Improvement Methods On Interface Bonding Of Hardened Cement Paste Of Oil Well

Oil well cement is a kind of special cements used for lining boreholes, which is predominantly used for the extraction of crude oil or natural gas. In the process of cementing wells, most of the problems in the boreholes are interface deterioration due to annulus breakthrough flow through the weak interface between steel casing and hardened cement paste. Therefore, the interface bonding is very important to the drilling engineering. As for the bonding properties, it has come to certain conclusions by the predecessors, but not on how to improve the bonding properties overall. In response to this situation, starting with a chosen lower water cement ratio, this paper choose several mineral mixture and two kinds of expanded admixture, to analyze the properties of fresh paste, mechanical properties and the bonding properties. By use of self-made test equipment for bonding properties, the effects of all above-mentioned admixtures on the interface bonding properties were discussed in this paper.The influence of mineral admixture on the fluidness of the cement paste mainly attributed to the nature of particles. That smaller particles absorbed large amount of water is the main reason to decrease the fluidness of cement paste. The order for admixtures to decrease the fluidness was: Bentonite> metakaolin> silica fume> slag> fly ash, and the expended agent decreased the fluidness in the order: MgO expansive> sulphoaluminate expanded agent.Mechanical properties increased with water-binder ratio decreasing, which the mineral admixture affected the strength of hardened cement paste in the order: silica fume> metakaolin> slag> fly ash> bentonite. The strength increased as the reaction activity of the mineral admixture increased.Metakaolin and bentonite can cause micro-expansion (0.01%, 0.02%), while silica fume, slag, fly ash don't make significant change because of the test conditions (RH100%). Sulphoaluminate expanded admixture(0.15%) led to apparent expansion of hardened cement paste, which mainly happened during the pre-hydration, MgO have a certain expansion effect (0.02%), but with a slow development, and it works mainly in the latter part of the hydration process. The temperature was an important factor.By using of the self-made bond strength test device, this paper examined the bond strength of cements with various proportions, combined with microscopic analysis of the interface and the cement shrinkage test, the degree and mechanism of the effect on how to improve performance cementation was analyzed, and it come to a conclusion: it will improve performance of the first bond interface with the increase of the water-binder ratio, the bonding capacity of second interface reduced. Improvement order on the performance of the cement admixture to improve the capacity: fly ash (200%)> metakaolin (175%)> silica fume (160%)> slag (140%)> standard cement > bentonite (<50%). The expansion producing micro-expansion will increase the capacity of the interface bond. Two expansive agent performance improvement order: sulphoaluminate expanders> MgO. At the same time, two interface models were come up.To conclude, the order of who enforce the bond strength most is sulphoaluminate expanders> fly ash> metakaolin> MgO> silica fume> slag> bentonite.