Abstract
A unique data set has been constructed based on the study of class G cement recovered from an oil well on the Norwegian continental shelf. The cement was placed between two casing sections and submitted to downhole conditions for 33 years. The aim of this research is to analyze the aging of the cement and its consequences for the well integrity. Additionally, the findings represent a baseline for future assessments of old age cement from oil and gas wells. The characterization includes: petrophysical properties (porosity and permeability); mechanical properties (uniaxial compressive strength (UCS) and Young’s Modulus); and compositional analysis using Computed Tomography (CT), Scanning Electron Microscopy (SEM), X-ray Fluorescence (XRF) and Quantitative X-ray Diffraction (XRD). Findings suggest that the overall decrease of cement integrity appears to be low, since the magnitudes of permeability and porosity fall within the ranges of early age cement. However, the chemical characterization shows that mud contamination may have an effect on the cement performance and is found to be relevant for a reduction in compressive strength. • In-depth characterization of class G cement exposed to downhole conditions for 33 years. • The results presented provide a baseline to assess the effect of aging on cement. • Findings suggest that the decrease in cement integrity appears to be low.
Highlights
Cement is the most important material used to construct barriers nowadays, for oil and gas wells, and for geothermal wells (Shadravan and Shine, 2015; Lohne et al, 2016) and underground CO2 storage facilities (Carey et al, 2007)
Mapping the consequence of these factors with a long-term perspective is a challenging and complex task but three main processes are identified as the most critical: increases in porosity and permeability, reduction of mechanical resistance and the creation of fluid migration paths that can allow the migration of fluids through a barrier creating well integrity problems such as Sustained Casing Pressure (SCP) (Bourgoyne et al, 1999; Wojtanowicz et al, 2001)
The porosity of the cement core samples varied from 46.7% in the bottom of the Transition Joint to between 35.9% and 41.1% for the core plugs retrieved from Fish # 11
Summary
Cement is the most important material used to construct barriers nowadays, for oil and gas wells, and for geothermal wells (Shadravan and Shine, 2015; Lohne et al, 2016) and underground CO2 storage facilities (Carey et al, 2007). In the oil industry, during well construction, cement is pumped into annular spaces to obtain well integrity, provide zonal isolation along the annulus, maintain mechanical support and prevent communication between the surroundings and the well fluids. It is subject to changes in pressure and temperature These could negatively affect the cement properties with adverse consequences concerning its effectiveness as a barrier in a long-term perspective. The recognized factors that potentially affect the cement properties include: water penetration, gas migration, chemical attack and variations in pressure and temperature that are able to generate thermo-mechanical failures such as shrinkage, cracking and debonding (Kiran et al, 2017; Andrade et al, 2016; Ferreira et al, 2019). Mapping the consequence of these factors with a long-term perspective is a challenging and complex task but three main processes are identified as the most critical: increases in porosity and permeability, reduction of mechanical resistance and the creation of fluid migration paths that can allow the migration of fluids through a barrier creating well integrity problems such as Sustained Casing Pressure (SCP) (Bourgoyne et al, 1999; Wojtanowicz et al, 2001)
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