Analysis of the Performance of Horizontal Wells in the Long-Term CO2 Sequestration in Saline Aquifers

Abstract

Abstract The objectives of this study are (a) to analyze the CO2 sequestration performance of horizontal wells in saline aquifers during both the injection period and the subsequent long ‘rest’ period of inactivity, (b) to investigate the potential impact of salinity on the well injectivity, (c) to evaluate the effects of key well and aquifer properties, system conditions, and injection practices, and (d) compare the sequestration performance of horizontal wells to that of vertical wells. The study accounts for all known non-isothermal multiphase flow and transport processes associated with CO2 sequestration, including the potential salt (halite) precipitation, and uses a high-resolution grid to accurately capture the possible emergence and effects of halite near the well. The study models continuous CO2 injection at realistic rates into varying-salinity aquifers until a geomechanically safe maximum pressure is reached, followed by 100 years of shut-in. The results show that (a) large-scale CO2 storage in saline aquifers using horizontal wells is technically feasible, (b) the storage potential of CO2 dissolved in the aqueous phase is substantial (both in terms of the magnitude of the dissolved concentration and its spatial extent) and increases over time during the rest period, but its benefit decreases rapidly with an increasing salinity, (c) unlike the case of vertical wells, salinity appears to have practically no effect on the well injectivity as halite precipitation is negligible even at high CO2 injection rates, requiring no mitigation measures, and (d) horizontal wells appear significantly (if not overwhelmingly) superior to vertical wells for CO2 sequestration in saline aquifers. The results of the study indicate that the overall CO2 sequestration performance of horizontal wells (a) is significantly greater for deeper saline aquifers and, surprisingly, for more permeable upper boundaries, and (b) improves modestly with a decreasing CO2 injection rate, an increasing aquifer permeability, an increasing aquifer salinity, and by positioning the horizontal well closer to the base of the aquifer, but (c) appears practically insensitive to the temperature of the injected CO2. With the exception of the cases of very high-salinity and of a more permeable upper boundary, the long-term amount of CO2 dissolved in the aqueous phase routinely exceeds 45-50% of the total CO2 amount stored in the aquifer. Additionally, with the exception of the cases of more permeable upper boundaries, the long-term losses into the overburden never exceed 1% of the injected CO2. More permeable overburdens may register losses as high as 5%, but these can be acceptable because they occur mostly in the aqueous phase, stagnate very rapidly as they ascend in the subsurface profile—where they pose a limited risk for CO2 exsolution because of lower temperatures—and are associated with much larger amounts of sequestered CO2.