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

Guilherme Fernandes Vasquez, Marcio Jose Morschbacher and Julio Cesar Ramos Justen
Experimental efforts to access 4D feasibility and interpretation issues of Brazilian presalt carbonate reservoirs
Interpretation (Tulsa) (August 2019) 7 (4): SH1-SH18

Abstract

Brazilian presalt reservoirs comprise carbonate rocks saturated with light oil with different amounts of CO (sub 2) 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.


ISSN: 2324-8858
EISSN: 2324-8866
Serial Title: Interpretation (Tulsa)
Serial Volume: 7
Serial Issue: 4
Title: Experimental efforts to access 4D feasibility and interpretation issues of Brazilian presalt carbonate reservoirs
Affiliation: Petrobras, Research and Development Center, Rio de Janeiro, Brazil
Pages: SH1-SH18
Published: 20190812
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 47
Accession Number: 2019-079969
Categories: Economic geology, geology of energy sourcesApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
S34°00'00" - N05°15'00", W74°00'00" - W34°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
Update Code: 201942
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