The strength retrogression of special class Portland oilwell cement

Abstract

Temperatures in excess of 110 ºC result in phase transformations of cement, significantly decreasing its compressive strength. This effect is referred to as strength retrogression. It is frequently observed in cement sheaths of heavy oil wells submitted to steam injection. The present study evaluated the mechanical behavior of Special Class Portland Oilwell Cement (SCPOC) slurries containing silica flour to prevent retrogression. A factorial statistical planning was used to assess the effect of the main variables on the mechanical behavior of cement slurries, i.e., mechanical testing temperature (30, 100, 120, 180 and 230 ºC); contents of silica flour replacing cement (0-18 and 36%) and curing time for rupture (12 h and 7 days). The results revealed that slurries containing 18% of silica flour tested at 230 ºC depicted an increase in compressive strength up to 30% after curing for 12 h and 10% after curing for 7 days, indicating retrogression. On the other hand, testing slurries containing silica flour at temperatures up to 180 ºC revealed strength increase of just 10%, suggesting the mechanical stability of the SCPOC, which prevents retrogression. Such behavior was probably related to the relatively low content of C3A and low specific area of the material. Therefore, strength retrogression at typical bottom hole temperatures of up to 180 ºC can be controlled by small additions of silica flour, economically contributing to the use of SCPOC cementing.