Enhanced NMR logging methods for accurately measuring volumes of gas and light oil in hydrocarbon reservoirs

摘要

Using nuclear magnetic resonance (NMR) logging to determline reservoir porosity occupied by gas or light oil currently requires data simultaneously acquired from at least two Carr-Purcell-Mekboom-Gill (CPMG) sequences having different wait-times. This dual-wait-time and dual-frequency method has been successfully applied to determine gas volumes in both clean and shaley sand formations. The success of the method relies on two factors. First, adequate signal-to-noise levels in an echo train difference must be maintained so that the gas-filled plorosity and its transverse relaxation time (T sub 2) can be accurately characterized. Second, methods must be available to reliably estimate the longitudinal relaxation time (T sub 1) of the hydrocarbon phase needed to apply a necessary amplitude correction to the apparent hydrocarbon filled porosity. A new triple-wait-time and multifrequency method has been developed for determining gas- and light-oil-filled porosity over a broader range of reservoir conditions. When compared with the dual-wait-time, dual-frequency logging technique, the new method offers two advantages. First, it improves the signal-to-noise ratio of the echo train difference. Second, it significantly improves the determination of T sub 1 values for the hydrocarbons, especially when the resevoir contains oil, gas, and water. The new method thus enables a more accurate determination of the pore volumes occupied by each hydrocarbon phase as well as the pore volume occupied by water. Results from laboratory experiments, performed on bulk fluids and a sandstone core, are presented to demonstrate the new method. In addition, a set of charts is provided as an aid in the selection of optimum acquisition parameters for logging applications.