VARIATION IN RESERVOIR FLUID CHEMISTRY DURING AND FOLLOWING CO2 INJECTION AT CITRONELLE OIL FIELD, ALABAMA

Citronelle Oil Field, located approximately 30 miles north of Mobile, Alabama, has been a major oil producer for the state since its discovery in 1955. The reservoir units within the Donovan Sand of the Rodessa Formation have been in secondary waterflood production since 1961. Between December 2009 and September 2010, the early Cretaceous Donovan Sand was the target for CO₂ tertiary recovery and sequestration evaluation. During this period, 8036 tons of CO₂ were injected into the reservoir and waterflood was subsequently resumed.

Aqueous samples were collected from four producing wells (B-1907, B-1908, B-1909, B-1911) surrounding the injection well from June 2010 until February 2011 to evaluate chemical changes induced by the injection of supercritical CO₂ into the reservoir. The samples were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) for cation concentrations, and by ion chromatography (IC) for anion concentrations.

Within the well data, several element trends and concentration patterns can be seen. Aluminum concentrations show a similar general trends and changes between all wells, although with varying magnitude. Chloride and sodium concentrations display similar temporal trends for wells B-1908 and B-1909 and a different trends for wells B-1907 and B-1911. Fluid Ca and Sr concentrations strongly correlate and have nearly identical temporal trends for individual wells; however, wells B-1908/B-1909 show increasing Ca and Sr concentrations during the study period, while wells B-1907/B-1911 show decreasing concentrations. Wells B-1907/B-1909 have much higher finite Na, Cl, Ca, Sr and Br concentrations than wells B-1908/B-1911. A pH increase can be seen in conjunction with the initial rise of Al, Ba, sulfate, Fe, and a few trace element concentrations, although these are inconsistent across wells. Many temporal element concentration changes appear to be linked to fluid breakthrough curves, the change from CO₂ injection back to waterflood, and/or the temporary halt of waterflood. Similar fluid element concentrations may indicate wells with similar flow regimes and connectivity within the heterogeneous reservoir. PHREEEQC models may provide insight into fluid-mineral reactions that may control the concentrations of elements that do not correlate with fluid pH.