As a complex composite material, the macroscopic mechanical property of oilwell cement is determined by its microscopic mechanical properties. During this study, the properties of oilwell cement specimens cured under high pressure and high temperature (HPHT) conditions were evaluated at both macro- and microscales. Short-term sonic strength analysis suggested that the HPHT strength retrogression occurred in the early stage. Specimens were cured at 200°C and 150MPa for different time periods of 14d, 30d, and 69d, to study the long-term mechanical property stability of silica-enriched oil well cement systems. Uniaxial compression tests were conducted on 25mm × 50 mm cylindrical specimens for macroscopic characterization of strength, modulus, and Poisson's ratio. Furthermore, microscopic testing was conducted using X-ray diffraction, mercury intrusion, and nano-indenter. Test results revealed that the damage during experiment may significantly influence the macroscopic properties of oilwell cement, but it had little effect on the microscopic level. On the micro-scale, the deterioration is characterized by increases in the proportion of a low-modulus and low-hardness phase. Additionally, the dependences of nanoindentation test results on testing parameters such as maximum load and appropriate grid size were also evaluated.
Oilwell cement is a highly heterogeneous composite material. Its reaction with water results in various hydration products under different curing conditions (Pang et al., 2020; Krakowiak et al, 2015). Particularly, the hydration temperature had a significant effect on the mechanical properties and microstructure of cement slurry (Pernites and Santra, 2016; Bahafid et al., 2017). At temperatures below 90°C, the main hydration products of oilwell cement are calcium–silicate–hydrate (C–S–H) gel or semi-crystalline C–S–H and Ca (OH)2, which have compact structures and good mechanical properties; while at temperatures above 110°C, semi-crystalline C-S-H is gradually converted to crystalline phases such as α-C2SH and hillebrandite, which lead to a decrease in mechanical properties of the set cement. This phenomenon is called strength retrogression. About 35-40% by weight of cement (bwoc) crystalline silica powder is usually added to the cement blend to form the stable non-retrogressing phases; however, deterioration of strength over time has been observed even for silica-enriched cement systems when curing temperature is above 150°C (Eilers and Nelson, 1979; Wang et al., 2011; Lu et al., 2017; Zhu, 2019, Pang et al., 2021). Uniaxial compressive strength testing is generally used to evaluate macroscopic mechanical properties (e.g., compressive strength, Young's modulus, and Poisson's ratio) of cement. The macroscopic mechanical properties of cement are determined by its microscopic hydration products and structure (Reddy et al., 2016; Krakowiak et al., 2015). As a typical multiscale (Lin and Christian, 2007), multiphase (Garcia et al., 2018), and hybrid amorphous-crystalline (Li et al., 2020) composite material, cement has unique microscopic mechanical properties, and it is very challenging to characterize its complex properties. In the past three decades, nanoindentation technology has been extensively used to determine the microscale mechanical properties (i.e., mostly Young's modulus and hardness) of various materials, such as cartilage (Shi et al., 2020), metal (Zhen et al., 2020), and wood (Yu et al., 2006). A statistical nanoindentation approach with pertinent data analytics was developed to probe the micro-mechanical behaviors of cement hydration products, especially C–S–H, which is highly sensitive to the hydration temperature (Bahafid et al., 2017). In the field of petroleum exploration, cement and shales have been fully probed by this technology (Vandamme and Ulm, 2013; Liu et al., 2016; Shukla et al., 2013). Ulm and colleagues have developed a grid indentation method with deconvolution to distinguish mechanical properties of different phases (Constantinides and Ulm, 2004, 2006; Miller et al., 2008). Several studies measured the mechanical properties and the fractions of individual phases by using nanoindentation and back-scattered electron (BSE) image analysis. The test results were used as an input in homogenization modeling to upscale mechanical properties from microscale level to a macroscale level (Gao et al., 2018; Liang et al., 2017). To obtain the true mechanical properties of individual phases, the indents number, indentation depth, distance between the grids, and bin sizes have been thoroughly discussed by various researchers (Gao, 2018 et al; Liang et al., 2017; Liu et al., 2018; Luo et al., 2020). Inadequate indentation points may cause unreliable and misleading deconvolution results (Luo et al., 2020). Liu et al. (Liu et al., 2018) identified the minimum representative grid size of nanoindentation tests by using a box-counting method based on the energy-dispersive X-ray spectroscopy (EDX) mineral mapping (a matrix of 225 indents covering 300 μm × 300 μm). Increasing the number of indent points can help to separate various phases in the composite more precisely.
