Abstract
Production-induced subsidence due to compressibility and fluid property changes in a Niger delta field has been investigated using well log and 4D seismic data sets. The objective of the study is to evaluate changes in time lapse seismic attributes due to hydrocarbon production and infer to probable ground subsidence. Petrophysical modeling and analysis of well data revealed that Density (ρ), Lambda rho (λρ) and Acoustic impedance (Ip) are highly responsive to changes in reservoir properties. These properties and water saturation attribute were subsequently, extracted from time-lapse seismic volumes in the immediate vicinity of well locations. Result show that monitor horizon slices exhibit appreciable increases in ρ, λρ, Ip and water saturation values compared to the base data, especially around the well locations. These increases in relative values of rock/attribute properties between the time-lapse surveys for a constant overburden stress are obvious indications of pore pressure and fluid depletion in the reservoir. Depletion in these properties increases the effective stress (pressure) and the grain-to-grain contact of the reservoir matrix, with a corresponding decrease in compressibility. Consequently, pore and matrix volume decreases, the reservoir compacts and the ground subsides. However, this is suspected to be small and at the reservoir scale due to low initial reservoir porosity and the relatively large lateral dimension compared to the thickness of the reservoir.
Highlights
As the demand for hydrocarbons continue to rise astronomically globally due to technology and population, tertiary recovery methods have been developed and advanced for poorly producing reservoirs, while already producing reservoirs are subjected to increasing pressure to meet global demand
The present study aims at evaluating changes in time-lapse seismic attributes from inverted 4D post stack depth migrated seismic data
The monitor horizon slices show appreciable increases in values of ρ, λρ, Ip and water saturation properties compared to the base data
Summary
As the demand for hydrocarbons continue to rise astronomically globally due to technology and population, tertiary recovery methods have been developed and advanced for poorly producing reservoirs, while already producing reservoirs are subjected to increasing pressure to meet global demand. These undoubtedly, will affect the effective stress (pressure) regime on the reservoir and adjacent formations and impact on reservoir saturation, pressure, structure, geometry and compressibility properties (Hermansen et al, 2000; Gambolati and Teatini, 2015). The reservoir compacts resulting in changes in bulk volume, pre-existing faults are reactivated and surficial subsidence is induced (Segall, 1989; Civasso, 1992; Rutledge et al, 1998; Dyer et al, 1999; Scot et al, 2000; Chan et al, 2007)
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