PNNL

Abstract

Portland cement, a common sealing material for wellbores for geological carbon sequestration was reacted with CO2 in supercritical, gaseous, and aqueous phases at various pressure and temperature conditions to simulate cement-CO2 reaction along the wellbore from carbon injection depth to the near-surface. Hydrated Portland cement columns (14 mm diameter × 90 mm length; water-to-cement ratio = 0.33) including additives such as steel coupons and Wallula basalt fragments were reacted with CO2 in the wet supercritical (the top half) and dissolved (the bottom half) phases under carbon sequestration condition with high pressure (10 MPa) and temperature (50 °C) for 5 months, while small-sized hydrated Portland cement columns (7 mm diameter × 20 mm length; water-to-cement ratio = 0.38) were reacted with CO2 in dissolved phase at high pressure (10 MPa) and temperature (50 °C) for 1 month or with wet CO2 in gaseous phase at low pressure (0.2 MPa) and temperature (20 °C) for 3 months. XMT images reveal that the cement reacted with CO2 saturated groundwater had degradation depth of ~1 mm for 1 month and ~3.5 mm for 5 month, whereas the degradation was minor with cement exposure to supercritical CO2. SEM-EDS analysis showed that the carbonated cement was comprised of three distinct zones; the innermost less degraded zone with Ca atom % > C atom %, the inner degraded zone with Ca atom % ˜ C atom % due to precipitation of calcite, the outer degraded zone with C atom % > Ca atom % due to dissolution of calcite and C-S-H, as well as adsorption of carbon to cement matrix. The outer degraded zone of carbonated cement was porous and fractured because of dissolution-dominated reaction by carbonic acid exposure, which resulted in the increase in BJH pore volume and BET surface area. In contrast, cement-wet CO2(g) reaction at low P (0.2 MPa)-T (20°C) conditions for 1 to 3 months was dominated by precipitation of micron-sized calcite on the outside surface of cement, which resulted in the decrease in BJH pore volume and BET surface area. Cement carbonation and pore structure change are significantly dependent on pressure and temperature conditions as well as the phase of CO2, which controls the balance between precipitation and dissolution in cement matrix. Geochemical modeling result suggests that ratio of solid (cement)-to-solution (carbonated water) has a significant effect on cement carbonation, thus the cement-CO2 reaction experiment needs to be conducted under realistic conditions representing the in-situ wellbore environment of carbon sequestration field site. Total porosity and air permeability for a duplicate cement column with water-to-cement ratio of 0.38 measured after oven-drying by Core Laboratories using Boyle’s Law technique and steady-state method were 31% and 0.576 mD. A novel method to measure the effective liquid permeability of a cement column using X-ray micro-tomography images after injection of pressurized KI (potassium iodide) is under development by PNNL. Preliminary results indicate the permeability of a cement column with water-to-cement ratio of 0.38 is 4~8 mD. PNNL will apply the method to understand the effective permeability change of Portland cement by CO2(g) reaction under a variety of pressure and temperature conditions to develop a more reliable well-bore leakage risk model.