A feasibility study for pore-pressure prediction using seismic velocities in the offshore Nile Delta, Egypt

Abstract

Overpressure in a formation, caused by abnormally high fluid pressures, is a concern during all phases of oil field operations—exploration, drilling, casing, completion, and reservoir evaluation. Accurate knowledge of formation pore pressure and fracture pressure is essential for drilling efficient and safe wells with optimum mud weights. Furthermore, knowledge of these pressures aids understanding of fluid migration pathways, sealing potential, and probability of fault leakage. Overpressure by definition occurs when pore pressure exceeds normal hydrostatic pressure and is related to certain environmental conditions in a given earth section. In the offshore Nile Delta, for example, low permeability shale in the Pliocene can trap fluids and cause overpressured shale as a result of undercompaction. Overpres-sured sediments also can be caused by fluid expansion mechanisms (e.g., heating, hydrocarbon maturation and expulsion of intergranular water during clay transformation). Local tectonic compression can also generate overpressured sediments. Given the young age and shallowness of overpressured sediments in the offshore Nile Delta, the observed pore pressure is largely attributed to undercompaction. Several methods for detecting/estimating overpressured formations are based on interpretation of drilling data, wireline logs, and geophysical data. Drilling and wireline log data are obtained while the well is drilled. They cannot, therefore, be used for predrill pore-pressure prediction. This paper describes a feasibility study to predict pore pressure before drilling and the subsequent calibration of pore pressure and seismic velocities in a key well. Port Fouad Field was discovered in 1982 when exploratory well Port Fouad-1 penetrated gas-bearing formations in the late Miocene. The field is approximately 35 km northeast of Port Said (Figure 1). Figure 1. Map of Port Fouad Field. Gas is present at shallow and deep depths. Shallow zones, less than 2200 m, belong to the Kafr El Sheikh Formation of Pliocene age which is characterized by high porosity and permeability. The …