Pressure estimation using time-lapse seismic in compacting reservoirs
Abstract
This thesis focuses on developing a new approach to estimate pressure changes from 4D amplitude
attributes in compacting reservoirs. The time-lapse seismic signal in these types of reservoirs results
from the combination of pressure depletion, rock compaction and stress redistribution within the
reservoir and throughout the surrounding rocks. Simulations using iterative coupling are performed
to understand the link between geomechanics, fuid fow and the seismic response. The analysis
of synthetic data defines a power law equation (eq.1) which relates pressure changes (¢P) to 4D
amplitude attributes (¢A). The coefficients (C1 and C2) are a function of initial porosity. The
pressure predictions show an agreement with the output from the reservoir modelling. However the
results indicate that the rock compaction has considerable effect on the normal average stress, and
the 4D seismic response shows a stronger correlation with effective stress than with pore pressure.
¢A = C1 ¤ (¢PC2) (1)
The technique is applied to the south east flank of the Valhall Field, Norwegian North Sea. The areas
where the initial porosities are above 38 % show a good correlation between the pressure changes
predicted from 4D amplitude and the pressure changes from the reservoir model. However, major
differences between both outputs occur in areas where no 4D signal is observed; these areas are
correlated with low porosity zones where the porosity reduction has not been significant enough to
enhance the 4D signal. Furthermore, the pressure predictions from the 4D seismic identify areas
where the reservoir has not been properly drained.
The impact of geological structure and gas saturation on the technique is assessed. Strong thickness
variations within the reservoir interval increase the errors on the pressure prediction. This is mitigated
if relative values instead of absolute values are used to estimate pressure changes, i.e. equation 1
becomes equation 2. Furthermore, the presence of gas on the reservoir requires a modification of the
i
equation 1 in order to accurately predict the pressure changes and account for the presence of free
gas on the reservoir (eq. 3).
¢A
A1
= C1 ¤ (
¢P
P1
)C2 (2)
¢A = (C3 ¤ ¢P) + C4 (3)
Finally, sensitivity analysis suggests that uncertainties in the elastic properties of the overburden
(rarely measured with accuracy) have little impact on the reservoir 4D amplitude response. Synthetic
models show that variations between 10 % to 15 % in the Poisson's ratio and Young's modulus of the
layer immediately above the reservoir causes negligible changes (less than 4 %) in the 4D amplitude
response.