Calibrating large-scale injection: Using saltwater disposal experience to predict CCS performance on the Texas Gulf Coast

Abstract

Over 50 geologic carbon storage (GCS) projects are now advancing on the US Gulf Coast. Comparing their stated goals with the number of currently permitted wells suggests some planned injection rates over 5Mtpa/well. Modelling supports these numbers, but Gulf Coast reservoirs are structurally and stratigraphically complicated, with potential for compartmentalization that may lead to unanticipated pressure buildup and premature loss of injectivity. We seek to calibrate that risk by looking at historical saltwater disposal (SWD) on the Texas Gulf Coast. From 1990 to 2020, over 20 billion barrels of brine (∼2 Gt CO₂-equivalent) were injected into non-productive reservoirs, largely without adverse effect. Analysis of injectivity index for these wells shows that most are poor performers in lifetime average terms, with few wells clearly capable of 1Mtpa CO₂-equivalent. However, intermittent injection, not captured by the data, may explain apparently poor performance. Analysis of monthly injectivity index shows that over half the wells had at least one month indicative of >1Mtpa CO₂-equivalent capability. Anecdotal evidence, and even our analysis, suggests that all wells eventually lose injectivity, but we can see no evidence of pressure buildup or declining performance at the formation level. More commonly, the cause seems to be formation damage, apparently mostly resulting from impurities in the injected brines—microbes, incompatible chemistry and/or entrained solids. We believe that GCS wells are likely to perform better than SWD wells, owing to better location, superior planning, and a cleaner injection stream. These results suggest cautious optimism for large-scale GCS on the Gulf Coast.