4D Seismic Pore Pressure Inversion – The Effect of Production Induced Overburden Changes

Time-lapse seismology has proven to be a useful method for monitoring reservoir fluid flow, identifying unproduced hydrocarbons and injected fluids, and improving overall reservoir management decisions. The large magnitudes of observed time-lapse seismic anomalies associated with strong pore pressure increases are sometimes not explainable by velocity-pressure relationships determined by fitting elastic theory to core data. This can lead to difficulties in interpreting time-lapse seismic data in terms of physically realizable changes in reservoir properties during injection. It is commonly assumed that certain geologic properties remain constant during fluid production/injection, including rock porosity and grain cementation. We have developed a new nonelastic method based on rock physics diagnostics to describe the pressure sensitivity of rock properties that includes changes in the grain contact cement, and we applied the method to a 4D seismic data example from offshore Australia. We found that water injection at high pore pressure may mechanically weaken the poorly consolidated reservoir sands in a nonelastic manner, allowing us to explain observed 4D seismic signals that are larger than can be predicted by elastic theory fits to the core data. A comparison of our new model with the observed 4D seismic response around a large water injector suggested a significant mechanical weakening of the reservoir rock, consistent with a decrease in the effective grain contact cement from 2.5% at the time/pressure of the preinjection baseline survey, to 0.75% at the time/pressure of the monitor survey. This approach may enable more accurate interpretations and future predictions of the 4D signal for subsequent monitor surveys and improve 4D feasibility and interpretation studies in other reservoirs with geomechanically similar rocks.

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Integration of geophysical and geomechanical data to understand the depletion of the Marlim field, Campos Basin

Geologia USP Série Científica ◽

10.11606/issn.2316-9095.v21-166922 ◽

2021 ◽

Vol 21 (1) ◽

pp. 45-55

Author(s):

Larissa Furtado Torres ◽

Emílio Velloso Barroso

Keyword(s):

Pore Pressure ◽

3D Models ◽

Pressure Gradients ◽

Campos Basin ◽

Sea Level Variations ◽

Pore Pressure Gradient ◽

4D Seismic ◽

Seismic Anomalies ◽

Potential Fluid ◽

Field Area

Located in the Campos Basin, Brazil, the Marlim field, consists of two turbidite systems deposited during eustatic sea-level variations in the Oligocene/Miocene. The reservoir was discovered in 1985, and its production started to decline in 2002. One of the techniques selected to assist in the recovery of oil from the reservoir was the 4D seismic. However, its interpretation can be complex. In order to help address this issue, the present study proposed an analysis of the depletion of a small field area from 1997 to 2010, combining geophysical (4D seismic) and geomechanical (pore pressure) data through the construction of pore pressure 3D models for both years, which can be subtracted and compared to seismic anomalies. The results obtained were: an average depletion of 0.42 ppg (50.33 kg/m3) of pore pressure gradient in the field; the identification of potential fluid-flow barriers, such as an NW-SE-oriented channel and sealing faults; and the detection of two areas with an expressive presence of 4D seismic anomalies, one of them showing a quite evident difference between pore pressure gradients, suggesting field depletion. The use of very old and noisy seismic data hindered the application of this methodology. Nevertheless, this research demonstrated the relevance of estimating pore pressure in the reservoir and how this geomechanical parameter can be useful in assessing the level of field depletion.

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Stress-path-dependent velocities in shales: Impact on 4D seismic interpretation

Geophysics ◽

10.1190/geo2017-0652.1 ◽

2018 ◽

Vol 83 (6) ◽

pp. MR353-MR367 ◽

Cited By ~ 6

Author(s):

Rune M. Holt ◽

Andreas Bauer ◽

Audun Bakk

Keyword(s):

Pore Pressure ◽

Laboratory Data ◽

Stress Path ◽

P Wave ◽

Seismic Velocities ◽

In Situ Conditions ◽

Overburden Stress ◽

Pressure Changes ◽

4D Seismic

Overburden stresses and pore pressure are altered by depletion or inflation of a subsurface reservoir, leading to seismic traveltime and reflectivity changes that may be interpreted as footprints of reservoir drainage or injection. Our objective is to contribute to the quantification of expected 4D seismic time shifts and reflectivities by understanding how overburden stresses and strains change and how seismic velocities depend on these stress and strain changes. The stress sensitivity of ultrasonic velocities has been obtained from controlled laboratory experiments in which field shale cores are brought to in situ conditions and then probed with different stress paths, i.e., different ratios between the horizontal and vertical stress change. The tests are performed in undrained conditions, and pore-pressure changes are recorded. The experiments indicate that the velocity and pore-pressure changes depend linearly on the stress path. The latter is a verification of the applicability of Skempton’s law from soil mechanics for shales. Overburden stress paths are, through analytical and numerical geomechanical modeling, seen to depend on the aspect ratio of the depleting or inflating zone, on the elastic contrast between the overburden and the reservoir, and on the reservoir tilt. By combining laboratory data and simulated overburden stress paths, the response of in situ wave velocities to reservoir pore-pressure change can be estimated. The calculated in situ stress dependence of the vertical P-wave velocity shows significant dependence on stress path. The strain sensitivity, expressed by the dilation parameter, or [Formula: see text] factor, increases strongly with the stress path. This expresses the explicit sensitivity of [Formula: see text] to vertical in situ strain. The results also indicate that the time-lapse overburden response may be significantly influenced by pore-pressure changes in the overburden.

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