Study on Nuclear Magnetic Resonance Logging T(2) Spectrum Shape Correction of Sandstone Reservoirs in Oil-Based Mud Wells

The oil-based mud filtrate will invade the formation under the overbalanced pressure during drilling operations. As a result, alterations will occur to the nuclear magnetic resonance (NMR) response characteristics of the original formation, causing the relaxation time of the NMR T(2) spectrum of the free fluid part to move towards a slower relaxation time. Consequently, the subsequent interpretation and petrophysical evaluation will be heavily impacted. Therefore, the actual measured T(2) spectrum needs to be corrected for invasion. For this reason, considering the low-porosity and low-permeability of sandstone gas formations in the East China Sea as the research object, a new method to correct the incorrect shape of the NMR logging T(2) spectrum was proposed in three main steps. First, the differences in the morphology of the NMR logging T(2) spectrum between oil-based mud wells and water-based mud wells in adjacent wells were analyzed based on the NMR relaxation mechanism. Second, rocks were divided into four categories according to the pore structure, and the NMR logging T(2) spectrum was extracted using the multidimensional matrix method to establish the T(2) spectrum of water-based mud wells and oil-based mud wells. Finally, the correctness of the method was verified by two T(2) spectrum correction examples of oil-based mud wells in the study area. The results show that the corrected NMR T(2) spectrum eliminates the influence of oil-based mud filtrate and improves the accuracy of NMR logging for calculating permeability.