Applications of fast NMR logging in shaly gas-bearing sands invaded by oil filtrate

摘要

Recent papers have described the use of NMR data acquired at slow speed and long wait time to enhance the accuracy of formation evaluation in shaly gas-bearing sands invaded by oil-based mud filtrate. Until now, the physics of the measurement have limited the use of NMR data obtained with fast passes and short wait time to determining bound fluid volume, and thus irreducible water saturation, when combined with other log data, such as density and neutron logs. However, recent NMR logs acquired in South-East Asia suggest that, against expectation and common wisdom, it may actually be possible to obtain a reliable porosity with such fast passes even in the very adverse conditions of oil filtrate and gas-filled pores.rnThis paper first presents an example of NMR data used in the computation of irreducible water saturation, with comparison to core results and traditional resistivity interpretation. Such comparison should greatly enhance the confidence of the oil companies in the accuracy of booked reserves.rnSecond, a method is described to effectively correct for the incomplete polarization of the gas phase and the oil filtrate phase in fast passes, equivalent to standard logging speed, with short wait time. It is shown that a reliable total porosity can be derived from the combination of NMR and density data. That porosity can, in turn, be used in the computation of the permeability from the Timur-Coates equation.