The presence of gas in hydrocarbon (HC) reservoirs has a distinct effect on commonly used porosity logging measurements such as density, neutron, and nuclear magnetic resonance (NMR). Although the effects of gas on NMR measurements are well understood, it remains difficult to estimate true formation porosity and hydrocarbon saturation exclusively from NMR data in gas-bearing formations. The hydrogen index (HI) of gas is much smaller than one and causes an underestimation of the measured NMR porosity. Unless temperature, pressure, hydrocarbon composition and saturation are known accurately, the gas effect is not easily compensated for. This paper introduces more robust approaches, integrating NMR logging data with data from downhole and surface logging, and connects the results to a full petrophysical reservoir description.
Data and results from two wells drilled in a North Sea clastic reservoir are presented. The reservoir is dominated by thick units of sandstone, deposited in a sub-marine turbidite fan, with high porosity and high permeability. The wells were logged with conventional triple-combo (i.e., gamma ray, density, neutron, and resistivity), NMR and formation tester tools, while drilling across gas, oil, and water intervals. Basic mud gas data are available from surface logging.
The stand-alone evaluation of NMR data by a T2 cutoff approach and by dual wait-time (DTW) data processing yields results with zones of under- and overcorrection of the gas effect. Combining NMR DTW data with density improves the results and reduces uncertainties in porosity and saturation significantly. Compositional information from the mud gas data validates an inferred trend of fluid property variation in gas and oil zones across the reservoir and is used to estimate a continuous HI log to further improve porosity and gas saturation estimation. Complementary to the results of the NMR and triple-combo data evaluation, we show results from mud gas data evaluation using a recently implemented method that provides independent porosity, permeability, and saturation indexes. Adverse conditions for the different approaches, including invasion, variations in mineralogy, and the limited vertical resolution of mud gas data, are discussed. Finally, the benefits of combining NMR, conventional and mud gas data are summarized.