Study on the Cementation Mechanism and Interface Enhancement Path of the Second Interface of Oil and Gas Well Cementing

Residual mud cake on the wellbore significantly compromises the cement–formation interfacial cementation quality. However, the research on the weak cementation mechanism of the cementing interface caused by mud cake properties is insufficient. In this paper, laboratory experiments and theoretical analysis were conducted to investigate the influence of mud cake properties on interfacial cementation strength. The results show that the main mechanism of weak cementation of the cement–formation interface caused by mud cake has three aspects: the thickness of mud cake is large, the structure is loose, the strength is low, the bearing capacity is insufficient, and the deformation–compression behavior is small and easily sheared. Based on this, three interfacial strengthening methods, chemical thinning by anhydrous sodium silicate, density enhancement by wollastonite and deformation–compression regulation using sepiolite fibers, were proposed to improve the cementation strength. The addition of anhydrous sodium silicate reduced mud cake thickness by up to 93.5% and increased interfacial cementation strength by 2.43 times. Wollastonite increased mud cake structural stability from 69 to 284 s·mm−1 and improved interfacial cementation strength by up to 2.27 times. Sepiolite fibers increased the deformation–compression coefficient from 1.26 to 1.83, and the maximum interfacial cementation strength was achieved at R ≈ 1.64. In addition, the proposed additives improve the performance of mud cake and have good compatibility with drilling fluid. This study provides a theoretical basis for improving the cementing quality of the cementing interface.