When Portland cement sheath is submitted to temperatures above 110 °C in oil well wellbore conditions, the compressive strength retrogression phenomenon is observed. This occurs due to the conversion of calcium silicate hydrate (C-S-H) to unstable phases, resulting in low compressive strength and high permeability. To minimize that compressive strength issue, guidelines suggests the addition of 35-40% BWOC (By Weight Of Cement) of an extra silica source to balance the relation CaO/SiO2 and convert it into more stable calcium silicate phases. Silica flour (SF) is the silica source worldwide used as anti-strength retrogression agent in cementing operations. The present work presents rice husk ash (RHA) as a sustainable silica source, in alternative to SF, for temperature of the environments where compressive strength retrogression is pronounced. Four compositions of cement slurries were tested: (i) a 15.6 ppg slurry with no silica addition (SF0); (ii) a 15.6 ppg slurry with 35% BWOC of SF addition (SF35); (iii) a 15.0 ppg slurry with 35% BWOC RHA addition (RHA-1) and (iv) a 14.5 ppg with 35% BWOC of RHA (RHA-2). The samples were submitted to 60 °C and 110 °C at atmospheric and 2,000 psi curing pressure, respectively, during 7 days. A uniaxial compression test was performed to evaluate RHA as anti-strength retrogression agent. All experimental procedures were performed in accordance to API RP 10B-2. X-ray diffraction (XRD), scanning electronic microscopy (SEM) and thermogravimetric analyses (TGA) were carried out to analyze the samples after temperature exposure. Results have shown that RHA samples developed more compressive strength in relation to the usual SF sample. Among the samples cured at 110 °C, RHA-1 presented the higher value of compressive strength (44.6 MPa), followed by sample SF35 (40.4 MPa) containing SF in its composition. The sample RHA-2 with 37.9 MPa of compressive strength was similar to SF35 and, as expected, the compressive strength of control sample SF0 was the lower with 28.3 MPa, due to the retrogression phenomenon. The samples cured at 60 °C showed the same tendency of 110 °C samples. XRD analysis showed the presence of typical stable crystalline phases such as xonotlite and tobermorite in cement samples containing RHA. The SEM images and TGA analyses were in accordance to XRD evaluations. As observed, RHA showed great potential as anti-strength retrogression agent even in weighted (15.0 ppg) or light weight (14.5 ppg) slurries. The sustainability of a renewable silica source makes the RHA an interesting alternative to the usual silica flour.

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