NMR INVERSION METHODS FOR FLUID TYPING
Publication Date: 2002-Oct-08
The IP.com Prior Art Database
Recent developments of NMR logging technology allow us to acquire CPMG echo trains with multiple wait-times and multiple echo spacings. Since the T1 relaxation time and diffusion coefficients of different fluids are different, the resultant apparent T2 distribution at different polarization times and echo spacings varies with fluid type in porous media. We devise two new NMR inversion methods that allow us to extract fluid saturations by analyzing the variation of T2 distribution as a function of echo spacing and wait-time. The first method, tentatively named as FET (Fluid typing by Editing T2 distributions), connects the T2 distribution of each fluid with the overall T2 distribution using a shift matrix. Each fluid's saturation and T2 distribution are extracted by minimizing the difference between the model T2 distributions and measured T2 distributions using the Singular Value Decomposition (SVD) algorithm. The second method, called GIFT (Global Inversion for Fluid Typing), relates a model T2 distribution of each fluid with CPMG echo trains using a global evolution matrix that governs the evolution of magnetization under T1, T2 relaxation, and diffusion. Each fluid's saturation and T2 distribution are extracted by minimizing the difference between the model echo trains and the measured CPMG echo trains. Based on these new methods, we further introduce a weight matrix derived from resistivity and nuclear logs for each fluid to improve the accuracy of the inversion using FET and GIFT. Integrating NMR, resistivity, and nuclear log analysis into one model using the weight matrix gives a novel solution to the formation evaluation of the reservoirs.