Mechanism for the promotional effect of a novel solidifiable spacer fluid system on the cementation quality of cement sheath/formation interface

Abstract

A cement-formation rock interface is the weakest part of an oil and gas well where the channeling of formation oil, gas, and water is most likely to occur. In particular, the channeling problem is most prominent at the interface between the cement sheath and the formation. Owing to the existence of a layer of thick virtual filter cake on the wall of an oil and gas well, the cementation quality of the cement sheath/formation interface is generally unsatisfactory, which increases the risk of well integrity failure. A solidifiable spacer fluid system was proposed in this paper to improve the cementation quality of the cement sheath/formation interface. The macroscopic mechanism governing the promotional effect of the proposed system on the shear strength of the cement sheath/formation interface was analyzed through shear strength experiments and compatibility experiments. Moreover, scanning electron microscopy, X-ray diffraction, and computed tomography tests were conducted to provide an in-depth characterization of the interfacial morphologies, products, and pores. The results showed that a solidifiable spacer fluid had good compatibility with the drilling fluid and cement slurry, which ensured that the interfacial zone could be solidified as a whole. Compared with specimens prepared without a spacer fluid and using a field spacer fluid, specimens that were prepared using a solidifiable spacer fluid and allowed to cure at 70°C for 2 days showed an increase in shear strength by 388% and 110%, but a decrease in interfacial void fractions by 248% and 46%, respectively. The C-S-H gel (hydrated calcium silicate gel) formed by the solidification of the solidifiable spacer fluid system could be filled between the particles of clay minerals and the weighting materials to prevent the formation and extension of cracks in the interfacial zone effectively, thereby improving the cementation quality of the cement sheath/formation interface.