Diffusion-editing: New NMR measurement of saturation and pore geometry

Nuclear magnetic resonance (NMR) diffusion measurements have become an important tool for fluid characterization in the near-wellbore region. Presently, all commercial NMR tools use a single type of pulse sequence, the Carr-Purcell-Meiboom-Gill (CPMG) sequence. Diffusion effects are detected by comparing multiple measurements with different echo spacings. High diffusion leads to a faster signal decay when the echo spacing is increased. In practice, it can be challenging to invert the measurements in the presence of wide, overlapping T₂ distributions of the different phases and uniquely separate their contributions without making additional assumptions. In addition, increasing the echo spacing reduces data density, leading to degradation of the signal-to-noise ratio. This paper presents a new NMR technique, called diffusion-editing, which overcomes these limitations. In this technique, the standard short-echo-spacing CPMG sequence is preceded by a brief editing sequence that varies the sensitivity to diffusion. A simple example of such an editing sequence consists of three pulses with significantly longer spacing than that of the following refocusing pulses. This editing sequence reduces, or edits, the amplitude of the signal according to diffusion during the initial time. The remaining sequence detects the amplitude and T₂ distribution of this diffusion-edited signal. In this way the effects of diffusion and relaxation are effectively orthogonalized. By comparing a series of diffusion-edited signals with varying editing parameters to the standard CPMG signal, diffusion and relaxation effects can be directly and accurately separated. A diffusion-relaxation correlation map can be generated, which is fundamentally new information. By studying specific aspects of the diffusion-relaxation map we can easily extract relevant reservoir parameters such as water saturation, oil viscosity, wettability state and hydrocarbon-corrected bound-fluid volume. By increasing the diffusion time, these sequences can also be used to probe pore geometry by detecting the effects of restricted diffusion. This new technique has been tested both with laboratory and downhole measurements. We demonstrate that even in cases with a strong overlap in the T₂ distributions of the oil and water phases, diffusion-editing results in a clear separation of the two signals. Quantitative results from the downhole measurements are in excellent agreement with surface laboratory analysis and other downhole information.