Micro-annulus generation under downhole conditions: Insights from three-dimensional staged finite element analysis of cement hardening and wellbore operations

A micro-annulus(MA)is defined as a high permeability zone or gap initiating/occurring at the casingcement and cement-formation interfaces during the wellbore life span.An MA can significantly compromise wellbore integrity by establishing enhanced fluid flow pathways.This study uses a staged finite element approach to simulate wellbore integrity during various loading steps of wellbore operations under downhole conditions.Particular emphasis is placed on the processes of cement poro-elastic property evolution,volume variation,and pore pressure variation as part of the cement hardening step.The resulting state of stress during the life cycle of a typical injection well(i.e.hardening,completion,and injection)is analyzed to assess the onset and evolution of micro-annuli at various interfaces of the composite wellbore system under downhole conditions.The results show that cement shear failure is observed at the casing-cement interface during pressure testing(excessive wellbore pressure);and tensile debonding failure initiates at the cement-formation interface due to cement shrinkage during hardening and injection-related cooling(thermal cycling).Sensitivity analyses considering several parameters show that:(1)the degree of poro-elastic bulk shrinkage has significant implications for both shear and tensile failure initiation e the less the cement shrinks,the less likely the failure initiation is;(2)cement integrity increases with increasing depth;(3)cement pore pressure evolution has significant implications for tensile failure e if cement pore pressure decreases more,higher temperature differences can be sustained before an MA occurs;and(4)cement temperature fluctuations during hardening promote initiation of debonding failure.In summary,the results presented indicate that establishing downhole conditions to quantitatively analyze MA generation is necessary.The results are different compared to laboratory studies without considering/simulating downhole conditions.The knowledge from this study can raise the awareness of predicting and evaluating MA under downhole conditions and can be used to supplement and improve future laboratory experiments.

Effect of Curing Regime on Degree of Al^(3+) Substituting for Si^(4+) in C-S-H Gels of Hardened Portland Cement Pastes

The effect of curing regime on degree ofAl3＋ substituting for Si^4＋ （Al/Si ratio） in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning （MAS） nuclear magnetic resonance （NMR） with deconvolution technique. The curing regimes included the constant temperature （20, 40, 60 and 80 ℃） and variable temperature （simulated internal temperature of mass concrete with 60 ℃ peak）. The results indicate that constant temperature of 20 ℃ is beneficial to substitution ofAl3＋ for Si4＋, and AI/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 ℃is less than that at 20℃ for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 ℃ curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of AI/Si ratio which is still lower than that at constant temperature regime of 20 ℃ for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length （MCL） of C-S-H gels, furthermore, the amount ofAl3＋ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.

Sulfate Ions Immobilization of Calcined Layered Double Hydroxides in Hardened Cement Paste and Concrete

The sulfate ions immobilization behavior of calcined layered double hydroxides(CLDHs) in the hardened cement paste was investigated. The experimental results show that the sulfate ions in cement paste are immobilized by CLDHs to reconstruct the layered structure and aggregate around CLDHs. The immobilization amount of sulfate ions by CLDHs reaches 4.74×10^-3 mol/g, while the increasing amount indicates non-linear relation with the addition of CLDHs. The incorporation of CLDHs decreases the amount of ettringite formed to limit the expansion of cement paste, which decays the sulfate reaction to enhance the sulfate resistance of concrete.

Effects of Carbonation on Micro Structures of Hardened Cement Paste

In order to investigate the effects of carbonation on the microstructure of cement concrete,the carbonation depth and microstructure of cement paste with 0.3,0.4 and 0.5 water/cement ratio after 7,14,21 and 28 d accelerated carbonation were studied respectively.The results showed that with the increase of waterto-cement ratio and carbonation age,the carbonation depth was deepened with faster early carbonation speed and slower later carbonation rate.Carbonation densified the structure of hardened cement stone with refinement of pore structure and reduced porosity.Then,during the carbonation process from the surface to the inside of carbonation area,it was prone to form micro-cracks extending to the interior specimen,resulting in cement paste carbonation depth uneven.It is further illustrated that the color reaction method using phenolphthalein solution combined with X-CT and X-ray diffraction analysis is much more reasonable to evaluate the cement concrete carbonation degree.Moreover,during carbonation process sulfur element in cement paste migrated to the area un-carbonated and the concentrated shape of sulfur element is consistent with the coloring region in carbonation interface.Finally it was identified that carbonation not only reduced the p H value in cement concrete but also made prone to crack in carbonation zone,which increased the probability of reinforcement corrosion.

Hydration Mechanism of Silica Fume- sulphoaluminate Cement

Setting time and strength of sulphoaluminate rapid hardening cement （SAC） incorporated in the presence and absence of silica fume （SF） were determined. Combined with the techniques of＂ isothermal calorimeter, XRD and FSEM, the hydration kinetics of the two systems and the effect mechanism of SF on SAC were investigated. The experimental results showed that SF was proved to be beneficial for SAC system, in terms of setting time and late strength gain. Evidence of accelerator effect of silica fume was found during the first 8 hours of hydration. The formation of AFt was accelerated and the microstructure of the hydration products grew denser with incorporation of SF. SF was proved to play the role of dispersion and setting control at early age and had a greater contribution to later strength due to the increment of crystal nucleation point and the pozzolanic activity. Therefore, SF can be used to not only control the hydration kinetics of SAC, but also develop the late strength and improve the microstructure.

Degradation of Pore Structure and Microstructures in Hardened Cement Paste Subjected to Flexural Loading and Wet-dry Cycles in Sea Water

Hardened cement paste was subjected to the flexural loading and wet-dry cycles in sea water. The degradation of microstructures was obtained using scanning electron microscope （SEM）, and the energy dispersive spectrum （EDS） analysis was carried to analyze the local composition. Mercury intrusion porosimetry （Poremaster GT-60） was used to analyze the degradation of pore structures. The experimental results show that the synergistic action of the flexural loading, wet-dry cycles and sea water leads to significant deterioration of hardened cement paste. The degradation of microstructures in the tensile region is more serious than that in the compressive region. The flexural loading and wet-dry cycles accelerate the chemical attack of sea water.

Statistical Characteristics of Microhardness of Hardened Cement Paste

Due to the intrinsic nature of a heterogeneous and anisotropic microstructure of cement based materials and the small indentation size, the measured microhardness values are subject to considerable variability. This work presents an in-depth assessment of microhardness of hardened cement pastes（HCPs） from a statistical perspective. Hundreds of microhardness measurements were conducted on an HCP sample using a Vickers Microhardness Tester. The results showed that the microhardness measured from the HCP sample significantly scattered with a large standard deviation, varying from tens to hundreds. The data-set of microhardness values was not normally distributed but fit best with a three-parameter lognormal model. By using a statistical software, the probability density function of the microhardness distribution can be readily obtained. The arithmetic mean and its 95% confidence intervals of the measured microhardness values can be used to best represent the microhardness characteristics of HCPs.

Electrokinetic Nanoparticle Treatment Success Factors

Electrokinetic pozzolanic nanoparticle treatments have been reported in the literature to achieve rapid porosity reductions and deeply penetrating strength enhancement of cement and concrete. The high electric fields required to achieve these results have tended to be accompanied by particle suspension instability. Coagulation is an instability that can limit the efficiency and effectiveness of a treatment by removing particles from suspension. The current study examines how electro-coagulation impacts electrokinetic treatment effectiveness. The nano-pozzolan suspension used in this study was Nalco 1056, alumina-coated silica (24-nm). A threshold electric field strength of 0.4 V/cm was identified for avoiding direct electro-coagulation. Treatments conducted at this threshold value exhibited a 50% strength increase. Treatments conducted above this threshold value exhibited significant particle electro-coagulation losses and strength increases of only 25%. This study found that electro-coagulation was influenced by the electric field strength, through a combination of particle crowding at pore entrances and pH shifts as driven by electrolysis.

Recycling of End of Life Concrete Fines into Hardened Cement and Clean Sand

One of the massive by-products of concrete to concrete recycling is the crushed concrete fines, that is often 0 - 4mm. Although the construction sector is to some extent familiar with the utilization of the recycled coarse fraction (>4 mm), at present there is no high-quality application for fines due to its moisturized and contaminated nature. Here we present an effective recycling process on lab scale to separate the cementitious powder from the sandy part in the crushed concrete fines and deliver attractive products with the minimum amount of contaminants. For this study, a lab scale Heating-Air classification system was designed and constructed. A combination of heat and air classification, resulted in a proper separation of finer fraction (0 - 0.250 mm), from coarser fractions. Heating of the materials was followed by ball milling to enhance the liberation of the cementitious fraction. Experiments were carried out at different heating temperatures and milling durations. Experimental results show that by heating the materials to 500℃ for 30 seconds, the required time of ball milling is diminished by a factor of three and the quality of the recycling products satisfies well the market demand. In addition, the removal of contaminants is complete at 500℃. The amount of CaO in the recovered finer fraction from the recycling process is comparable with the amount of CaO in low-quality limestone. By using this fraction in the cement kiln as the replacement of limestone, the release of the chemically bound CO2 could be reduced by a factor of three.

Relationship between chloride diffusivity and pore structure of hardened cement paste

Based on effective media theory, a predictive model, relating chloride diffusivity to the capillary pores, gel pores, tortuosity factor, and pore size distribution of hardened cement, is proposed. To verify the proposed model, the diffusion coefficient of chloride ions, the degree of hydration, and peak radius of capillary pores of cement paste specimens were measured. The predicted results for chloride diffusivity were compared with published data. The results showed that the predicted chloride diffusivity of hardened cement paste was in good agreement with the experimental results. The effect of the evolution of pore structures in cement paste on chloride diffusivity could be deduced simultaneously using the proposed model.