Effects of brine and CO2 exposure on the mechanical and chemical properties of class G cement in oil well applications

Cementing is one of the most critical stages in oil well construction, as it ensures the proper fixation of the casing and prevents fluid migration through permeable zones.Class G cement, used in this process, is subjected to harsh conditions, particularly at great depths, where both temperatures and pressures are elevated.Exposure to brine and CO 2 under extreme conditions can compromise the durability of the cement and the well's integrity by altering its physical, chemical, and mechanical properties.Given the significance of these factors, it is essential to investigate their influence on the cement sheath.This study exposed Class G cement samples for three months in autoclaves under high pressure (20 MPa) and temperature (88 C) in environments saturated with brine and brine with CO 2 .Uniaxial and triaxial compression tests, porosity analysis, X-ray diffraction, and pH measurements were conducted.The results showed that confining pressure significantly impacted samples exposed to brine+CO 2 , leading to plastic deformations at pressures above 20 MPa, even before the application of deviatoric stresses.Exposure to brine+CO 2 reduced compressive strength by 45% compared to the reference samples, likely due to micro defects forming during curing and chemical reactions with acidic gases.To determine whether the contribution of micro defect formation caused by the loading and unloading during the curing procedure was more significant than the chemical reactions, samples cured under the same pressure and temperature conditions but exposed only to brine were tested.The results showed that the reduction in strength and alterations in the material's elastic properties were more closely related to the adopted curing procedure than to the chemical reactions resulting from the acidic environment.