Study on the Method of Identifying Low Resistivity Pay by LWD NMR Water Spectrum Reconstruction Based on Permeability Model in the Guantao Formation, Bohai Basin

Low resistivity pay (LRP) has become a valuable resource globally. Estimated reserve of more than 100 million cubic meters of explored hydrocarbon is stored in LRP zones in the N1g formation in Bohai Bay. It is important to identify and evaluate them in order to improve reservoir productivity. In the N1g layer, production and well testing data have shown the resistivity of the pay zone is normally lower than the adjacent water zones. As a result, identification of these LRP zones can be quite challenging, particularly in wells where the spontaneous potential and micro-resistivity logs are not available. The purpose of this paper is to document the best practice of evaluating LRP using only LWD data. The key step in our workflow is to perform fluid substitution on measured NMR data, such that the resulting T2 distribution removes all the hydrocarbon signal and replaces it with water signal. This reconstructed T2 represents the pore size distribution of the rock. When the rocks contain hydrocarbon, the measured T2 distribution will typically exhibit a longer T2 component. Therefore, by comparing the measured T2 against the 100% water T2, we can identify these LRP intervals. We used core data to determine the T2 cutoff and pick the T2 log mean (T2LM) of pure water signal. Main methodology is as follows: First step, We used irreducible water saturation (Swirr) from capillary pressure data to determine the T2 cutoff. In part two, We used core and fluid samples in N1g layer to estimate the T2LM of brine in the hydrocarbon layers in a studied field. Part three, We then used the T2 cutoff from step1 to get a continuous Swirr from LWD NMR logging data and reconstruct a T2 spectrum through Gaussian distribution centered at T2LM. And finally, We study the difference between reconstructed T2LM of pure water and the logging data to identify LRP precisely. The proposed method addressed the challenge of differentiating LRP from water zones. By using this method, the new discovered reserve in N1g layer reached 12 million tons of oil. More than 95% of the evaluation results of N1g reservoir agreed with production data. So far, operator has drilled 44 high production wells, cumulative oil production reached 0.75 million cubic meters, which facilitated the successful deployment of oil development plan.