Experimental Assessment of Cement Integrity under Thermal Cycle Loading Conditions in Geopressured Geothermal Reservoirs

The number of well integrity issues increase as wells are exposed to severe downhole conditions and have longer lifetimes. 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 influence cement sheaths causing them to fail. Failure of annular cement sheaths can introduce well integrity issues and subsequently lead to sustained casing pressure. This study measures the effect of cyclic thermal loading of cement slurry designs in salt brines. Grain volume porosimeter and Liquid Pressure-pulse Decay Permeameter was used to quantify the presence of thermal fractures as it is capable of measuring brine permeability of cement under reservoir conditions. Scanning Electron Microscopy micrographs with Energy Dispersive Spectroscopy capabilities, Thermogravimetric analysis and X-Ray Photoelectron Spectroscopy were used to study the physical and chemical changes in the cement slurry designs. Five cement designs with a range of chemical additive were subjected to 100 thermal cycles of 40⁰C at 100% relative humidity in salt brine. The experimental result indicates leaching of Ca(OH)2 will occur from the cement irrespective of cement composition which causes the porosity and permeability of the cement sheath to increase. Due to the thermal cycling, the strength of the cement sheath decrease. The study also shows that steel fiber can be added to the design to improve the permeability and increase the strength of the cement sheath under thermal cycle loading conditions. Future work is essential in order to fully understand within which temperature ranges a particular well can be operated, without leaks along the annular cement sheaths. This can be obtained by conducting tests varying the different materials in the cement mix. In addition, experimental tests determining the effect of exposing the formation to drilling fluids prior to cementing and further thermal cycling can be conducted. Effect of various wellbore scaling ratios is also important, as the effects of the total volumes on the obtained results are unknown