The present disclosure relates generally to a downhole nuclear magnetic resonance (NMR) apparatus, data processing, and interpretation methods for evaluating a characteristic of a region, and particularly for detecting and quantifying a gas-bearing earth formation in a subterranean region.
NMR well logging is a technique used to investigate subterranean regions that may contain water, oil and/or gas reserves. The nuclei of chemical elements have a characteristic angular momentum (spin) and a magnetic moment, and by detecting and analyzing the reaction of the nuclei to applied magnetic fields, the characteristics of specific nuclei may be deduced. In the presence of an externally applied static magnetic field (B0), the nuclei spins become magnetized and align themselves parallel to the B0 field. By applying a radio frequency (RF) pulse train of a specific frequency to the magnetized nuclei, a pulsed RF magnetic field (B1) is generated that tips, or flips, the spins away from the direction of the B0 field. If the RF frequency (ω) substantially matches the condition for NMR (ω=γB0), where γ is the gyromagnetic ratio, then the first pulse (herein referred to as A pulse) reorients the magnetization to start precession and subsequent pulses (herein referred to as B pulses) generate spin-echo signals. A RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence is typically used for well logging.
At the end of an A pulse, the spins are oriented transverse to the B0 field and precess around the direction of the B0 field at the Larmor frequency (ω0=γB0), and the transverse magnetization dephases with a transverse relaxation time constant (T2), also known as the spin-spin relaxation time. Repeated tipping and relaxation of the spins results in the NMR spin-echo signal, which may then be detected and analyzed for oil and/or gas field exploration.
Existing methods use dual wait-time logs and the T1 contrast between gas and other formation fluids for gas detection and for gas saturation estimation. One such method is based on the assumption that water signals are fully polarized at both short and long wait time, TW, but gas signal is only partially polarized. So the difference between the two is contributed from gas only. However, if three phases coexist in the NMR detected sensitive volume, especially if slow relaxing water signal and light oil or oil-based mud filtrates are present in the formation, detection may be difficult or limited. Other methods for acquiring and processing multiple wait time data for T1 estimation employ improved log quality data using a summation of echoes approach, which is more useful if all of the partially polarized signal is gas. However, the certainty of the signal is reduced for discerning gas using this technique for wells drilled with OBM (oil based mud), and the subsequent invasion of the OBMF (oil based mud filtrate) into the sensitive volume, or for formations containing large-pore water or light oil. Thus skilled human interpretation is required in order to use existing art techniques. Accordingly, there is a need in the art for a robust NMR detection and analysis method that overcomes these drawbacks.