Influence of the Particle Size Distribution of Silica Flour on the Mechanical and Microstructural Properties of Oil Well Cement Paste Exposed to HTHP Conditions

Abstract

Silica flour is used to mitigate the strength retrogression and microstructure degradation problems of oil well cement paste in high temperature wells. This study investigates the effects of the particle size distribution of silica flour on the thermal stability of cement paste. After 7 days of hydrothermal curing at 80 °C, the cement paste was exposed to further aging for 7 or 28 days under high temperature and high pressure (HTHP) conditions in a sealed chamber at 260 °C (500 °F) and 21 MPa (2900 psi). The results show that 43% silica flour by weight of cement (BWOC) mitigates the strength degradation of silica-stabilized Portland cement (SSPC), and course silica flour (63.13 and 48.25 µm) maintains the strength stability of SSPC better than fine silica flour (3.25 µm). The transformation of C-S-H gel to xonotlite in SSPC was influenced by the silica particle size. The relatively fine silica flour (d 50 = 3.25 µm) accelerates the consumption of calcium hydroxide during the pozzolanic reaction, resulting in a poorer strength stability of SSPC than the courser silica flour (d 50 = 63.13 and 48.25 µm). The silica flour with an average particle size of 48.25 µm is more conducive to crystallization of xonotlite on the crystal plane corresponding to a diffraction angle of 25.5° than the other size silica flour.

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

“…In the petroleum industry, service companies came up with slightly different definitions for HPHT conditions. Baker Oil Tools' HPHT classification was used in Figure 1 [18], and the curing conditions of previous publications for strength retrogression were plotted [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In those studies, silica-enriched systems were often used to ensure the integrity of the wellbore.…”

“…With the increasing activities of deep well drilling and completion, significant efforts have been made to study the physical principles of the strength retrogression of oil well cement under high pressure and high temperature (HPHT) conditions. Silica has traditionally been the most commonly used material to mitigate strength retrogression [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, it has been [14,15] revealed recently that silica can become ineffective during long-term curing under simulated deep well cementing conditions (set and cured under HPHT conditions, i.e., the so-called "one-step" curing regime), which is in great contrast to the test results under simulated steam injection well conditions (set at low temperature and cured at HPHT conditions, i.e., the so-called "two-step" curing regime).…”

Belite hydration at high temperature and pressure by in situ synchrotron powder diffraction

“…In such conditions, α-C2SH stability will be favoured respect to that of Jaffeite. The transformation to higher density phases increases the porosity and decreases the mechanical strength (behaviour known as strength retrogression) [12][13][14][15][16][17][18]. Moreover, CSH gel crystallization at HPHT has been reported to yield tobermorite and related minerals [19][20][21][22] and to change its microstructure by densification and hence porosity increase [22,23].…”