A South
B Antenna
C
D
Echo T2 decay T1 buildup
TE
North WT
> Basic NMR theory. Hydrogen nuclei behave like tiny bar magnets and tend to align with the magnetic field of permanent magnets, such as those in an NMR logging tool (A). During a set wait time (WT), the nuclei polarize at an exponential buildup rate, T1, comprising multiple components (C). Next, a train of RF pulses manipulates the spins of the hydrogen nuclei causing them to tip 90° and then precess about the permanent magnetic field. The formation fluids generate RF echoes between successive pulses, which are received and measured by the antenna of the NMR tool (B). The time between pulses is the echo spacing (TE) (D). The amplitudes of the echoes decay at a superposition of exponential relaxation times, T2, which are a function of the pore-size distribution, fluid properties, formation mineralogy and molecular diffusion (E). An inversion technique converts the decay curve into a distribution of T2 measurements (F). In general, for brine-filled rocks, the distribution is related to the pore sizes in the rocks (G).
CPMG sequence
E
Total porosity Small pores
Large pores
The echoes occur between RF bursts. The time between bursts is the echo spacing, TE. The amplitude of the echoes is proportional to the net magnetization in the plane transverse to the static field created by the permanent magnets. The amplitude of the initial echo is directly related to the formation porosity. The strength of the subsequent echoes decreases exponentially during the measurement cycle. The exponential decay rate, represented by the relaxation rate, T2, is primarily a function of pore size, but also depends on the properties of the fluid in the reservoir, the presence of paramagnetic minerals in the rock and the diffusion effects of the fluids. In typical cases, the decay of the echo amplitudes is governed by a distribution of T2 times, similar to the T1 times found in the buildup curve. An inversion technique fits the decay curve with discrete exponential solutions. These solutions are converted to a continuous distribution of
A key feature of this sequence is alternating the polarity of the received signal to eliminate electronics-related artifacts. During the CPMG measurement cycle, the hydrogen nuclei in the formation generate detectable RF echoes at the same frequency used to manipulate them.3
After a given WT, a train of electromagnetic RF pulses manipulates the magnetic moments of the hydrogen nuclei and tips their direction away from that of the B0 field. The process of sending long trains of RF pulses is referred to as a CPMG sequence.2
relaxation times representative of the fluid-filled pores in the reservoir rock (above).4 When the fluid in the sensed region is brine,
the T2 distribution is generally bimodal, particularly in sandstones. Small pores and bound fluid have short T2 times, and free fluids in larger pores have longer relaxation times. The dividing line between bound and free fluid is referred to as the T2 cutoff. Oil and gas in the pore spaces introduce a few complications into the model.
The three primary mechanisms that influence T2 relaxation times are grain surface relaxation, relaxation by bulk-fluid processes and relaxation from molecular diffusion.5
Grain
surface relaxation is a function of pore-size distribution. Relaxation effects from molecular diffusion and bulk-fluid properties are directly related to the type of fluid in the pores. Tar has an extremely short relaxation time and may not be measurable with downhole NMR tools. Heavy oils have short relaxation times, similar to those of clay- and capillary-bound fluids, but may also be too short for NMR acquisition (next page). Lighter oils have longer T2 times, similar to those associated with free fluids. Gas has an even longer relaxation time than oil. During the measurement process, oil and gas signals are detected along with signals from movable and irreducible water. While the T2 times from the oil and gas signals may have no relationship to the producibility of the
G
Clay- bound water
Capillary- bound water
Free water
F
Time
Inversion
Time
T2
6
Oilfield Review
Amplitude
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Amplitude
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