Various admixtures to mitigate the long-term strength retrogression of Portland cement cured under high pressure and high temperature conditions

Mechanism of long-term strength retrogression of silica-enriched Portland cement assessed by quantitative X-ray diffraction analysis

“…The filled molds were put in an HPHT curing autoclave (made by Hai'an Petroleum Instrument Co., Ltd.), and the autoclave's temperature and pressure were ramped up to 200 °C/50 MPa and held for a predetermined time. The temperature and pressure of the autoclave were then decreased to ambient temperature and pressure by natural cooling, which took about 48 h. More detailed information about the sample curing method can be found in our two previous publications (Pang et al, 2021;Qin et al, 2022).…”

“…Among these materials, silica flour is the most commonly used anti-strength retrogression agent and it has been proven to be quite effective for thermal recovery wells where the cement was set at a low temperature before high temperature exposure (Jiang et al, 2021;Santiago et al, 2021). However, several of our recent studies (Pang et al, 2021;Liu et al, 2022;Qin et al, 2022) have shown that silica flour is ineffective in preventing long-term strength retrogression at deep well conditions where the cement was set and cured both at 200 °C. While it is clear that the long-term strength retrogression is accompanied by cement microstructure coarsening, some inconsistent conclusions have been drawn with regard to what caused microstructure coarsening, primarily due to uncertainties in X-ray diffraction analysis.…”

“…For example, based on the work of Pang et al (2021), it appears that transformation of amorphous C-S-H to tobermorite and xonotlite was the driving force of strength retrogression. However, based on the work of Qin et al (2022), it appears that the transformation of tobermorite to xonotlite and possibly structural change of amorphous C-S-H itself may be the driving force of strength retrogression. Therefore, this study will investigate different methods of performing quantitative X-ray diffraction (QXRD) analysis on cement cured at HPHT conditions and the influence factors of analysis results to further clarify the long-term phase changes in set cement and the driving mechanism of cement high temperature strength retrogression.…”

Effects of Curing Pressure on the Long-Term Strength Retrogression of Oil Well Cement Cured under 200 °C

“…Energies 2022, 15, 6071 2 of 19 can significantly improve the long-term stability of Young's modulus but cannot prevent strength decline or permeability increase [16]; (6) long-term strength retrogression may be caused by crystallization or structural coarsening of amorphous C-S-H [14,16,17].…”

“…The long-term strength decline of silica-enriched oil well cement often exceeds 80%, while its permeability can increase by two orders of magnitude. We have conducted a series of studies to understand such behavior of oil well cement and to optimize the cement composition design to improve its properties under the "one-step" curing regime (at the specific condition of 200 • C and 50 MPa) [14][15][16][17]. The main findings include: (1) the optimal dosage of silica flour is 60-80% based on 14 d strength [17]; (2) reducing the particle size of silica is beneficial for strength stability, at least during the first 30 d [16]; (3) addition of silica fume can improve strength at early age (2 d) but accelerates the strength decline afterward [16]; (4) addition of nanoparticles (such as nanosilica and nano-iron oxide) can reduce permeability at early age, but probably has little effect in the long term [16,17]; (5)…”