THE IMPACT OF T_1/T_2 RATIO ON POROSITY ESTIMATION

摘要

NMR logging data are often acquired by a long CPMGsequence followed by a set of short bursts. These shortbursts have a wait times of the order of 20 ms and theyare primarily used to improve the porosity accuracy ofshort T_2 components. A joint inversion method is thenapplied to derive an accurate NMR total porosity. Thisjoint inversion technique also allows us to optimize theT_1/T_2 ratio. Because the difference between the waittime of the regular CPMG echo train (1～10 seconds)and that of the short burst echo trains is quite large, ournumerical simulations show that the derived T_1/T_2 ratiodepends strongly on noise profiles and consequentlycan have a huge impact on NMR porosity, especiallyfor formations such as carbonates with long T_2relaxation times. We also found that for formationswith large paramagnetic impurities, the range foroptimizing the T_1/T_2 ratio should be increased to 1 to 20from the 1 to 3 typically used. To investigate thisproblem further, we performed NMR coremeasurements using T_1-T_2 2D NMR and found thatT_1/T_2 ratio strongly depends on T_2 relaxation time. Wehave incorporated this functional dependence of T_1/T_2ratio into our inversion code and greatly improved theaccuracy of NMR total porosity in both carbonateformations and formations with large amount ofparamagnetic impurities.