Effective Stress Coefficient for Seismic Velocities in Carbonate Rocks: Effects of Pore Characteristics and Fluid Types

2018 ◽  
Vol 176 (4) ◽  
pp. 1467-1485 ◽  
Author(s):  
Gautier Njiekak ◽  
Douglas R. Schmitt
Keyword(s):  
Effective Stress ◽  
Carbonate Rocks ◽  
Seismic Velocities ◽  
Pore Characteristics ◽  
Stress Coefficient

scholarly journals Influence of microheterogeneity on effective stress law for elastic properties of rocks

Geophysics ◽  
2008 ◽  
Vol 73 (1) ◽  
pp. E7-E14 ◽  
Author(s):  
Radim Ciz ◽  
Anthony F. Siggins ◽  
Boris Gurevich ◽  
Jack Dvorkin
Keyword(s):  
Effective Stress ◽  
Seismic Data ◽  
Elastic Moduli ◽  
Seismic Velocity ◽  
Time Lapse ◽  
Seismic Velocities ◽  
Laboratory Measurements ◽  
Hydrocarbon Production ◽  
Stress Coefficient ◽  
Seismic Measurements

Understanding the effective stress coefficient for seismic velocity is important for geophysical applications such as overpressure prediction from seismic data as well as for hydrocarbon production and monitoring using time-lapse seismic measurements. This quantity is still not completely understood. Laboratory measurements show that the seismic velocities as a function of effective stress yield effective stress coefficients less than one and usually vary between 0.5 and 1. At the same time, theoretical analysis shows that for an idealized monomineral rock, the effective stress coefficient for elastic moduli (and therefore also for seismic velocities) will always equal one. We explore whether this deviation of the effective stress coefficient from unity can be caused by the spatial microheterogeneity of the rock. The results show that only a small amount (less than 1%) of a very soft component is sufficient to cause this effect. Such soft material may be present in grain contact areas of many rocks and may explain the variation observed experimentally.

Experimental efforts to access 4D feasibility and interpretation issues of Brazilian presalt carbonate reservoirs

2019 ◽  
Vol 7 (4) ◽  
pp. SH1-SH18
Author(s):  
Guilherme Fernandes Vasquez ◽  
Marcio Jose Morschbacher ◽  
Julio Cesar Ramos Justen
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Carbonate Rocks ◽  
Time Lapse ◽  
Carbonate Reservoirs ◽  
Fluid Replacement ◽  
S Wave ◽  
Confining Stress ◽  
Reservoir Properties ◽  
Stress Coefficient

Brazilian presalt reservoirs comprise carbonate rocks saturated with light oil with different amounts of [Formula: see text] and excellent productivity. The occurrence of giant-size accumulations with such productivity generates the interest in production monitoring tools, such as time-lapse seismic. However, time-lapse seismic may present several challenges, such as imaging difficulties, repeatability, and detectability of small variations of reservoir properties. In addition, when assessing time-lapse seismic feasibility, the validity of Gassmann’s modeling for complex, heterogeneous carbonate rocks is arguable. Other questions include the pressure variation effects on the seismic properties of competent rocks. The effective stress is a linear combination of confining stress and pore pressure that governs the behavior of physical properties of rocks. Many applications assume that the effective stress for elastic-wave velocity is given by the difference between confining stress and pore pressure, whereas another common approach uses the Biot-Willis coefficient as a weight applied to the pore pressure to estimate the effective stress. Through a series of experiments involving ultrasonic pulse transmission on saturated core plugs in the laboratory, we verified the applicability of Gassmann’s fluid substitution and estimated the empirical effective stress coefficients related to the P- and S-wave velocities for rock samples from two offshore carbonate reservoirs from the presalt section, Santos Basin. We observed that Gassmann’s equation predicts quite well the effects of fluid replacement, and we found that the effective stress coefficient is less than one and not equal to the Biot-Willis coefficient. Moreover, there is a good agreement between the static and dynamic Biot-Willis coefficient, which is a suggestion that the presalt rocks behave as a poroelastic media. These observations suggest that more accurate time-lapse studies require the estimation of the effective stress coefficient for the particular reservoir of interest.

Seismic velocities in carbonate rocks

1991 ◽  
Vol 28 (6) ◽  
pp. A354 ◽  
Author(s):  
Z. Wang ◽  
W.K. Hirsche ◽  
G. Sedgwick
Keyword(s):  
Carbonate Rocks ◽  
Seismic Velocities
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