Residual Oil Saturation in a Pure Gravity Drainage Regime
Authors: Delshad Mohammad , Nedrud Michael C. 
Published: December 2007

Abstract
Accurate quantification of the residual oil saturation impacts the reserves calculations. This investigation which addresses the role of gravity drainage in lowering the residual oil saturation was a part of an overall focus to improve the understanding of various recovery mechanisms in production of oil from mature steamflood projects.
Gravity drainage of oil was studied using hot nitrogen, steam condensate, and hot water in 183 cm (6 foot) vertical cells, packed with sand retrieved from the steam chest regions of the Kern River Field. Sandpacks were saturated with Kern River crude (13 API) at irreducible water saturation at about 94 C (200 F) before initiation of gravity drainage process. Water and crude oil were allowed to drain by gravity from the bottom of the cell for about five to nine months in each test. The drained fluids were replaced at the top of the cell by nitrogen, steam, or water.
The residual oil saturation to steam condensate was 10% 5% and was slightly higher, 16.5% 1%, to hot water draining along the oil column. The average oil saturation to hot nitrogen was 22% 3%. The average residual oil saturation in the recent cores from steamed zones of the Kern River field was 6% 3%. This indicates that in addition to drainage time, other mechanisms such as compositional changes of the oil may be responsible for the further reduction of the oil saturation in the field.
The oil drainage rate shows a short and high initial period followed by a long and slow declining rate. During the initial period, the main flow channels are established for the gas or water to flow and to replace the drained oil. After this period, the oil production is by film drainage. Drainage rate for all tests were similar after the fifth day which implies that film drainage rate is not a function of the oil permeability or what (gas, water) is replacing the drained oil.
In all cases the residual oil saturations are lower than those previously observed in the displacement tests. This indicates that there is a gravity/time contribution to the residual oil saturation that is not measured in the viscous displacement tests. Gravity drainage results also indicate that low recoveries from the displacement tests may be due to high rates used for the injected fluids. This may cause separation and isolation of oil drops that are more difficult to mobilize. Under gravity drainage, the oil phase remains continuous and can drain to thin layers (films) with time.

Introduction
In order to evaluate the effect of gravity drainage alone in lowering the residual oil saturation in a steamflood process, experiments should be designed to isolate this mechanism. Sandpacks should have significant regions uninfluenced by end-effect or by capillary transition zone to ensure that the density difference is the only driving force. In addition the drained water and oil should be replaced with the new fluid instead of being displaced by injection. Under these conditions alone the pure gravity drainage phenomena can be studied.
Previous gravity drainage investigations mostly involve injection of a gas (air, nitrogen, or steam). Very few researchers have investigated pure gravity drainage phenomena. Some of these investigations are briefly reviewed here.
Vizika and Blunt et al. studied the equilibrium thickness of a flat oil layer between water and gas. Blunt et al. performed gravity drainage experiments in 122 cm, 48-darcy sandpacks starting with waterflooded conditions for a day of free drainage followed by air injection. They reported final oil saturations of 4% and 10.5% for high and low interfacial tension systems, respectively. It was concluded that the oil film ensures pressure continuity in the oil phase, and that allows efficient oil drainage under gravity. A low oil/water interfacial tension causes stable oil layers and results in lower saturations than from the high interfacial fluids. In core floods, however, thick oil layers stabilized by capillary forces are formed as the oil bank moves through the system.
P. 395^