Comparison of laboratory-and field-observed CO₂ tertiary injectivity

Although CO₂ injectivity should be significantly greater than brine injectivity because CO₂ has a much lower viscosity than brine, this behavior is not always seen, as shown in a Denver Unit field test. This paper examines features that cause differences in CO₂ injectivity with a model that uses simple nondispersive flow with a series of constant-composition slugs to approximate the analytical solution (normally a sequence of shocks and tails) in a sequence of noncommunicating layers. Because of its simplicity, this model identifies the primary features that result in the different observed CO₂ injectivities more clearly than the finite-difference model. This paper shows that the qualitative differences between Cedar Creek anticline corefloods and field behavior result solely from differences in geometry. That is, a single set of centrifuge-measured, quasinative-state, secondary-drainage relative permeabilities can be used to predict both laboratory and field behavior. Primary factors that contribute to the differences between the two field tests are fluid/rock properties, effective wellbore radius (or skin), and heterogeneity in the layering.