Geopressure Prediction Uncertainties in Deepwater -Part 1: Pre-Drill Estimation
By
Saad T Saleh1
(1) Knowledge Systems, Inc, Stafford, TX
Risk is an important element of pore pressure prediction in deepwater. In deep water, the margin between the pore pressure and fracture gradient can be very narrow, requiring several casing points in the shallow portion of the wellbore to stay within gradient limits.
Pore pressure derived from known seismic
velocity may only represent the
dominant lithology and does not reflect changes in pore pressure in sand bodies
and may not be sensitive to geologic conditions such as uplifting, erosion,
digenesis, etc. The basin
modeling
approach is perhaps best suited for
accounting of some of the major geologic and structural effects. Often, in an
exploration environment, the lack of sufficient offset well data to build a
basin model limit its application.
We will address the effect of seismic
data quality on the predicted pore
pressure in the absence of offset wells. A Monte Carlo simulation model has been
developed to generate
seismic
-based maximum and minimum pore pressures bounds
along with the “mean” pore pressure value at any given depth. Armed with this
data, the drilling engineer can consider the uncertainty in pore pressure
prediction and its implication on well planning. In addition, the results can be
used to justify the additional costs that are required to obtain more certain
prediction results. For example, the cost of reprocessing
seismic
data, or
obtaining a high resolution
seismic
hazard survey might be justified if a more
certain pressure prediction make it possible to eliminate a casing string.