A dynamic elastic model for squirt-flow effect and its application on fluid-viscosity-associated velocity dispersion in reservoir sandstones

Geophysics ◽  
2020 ◽  
Vol 85 (4) ◽  
pp. MR201-MR212
Author(s):  
Zhi-Qiang Yang ◽  
Tao He ◽  
Chang-Chun Zou
Keyword(s):  
Velocity Dispersion ◽  
Pore Space ◽  
Pore Fluid ◽  
Quantitative Prediction ◽  
Elastic Model ◽  
Fluid Viscosity ◽  
Porous Rocks ◽  
S Wave ◽  
Flow Effect ◽  
Reservoir Sandstones

Velocity dispersion is a common phenomenon for fluid-charged porous rocks and carries important information on the pore structure and fluid in reservoir rocks. Previous ultrasonic experiments had measured more significant non-Biot velocity dispersion on saturated reservoir sandstones with increasing pore-fluid viscosity. Although wave-induced local squirt-flow effect could in theory cause most of the non-Biot velocity dispersion, its quantitative prediction remains a challenge. Several popular models were tested to predict the measured velocities under undrained conditions, but they either underestimated the squirt-flow effect or failed to simultaneously satisfy P- and S-wave velocity dispersions (especially for higher viscosity fluids). Based on the classic double-porosity theory that pore space is comprised of mainly stiff/Biot’s porosity and minor compliant porosity, an effective “wet frame” was hypothesized to account for the squirt-flow effect, whose compliant pores are filled with a hypothesized fluid with dynamic modulus. A new dynamic elastic model was then introduced by extending Biot theory to include the squirt-flow effect, after replacing the dry-frame bulk/shear moduli with their wet-frame counterparts. In addition to yielding better velocity predictions for P- and S-wave measurements of different fluid viscosities, the new model is also more applicable because its two key tuning parameters (i.e., the effective aspect ratio and porosity of compliant pores) at in situ reservoir pressure could be constrained with laboratory velocity measurements associated with pore-fluid viscosities.

Estimates of velocity dispersion between seismic and ultrasonic frequencies

Geophysics ◽  
1986 ◽  
Vol 51 (1) ◽  
pp. 183-189 ◽  
Author(s):  
Kenneth W. Winkler
Keyword(s):  
Velocity Dispersion ◽  
Low Frequency ◽  
Degree Of Saturation ◽  
Fluid Viscosity ◽  
Seismic Velocities ◽  
S Wave ◽  
Local Flow ◽  
Saturated Rock ◽  
Analysis Technique ◽  
Zero Frequency

It is generally accepted that acoustic velocities in fluid‐saturated rocks vary with frequency. Evidence comes from experimental measurements and from theoretical causality arguments. We have developed a simple analysis technique that gives estimates of total velocity dispersion between zero frequency and any measurement frequency. The technique requires compressional (P) and shear (S) wave velocity measurements on dry and fully saturated rock. Assuming that the dry velocities are independent of frequency, the Biot‐Gassmann equations are used to calculate the zero‐frequency velocities in the fully saturated rock. Any difference between the measured velocities and the calculated zero‐frequency velocities is interpreted as evidence of dispersion. Application of this analysis technique to a variety c ultrasonic data sets gives consistent results. In many rocks, dispersion between zero frequency and ultrasonic frequencies is on the order of 10 percent at low effective stress, and it decreases to only a few percent at higher stresses. Dispersion varies with degree of saturation and with fluid viscosity in the same way as do low‐frequency attenuation measurements. The results are readily interpreted in terms of the same local‐flow absorption/dispersion mechanism that has been used to explain recent laboratory attenuation measurements. This apparent dispersion places upper bounds on seismic‐to‐sonic velocity differences. It also points out possible discrepancies between seismic velocities and ultrasonic laboratory measurements.

Estimating grain‐scale fluid effects on velocity dispersion in rocks

Geophysics ◽  
1991 ◽  
Vol 56 (12) ◽  
pp. 1940-1949 ◽  
Author(s):  
Gary Mavko ◽  
Diane Jizba
Keyword(s):  
High Frequency ◽  
Velocity Dispersion ◽  
Rock Properties ◽  
S Wave ◽  
Unknown Parameters ◽  
Local Flow ◽  
Essential Information ◽  
Flow Effect ◽  
Total Dispersion ◽  
Aspect Ratios

The magnitude of the grain‐scale local flow effect on velocity dispersion in saturated rocks is quantified, by estimating the high‐frequency unrelaxed shear and bulk frame moduli, which are then combined with the Biot formulation to predict total dispersion. The method is relatively independent of assumptions about idealized pore geometries and unknown parameters such as pore aspect ratios. The local flow effect depends on the heterogeneity of pore stiffness, in particular the presence of compliant cracks and grain contacts; the pressure dependence of the dry rock properties is shown to contain the essential information about the distribution of pore stiffnesses needed to estimate the high‐frequency saturated behavior. To first order, the unrelaxed wet frame compressibility at any given pressure is shown to be approximately the dry frame compressibility at very high pressure; second order corrections add the additional compressibility gained by replacing an amount of mineral equal to the compliant pore volume with fluid. The method predicts that the difference between relaxed and unrelaxed shear compliance is simply proportional to that in bulk. The results for total dispersion (local flow plus Biot) explain quite well the measured P- and S-wave dispersion for a variety of saturated rocks.

Laboratory study of fluid viscosity induced ultrasonic velocity dispersion in reservoir sandstones

2010 ◽  
Vol 7 (2) ◽  
pp. 114-126 ◽  
Author(s):  
Tao He ◽  
Chang-Chun Zou ◽  
Fa-Gen Pei ◽  
Ke-Ying Ren ◽  
Fan-Da Kong ◽  
...  
Keyword(s):  
Ultrasonic Velocity ◽  
Laboratory Study ◽  
Velocity Dispersion ◽  
Fluid Viscosity ◽  
Reservoir Sandstones

Pore fluid viscosity effects on P- and S-wave anisotropy in synthetic silica-cemented sandstone with aligned fractures

2014 ◽  
Vol 62 (6) ◽  
pp. 1238-1252 ◽  
Author(s):  
Philip Tillotson ◽  
Mark Chapman ◽  
Jeremy Sothcott ◽  
Angus Ian Best ◽  
Xiang-Yang Li
Keyword(s):  
Pore Fluid ◽  
Fluid Viscosity ◽  
S Wave ◽  
Viscosity Effects

Inverse rock physics modeling for reservoir quality prediction

Geophysics ◽  
2013 ◽  
Vol 78 (2) ◽  
pp. M1-M18 ◽  
Author(s):  
Tor Arne Johansen ◽  
Erling Hugo Jensen ◽  
Gary Mavko ◽  
Jack Dvorkin
Keyword(s):  
Rock Physics ◽  
Pore Fluid ◽  
Quality Data ◽  
Quantitative Prediction ◽  
Reservoir Quality ◽  
S Wave ◽  
Data Set ◽  
Seismic Parameters ◽  
Rock Physics Modeling ◽  
Physics Modeling

Seismic reservoir characterization requires a transform of seismically derived properties such as P- and S-wave velocities, acoustic impedances, elastic impedances, or other seismic attributes into parameters describing lithology and reservoir conditions. A large number of different rock physics models have been developed to obtain this link. Their relevance is, however, constrained by the type of lithology, porosity range, textural complexity, saturation conditions, and the dynamics of the pore fluid. Because the number of rock physics parameters is often higher than the number of seismic parameters, this is known to be an underdetermined problem with nonunique solutions. We have studied the framework of inverse rock physics modeling which aims at direct quantitative prediction of lithology and reservoir quality from seismic parameters, but where nonuniqueness and data error propagation are also handled. The procedure is based on a numerical reformulation of rock physics models so that the seismic parameters are input and the reservoir quality data are output. The modeling procedure can be used to evaluate the validity of various rock physics models for a given data set. Furthermore, it provides the most robust data parameter combinations to use for either porosity, lithology, and pore fluid prediction, whenever a specific rock physics model has been selected for this cause.

Effects of fluid viscosity on shear‐wave attenuation in partially saturated sandstone

Geophysics ◽  
1991 ◽  
Vol 56 (8) ◽  
pp. 1252-1258 ◽  
Author(s):  
Dung Vo‐Thanh
Keyword(s):  
Shear Wave ◽  
Velocity Dispersion ◽  
Wave Attenuation ◽  
Pore Space ◽  
Viscoelastic Model ◽  
Fluid Viscosity ◽  
Berea Sandstone ◽  
Frequency Range ◽  
Partially Saturated ◽  
Viscous Shear

Shear‐wave attenuation and velocity have been measured in the kiloHertz frequency range at temperatures varying from −80°C to 80°C in a sample of Berea sandstone partially saturated with glycerol. I investigated 7 saturation states ranging from 0 to 62 percent of the pore space. Plots of attenuation versus temperature show squirt and viscous shear peaks, even at low saturation. Their amplitudes and half‐widths increase with increasing saturation. The maxima of the peaks progressively move to higher temperatures (about 4°C for viscous shear peak and 30°C for squirt peak) with increasing saturation from 7 to 62 percent. The velocity dispersion between −80°C and 80°C progressively increases from 700 to 1200 m/s with increasing saturation from 7 to 62 percent. By introducing the crack saturation parameter, a simple viscoelastic model based on O’Connell and Budiansky and using a Cole‐Cole distribution of cracks, is proposed for calculating the shear modulus in partially saturated rocks. This model partially interprets the experimental data.

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