Effect of the Solidifiable Spacer Fluid System on Hydraulic Sealing Ability of the Cement Sheath-Formation Interface

Drilling fluid retained on the wellbore wall is prone to forming seepage channels at the interface, rendering the cement sheath-formation interface the weakest point in the sealing integrity of the cement sheath. The solidifiable spacer fluid system due to its excellent compatibility with drilling fluid and integrated curing capacity has been demonstrated to enhance the shear bond strength at the cement sheath-formation interface. However, relying solely on shear bond strength tests is inadequate to characterize the isolation performance at the interface, so this study further adopts flushing efficiency tests and hydraulic sealing tests to directly evaluate the effect of the solidifiable spacer fluid on the hydraulic sealing ability of the cement sheath-formation interface. The results show that the flushing efficiency of the solidifiable spacer fluid for the filter cake on the rock core surface is significantly superior to that of conventional oilfield spacer fluids, and the solidifiable spacer fluid remarkably improves the maximum sealing pressure at the cement sheath-formation interface, with performance further enhanced as curing time extends. The enhancement mechanism is further elucidated by X-ray computed tomography, which provides microscale evidence of a consolidated interface with reduced porosity after the solidifiable spacer fluid. Leveraging its characteristically strong alkaline environment, the alkali-activated solidifiable spacer fluid facilitates rapid amide group hydrolysis, producing carboxylate groups (-COONa) that enhance water solubility. This leads to a 12.9% improvement in filter cake decomposition and removal efficiency compared to conventional spacers. The resulting denser filter cake, together with the fluid's ability to cosolidify with drilling fluid, substantially improves the interfacial sealing capacity. Specifically, the maximum sealing pressure gradient increased from 0.48 to 20.36 MPa/m after just 3 days of curing. This study lays a foundation for the field application of solidifiable spacer fluid and facilitates the improvement of cement sheath integrity.