Experimental study of the effects of CO2 concentration and pressure at elevated temperature on the mechanical integrity of oil and gas well cement

Abstract

Oil and gas well cement reacts with carbonated brine thereby resulting in mechanical degradation after the cement is placed behind the casing. CO2 concentration and pressure influence the rate of the reaction. This study examines the effect of these downhole parameters on the mechanical degradation of well cement.Classes G and H cements were aged in CO2-saturated brine. CO2 concentration (i.e. partial pressure) was varied from 10% to 100%, total test pressure was varied from 21 MPa to 62 MPa, and temperature was kept constant at 107 °C. Compressive strength and transport properties of the specimens were measured before and after exposure to quantify the effect of CO2 attack on cement integrity. In the analysis, these measurements are augmented by results of microstructure and mineralogy studies performed using Fourier transform infrared, X-ray diffraction, energy-dispersive X-ray, and scanning electron microscopy.Results show that the concentration of CO2 and test pressure affect the mechanical behavior of well cement after exposure. The major governing mechanisms are carbonation reaction between cement hydrates and dissolved CO2, and the subsequent leaching of the product of carbonation. Although the specimens show significant physical and chemical degradation, the mechanical strength is improved. The mechanical performance is generally consistent with the evolution of transport properties and FTIR, SEM, EDX, and XRD mineralogy. Comparison of the performance of standard formulations of classes G and H cement shows that the strength of Class H cement is improved than the strength of Class G cement as CO2 concentration and pressure increase.Investigations of the effect of CO2 on the mechanical performance of well cement under extreme downhole conditions is limited. This study expands the understanding on how hostile and severe downhole conditions influence the degradation of well cement.