Research On Activation Mechanism Of Mud Cake At Cement-Formation Interface

At present, the fluid channeling at cement-formation interface which plagued petroleum engineering for a long time is a complexity engineering technical problems needed to resolve. The appearance of fluid channeling after cementing is the fact of isolation failure at cement-formation interface, which will result in the isolation failure of stratigraphic trap fluid (formation water and gas) effectively. The phenomenon of moisture content increase greatly and fluid channeling at interlayer will occur during the production, even the watered out and blowout accident, and leading to great economic loss and resources waste. The form of mud cake at cement-formation interface is essential to drilling operation, and it is necessary to the fluid loss, maintaining borehole stability and the protection of oil and gas reservoir. But it goes against to the cementing operation, and hinders the direct contact between sidewall and cement sheath, and difficulty form an integrated solidification and cementation. Then it is vulnerable to form micro cracks with the effect of formation water and fracturing fluid. So the mud cake at cement-formation interface become the weak link of isolation systems between sidewall and cement sheath, it is easily cause the channeling-path that resulting in the isolation failure at cement-formation interface, and leading to the appearance of oil-gas well fluid channeling problem frequently.With the rapid development of economic construction in china, the shortage of oil-gas resources has been a major bottleneck of economic sustainable development. Improving oil gas recovery factor, it is an effect method to solve the isolation failure of cement-formation interface, which has a great economic values and practical significance. Aiming at this cementing problem, a new anti-gas channeling cementing technology of using mud cake solidified agent as cementing ahead fluid is creatively proposed without changing the drilling fluid and cement slurry systems. The core mechanism of this technology is that, through solidified mud cake and interfacial crosslink realize the cement-formation interface integrated solidification. So the research on activation mechanism of mud cake at cement-formation interface is particularly urgent.The study of activation mechanism of mud cake at cement-formation interfac is the emphasis, and lots of relevant experiments have been conducted. Firstly, the paper analyzed the basic physical and chemical performance of mud cake curing agent, researched variation of cementing strength and compared the cementing situation by observing macroscopic morphology at cement-formation interface in conventional technique system and mud-cake-solidification system. Then combined with modern microcosmic analyzing testing technique, such as XRD component analysis, ESEM-EDX morphology observation and TG-DSC testing analysis, the microcosmic proof of cementing strength improvement at cement-information interface have been obtained, which lay a foundation for the study of chemical activation mechanism of mud cake solidification agent. By magic angle spinning nuclear magnetic resonance (MAS-NMR) testing technique, chemical activation mechanism of clay mineral has been discussed deeply. Finally, from the perspective of mineralogy, chemical reaction equations in the process of mud cake activation have been summarized, and the physical and chemical dynamic changing process of making mud cake solidification has been derived,, and revealed variation of the structure, structure and crystal transformation in the curing process. The paper offered important scientific foundation and theoretical basis for the research of integrated solidification and cementing at cement-formation interface, which was used to solve interlayer channeling problem in deep wells.This paper obtained the main conclusions are as follows:1. Through the experiments of cementing strength at cement-formation interface, following results can be concluded. Comparing with conventional technique system, the cementing strength at cement-formation interface remarkably increased by657.10%～2565.00%(3d),421.74%～3067.83%(7d) and288.96%～2581.61%(15d), respectively. The cementing strength at cement-formation interface in the mud-cake-solidification system is greatly improved, which shows the mud cake solidification agent can significantly improve the cementing strength at cement-formation interface.2. Base on the comparison observation on macroscopic morphology, the mud cake at cement-formation interface in the conventional technique system is apart from cement column, which makes the interface loose and porous. Thus, the cement-formation interface cannot solidify and cement integrally. And mud cake in the mud-cake-solidification system have cemented closely with cement column, and the mud cake have been solidified to agglomerated cake, realizing the integrated solidification and cementation of the mud cake, cement column and interlayer.3. Composition analysis and micro morphology analysis show that, in the conventional technique system, no new chemical composition form in the mud cake at cement-formation interface with curing time. However, by the comprehensive analysis of XRD, TG-DSC and ESEM-EDX, in the mud-cake-solidification system, some new kinds of gel can be found both in the mud cake and at the cement-formation interface, such as ettringite, zeolite. C-S-H. These gels cover the particles in the mud cake and also form in the pores and fractures, thus they can connect the panicles together and cementing the mud cake with the cement plaster, leading to a solidiiied mud cake and the integral solidification and cementing of the cement-formation interface. 4. MAS-NMR analysis illuminates that the percentage of four-coordinated Al increases firstly and then decreases with curing time. On the contrary, the percentage of six-coordinated Al decreases firstly and then increases with curing time. Compared to the conventional technique system, the tendency is much obvious in the mud-cake-solidification system. The results indicate that the mud cake has corroded by the slurry filtration and mud cake solidification agent could promote this process. Corrosion reactions stop when solution is depleted with the curing time. Then lots of four-coordinated Al units begin to combine with the silicate units and form new kinds of gels. These gels connect the clay particles and cuttings together, leading to a three-dimension gel structure, thus realizing the integrated solidification and cementation of the mud cake, cement column and interlayer.5. The activation process of mud cake solidification agent to mud cake at cement-formation interface is the depolymerization to clay mineral, its main mechanism is dissolution of clay minerals with alkali excitation, generate the oligomers with chemical reaction activity. These oligomers provide the raw material to the later gelation solidification reaction, organically bond the solid phase particles together, realize the integrated solidification and cementation of mud cake at cement-formation interface. The activation reaction of clay mineral is a complex process of physical and chemical reaction, and the activation reaction mechanism mainly contain5steps: the dissolution depolymerization stage, the products diffusive migration, the accumulation of active monomers, the generation of gelation material and the crystal transformation of gelation material.