Summary To investigate the mechanism of abnormal thickening behavior of oilwell cement slurries containing hydroxycarboxylic acid and sugar-based retarders, the effects of citric acid, maltodextrin, and glucose on the early hydration of oilwell cement were studied using isothermal calorimetry (IC) and thickening time (TT) tests at 70°C. Cement slurries with a higher dosage of any of the three retarders typically exhibited abnormal thickening (premature thickening) during the early TT test period, characterized by a “stepwise increase” in consistency. Higher dosage of any of the three retarders can fundamentally alter the typical shape of the cement hydration heat flow curve, resulting in an additional exothermic peak before the main hydration peak. Based on its timing and on previous literature, this peak is tentatively attributed to accelerated hydration of aluminate/ferrite phases. The duration of the initial hydration peak shows excellent agreement with the duration of the stepwise rise in cement slurry consistency, which establishes a clear correlation between macroscopic cement slurry thickening behavior and microscopic cement hydration kinetics. Therefore, the abnormal thickening behavior of the cement slurries is suggested to be associated with early accelerated hydration of aluminate and ferrite phases before the main hydration peak. As a result, a threshold hydration extent of aluminate and ferrite phases may exist beyond which a cement slurry will exhibit abnormal thickening regardless of the retarder’s ability to inhibit the tricalcium silicate (or alite, C3S) main hydration peak.

This study conducted a multi-scale experimental investigation to understand the combined influence of Ca content and Si/Al molar ratios on the performance of metakaolin-ground granulated blast furnace slag (MKGGBFS) based geopolymer binders. The investigation focused on the geopolymer pastes with 10-25 % Ca content and Si/Al ratios ranging between 1.75 and 2.5. The detailed phase composition, microstructure, pore-related properties, and micromechanical properties of the specimens were characterized using X-ray diffraction, scanning electron microscopy, mercury intrusion porosimetry, and nanoindentation tests. The results indicate that the optimal performance is achieved given 20 % Ca and a Si/Al ratio at 2.0. Microstructural analysis revealed that Ca could promote the formation of high-modulus C-A-S-H gels, thereby driving the evolution of gel phases. In contrast, the Si/Al ratio governs the structural quality and homogeneous distribution of gel phases. At this optimal formulation, the interaction between the Ca content and Si/Al ratio optimizes the gel network and promotes the formation of an intermediate gel phase (N-C-A-S-H), featuring optimized chemical composition, microstructure, and macroscopic properties. This work establishes a multiscale mechanism linking chemical formulation with micromechanical behavior and macroscopic performance, providing a theoretical foundation for performance-based design and optimization of geopolymers.

Well cement is subjected to complex curing histories, in-situ stresses, and moisture conditions during wellbore service. However, the combined effects of these factors on its mechanical response and failure criteria remain insufficiently understood. This gap limits the reliable evaluation of cement sheath integrity in deep and complex wells. In this study, a comprehensive triaxial investigation was conducted on conventional-density and low-density well cement systems. Samples were cured at 50 ℃, 100 ℃, and 150 ℃ under different pressure conditions, and then tested at ambient temperature under confining pressures ranging from 0 to 80 MPa. Both dry and saturated conditions were considered to clarify the coupled effects of curing temperature, confinement, and moisture state. Under dry conditions, all well cement systems exhibited a brittle-to-ductile transition as confining pressure increased. In contrast, saturated conventional-density cement exhibited predominantly brittle failure and reduced frictional sensitivity. Low-density well cement containing hollow glass microspheres consistently demonstrated brittle-to-ductile transitions and near-elastoplastic behavior at high confining pressures, highlighting the role of microstructural design in enhancing mechanical performance. Fitting of the present experimental data showed that the linear Mohr–Coulomb criterion provided an acceptable description of the strength data for conventional-density cement. The parabolic Drucker–Prager criterion reasonably captured the low-density cement data. The cap model with a single unified equation reproduced the observed yield stress trends of both cement types under dry test conditions. However, it showed limited agreement with the saturated-test data, which may partly reflect potential pore-pressure effects that were not explicitly represented in the model.

This study aims to investigate the influences of temperature and critical additives (suspension aid and dispersant) on the rheological performances of oil well cement slurries. A high-temperature and high-pressure rheometer was used to evaluate the rheology of high-temperature cement slurries at eight different temperatures (30 degrees C, 60 degrees C, 90 degrees C, 120 degrees C, 150 degrees C, 180 degrees C, 210 degrees C and 220 degrees C) using multiple "ramp and hold" testing schemes. Test results reveal that almost all slurries exhibit thermal thinning and shear thinning behavior and that the shear thinning effect is enhanced at high temperatures. However, increases in slurry viscosity with increasing temperature at certain temperature ranges are also observed, typically at relatively low shear rates (<100 s-1), which can be attributed to both structural changes of polymer additives and cement hydration reactions. Apparent viscosity of cement slurries generally increases with increasing dosage of suspension aid and decrease with increasing dosage of dispersant. It is found that all the seven rheological models investigated in this study fit well with the experimental results at temperatures between 30 degrees C and 120 degrees C, but the fitting quality deteriorates at higher temperatures, especially for the two-parameter models (such as the Bingham-plastic and Power-law models). The three-parameter Hershel-Bulkley (H-B) model is observed to provide the best fitting quality to experimental data at all temperatures. The variations of all three H-B model parameters as functions of temperature can be divided into two stages (30 degrees C-120 degrees C and 150 degrees C-220 degrees C) due to increases in slurry viscosity at approximately 150 degrees C and each stage can be fitted reasonably well with a quadratic equation. The yield stress obtained by the H-B model typically show an increasing trend with increasing temperature and increasing dosage of suspension aid. The rheology test results of water solutions of pure suspension aid and dispersant exhibit apparent different behaviors compared to that of cement slurries, which can be attributed to the influences of cement hydration as well as its interactions with cement additives.

A comprehensive investigation of the impact of temperature on C-S-H properties over 5-90 degrees C was conducted using six model cement systems prepared with different C3S/C2S ratios. A fast DTG deconvolution method and a novel algorithm based on water-vapor adsorption were proposed to guarantee the accuracy and accessibility of the derived C-S-H properties. Test results indicate that the Ca/Si ratio of C-S-H generated by hydration of silicate cement with different C3S/C2S ratios increases with increasing C3S content. The Ca/Si ratio (1.73-1.93) and the chemically bound water content (H2O/Si ratio of 1.56-1.89) of C-S-H remain relatively stable within 30-70 degrees C. The total water (H2O/Si ratio of 2.57-4.10), gel water (H2O/Si ratio of 1.01-1.96), and intrinsic porosity (0.18-0.29) of saturated C-S-H are found to decrease with rising temperature, exhibiting a density range of 2.11-2.38 g/cm3, with maxima at 5 degrees C and 90 degrees C and minima at 30 degrees C and 70 degrees C. The density of C-S-H solid phase is determined to be 2.59 +/- 0.12 g/cm3. The water-vapor adsorption results indicate that dried C-S-H possesses a specific surface area ranging from 91 to 502 m2/g. The obtained C-S-H parameters at varying temperatures provide essential inputs for analyzing the engineering properties of a wide range of cement systems.

This study utilized low-angle high-resolution X-ray diffraction (XRD) and conventional X-ray diffraction techniques to systematically investigate the C-S-H interlayer spacing and phase composition of synthetic oil well cement with and without the addition of silica flour at temperatures ranging from 110 to 200 °C. Test results indicate that the basal spacing of C-S-H in high-temperature well cement without silica flour is 10.44 ± 0.13 Å and exhibits minimal variation with changing relative humidity (RH). Furthermore, as the temperature rises, the lower-density major phases of C-S-H and portlandite in the cement without silica flour gradually disappear and are substituted by higher-density phases of α-C2SH, kilchoanite, and reinhardbraunsite. This phase transition results in an increase in the total porosity of well cement, which is a primary contributor to both strength retrogression and deterioration of sealing performance in oil well cement. In contrast, the basal spacing of saturated C-S-H in high-temperature silica-enriched well cement measures 11.30 ± 0.15 Å and retains the characteristic of increasing value with rising RH. However, this variation in C-S-H basal spacing decreases with increasing temperature. The incorporation of silica flour ensures that C-S-H remains the major phase in well cement, even at 200 °C. This addition effectively slows both the phase transition rate and the rate of increase in total porosity of the cement. The metastable phase diagram of high-temperature silica-enriched cement developed in this study, along with the associated C-S-H basal spacing data, provides critical insights for the analysis and prediction of the engineering performance of cement sheaths and wellbore integrity in high-temperature high-pressure wells.

To study the working mechanism of Portland cement retarders, the influences of various types of sugars (sucrose, glucose, and maltodextrin) on the hydration kinetics of oil well cement was evaluated primarily using isothermal calorimetry (IC) test method in the temperature range from 60 degrees C to 89 degrees C. By thorough analysis of the experimental data of this study and other relevant studies in the literature, a new retardation mechanism is proposed. Sugars inhibit the hydration of oil well cement by primarily retarding the hydration of silicate phases (mainly C3S). The hydration of aluminate phases, which is normally inhibited by the silicate hydration, can be accelerated when the silicate phases are strongly inhibited under high dosage of retarders. Indeed, strong aluminate hydration peaks are observed during the pre-induction period with sugar dosages of 0.2% and higher, and the effect intensifies at high temperatures. The initiation of the silicate hydration peak (main peak) is simultaneously controlled by the overall hydration extent of the cement (which can disrupt the retarder action) and the adsorption stability of the retarder (which is influenced by both retarder type and dosage). Therefore, high sugar dosages (>= 0.3%) often causes strong inhibition of C3S hydration initially, but the resulted strong acceleration of aluminate hydration can cause earlier initiation of the main peak, resulting in decreased induction period with increasing sugar dosage, especially at elevated temperatures (>= 80 degrees C). The proposed mechanism is further verified using thermogravimetric analysis (TGA) and quantitative XRD (QXRD) analysis of partially hydrated cement.

Sulphoaluminate cement is a type of cement independently developed in China during the last century, primarily composed of calcium aluminate hydrate and dicalcium silicate. Owing to its rapid hydration rate and swift development of early mechanical properties, it is also known as early-strength cement. Sulphoaluminate cement has been widely used in the construction cement sector. However, due to its excellent performance in low-temperature environments and its potential for low-temperature cementing, it has garnered significant attention and gradually become a research hotspot. Nevertheless, the rapid early-stage hydration rate of sulphoaluminate cement makes its setting time difficult to meet the requirements of cementing operations, thereby limiting its application in cementing projects. To regulate the thickening properties of sulphoaluminate cement, this study investigates the regulatory effects and underlying mechanisms of three representative retarders (glucose, boric acid, and borax) on the thickening and mechanical properties of sulphoaluminate cement. The results show that: in terms of mechanical properties, all three retarders promote the increase in 1-day compressive strength of the cement paste without significantly affecting later-stage strength. Regarding retarding effects: boric acid > glucose > borax, and none of them alter the low-temperature right-angle thickening characteristics of sulphoaluminate cement. The addition of these three retarders does not alter the types of early-stage hydration products in sulfoaluminate cement but only affects the rate of their formation. At low dosages, they promote the formation and development of calcium aluminate hydrates (cahs), while at higher dosages, they inhibit the hydration reaction, leading to a significant decrease in the formation rate of cahs.

Neat Portland cement suffers severe strength retrogression at temperatures above 110 °C, and silica addition is commonly used to improve its high-temperature performance. This study investigates the long-term evolution of silica-cement systems containing 40% and 70% silica cured at 150–240 °C for up to 90 days. Mechanical properties, pore structure, bound water content, and mineral composition were systematically evaluated. Results show that the severity of strength retrogression first increases and then decreases with curing temperature. The lowest long-term strengths were observed at 180 °C for the 40% silica system and 210 °C for the 70% silica system. Severe strength retrogression was accompanied by microstructural coarsening and significant bound water loss during prolonged curing. Increasing silica dosage markedly enhanced early strength, but its benefit diminished over time because of aggravated strength retrogression. Mineralogical analyses indicate that strength retrogression is associated with the transformation of amorphous C–S–H and tobermorite into crystalline phases, including xonotlite, reyerite, and gyrolite, together with structural changes in amorphous hydrates. The reduced retrogression at 240 °C is mainly attributed to the lower initial contents of amorphous C–S–H and tobermorite. These findings provide guidance for the design of high-temperature-resistant oilwell cement systems for deep hydrocarbon wells.

In the study of ultra-high temperature cementing systems, it is generally believed that the slurry performance of well cement is primarily determined by chemical additives, while its physico-mechanical properties are influenced by the incorporation of mineral admixtures. This study investigates the effects of different additives on the physico-mechanical properties of well cement systems after curing at 240 °C for various durations. The results show that chemical additives can significantly affect the physico-mechanical properties of cement by altering its setting temperature. Among the three formulations tested, the well cement formulation H70, which includes a complete set of chemical additives such as suspending agents, retarders, dispersants, and fluid loss reducers, and satisfies the high-temperature setting and hardening conditions at 240 °C, achieved a 2-day compressive strength of 44 MPa. However, during the 90-day long-term curing process, the cement exhibited notable pore coarsening, increased porosity, and strength retrogression. In contrast, the cement formulation H70-R, which contains all additives except the retarder, reached a 2-day strength of only 15.5 MPa. Over the 90-day curing period, it showed an increase in strength and a reduction in pore size, but porosity increased. The cement formulation H70-A, which only contained the suspending agent as chemical additive, achieved a 2-day strength of 32 MPa. During the 90-day curing process, its strength continued to improve, with pore refinement and nearly unchanged porosity. Through high-temperature, high-pressure thickening time experiments simulating the heating process in curing vessels, it was found that the setting temperatures for formulations H70, H70-R, and H70-A were 240 °C, 160 °C, and 90 °C, respectively. This difference in setting temperature is the primary factor responsible for the significant variations in the physico-mechanical properties of cement paste in similar systems.

This paper introduces a general method for extracting the profile of C-S-H with Ca/Si ratios at and beyond 1.7 from X-ray diffraction (XRD) patterns of synthetic cement composed of alite and belite at 6 ratios (100/0, 80/20, 65/35, 50/50, 35/65, and 0/100) hydrated between 5 degrees C and 90 degrees C. This method integrates the initial construction of a C-S-H model via asymmetric least squares (ALS) and partial or no known crystal structure (PONKCS) techniques, followed by iterative refinement using Rietveld full-pattern fitting and an external standard K-factor approach until convergence criteria are met, with thermogravimetric analysis employed as a crosscheck to validate model accuracy. Deconvolution-based peak profile analysis reveals that C-S-H structurally resembles low-symmetry defect-bearing tobermorite, with low-temperature (<= 30 degrees C) C-S-H resembling anomalous type and high-temperature (>= 50 degrees C) C-S-H resembling normal type. Insights from crystal defect, main peak positions and intensities, and fingerprint zone reveal that the XRD patterns/structures of each C-S-H exhibit their unique characteristics, mainly as functions of cement composition and curing temperature. Elevated curing temperatures, high-belite contents, and low relative humidity were more favorable to yield C-S-H interlayer spacing data (11 +/- 0.6 & Aring;). The crystallinity of C-S-H improves as the temperature increases (excluding pure belite system). The morphology of C-S-H crystallites ranges from stable rods in most cement systems to metastable bricks formed from pure alite or belite, and occasionally to platelets, with all crystallites having a maximum dimension of 24.2 nm. Distinct from pure C-S-H, the XRD patterns of C-S-H formed in the hydration of synthetic cement obtained in this study provide reference data for quantitative analysis and molecular modeling of C-S-H in various types of calcium silicate cement.

Casing pressure fluctuations represent a primary cause of wellbore integrity failure, with the mechanical properties of cement playing a crucial role in determining sealing performance. This study employs finite element analysis to compare the effects of cement mechanical parameters under both ideal elastoplastic and elastoplastic damage constitutive models. The results indicate that, under the ideal elastoplastic model, a lower elastic modulus and a higher Poisson's; ratio help mitigate micro-annulus formation and reduce circumferential tensile stress. However, the elastoplastic damage model reveals that cement performance is governed not only by a low modulus but also by the maximum elastic strain and the strain corresponding to tensile and compressive strength. High-ductility cements, characterized by a low modulus combined with large deformation capacity, exhibit superior integrity under pressure cycling. This study proposes a novel dual-model framework for cement optimization, emphasizing that enhancing tensile and compressive deformation capacities alongside modulus reduction is essential for ensuring long-term sealing integrity in wells subjected to significant pressure variations.

In this study, various raw materials, including silica sand, silica fume, calcium hydroxide, α-alumina, and nano-activated alumina, were used to produce hydroceramic systems with varying Ca/Si/Al ratios to optimize their high-temperature resistance. The hydroceramic slurries, with a constant density of 1.65 g/cm3, were all designed to have a setting time of more than 4 h at the condition of 240 °C and 50 MPa and then cured at the same condition for 2, 30, and 90 days to evaluate their long-term performances. Subsequently, compressive strength, water permeability, mercury intrusion porosimetry, thermogravimetry, and X-ray diffraction tests were conducted on set samples at various curing times to analyze the hydroceramic systems’ long-term stability and the underlying mechanism. The results indicated that the hydration reaction of α-Al2O3 was minimal, and its inclusion reduced the incorporation of silica sand in the hydration process. Nano-activated alumina improved the macroscopic properties of the hydroceramic systems and promoted the formation of a significant amount of tobermorite 11 Å. The addition of silica fume can enhance the system’s macroscopic properties and the long-term stability, promoting the reaction of silica sand. The long-term stability of slurries with a Ca/Si ratio of 1 was significantly better than that of slurries with a Ca/Si ratio of 0.5. The best-performing slurry can maintain a compressive strength of more than 19 MPa after being cured at 240 °C for 90 days.

In order to investigate the fatigue damage characteristics of oil well cement under different cyclic loading rates, the fatigue experience of oil well cement was conducted under various loading rates and testing conditions. The results reveal that increasing the loading rate prolongs the fatigue life of oil well cement, enhances deformation capacity, and decreases the loading modulus. Furthermore, within the comparable plastic deformation range, oil well cement exhibits the lower dissipative energy when the loading rate is relatively higher. Notably, under high-temperature and high-pressure conditions, the oil well cement exhibits remarkable deformability but a shortened fatigue life, the energy absorbed during the loading test is primarily converted into dissipative energy. Fatigue failure occurs shortly after the volumetric strain reaches the inflection point, typically occurring after three cycles. Regardless of the test conditions, the inflection point value can serve as a reliable warning indicator for the onset of fatigue failure in oil well cement.

The short thickening time of geopolymers under high-temperature conditions and the difficulty in controlling it with retarders limit their application in oil and gas well cementing engineering. To address this question, the chemical principle that Na2SO4 reacts with Ca(OH)2 to form NaOH was applied. Na2SO4 and Ca(OH)2 were used to replace the strong alkali NaOH as the alkaline activator in this paper. Moreover, the reaction rate of Na2SO4 and Ca(OH)2 is controlled by a retarder to regulate geopolymer thickening time. Fly Ash Sink Beads (FASB) and Ground Granulated Blastfurnace Slag (GGBS) were employed as precursors in a 8:2 mass ratio. A comparative study was conducted on the thickening time adjustability and strength characteristics of geopolymers activated by NaOH and Na2SO4+Ca(OH)2 respectively. Experimental results indicate that when ethylene diamine tetra methylene phosphonic acid sodium (EDTMPS) was employed as a retarder, Na2SO4+Ca(OH)2 - activated geopolymers are superior in regulating thickening time behaviour. At 75 degrees C, NaOH-activated geopolymers showed a limited thickening time extension from 8 to 63 min even with a large retarder dosage increase from 0.18 % to 1.6 %, which is far from meeting the actual construction requirements. By contrast, Na2SO4+Ca(OH)2 - activated geopolymers achieved significant thickening time control (151-240 min) with a much smaller retarder dosage increase (0.06-0.18 %), showing better adjustability. Moreover, at a high temperature of 150 degrees C, the thickening time of Na2SO4+Ca(OH)2 - activated geopolymers could also be effectively regulated by the retarder. In addition, the geopolymer activated by Na2SO4 and Ca(OH)2 displays excellent strength characteristics, with its compressive strength reaching 17.4 MPa after being cured at 75 degrees C for one day. SEM analysis shows that the microstructure of the geopolymer activated by Na2SO4 and Ca(OH)2 is similar to that of the NaOH-activated geopolymer. This indicates that using Na2SO4 and Ca(OH)2 as the activator has little effect on the geopolymer's microstructure.

Recently, the calcium oxide (CaO)-aluminum oxide (Al2O3)-silicon dioxide (SiO2)-water (H2O) hydroceramic system has demonstrated strong potential for use as a cementing material for deep wells due to its stability at high temperatures. However, the typically used aluminum source alpha-alumina (alpha-Al) has extremely low hydration reactivity. With this study, we aim to improve the participation of Al2O3 in the hydroceramic reaction by replacing alpha-Al with calcium aluminate (CA). CaO-Al2O3-SiO2-H2O hydroceramic systems with various Ca/Si/Al molar ratios were prepared to have a constant density of 1.65 g/cm(3) and sufficiently retarded to meet the placement requirements in deep wells. The hydroceramic systems were then cured at 240 degrees C for 2 days, 30 days, and 90 days, respectively, to study their long-term stability by various evaluation methods, including compressive strength test, water permeability test, mercury intrusion porosimetry (MIP), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) analysis. Test results indicated that CA had significantly higher reactivity than alpha-Al, and the total hydration heat of the hydroceramic system showed a strong correlation with its compressive strength at 2 days. Overall, the high calcium systems had markedly superior performances compared with the low calcium systems, primarily due to increased silica reactivity. For the low calcium case, hydroceramic systems with a Ca/Si/Al ratio of 1:2:1 exhibited inferior performance than those with a Ca/Si/Al ratio of 2:4:1, primarily due to alumina's competition with silica for calcium sources, which led to the formation of the wairakite (CaAl2Si4O122H(2)O) phase. For the high calcium case, hydroceramic systems with a Ca/Si/Al ratio of 2:2:1 exhibited similar performance as those with a Ca/Si/Al ratio of 4:4:1, suggesting that the content of reactive alumina had a relatively small effect in these systems. Although various systems showed complex strength retrogression behavior, the use of CA to replace alpha-Al improved the compressive strength by 88.4% on average in the high-calcium hydroceramic systems.

To address the problem that conventional plugging materials for fractured reservoirs are hard to remove, magnesium oxychloride cement (MOC) has unique advantages for reservoir plugging due to its fast setting speed and good acid solubility. However, medium-high reservoir temperatures make traditional thixotropic agents incompatible with the MOC system, so developing a new temperature-resistant thixotropic agent suitable for MOC is urgent. In this study, diutan gum (DG) was modified via free radical polymerization with acrylic acid (AA) and N-vinylpyrrolidone (NVP) as monomers. Modified diutan gum (MDG) was obtained by optimizing the process through single-factor experiments and response surface methodology. XRD, FTIR, 1H NMR, XPS, and TGA confirmed its synthesis: the dissolution time in water decreased from 80 min to 15 min, viscosity loss in 20 % salt solution was only 13 %, and maximum thermal decomposition temperature rose to 422 degrees C, significantly improving solubility, salt resistance, and temperature resistance. MDG was compounded with magnesium aluminum silicate (MAS) to develop MDG-MAS, a temperature-resistant thixotropic agent for MOC. At 3 % dosage, the cement slurry showed excellent thixotropy without affecting cement stone's mechanical strength. The mechanism was revealed as the physical-chemical crosslinking synergy between MDG molecular chain winding and MAS sheet delamination, forming a reversible 3D network. This study offers fundamental insights and practical guidelines for reservoir fast-plugging cement slurry systems.

During offshore cementing at shallow depth, the low-temperature environment at the bottom of the sea and the low-density requirement of the cement slurry significantly hinder the strength development of oil well cement systems. Hence there is always a strong need to take various measures to enhance the strength development of low-density oil well cement systems. During this study, potential synergistic effects of silica fume, nanomaterials (C-S-H nano-seeds, nano-silica, nano-alumina), and inorganic salts (CaCl2, NaCl, Na2SiO3) to improve the strength of low-density well cement slurry were investigated. Water-to-cement ratio (w/c) was varied between 1.04 and 1.28 to obtain a constant slurry density of 1.5 g/cm3. Test results revealed that the addition of silica fume altered the rheology and flow behavior of low-density cement slurries, resulting in flat rheology profiles at high shear rates. The Bingham plastic model can describe the rheological behavior of cement slurries without silica fume, whereas the Power-law model is more suitable to cement slurries with silica fume. High-dosage silica fume (30%) is shown to have similar acceleration capability as the strongest nanomaterial accelerator (i.e. C-S-H nano-seeds) at 2% dosage. However, adding nanomaterials to silica-fume-enriched slurries cannot further increase the hydration rate of cement (i.e. no synergistic effect), possibly due to their similar acceleration mechanism. In contrast, adding chloride-based inorganic salts to silica-fume-enriched slurries further increased the hydration rate of cement significantly, exhibiting a strong synergistic effect. Based on the 7-day compressive strength test results at 15°C, the addition of silica fume or nanomaterials individually can increase the strength of neat cement by up to 92%, while the combined addition of silica fume and NaCl can increase its strength by 306%.

This study investigates the effect of incorporating modified calcium carbonate whiskers, treated with tetraethyl orthosilicate (TEOS), to enhance the mechanical properties and sealing integrity of oil well cement under high-temperature and high-pressure (HTHP) conditions. Traditional cement systems are prone to brittleness and cracking under dynamic loads, leading to compromised wellbore sealing performance. Our findings demonstrate that fiber-toughened cement slurry improves the toughness and sealing performance of the cement annulus, maintaining gas tightness and mechanical integrity under cyclic alternating pressures at 150 °C. Specifically, the inclusion of 5% modified whisker fibers improves compressive strength by 24.5% and flexural strength by 43.3% while maintaining stable rheological and thickening properties. These results support the hypothesis that modified whisker fibers enhance the durability and sealing integrity of cement wellbores under extreme conditions, providing a practical solution for challenging cementing applications.