The difficulties encountered in the interpretation of water saturation in carbonate rocks from conventional logs and Archie formulae have been the subject of many publications. The impact of diagenesis and rock wettability variations on Archie's parameters is difficult to quantify throughout the reservoir.
On a different scale, well geometries and reservoir morphologies can make resistivity measurements for fluid evaluation misleading. Moreover, an ultra-mature environment with secondary recovery development schemes, varying oil properties, heavy oil deposits, and a complex oil-filling history make the evaluation of producible volumes a challenging task.
More recently, 3D nuclear magnetic resonance (NMR) acquisition has been shown to provide a resistivity- and salinity-independent estimate of hydrocarbon saturation and hydrocarbon properties in carbonate reservoirs. The method, however, suffers from shallow depth-of-investigation (DOI) and is easily affected by reservoir alteration caused by the invading mud filtrate.
In this paper, a comprehensive welldata-acquisition strategy is described, based on downhole pumping and NMR measurements. This method aims to quantify fluids-in-place more accurately and, therefore, to evaluate the remaining potential in an ultra-mature carbonate environment with varying oil properties and bypassed oil pockets linked to sub-seismic events.
