Barite settling in the annulus behind the casing is an undesired yet common occurrence during the life of a well. Over time, the drilling fluid left in the annulus settles, leaving behind solidified barite that can hinder slot recovery and plugging and abandonment (P&A) by impeding the cut and pull process.
During P&A, Equinor acquired settled barite samples from a North Sea well where the casing was held back by these weighting agents, introducing overpull and prolonging the cut and pull operation. A laboratory analysis program that included determination of the particle-size distribution, electrokinetic potential of particles (zeta potential), crystallography [X-ray diffraction (XRD)], chemical composition [X-ray fluorescence (XRF)], thermogravimetric analysis, and microstructure evaluation [scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and quantitative evaluation of materials by scanning electron microscopy (QEMSCAN)] was conducted on these settled barite samples to understand their properties.
The results of this study provided valuable insights into the composition and characteristics of the settled material in the annulus. The solidified barite within the annular space exhibited no signs of chemical reactions. XRD analysis confirmed the barite to be the sole predominant solid in the solidified material, aligning with expectations. However, additional analytical techniques, including XRF, QEMSCAN, and EDS, detected interconnections between barite particles, primarily facilitated by iron or quartz particles. Additionally, trace amounts of calcite, iron oxide, and mixed compositions involving silicon, sulfur, iron, barium, oxygen, and chlorine were identified within the solidified material.
SEM results indicated that the particles exhibited strong compaction characteristics but lacked cementation, retaining some porosity. Notably, the absence of bentonite or other clays was consistently observed in all analyses.
This study highlights the process of solidification observed in settled barite, suggesting that factors other than chemical reactions may be responsible for this phenomenon. The potential mechanisms contributing to solidification include physical aggregation, compaction, and alterations in surface charge under downhole conditions.
This enhanced understanding of the solidification process will contribute to the development of solutions for efficient casing removal and even the usage of settled barite as a barrier material.
This abstract is taken from paper SPE 218460 by H. B. Yousuf, M. Khalifeh, and A. Saasen, University of Stavanger; R. Godøy, H. C. Karlsen, M. Naumann, Z. Ibragimova, and C. Kruber, Equinor; and R. V. Noort, Institute of Energy Technology. The paper has been peer reviewed and is available as Open Access in SPE Journal on OnePetro.