Gravity effects in miscible displacement

Abstract

Segregation of hydrocarbons and water can severely reduce the ultimate vertical sweepout efficiency of miscible displacement processes when a mobile water phase is present in all or part of the reservoir. Computer studies have shown that the decreased miscible sweep resulting from segregated flow can result in uneconomic operations in some fields in whi,ch economic operations were predicted for non-segregated flow. Gravity override is enhanced as a result or segregation, and once the reservoir fluids segregate, the high relative permeability of the lighter fluids at the top of the reservoir increases the gravity override with the result that oil in the lower portion of the reservoir is not displaced miscibly. The shape and movement of the oil bank formed during secondary and tertiary projects can be paramount to a successful field operation. Results from our computer model study show that an oil bank formed ahead of an overriding miscible fluid is forced down below the solvent tongue and moved relatively slowly toward the producing well. The oil bank is slowly recovered immiscibly at an extremely low rate by the water injected along with the gas. Thus at the economic limit of the field operation, a substantial, amount of the oil displaced by the miscible fluid early in the reservoir life is left unswept at abandonment. Most of the above information has been obtained from a two-phase flow, two-dimensional mathematical model, which includes the effect of the segregation of all hydrocarbon fluids in the presence of a denser, mobile, immiscible water phase present in watered-out systems on the sweepout behavior of the oil-bank front and miscible front. This behavior was previously unattainable from our physical [laboratory] flow models in which miscible fluids were used.