Tailoring Barrier Design Process to Assess Cement Integrity in CCS Wells

ABSTRACT: In CCS wells, the hydration products in the cement sheath can react with CO2 and alter the composition of the barrier. Therefore, cement sheath mechanical properties can change spatially and temporally. Designing an appropriate barrier should account for these changes when assessing the risk of isolation failure. The proposed design process comprises of four steps: (i) Predict the extent of alteration between CO2 and the cement sheath, (ii) Predict mechanical property changes in the cement sheath due to alteration, (iii) Determine stresses and temperatures in the modified cement sheath due to CCS loads, and (iv) Assess the risk of cement sheath failure. The extent of alteration and mechanical property (compressive strength, tensile strength, Young's modulus, and Poisson's ratio) models are developed using micro-CT and destructive mechanical testing data generated on CO2 exposed cement systems. The test matrix consisted of more than ten cement compositions encompassing neat cement and reduced Portland designs exposed to a wide range of temperature, pressure and CO2 state (Dry vs. Wet vs. Carbonic) for seven days to seven months. These systems used a variety of Pozzolanic materials and additives. The models in the proposed design method incorporates variables describing composition and exposure conditions. Consequently, this vital tool enables tailoring cement compositions to minimize risk of loss in cement sheath integrity. 1. INTRODUCTION Well construction and operation will invariably cause changes in the near wellbore (NWB) temperatures, pore pressures and three-dimensional stress state. These changes govern the risk of cement sheath failure after successful placement. For many years, the oil and gas industry has successfully relied on design tools (Bosma et al. (1999), Thiercelin et al. (1997), Onaisi et al. (2017), Bois et al. (2015)) to quantify and mitigate the cement sheath failure risk posed by well construction and operation loads. Risk mitigation is attained through the process of tailoring cement composition in the laboratory until its thermo-mechanical properties meet the recommendations from the design tool. These design tools generally model the cement sheath as a material with constant thermo-mechanical properties. In case of carbon capture and sequestration (CCS) wells, the injected CO2 undergoes a time and space dependent reaction with the hydration products in Portland cement. These reactions cause time and space dependent changes to cement's thermo-mechanical properties. Tools that do not account for these changes can mislead a design engineer trying to tailor cement composition for the purpose of NWB integrity.