Many heavy oil and oil sands formations in Canada have mobile water zones in communication with the oil zone. The high water saturation zones are usually below the oil zone, but may also occur above. Under such circumstances, the heavy oil is difficult to exploit by any recovery method. Even under primary recovery, the water cut climbs rapidly to nearly 100% water.
This study is devoted to steamflooding very viscous and moderately viscous oils when a mobile water zone is present. A steamflood numerical model was used for the runs conducted. Selected results are also given for a scaled physical model for bottom water.
The oil considered in this study is the Aberfeldy crude, which represents a typical Saskatchewan oil. Water-to-oil zone thickness ratios were varied from zero, i.e. no water, to one tenth. Two steam injection rates, and several vertical permeabilities and oil saturations were considered.
It was found that it may be feasible to conduct a steamflood when only a thin bottom water zone is present, and/or the vertical permeability is very low. Partially penetrating wells may offer a solution in some cases. [n general, each bottom water situation should be examined carefully, for in some seemingly adverse cases a steamflood may be economically viable. Numerical simulators are best situated for such studies.
The presence of a high water saturation zone or a water leg in heavy oil formations is a common occurrence in Saskatchewan heavy oil reservoirs. Steam injection is often inefficient under such conditions, because the injected steam is channelled into the water zone, due to its higher flow conductivity. As a result, the water zone is heated to a greater extent than the target oil zone. In time, some of the: heat will be conducted into me overlying oil zone; the effectiveness of such heat transfer would depend on a number of factors, such as water-to-oil zone thickness ratio, permeabilities, oil saturations of the two zones, vertical permeability, steam injection rate, and time.
Relatively few studies have dealt with the effect of a bottom water layer on steamflood efficiency. Here, "bottom water" is considered [Q be a high water saturation zone, containing residual oil. In practice, bottom water may consist of a high water saturation zone, a water leg, a transition zone, or even a water drive. Such a zone may occur above the oil zone in some instances. Generally speaking, presence of bottom water is undesirable in a steamflood. However, in some cases it may be helpful, as in sleamflooding a highly viscous oil, where the water zone would provide initial injectivity.
Several investigators have conducted laboratory experiments with bottom water steamfloods. Pursley (1) carried out experiments with a scaled model to simulate a 1.2 acre pattern. The sand thickness was DO ft, with 15% bottom water and oil viscosity was 100,000 cp. Upon injection of one pore volume of steam, 36% recovery was obtained. Steam override was observed, therefore, it contacted a large proportion of the oil.