Compositional Effects in Gravity-Dominated Nitrogen Displacements

Abstract

Summary In this paper, the role of compositional effects during secondary, gravity-stable nitrogen displacements in (dipping) light-oil reservoirs, such as the Statfjord reservoir in the Brent field, is discussed. Displacements of both a gas condensate from the top of the Statfjord hydrocarbon column and a volatile oil from a more downdip location in this column are discussed. The main compositional effects in such vaporizing-gas-drive-type displacements are the following. The displacing vapor at the displacement front is not nitrogen, but a rich hydrocarbon gas liberated from the reservoir fluid upon contact with nitrogen. This hydrocarbon vapor is miscible with the reservoir fluid in the case of the gas condensate, but just fails to develop miscibility in the case of the oil-like reservoir fluid. The remaining oil is a degassed, more viscous oil than the original reservoir fluid. In addition to equation-of-state (EOS) programs, both a compositional simulator and an analytic, dispersion-free model for vaporizing-gas drives have been used to quantify the effect of phase behavior on recovery efficiency. These compositional calculations indicated that next to the phase behavior, the longitudinal dispersion level and, in the case of the oil-like fluid, the degree of gravity segregation at the displacement front are key factors determining displacement efficiency. Results are presented of a systematic investigation into the relative contributions of phase behavior, physical dispersion, and gravity drainage to the recovery process. A comparison of predicted results and slim-tube and coreflood experiments is made. The extent to which these laboratory experiments might be representative for reservoir-scale displacements is also briefly discussed.