Seismic study of the low-permeability volume in southern France karst systems

Geophysics ◽  
2014 ◽  
Vol 79 (1) ◽  
pp. EN1-EN13 ◽  
Author(s):  
Pierre-Yves Galibert ◽  
Rémi Valois ◽  
Manuela Mendes ◽  
Roger Guérin
Keyword(s):  
Seismic Attenuation ◽  
Low Permeability ◽  
Driving Mechanism ◽  
Refraction Tomography ◽  
Seismic Amplitudes ◽  
Karst Systems ◽  
Water Saturated ◽  
Low Amplitude ◽  
Wave Induced

Locating groundwater in deep-seated karst aquifers is inherently difficult. With seismic methods, we studied the upper epikarst and the underneath low-permeability volume (LPV) of several karst systems located in the southern Quercy and Larzac regions of France and found that refraction tomography was effective only in the epikarst and not in the LPV. We evaluated a 3D case study using a combination of surface records and downhole receivers to overcome this limitation. This 3D approach unveiled a set of elongated furrows at the base of the epikarst and identified heterogeneities deep inside the LPV that may represent high-permeability preferred pathways for water inside the karst. To achieve the same result when no borehole was available, we studied seismic amplitudes of the wavefield, recognizing that wave-induced fluid flow in low-permeability carbonates is a driving mechanism of seismic attenuation. We developed a workflow describing the heterogeneity of the LPV with spectral attributes derived from surface-consistent decomposition principles, and we validated its effectiveness at benchmark locations. We applied this workflow to the 3D study and found a low-amplitude signal area at depth; we interpreted this anomaly as a water-saturated body perched above the aquifer.

Seismic attenuation and pore‐fluid viscosity in clay‐rich reservoir sandstones

Geophysics ◽  
1995 ◽  
Vol 60 (5) ◽  
pp. 1386-1397 ◽  
Author(s):  
Angus I. Best ◽  
Clive McCann
Keyword(s):  
Seismic Attenuation ◽  
Pore Fluid ◽  
Low Permeability ◽  
Biot Theory ◽  
Fluid Viscosity ◽  
High Permeability ◽  
Loss Mechanism ◽  
Local Flow ◽  
Flow Loss ◽  
Water Saturated

The frequency dependence of seismic attenuation in a suite of clay‐rich reservoir sandstones was investigated in the laboratory. Compressional‐ and shear‐wave velocities ([Formula: see text] and [Formula: see text]) and quality factors ([Formula: see text] and [Formula: see text]) were measured as functions of pore‐fluid viscosity at an effective pressure of 50 MPa and at an experimental frequency of about 0.8 MHz using the pulse‐echo technique. The experimental viscosity ranged from 0.3 to 1000 centipoise, which gives equivalent frequencies for a water‐saturated sandstone of 2.6 MHz to 780 Hz, assuming a global‐flow loss mechanism. Two types of behavior were observed: high permeability (greater than 100 millidarcies) sandstones tend to show variable [Formula: see text] and [Formula: see text] which are similar in magnitude to those predicted by the Biot theory over the viscosity range 0.3 to about 20 centipoise (equivalent frequency range 2.6 MHz to about 39 kHz); low permeability (less than 50 millidarcies) sandstones tend to show almost constant [Formula: see text] and [Formula: see text] over the experimental viscosity range that are not predicted by the Biot theory. The Biot theory does not predict the observed [Formula: see text] and [Formula: see text] values in the high permeability sandstones for viscosities greater than about 20 centipoise, where the observed [Formula: see text] values are generally much lower than the Biot predicted values. High permeability sandstones show small velocity dispersions with changing pore‐fluid viscosity that are consistent with the Biot theory. Low permeability sandstones show relatively large increases in velocity with increasing viscosity not explained by the Biot theory, which are consistent with a local flow loss mechanism. The results indicate the presence of two dominant loss mechanisms: global flow (at least down to about 39 kHz in water‐saturated rocks) in high permeability sandstones with only small amounts of intrapore clay, and local flow at ultrasonic frequencies in low permeability, clay‐rich sandstones.

Pore fluid effects on S-wave attenuation caused by wave-induced fluid flow

Geophysics ◽  
2012 ◽  
Vol 77 (3) ◽  
pp. L13-L23 ◽  
Author(s):  
Beatriz Quintal ◽  
Holger Steeb ◽  
Marcel Frehner ◽  
Stefan M. Schmalholz ◽  
Erik H. Saenger
Keyword(s):  
Fluid Flow ◽  
Wave Attenuation ◽  
Pore Space ◽  
Seismic Attenuation ◽  
P Wave ◽  
S Wave ◽  
Hydrocarbon Reservoir ◽  
Saturated Media ◽  
Water Saturated ◽  
Wave Induced

We studied seismic attenuation of P- and S-waves caused by the physical mechanism of wave-induced fluid flow at the mesoscopic scale. Stress relaxation experiments were numerically simulated by solving Biot’s equations for consolidation of 2D poroelastic media with finite-element modeling. The experiments yielded time-dependent stress-strain relations that were used to calculate the complex moduli from which frequency-dependent attenuation was determined. Our model consisted of periodically distributed circular or elliptical heterogeneities with much lower porosity and permeability than the background media, which contained 80% of the total pore space of the media. This model can represent a hydrocarbon reservoir, where the porous background is fully saturated with oil or gas and the low-porosity regions are always saturated with water. Three different saturation scenarios were considered: oil-saturated (80% oil, 20% water), gas-saturated (80% gas, 20% water), and fully water-saturated media. Varying the dry bulk and shear moduli in the background and in the heterogeneities, a consistent tendency was observed in the relative behavior of the S-wave attenuation among the different saturation scenarios. First, in the gas-saturated media the S-wave attenuation was very low and much lower than in the oil-saturated or in the fully water-saturated media. Second, at low frequencies the S-wave attenuation was significantly higher in the oil-saturated media than in the fully water-saturated media. The P-wave attenuation exhibited a more variable relative behavior among the different saturation degrees. Based on the mechanism of wave-induced fluid flow and on our numerical results, we suggest that S-wave attenuation could be used as an indicator of fluid content in a reservoir. Additionally, we observed that impermeable barriers in the background can cause a significant increase in S-wave attenuation. This suggests that S-wave attenuation could also be an indicator of permeability changes in a reservoir due to, for example, fracturing operations.

Testing the validity of different synthetic scenarios for flow and transport simulation in karst systems using a real case study application. 

2021 ◽  
Author(s):  
Joanna Doummar ◽  
Nidal Farran ◽  
Marwan Fahs ◽  
Benjamin Belfort ◽  
Thomas Graf
Keyword(s):  
Real Case ◽  
Richards Equation ◽  
Model Quality ◽  
Transient Model ◽  
Flow And Transport ◽  
Event Time ◽  
Model Flow ◽  
Karst Systems ◽  
The Impact

<p>Climate change and pollution are posing additional unprecedented threats to existing water resources, especially to water supply from karst aquifers in Mediterranean and semi-arid regions. A numerical model considering the most important key hydraulic parameters can forecast the impact of any given input on model quality and quantity output. In this work, we propose to model flow and transport using Comsol multiphysics in a synthetic model and to apply it to a simplified real case study (Jeita spring in Lebanon supplying water to 1.5 million inhabitants). The model geometry consists of a 5300 m long variably saturated horizontal conduit portrayed as 1) 2-D continuum and/or 2) a channel draining a porous equivalent matrix (400 m thick). Flow is simulated using the Richards Equation in both saturated and unsaturated medium. Recharge is applied vertically as both diffuse and point source in a shaft linked to the conduit. Percentages of fast infiltration rates are obtained from the analysis of event time series recorded at the spring (electrical conductivity and discharge). Flow rates at the outlet are used for transient model calibration. Mean velocities, dispersivities, and phreatic conduit diameters obtained from tracer experiments under various flow periods are used for transport validation in the channel. The aim is to test the validity of a functional simplified flow model on a complex real case and to identify based on a sensitivity analysis the key parameters that allow an optimal calibration of such a model. </p>

scholarly journals Fluid Saturation Predictions in a “Transition Zone” Carbonate Reservoir, Abu Dhabi

GeoArabia ◽  
1996 ◽  
Vol 1 (4) ◽  
pp. 551-566
Author(s):  
Anthony Kirkham ◽  
Mohamed Bin Juma ◽  
Tilden A.M. McKean ◽  
Anthony F. Palmer ◽  
Michael J. Smith ◽  
...  
Keyword(s):  
Transition Zone ◽  
Free Water ◽  
Analysis Data ◽  
Carbonate Reservoir ◽  
Rock Types ◽  
Fluid Saturation ◽  
Porosity And Permeability ◽  
Capillary Pressures ◽  
Seismic Amplitudes ◽  
Low Amplitude

ABSTRACT The field is a low amplitude structure with a chalky, Lower Cretaceous, Thamama reservoir characterised by a large hydrocarbon transition zone. Porosity generally decreases with depth within the trap although porosity versus depth trends are skewed by tilting. Porosity and permeability mapping was therefore achieved using templates based on seismic amplitudes. Special core analysis data were used to construct algorithms of Leverett J functions versus saturation for a variety of rock types mapped throughout the 3-D geological model of the field. The templated poroperms were then combined with capillary pressures to predict fluid saturations from these algorithms. The modelling of fluid distributions was therefore dependent upon heterogeneities imposed by the rock fabrics. Calibrating the model-predicted saturations against log-derived saturations at the wells involved regression techniques which were complicated by: notional structural tilting of the free water level, imbibition, hysteresis and permeability averaging procedures. Filtered “stick displays” proved useful in assessing the quality of the calibrations and were invaluable tools for highlighting and investigating data anomalies.

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