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SPE Member Abstract Reduced water injectivity after CO2 injection has frequently occurred in West Texas oil fields. One explanation proposed for this phenomena was a change in rock wettability by the CO2 and crude oil miscible bank. This paper describes an experimental study to determine the effects Of CO2 flooding on the wettability of West Texas dolomitic cores. This study examined the relative permeability characteristics of fresh-state San Andres and Grayburg dolomite core samples before and after passing a crude Oil/CO2 miscible front through the test core. Changes in the relative permeability characteristics were used to infer the effects Of CO2 flooding on rock wettability. The cores chosen were intermediate oil wet, intermediate, and intermediate water wet. The results showed that the cores became slightly more water wet. However, this change in wettability was near the limit of statistical confidence. Some data suggests that the tendency to becoming water wet was cause by slight rock dissolution by the CO2 and brine. Some observations concerning the compatibility of brines used in laboratory core floods with dolomites containing anhydrite are discussed. Introduction A common occurrence during field solvent floods has been a decline in fluid injectivity compared to presolvent injection performance. The Levelland, Slaughter Estates, and Wasson fields have experienced reduced injectivity after CO2 injection followed by water injection. Reduced solvent injection has also been noted when rich or lean gas injection was alternated with water injection in Amoco's Project "A" Exxon's Seeligson and Sonatrach's Hassi Messauod projects. Petal, et al., examined the effect of phase behavior, inorganic salt precipitates, and rock wettability characteristics as possible causes of reduced injectivity. They concluded that the predominate cause for reduced injectivity was mixed rock wettability. However, they commented that the phase behavior of crude oil could cause heavy-end deposition which could alter the rock wettability, as suggested by Ehrlich, et al. This paper reports the results of a study to determine how the wettability of West Texas dolomite is changed by flooding with crude oil and CO2. The wettability of preserved core samples was measured by evaluating water-oil relative permeability properties of the samples before and after a crude oil and CO2 flood. Wettability characteristics were inferred from the connate water saturation, the relative permeability at floodout, and the saturation at which water and oil relative permeabilities are equal, as suggested by Craig. EXPERIMENTAL Core Test Sample Description All core samples used in this study were selected from preserved fresh-state cores. The preserved cores were scanned with a X-ray fluoroscope to select uniform test samples that did not contain fractures, vugs, or heterogeneous mineral deposits. The test samples were horizontal plugs taken parallel to the bedding planes with the exception of the second test sample which was machined from a whole core perpendicular to the bedding planes. Each sample was approximately 2 inches (5.08 cm) in diameter by 3 inches (7.62 cm) Long. Test Samples No. 1 and No. 2 were taken from Levelland Unit cores from the San Andres formation. North Cowden Unit samples from the Grayburg formation were used in tests No. 4, No. 5, and No. 6. P. 183^ |
