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.