5E, N35E-to-N60E, N75E-to-N85E, and N20W-to-N55W. Crack orientations obtained from observed polarization orientations are in good agreement with independent field evidence, such as cracks in geological core data, tracer tests, locally mapped fractures, and the regional tectonic setting. Time delays range typically between 8 and 40 ms km,1, indicating crack densities well within the norm of fractured reservoirs. The sizeable collection of high resolution shear-wave splitting parameters shows evidence of prevalent vertical to nearly vertical fracture patterns in The Geysers field. At some locations, however, strong variations of SWS parameters with ray azimuth and incident angle within the shear-wave window of seismic stations indicate the presence of more complex fracture patterns in the subsurface. [source]

Processing, modelling and predicting time-lapse effects of overpressured fluid-injection in a fractured reservoir

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2002
Erika Angerer
Summary Time-lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs. We analyse two 4-D, 3C onshore surveys from Vacuum Field, New Mexico, USA, where the reservoir of interest is a fractured dolomite. In Phase VI, a time-lapse survey was acquired before and after a pilot tertiary-recovery programme of overpressured CO2 injection, which altered the fluid composition and the pore-fluid pressure. Phase VII was a similar time-lapse survey in the same location but with a different lower-pressure injection regime. Applying a processing sequence to the Phase VI data preserving normal-incidence shear-wave anisotropy (time-delays and polarization) and maximizing repeatability, interval-time analysis of the reservoir interval shows a significant 10 per cent change in shear-wave velocity anisotropy and 3 per cent decrease in the P -wave interval velocities. A 1-D model incorporating both saturation and pressure changes is matched to the data. The saturation changes have little effect on the seismic velocities. There are two main causes of the time-lapse changes. Any change in pore-fluid pressures modifies crack aspect ratios. Additionally, when there are overpressures, as there are in Phase VI, there is a 90° change in maximum impedance directions, and the leading faster split shear wave, instead of being parallel to the crack face as it is for low pore-fluid pressures, becomes orthogonal to the crack face. The anisotropic poro-elasticity (APE) model of the evolution of microcracked rock, calculates the evolution of cracked rock to changing conditions. APE modelling shows that at high overburden pressures only nearly vertical cracks, to which normal incidence P waves are less sensitive than S waves, remain open as the pore-fluid pressure increases. APE modelling matches the observed time-lapse effects almost exactly demonstrating that shear-wave anisotropy is a highly sensitive diagnostic of pore-fluid pressure changes in fractured reservoirs. In this comparatively limited analysis, APE modelling of fluid-injection at known pressure correctly predicted the changes in seismic response, particularly the shear-wave splitting, induced by the high-pressure CO2 injection. In the Phase VII survey, APE modelling also successfully predicted the response to the lower-pressure injection using the same Phase VI model of the cracked reservoir. The underlying reason for this remarkable predictability of fluid-saturated reservoir rocks is the critical nature and high crack density of the fluid-saturated cracks and microcracks in the reservoir rock, which makes cracked reservoirs critical systems. [source]

The 3D shear experiment over the Natih field in Oman: the effect of fracture-filling fluids on shear propagation

GEOPHYSICAL PROSPECTING, Issue 2 2001
C.M. Van Der Kolk
This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three-component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear-wave velocities. One is that the shear-wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear-wave velocity which senses both the fractures and the fracture-filling fluid. Using a new effective-medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear-wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear-wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P-wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective-medium modelling and the geology. With this new effective-medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional-wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S-wave data, as well as the conventional P-wave data, as a direct hydrocarbon indicator. [source]

A shear-dilation-based model for evaluation of hydraulically stimulated naturally fractured reservoirs

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 5 2002
M. K. Rahman
Abstract The role of shear dilation as a mechanism of enhancing fluid flow permeability in naturally fractured reservoirs was mainly recognized in the context of hot dry rock (HDR) geothermal reservoir stimulation. Simplified models based on shear slippage only were developed and their applications to evaluate HDR geothermal reservoir stimulation were reported. Research attention is recently focused to adjust this stimulation mechanism for naturally fractured oil and gas reservoirs which reserve vast resources worldwide. This paper develops the overall framework and basic formulations of this stimulation model for oil and gas reservoirs. Major computational modules include: natural fracture simulation, response analysis of stimulated fractures, average permeability estimation for the stimulated reservoir and prediction of an average flow direction. Natural fractures are simulated stochastically by implementing ,fractal dimension' concept. Natural fracture propagation and shear displacements are formulated by following computationally efficient approximate approaches interrelating in situ stresses, natural fracture parameters and stimulation pressure developed by fluid injection inside fractures. The average permeability of the stimulated reservoir is formulated as a function of discretized gridblock permeabilities by applying cubic law of fluid flow. The average reservoir elongation, or the flow direction, is expressed as a function of reservoir aspect ratio induced by directional permeability contributions. The natural fracture simulation module is verified by comparing its results with observed microseismic clouds in actual naturally fractured reservoirs. Permeability enhancement and reservoir growth are characterized with respect to stimulation pressure, in situ stresses and natural fracture density applying the model to two example reservoirs. Copyright © 2002 John Wiley & Sons, Ltd. [source]