Carbon dioxide ($CO_2$) sequestration in oil reservoirs can be one of the most effective strategies for long-tenn removal of greenhouse gas from atmosphere. This paper presents an advantage of the localized nonlinear approximation of integral equation solutions for inverting crosswell electromagnetic data, which are observed as a part of pilot project of $CO_2$ flooding at the Lost Hills oil field in central California, U.S.A. To monitor the migration of $CO_2$, we have used two-dimensional (2D) cylindrically symmetric and 2.5D tomographic inversion methods. These two schemes produce similar images if the borehole separation is large enough compared with the skin depth. However, since the borehole separation is much less than five skin depths in this $CO_2$ injection experiment, the 2.5D model seems to be more reliable than the 2D model. In fact, the 2.5D image derived from pre-injection data is more successfully compared with induction logs observed in the two wells than the 2D image. From the time-lapse crosswell imaging, we can confirm that the replacement of brine with $CO_2$ makes a decrease of conductivity.