Abstract
The objective of this work is to characterize the alteration on cement pastes exposed to 30+ years of natural ageing at downhole conditions. The cement samples used in this study were recovered from two production wells drilled in the Norwegian continental shelf completed in the late 1980's. Sandwich sections comprised of an inner and outer casing section with cement in between were recovered to surface prior to well abandonment. After recovery, cement samples were cut and studied to investigate signs of alteration and degradation. The characterization was performed using chemical and mineralogical characterization techniques such as: Thermogravimetric Analysis (TGA), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and X-Ray Fluorescence (XRF). Along with the two well cement samples, an additional class G cement, control sample, was produced and cured in control condition so that it could be used as a reference for a younger cement.
The cement samples studied in this article were not exposed to aggressive agents such as CO2 and H2S either at downhole conditions or in the lab, therefore the main driver for alteration or degradation was ageing. The results indicate that after an exposure time frame comparable with the service life of an oil well at relatively low temperatures, the conditions did not induce significant chemical or mineralogical changes. When comparing the recovered samples with the control sample, the main alteration identified was the higher level of carbonation process. This process indicates natural ageing of the cement during the 40-year period that finally led to a reduction of strength. This work supports the understanding of the durability of well cement and well integrity. The results presented help to establish a base line and to identify the alteration and degradation due to ageing through the end of the wells production life and the start of the abandonment phase.
