Effect of Water-Based-Mud Filter Cake on Zonal Isolation at the Interface Between Cement Sheath and Formation

To enhance the cementing quality in oil and gas wells, and to optimize the cementing process by quantifying the relationship between the drilling fluid filter cake (DFFC) thickness and the zonal isolation at the interface between the cement sheath and formation (also called the second interface (SI)), this study devised a DFFC structural force evaluation device to classify the DFFC structure and developed SI sealing capacity evaluation equipment and method. This study investigated the impact of different structural DFFCs on the SI sealing capacity (shear strength and channeling pressure). The results revealed that the DFFC can be divided into a virtual layer (3.0 to 4.0 mm), an underpressure layer (1.0 to 3.0 mm), and a dense layer (0 to 1.0 mm). The study found that the amount of DFFC residue had a significant effect on the SI shear strength and channeling pressure. When the amount of DFFC residue increased, the SI shear strength and channeling pressure decreased. For instance, when 15% (0.6 mm) of the DFFC residue was present, the shear strength and channeling pressure were 2.5 and 8.53 MPa, respectively, which were 733 and 1,118.6% higher than those with 100% (4.0 mm) DFFC residue (virtual layer), but slightly lower than those without DFFC residue. The DFFC’s dense layer (less than 15% residue (0.6 mm)) had little effect on the SI sealing capacity. Additionally, the curing time had a significant effect on the SI shear strength and channeling pressure. The SI shear strength increased with the extension of curing time and remained stable at 30 days of curing, while the SI channeling pressure first increased and then decreased, peaking at 7 days of curing and approaching that of the SI without DFFC residue. The decrease of channeling pressure in the later stage may be related to the increase of fluid channeling channels due to the volume shrinkage of cement stone. These conclusions can provide a basis for improving the SI sealing capacity during the cementing process.