Deep characterization of heavy-weighted oil well cement segregation by a novel nuclear magnetic resonance (NMR) technique

Oil well cement segregation is one of the catastrophic issues in drilling operations. It changes the properties of hardened cement, such as reducing its strength and increasing its permeability. This can lead to severe consequences, such as cement failure. Therefore, the early identification of cement segregation is crucial to provide some solutions to minimize it. However, conventional techniques that evaluated cement segregation need enhancing. In this work, four heavy-weight cement samples were prepared with a high density of 18 ppg using the four common weighting materials: barite, ilmenite, hematite and Micromax. To identify the potential of cement segregation, a new proposed approach by nuclear magnetic resonance (NMR) was utilized to characterize the cement segregation by measuring the porosity differences and heterogeneity in the hardened cement samples. Moreover, the NMR results were compared with the existing techniques, such as the direct weight and volume method and computer tomography (CT) scan. The results of the conventional direct weight and volume method and CT scan technique showed critical cement segregation of barite, ilmenite and hematite-based cement samples, which can result in several problems related to zone isolation. By applying the evaluation method involving NMR, the NMR approach confirmed the results of the conventional direct weight and volume method and CT scan technique in which severe cement segregation was found in the barite, ilmenite and hematite-based cement samples. To validate the ability of NMR to identify the cement segregation issue, the Micromax cement samples were also examined. The NMR method confirmed the results of the direct weight and volume method and CT scan technique, showing that Micromax had no segregation issue. NMR results showed that the porosity distribution of Micromax weighted cement was mainly the same in which there was no deviation window between its top, middle and bottom sections, while there was obvious variation in the porosity distribution of the barite, ilmenite, and hematite-based cement samples with large deviation window along the cement column. The NMR method allowed a rigorous investigation of the cement segregation in the hardened cement samples. The NMR successfully identified segregation by looking for the pore size distribution and its relation with porosity variation. The novelty of this work is applying a new approach to determine the heavy-weight cement segregation by NMR. Findings from this work have led to an effective method of determining cement segregation for basic laboratory tests of oil well cement.