Transverse Relaxation

Key Takeaways

• CPMG pulse sequence is the foundational measurement protocol for transverse relaxation analysis — the sequence applies a 90-degree initial pulse that creates the transverse magnetization, followed by a series of 180-degree refocusing pulses at regular intervals (typical echo spacing of 0.2-1.0 ms) that reverse the dephasing caused by static field inhomogeneities; the resulting echo train decays exponentially with the actual T2 of the formation rather than the much-shorter T2* of the uncorrected measurement; the CPMG protocol developed by Carr and Purcell (1954) and refined by Meiboom and Gill (1958) became the standard NMR measurement after its application to oilfield logging in the 1990s; modern NMR logging tools include sophisticated CPMG sequences with appropriate echo spacings and acquisition timing for various applications.
• Surface relaxation dominates T2 in most porous rocks with rate proportional to surface area to volume ratio — the surface relaxation rate 1/T2_surface = rho_2 × (S/V), where rho_2 is the surface relaxivity (a rock-specific parameter typically 1-50 micrometers/second for typical sandstones and carbonates) and (S/V) is the surface area to volume ratio of the pore space; smaller pores (high S/V ratio) have shorter T2 due to faster surface relaxation, while larger pores (low S/V ratio) have longer T2; the relationship between T2 and pore size supports the pore size distribution analysis from T2 distribution; for typical sandstones, T2 of 1-3 ms corresponds to clay-bound water in very small clay pores, T2 of 3-30 ms corresponds to capillary-bound water in small to intermediate pores, T2 of 30+ ms corresponds to free fluid in larger pores; the calibration of T2 vs pore size is rock-type specific and supported by laboratory measurements.
• Bulk and diffusion relaxation become important in specific applications — bulk relaxation contributes to T2 for fluids in very large pores or unconfined fluids, with bulk T2 of typical reservoir fluids being approximately 2-3 seconds for water at room temperature, 0.3-3 seconds for typical oils depending on viscosity, and shorter for gases; diffusion relaxation becomes significant in applications where the magnetic field gradient is substantial (typical for some specialty NMR tools or for applications targeting low-T2 detection), with the diffusion-related relaxation depending on the fluid's diffusion coefficient and the gradient strength; modern NMR interpretation accounts for these multiple relaxation mechanisms through systematic analysis of the T2 distribution combined with appropriate models.
• T2 cutoff for free fluid index (FFI) and bulk volume of irreducible water (BVI) calculation provides operational application of transverse relaxation analysis — the T2 cutoff is the threshold value that separates short-T2 (bound water, BVI) from long-T2 (free fluid, FFI) signal; for typical sandstones the cutoff is 33 ms; for carbonates the cutoff is typically 50-100 ms (reflecting the larger pores that store irreducible water in carbonates); the cutoff calibration is performed through laboratory measurements relating T2 to capillary pressure, with the resulting cutoff supporting the porosity decomposition into productive (FFI) and non-productive (BVI) components; modern petrophysical interpretation routinely applies T2 cutoffs to NMR data, providing the irreducible water saturation analysis that drives reservoir characterization.
• Multi-frequency and multi-echo NMR applications use T2 measurements at multiple acquisition parameters to support advanced analysis — for hydrocarbon typing, multi-T2 (with different echo spacings) measurements provide diffusion-based fluid discrimination because oil, water, and gas have different diffusion coefficients that affect their respective T2 relaxation differently; for pore size analysis in complex pore systems, multi-frequency NMR (using different operating frequencies that probe different pore size ranges) supports detailed pore structure characterization; modern NMR logging tools include flexible acquisition capability that supports these advanced applications, with the resulting integrated NMR analysis providing the comprehensive petrophysical characterization that conventional logging cannot provide.