Experimental study on the zonal isolation performance of cement sheaths with SBT logging

The integrity of the cement sheath's zonal isolation in oil and gas wells is a crucial safeguard for safe production and operations in oilfields. However, the zonal isolation integrity of the cement sheath depends on various factors. Currently, there are no established evaluation methods and workflows for quantitatively assessing the interlayer isolation performance of the cement sheath. We developed a full-scale cased-hole testing device for simulating downhole conditions and measuring the isolation performance of cement sheaths. Using this device, we quantitatively monitored the hydraulic sealing pressure differentials of conventional Class G cement versus lightweight cement. Additionally, a Segmented Bond Tool was installed on the inner wall of the casing to monitor the variation trend of the casing-wave amplitude and attenuation during the cement setting and isolation performance testing processes. The experimental results indicate that as the cement gradually sets, its strength increases and the attenuation progressively rises. After the cement strength stabilizes, pressure is applied through the injection port at the bottom of the first interface of the cement sheath. Once the pressure exceeds the hydraulic bonding strength at the interface between the casing and the cement sheath, debonding gradually occurs. The isolation pressure differential of conventional cement is greater than that of lightweight cement. Changes in casing-wave amplitude across various sectors demonstrate that the debonding fracture initiates from the injection port, through the entire circumference of the cement sheath, and then propagates upward along the axial direction. If the cemented sheath has a poorly bonded sector, channeling will initiate from this weakly bonded section and subsequently propagate circumferentially throughout the entire annulus. The experimentally measured hydraulic bonding strength or the isolation pressure differential of the cement sheath can provide reference data for on-site testing or the design of safe production pressure differential schemes.