REVISITING TWO PHASE NMR DATA WITH A NEW INVERSION ROUTINE USING A TRIANGULAR PORES MODEL FOR WETTABILITY ESTIMATION

摘要

NMR measurements are sensitive to the wettability because of the surface relaxivity of the pore surface that enhances the relaxation of the fluids inside the porous media. It has been shown in numerous publications that NMR can detect changes in wettability and many different approaches have been taken for finding the wettability by NMR. The NMR magnetic decay curve is commonly inverted to a T2 distribution. The distribution is then used to estimate wettability changes based on shifts in the distribution or specific T2 values such as the logarithmic mean of the T2 distribution. We have developed an inversion routine that fits the decay curve to a distribution of triangular pores. The triangular pores make it possible to model two immiscible fluids simultaneously present in the same pore. We get pore saturation and wettability as functions of pore size, contact angle and capillary pressure. The corresponding NMR signal is calculated based on a previously published correlation for the magnetic signal of two immiscible phases in triangular pore geometry. The resulting output from the inversion routine is a pore size distribution, saturation, separate oil and water distributions and a wettability index based on fluid-pore surface areas for the oil and water.