The Effect of Geofluids on Wellbore Cement Under Thermal Cycle Loading Conditions

Abstract

The number of well integrity abnormalities increase as wells are exposed to severe downhole conditions over longer production periods. In geopressured geothermal wellbores, cement is always in contact with slightly acidic formation brine at the cement-formation interface. The consequence of this is leaching of Ca2+ out of the cement as a result of inequlibrium between the concentration of the ions in the formation brine and the highly alkaline cement pore water. In addition, techniques for heat extraction from geopressured geothermal reservoirs involve production of hot water and injection of cold water which expose downhole materials to harsh cyclic temperature variations. Heating and cooling make the cement expand and contract as a result of thermal expansion. This volumetric change can cause cement sheaths to fail. The combination of the leaching process and cyclic temperature loading increase cement sheath porosity and permeability, exposing it to further degradation. For this study, five cement designs with a range of chemical additive were subjected to 100 thermal cycles of 40°C at 100% relative humidity in salt brine. Leaching of Ca(OH)2 from the cement was prominent in all the samples causing porosity of the cement matrix to increase. This study suggests part of the cement matrix will be lost to the formation brine irrespective of cement composition, although the rate of leaching could be altered by varying the cement composition. Permeability estimation using Kozeny-Carman equation shows that these chemical changes will also increase permeability which can lead to reduced zonal isolation and potential wellbore casing failure due to contact with corrosive geofluids.