Surface modification of calcium sulfate whiskers and its effect on cement-casing bonding in oil wells

The interfacial shear bonding strength between cement sheath and casing is a critical indicator for ensuring long-term wellbore integrity. To improve the bonding performance of the first interface between oil well cement and casing, calcium sulfate whiskers (CSW) were surface-modified using γ-aminopropyltriethoxysilane (KH-550) to fabricate modified calcium sulfate whiskers (MCSW). MCSW was incorporated into the oil well cement system as a reinforcing component. The microstructure evolution of whiskers before and after modification and the regulation mechanism of MCSW on the interfacial bonding characteristics of cement stone were systematically investigated. Characterizations including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) confirm that the silane coupling agent can be successfully grafted onto the whisker surface, which remarkably improves the dispersion and bonding property of CSW in the cement matrix. On this basis, the influence of MCSW dosage on the shear bonding performance of cement stone was investigated. Experimental results show that after 7 days of curing, the interfacial shear bond strength of cement stone incorporating 5% MCSW increases by 35.82 ± 4.57% compared with the blank sample. The underlying mechanism is that MCSW forms tight interfacial bonding with the hydrated cement matrix, which can restrain volume shrinkage of cement stone and optimize pore structure distribution simultaneously. This study provides a feasible technical strategy to strengthen the interfacial shear bonding during cementing operations.