We present a two-dimensional inversion method to obtain images of the ratio T1/T2apparent vs. T2apparent from NMR logs acquired with multiple wait times (TW. The technique is particularly useful and robust for detecting gas and retrograde condensates. With the inverted data we also quantify the gas and or condensate saturations.
The technique works best in situations where a large diffusion contrast and consequently a large T1/T2apparent contrast exists thus it is suitable for discerning gas from liquids (water and oil. Instead of inverting one dimensional T1 and T2 separately, or jointly two-dimensional T1 and T2, directly inverting T1/T2apparent vs. T2apparent has definite advantages. Firstly, the high T1/T2apparent contrasts of gas vs. liquid provides a distinctive signature on a T1/T2apparent vs. T2apparent image facilitating robust interpretation. Secondly, by selecting the frequencies (and, thus, the magnetic field gradients and/or interecho time, TE, we can keep the gas T2apparent in a narrow range, e.g., 50-150ms. Therefore, the gas signal location on the T1/T2apparent and T2apparent image is always narrowly-defined which makes interpretation substantially simpler. Thirdly, physical constraints, such as T1/T2apparent can be applied much easily thereby reduce some noise-induced uncertainties. Furthermore, it is often difficult to construct the T1/T2apparent ratio from T1 and T2 based on predetermined times (i.e., bins as the inversion artifacts and noise effects make bin-by-bin computations nearly impossible. Thus, bin-to-bin ratios work only for gas wells where significant amount of gas signals is apparent, but the new processing works even when gas-saturation are relatively small. In addition the new inversion processing works well even when the echo data are relatively noisy (e.g, salt-saturated mud well. From the recovered T1/T2apparent we can reconstruct T1 spectra from the corresponding T2apparent spectra.
Examples of OBM- and WBM-drilled gas wells demonstrated the advantage of using this technique over other 1D or 2D inversion techniques for gas well evaluation. In one clean-sand example, the gas intervals are identified by both this new technique and the conventional cross-over neutron-density porosities. In another complex lithology example, the neutron-density crossover is not obvious. Yet we are able to identify the gas using the T1/T2apparent method. Furthermore, by setting the threshold T1 and T1/T2apparent values, we are able to estimate the flushed zone gas saturation and the hydrogen index corrected porosity.
