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

Abstract

Ultrasonic velocities of a set of saturated sandstone samples were measured at simulated in-situ pressures in the laboratory. The samples were obtained from the W formation of the WXS Depression and covered low to nearly high porosity and permeability ranges. The brine and four different density oils were used as pore fl uids, which provided a good chance to investigate fluid viscosity-induced velocity dispersion. The analysis of experimental observations of velocity dispersion indicates that (1) the Biot model can explain most of the small discrepancy (about 2 -3%) between ultrasonic measurements and zero frequency Gassmann predictions for high porosity and permeability samples saturated by all the fl uids used in this experiment and is also valid for medium porosity and permeability samples saturated with low viscosity fluids (less than approximately 3 mP•S) and (2) the squirt flow mechanism dominates the low to medium porosity and permeability samples when fl uid viscosity increases and produces large velocity dispersions as high as about 8%. The microfracture aspect ratios were also estimated for the reservoir sandstones and applied to calculate the characteristic frequency of the squirt fl ow model, above which the Gassmann' s assumptions are violated and the measured high frequency velocities cannot be directly used for Gassmann's fluid replacement at the exploration seismic frequency band for W formation sandstones.

Simulating attenuation in saturated sandstones with Dvorkin-Mavko model

“…The core samples used in this modeling were taken from He et al, (2010) (Figure 1 and Table 1). Are sandstone from the W formation of the WXS Depression and covered low to nearly high porosity and permeability ranges.…”

“…Figure 1 -Data used in modeling. 𝑃 𝑑𝑖𝑓𝑓 is the differential pressure between confining pressure and pore fluid pressure at core collection depth; 𝜌 𝑓 , 𝐾 𝑓 and 𝐺 𝑓 are the estimated density, the bulk and shear moduli of solid matrix based on the thin section under the optical microscope; D is the diameter; L is the length; is the effective porosity; k is the permeability; 𝜌 𝑑𝑟𝑦 is the dry density calculated after cleaning and oven-drying; 𝑉 𝑝𝑑𝑟𝑦 and 𝑉 𝑠𝑑𝑟𝑦 are the measured compressional and shear velocities at dry conditions; and η is the estimated microfracture aspect ratio (He et al, 2010).…”

An Experimental Study of Wave Velocity Anisotropy in Sandstone Bearing Differential Pore Fluids from the Yanchang Formation

“…The compressional (P) and shear (S) wave velocity of the sandstone samples were measured by the pulse transmission method using the AutoLab series He et al, 2010;Purcell et al, 2010;Mur et al, 2011). The 2000 Laboratory System of New England Research, Inc. (NER) that can measure one compressional and two orthogonally polarized shear waves (SH and SV) (He et al, 2010;Purcell et al, 2010;Mur et al, 2011), and record these waveforms simultaneously under simulated in-situ high pressure and temperature conditions is used. For V samples, in fact only a polarized shear waves SH can be measured (Best, 2007).…”

The Uncertainty Analysis of the Key Factors That Affect the AVO Attributes in Sandstone Reservoir

“…In the W3 formation of WXS depression, the pore fluid of reservoir sandstone is the mixture of brine (26 g/L) and light oil (∼0.7 g/cm 3 ). Since the lack of S-wave acoustic logging in the study area and unknown oil saturation ratio for the measured P-wave acoustic logging, we built the empirical relationships of reservoir sandstone samples' physical properties and P-and S-wave impedances by measurements under a dry condition [16,17] , and then applied Gassmann equation [18] to calculate model velocities for different oil saturation ratios. There were 3 advantages in this method: (1) The relationship of porosity and P-and S-wave impedances is valid at any interpolated value.…”