You would need a suitable formation in terms of geological properties and then you'd need to drill and complete an injector and a producer well. Best case, you would probably lose half the stuff you're trying to store due to multiphase flow in reservoirs being a total drag. Worst case, the oil you're storing isn't compatible solubility-wise with the residual oil left in the reservoir, and you cause massive asphaltene precipitation and lose your entire storage well.