scholarly journals Numerical Modeling of Fluid Effects on Seismic Properties of Fractured Magmatic Geothermal Reservoirs

2016 ◽  
Author(s):  
Melchior Grab ◽  
Beatriz Quintal ◽  
Eva Caspari ◽  
Hansruedi Maurer ◽  
Stewart Greenhalgh
Keyword(s):  
Numerical Modeling ◽  
Velocity Dispersion ◽  
Seismic Velocity ◽  
Seismic Attenuation ◽  
Geothermal Reservoir ◽  
Modeling Technique ◽  
Hydraulic Permeability ◽  
Seismic Properties ◽  
Fluid Content ◽  
Geothermal Reservoirs

Abstract. Seismic investigations of geothermal reservoirs over the last 20 years have sought to interpret the resulting tomograms and reflection images in terms of the degree of reservoir fracturing and fluid content. Since the former provides the pathways and the latter acts as the medium for transporting geothermal energy, such information is needed to evaluate the quality of the reservoir. In conventional rock physics-based interpretations, this hydro-mechanical information is approximated from seismic velocities computed at the low frequency (field-based) and high frequency (lab-based) limits. In this paper, we demonstrate how seismic properties of fluid-filled, fractured reservoirs can be modeled over the full frequency spectrum using a numerical simulation technique which has become popular in recent years. It is based on Biot's theory of poroelasticity and enables the modeling of the seismic velocity dispersion and the frequency dependent seismic attenuation due to wave-induced fluid flow. These properties are sensitive to key parameters such as the hydraulic permeability of fractures as well as the compressibility and viscosity of the pore fluids. Applying the poroelastic modeling technique to the specific case of a magmatic geothermal system under stress due to the weight of the overlying rocks requires careful parameterization of the model. This includes consideration of the diversity of rock types occurring in the magmatic system and examination of the confining pressure-dependency of each input parameter. After the evaluation of all input parameters, we use our modeling technique to determine the seismic attenuation factors and phase velocities of a rock containing a complex interconnected fracture network, whose geometry is based on a fractured geothermal reservoir in Iceland. Our results indicate that in a magmatic geothermal reservoir the overall seismic velocity structure mainly reflects the lithological heterogeneity of the system, whereas indicators for reservoir permeability and fluid content are deducible from the magnitude of seismic attenuation and the critical frequency at which the peak of attenuation and maximum velocity dispersion occur. The study demonstrates how numerical modeling provides a valuable tool to overcome interpretation ambiguity and to gain a better understanding of the hydrology of geothermal systems, which are embedded in a highly heterogeneous host medium.

scholarly journals Numerical modeling of fluid effects on seismic properties of fractured magmatic geothermal reservoirs

Solid Earth ◽  
2017 ◽  
Vol 8 (1) ◽  
pp. 255-279 ◽  
Author(s):  
Melchior Grab ◽  
Beatriz Quintal ◽  
Eva Caspari ◽  
Hansruedi Maurer ◽  
Stewart Greenhalgh
Keyword(s):  
Numerical Modeling ◽  
Velocity Dispersion ◽  
Seismic Velocity ◽  
Seismic Attenuation ◽  
Geothermal Reservoir ◽  
Modeling Technique ◽  
Hydraulic Permeability ◽  
Seismic Properties ◽  
Fluid Content ◽  
Geothermal Reservoirs

Abstract. Seismic investigations of geothermal reservoirs over the last 20 years have sought to interpret the resulting tomograms and reflection images in terms of the degree of reservoir fracturing and fluid content. Since the former provides the pathways and the latter acts as the medium for transporting geothermal energy, such information is needed to evaluate the quality of the reservoir. In conventional rock physics-based interpretations, this hydro-mechanical information is approximated from seismic velocities computed at the low-frequency (field-based) and high-frequency (lab-based) limits. In this paper, we demonstrate how seismic properties of fluid-filled, fractured reservoirs can be modeled over the full frequency spectrum using a numerical simulation technique which has become popular in recent years. This technique is based on Biot's theory of poroelasticity and enables the modeling of the seismic velocity dispersion and the frequency dependent seismic attenuation due to wave-induced fluid flow. These properties are sensitive to key parameters such as the hydraulic permeability of fractures as well as the compressibility and viscosity of the pore fluids. Applying the poroelastic modeling technique to the specific case of a magmatic geothermal system under stress due to the weight of the overlying rocks requires careful parameterization of the model. This includes consideration of the diversity of rock types occurring in the magmatic system and examination of the confining-pressure dependency of each input parameter. After the evaluation of all input parameters, we use our modeling technique to determine the seismic attenuation factors and phase velocities of a rock containing a complex interconnected fracture network, whose geometry is based on a fractured geothermal reservoir in Iceland. Our results indicate that in a magmatic geothermal reservoir the overall seismic velocity structure mainly reflects the lithological heterogeneity of the system, whereas indicators for reservoir permeability and fluid content are deducible from the magnitude of seismic attenuation and the critical frequency at which the peak of attenuation and maximum velocity dispersion occur. The study demonstrates how numerical modeling provides a valuable tool to overcome interpretation ambiguity and to gain a better understanding of the hydrology of geothermal systems, which are embedded in a highly heterogeneous host medium.

Predicting Optimal Borehole Locations for Parameter Estimation in Geothermal Reservoirs Using Optimal Experimental Design

2021 ◽  
Author(s):  
Johanna Fink ◽  
Ralf Seidler
Keyword(s):  
Parameter Estimation ◽  
Experimental Design ◽  
Mathematical Optimization ◽  
Simulation Software ◽  
Geothermal Reservoir ◽  
Temperature Measurements ◽  
Optimal Experimental Design ◽  
Reservoir Engineering ◽  
Hydraulic Permeability ◽  
Geothermal Reservoirs

<p>Drilling boreholes during exploration and development of geothermal reservoirs not only involves high cost, but also bears significant risks of failure. In geothermal reservoir engineering, techniques of optimal experimental design (OED) have the potential to improve the decision making process. Previous publications explained the formulation and implementation of this mathematical optimization problem and demonstrated its feasibility for finding borehole locations in two- and three-dimensional reservoir models that minimize the uncertainty of estimating hydraulic permeability of a model unit from temperature measurements. Subsequently, minimizing the uncertainty of the parameter estimation results in a more reliable parametrization of the reservoir simulation, improving the overall process in geothermal reservoir engineering.</p><p>Various OED techniques are implemented in the Environment for Combining Optimization and Simulation Software (EFCOSS). To address problems arising from geothermal modeling, this software framework links mathematical optimization software with SHEMAT-Suite, a geothermal simulation code for fluid flow and heat transport through porous media. This contribution shows how to determine experimental conditions such that the uncertainty when estimating different parameters of model units from temperature measurements in the borehole is minimized. Numerical simulations of synthetic geothermal reservoir scenarios are presented to demonstrate the OED workflow and its applicability to geothermal reservoir modeling</p>

Effects of Fluid Saturation on Waves in Porous Rock and Relations to Hydraulic Permeability

1980 ◽  
Vol 20 (06) ◽  
pp. 450-458 ◽  
Author(s):  
Amos M. Nur ◽  
Joel D. Walls ◽  
Kenneth Winkler ◽  
John DeVilbiss
Keyword(s):  
Seismic Wave ◽  
Seismic Attenuation ◽  
Hot Water ◽  
Gradual Transition ◽  
Hydraulic Permeability ◽  
Partial Saturation ◽  
Pore Fluids ◽  
Geothermal Reservoirs ◽  
Wave Velocities ◽  
Saturated Rock

Abstract Detailed and very accurate measurements of compressional Vp and shear Vs wave velocities and their attenuation in dry, partly saturated and fully saturated clay-bearing sandstone were carried out at 20, 145, and 198 degrees C, under variable confining and pore pressures.The velocity results show that water saturated rock has high Vp and relatively low Vs not only at room temperature but also at 145 and 198 degrees C. Varying the pore pressure reveals that the transition from the fully saturated to the dry state involves a gradual transition of Vs and a very irregular transition of Vp - with a minimum in Vp relative to either the dry or the fully saturated cases.The attenuation of S and P waves - Qs(-1) and Qp(-1), respectively - shows similar patterns: in dry rock, both Qs(-1) and Qp(-1) are low. In fully saturated rock Qp(-1) is low and Qs(-1) is high. In partly saturated rock, Qs(-1) changes gradually with saturation, but Qp(-1) shows a very strong maximumsuggesting strong energy dissipation. The same behavior is observed at 145 degrees C, with the steam/hot-water transition. It was also found that attenuation of shear waves increases with permeability in some sandstones, consistent with squirting attenuation mechanism.The results suggest diagnostic characteristics for determining the degree of saturation in situ from seismic logs, with particular applicability to evaluation of steam content in geothermal reservoirs and of gas content in oil reservoirs. Introduction In recent years seismic attenuation has become of increasing interest to seismologists, including the study of bright spots in hydrocarbon exploration, reservoir evaluation, and well logging.Seismic methods are particularly important in exploration for and evaluation of geothermal reservoirs, including physical property determinationssuch as Poisson's ratio and seismic wave attenuationfor delineating the boundaries and state of reservoirs. The latter measurements are particularly interesting for the possible distinction between steam-bearing and hot-water domains. To make full use of seismic data, it is necessary to interpret attenuation in terms of the physical properties of rocks, and the mechanisms responsible for loss of energy in seismic waves. In this paper we show some of the important effects of fluids on wave velocities and attenuation in porous and cracked rocks. We further show that a fundamental physical link may exist between seismic wave attenuation in rocks and the hydraulic permeability in these rocks.The effects that pore fluids have on seismic velocities are well-documented. It is natural to suppose that pore fluids will also influence seismic attenuation, but very little experimental work has been done in this area. There is, however, no shortage of theoretical models of fluid loss mechanism. Flow models based on partial saturation of individual cracks have been developed by White and by Mavko and Nur, and a thermoelastic partial saturation mechanism has been presented by Kjartansson and Nur. The models show invariably that attenuation is related to the local hydraulic diffusivity. Thus, if the local and global hydraulic properties can be related, we may eventually be able to infer permeability from seismic measurements. SPEJ P. 450＾

The relation between seismic P‐ and S‐wave velocity dispersion in saturated rocks

Geophysics ◽  
1994 ◽  
Vol 59 (1) ◽  
pp. 87-92 ◽  
Author(s):  
Gary Mavko ◽  
Diane Jizba
Keyword(s):  
Wave Velocity ◽  
Large Scale ◽  
Velocity Dispersion ◽  
Seismic Velocity ◽  
Ultrasonic Velocities ◽  
S Wave ◽  
Local Flow ◽  
Wave Velocities ◽  
Shear Compliance

Seismic velocity dispersionin fluid-saturated rocks appears to be dominated by tow mecahnisms: the large scale mechanism modeled by Biot, and the local flow or squirt mecahnism. The tow mechanisms can be distuinguished by the ratio of P-to S-wave dispersions, or more conbeniently, by the ratio of dynamic bulk to shear compliance dispersions derived from the wave velocities. Our formulation suggests that when local flow denominates, the dispersion of the shear compliance will be approximately 4/15 the dispersion of the compressibility. When the Biot mechanism dominates, the constant of proportionality is much smaller. Our examination of ultrasonic velocities from 40 sandstones and granites shows that most, but not all, of the samples were dominated by local flow dispersion, particularly at effective pressures below 40 MPa.

scholarly journals Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: A case study for the COSC-1 borehole, Sweden

2020 ◽  
Author(s):  
Felix Kästner ◽  
Simona Pierdominici ◽  
Judith Elger ◽  
Christian Berndt ◽  
Alba Zappone ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Seismic Velocity ◽  
Metamorphic Rocks ◽  
Seismic Velocities ◽  
Mafic Rocks ◽  
Seismic Properties ◽  
The Core ◽  
Nappe Complex ◽  
Thrust Zone ◽  
Mountain Belts

<p>Deeply rooted thrust zones are key features of tectonic processes and the evolution of mountain belts. Exhumed and deeply-eroded orogens like the Scandinavian Caledonides allow to study such systems from the surface. Previous seismic investigations of the Seve Nappe Complex have shown indications for a strong but discontinuous reflectivity of this thrust zone, which is only poorly understood. The correlation of seismic properties measured on borehole cores with surface seismic data can help to constrain the origin of this reflectivity. In this study, we compare seismic velocities measured on cores to in situ velocities measured in the borehole. The core and downhole velocities deviate by up to 2 km/s. However, velocities of mafic rocks are generally in close agreement. Seismic anisotropy increases from about 5 to 26 % at depth, indicating a transition from gneissic to schistose foliation. Differences in the core and downhole velocities are most likely the result of microcracks due to depressurization of the cores. Thus, seismic velocity can help to identify mafic rocks on different scales whereas the velocity signature of other lithologies is obscured in core-derived velocities. Metamorphic foliation on the other hand has a clear expression in seismic anisotropy. To further constrain the effects of mineral composition, microstructure and deformation on the measured seismic anisotropy, we conducted additional microscopic investigations on selected core samples. These analyses using electron-based microscopy and X-ray powder diffractometry indicate that the anisotropy is strongest for mica schists followed by amphibole-rich units. This also emphasizes that seismic velocity and anisotropy are of complementary importance to better distinguish the present lithological units. Our results will aid in the evaluation of core-derived seismic properties of high-grade metamorphic rocks at the COSC-1 borehole and elsewhere.</p>

NUMERICAL MODELING OF A DESATURATING GEOTHERMAL RESERVOIR

1978 ◽  
Vol 1 (2) ◽  
pp. 203-216
Author(s):  
R. N. Home ◽  
M. J. O'Sullivan
Keyword(s):  
Numerical Modeling ◽  
Geothermal Reservoir

High-resolution seismic velocity analysis by sign-based weighted semblance

Geophysics ◽  
2021 ◽  
pp. 1-35
Author(s):  
M. Javad Khoshnavaz
Keyword(s):  
Seismic Velocity ◽  
Resolution Enhancement ◽  
Synthetic Data ◽  
Correlation Coefficients ◽  
Velocity Model ◽  
Seismic Attenuation ◽  
Velocity Analysis ◽  
Velocity Models ◽  
Amplitude Versus Offset ◽  
Amplitude Versus

Building an accurate velocity model plays a vital role in routine seismic imaging workflows. Normal-moveout-based seismic velocity analysis is a popular method to make the velocity models. However, traditional velocity analysis methodologies are not generally capable of handling amplitude variations across moveout curves, specifically polarity reversals caused by amplitude-versus-offset anomalies. I present a normal-moveout-based velocity analysis approach that circumvents this shortcoming by modifying the conventional semblance function to include polarity and amplitude correction terms computed using correlation coefficients of seismic traces in the velocity analysis scanning window with a reference trace. Thus, the proposed workflow is suitable for any class of amplitude-versus-offset effects. The approach is demonstrated to four synthetic data examples of different conditions and a field data consisting a common-midpoint gather. Lateral resolution enhancement using the proposed workflow is evaluated by comparison between the results from the workflow and the results obtained by the application of conventional semblance and three semblance-based velocity analysis algorithms developed to circumvent the challenges associated with amplitude variations across moveout curves, caused by seismic attenuation and class II amplitude-versus-offset anomalies. According to the obtained results, the proposed workflow is superior to all the presented workflows in handling such anomalies.

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