Integration of 3D Seismic in the Absence of Core Data to Mitigate Porosity Uncertainty in a Deep, Over Pressure Reservoir in Niger Delta

Abstract

Determining reservoir porosity from density logs is possibly the most common practice in the industry over the years. Computation of porosity from density is largely affected by the grain density and fluid density used in the models. There are many best practices available in the industry to compute the grain density and fluid density with reasonable certainty. There are also many different models available which takes into account other available log responses like neutron porosity, resistivity, and shale volume to account for the uncertainties in the porosity computation. However, one of the areas that need some special attention is to integrate non-petrophysical data like seismic which can help reducing the porosity uncertainty significantly especially when no other calibration reference is available. Log measurements from the padded tools like density, neutron porosity are generally quality checked against caliper and other lithology indicators like gamma ray, resistivity, sonic etc. This paper demonstrates that porosity computation could also go wrong in a true gauge hole where density log reading apparently looks good and also showing right kind of deflection for changes in lithology. The situation can get quite complicated if the reservoir is highly over pressured and if there is no core data or regional data available. In a deep, over pressured reservoir in the niger delta, with only two well penetrations, no core measurements, and no regional calibration reference, top quality recent 3D seismic data was integrated with the well data to mitigate uncertainties in reservoir porosity. The formation evaluation challenge was that log in one of the wells showed a relatively low density reading (hence high porosity) compared to the second well density over the same reservoir. However seismic amplitude did not support the porosity variation between the two wells hence leading to further investigation on identification and management of the observed uncertainties. This paper showcased in details how the seismic data was used to identify the wrong well log reading that lead to wrong prediction of porosity distribution and the accuracy of the integrated seismic and well log predicted properties when compared to actual well log result from a well that was later drilled in the same reservoir.