*Article* **Study on Nuclear Magnetic Resonance Logging** *T***<sup>2</sup> Spectrum Shape Correction of Sandstone Reservoirs in Oil-Based Mud Wells**

**Jianmeng Sun 1,2, Jun Cai 3, Ping Feng 2,\*, Fujing Sun 2, Jun Li 1, Jing Lu <sup>1</sup> and Weichao Yan <sup>2</sup>**


**Abstract:** 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*<sup>2</sup> 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*<sup>2</sup> 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*<sup>2</sup> spectrum was proposed in three main steps. First, the differences in the morphology of the NMR logging *T*<sup>2</sup> 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*<sup>2</sup> spectrum was extracted using the multidimensional matrix method to establish the *T*<sup>2</sup> spectrum of water-based mud wells and oil-based mud wells. Finally, the correctness of the method was verified by two *T*<sup>2</sup> spectrum correction examples of oil-based mud wells in the study area. The results show that the corrected NMR *T*<sup>2</sup> spectrum eliminates the influence of oil-based mud filtrate and improves the accuracy of NMR logging for calculating permeability.

**Keywords:** NMR; oil-based mud; invasion correction; permeability
