CHEMICAL EFFECTS OF CO2 INJECTION ON MORROW SANDSTONE GROUNDWATER IN THE FARNSWORTH, TEXAS HYDROCARBON FIELD

The Farnsworth hydrocarbon field in northern Texas is the site of a new study begun in 2013 of commercial-scale CO₂ sequestration through enhanced oil recovery by the U.S. Department of Energy sponsored Southwest Partnership. Samples of groundwater were collected from the Morrow Sandstone reservoir in order to characterize the groundwater’s present composition and to predict its response to CO₂ injection. Morrow Sandstone groundwater was found to be relatively dilute given its depth of about 2400 m, with a total dissolved solids content of about 3600 mg/L dominated by Na, Cl, bicarbonate, and Ca. Geochemical speciation modeling assuming a reducing log f_O2 value of −55 showed the Morrow formation water to be supersaturated with respect to multiple Mg silicates, carbonates, clay and mica minerals, Al hydroxides, zeolites, Zn sulfides, and Ni sulfides and selenides. Quartz was found to be close to saturation, consistent with the quartz-rich nature of the Morrow sandstone. The feldspars, albite and K-feldspar, were found to be slightly undersaturated, consistent with their ongoing alteration to clay. Reaction path modeling was carried out in two stages. In the first stage, CO₂ was titrated into the Morrow sandstone groundwater up to its solubility limit. The resultant fluid was then allowed in the second stage to react with the Morrow host rock assuming an initial porosity of 15%. Titration of CO₂ into the Morrow fluid in stage 1 led to a sharp decrease in pH from about 7 to 4 causing the complete dissolution of carbonate minerals from the host rock. Reaction of the CO₂-charged fluid with the Morrow host rock caused an increase in pH, the precipitation of carbonate minerals, diaspore, and muscovite, and a small net decrease in porosity of no more than 2%. The results show that mineral trapping could make a significant contribution to CO₂ sequestration in the Morrow Sandstone, despite its quartz-dominant lithology and the relative dilute character of its groundwater.