Recovering oil bypassed by a steam override zone

The invention is a method of recovering oil bypassed by a steam override zone. First, the production well is drilled below the oil producing zone into an underlying interval. The well perforations in the oil producing zone are closed off. The underlying interval is fractured to establish fluid communication between the underlying interval and the oil producing zone. The production well is completed for production through the fractured portion of the underlying interval. Finally, the steamflood is controlled to increase the pressure in the oil producing zone and drive the oil and steam downward to the new perforations in the underlying for production.

BACKGROUND OF THE INVENTION 
The invention concerns a method to recover a portion of oil bypassed by a 
steam override zone during a steamflood by recompleting a production well 
into the interval underlying the oil producing zone. 
It is well recognized that primary hydrocarbon recovery techniques recover 
only a portion of the petroleum in the formation. Thus, numerous secondary 
and tertiary recovery techniques have been suggested and employed to 
increase the recovery of hydrocarbons from the formations holding them in 
place. Thermal recovery techniques have proven to be effective in 
increasing the amount of oil recovered from the ground, particularly for 
heavy oils. Steamflooding has been the most successful thermal recovery 
technique yet employed in commercial practice. However, steamflooding may 
still leave up to 60% to 70% of the original hydrocarbons in place, 
depending on the formation and the quality of oil. 
When an oil reservoir is subjected to steam injection, steam tends to move 
up in the formation, and condensate and oil tend to move down due to the 
density difference between the fluids. Gradually, a steam override 
condition develops, in which the injected steam sweeps the upper portion 
of the oil zone, but leaves the lower portion untouched. Injected steam 
will tend to follow the path of least resistance from an injection well to 
a production well. Thus, areas of high permeability will receive more and 
more of the injected steam which further raises the permeability of such 
areas. This phenomenon exists to an even larger degree with low injection 
rates and thick formations. The steam override problem worsens at greater 
radial distances from the injection well because steam flux decreases with 
an increase in steam zone radius. 
Although residual oil saturation in the steam swept region can be as low as 
10%, the average residual oil saturation in the formation remains much 
higher due to poor vertical conformance. For these reasons, increasing 
vertical conformance in steamfloods by reducing the amount of oil bypassed 
by a steam override zone has long been a concern of the oil industry. 
SUMMARY OF THE INVENTION 
The invention is a method of recovering oil bypassed by a steam override 
zone during a steamflood which comprises a multi-step method of 
recompleting a production well in the interval underlying the oil 
producing zone, and altering the steamflood to drive oil and steam 
downward through a new path to the production well. First, the production 
well is drilled below the oil producing zone into an underlying interval. 
The well perforations in the oil producing zone are closed off to prevent 
oil and steam from entering the production well from the oil producing 
zone. The underlying interval is then fractured to establish fluid 
communication between the underlying interval and the oil producing zone. 
The production well is completed for production through the fractured 
portion of the underlying interval. Finally, the steamflood is controlled 
to increase the pressure in the oil producing zoe and drive the oil and 
steam downward through the fractures to the new perforations in the 
underlying interval. Hydrocarbons and other fluids are then recovered 
through the production well.

DETAILED DESCRIPTION 
Oil production is improved by the use of the invention method in a 
steamflood. A greater volume of oil is recovered because of two main 
factors. First, the path of the steam override zone is changed to channel 
through oil previously bypassed to reach the new location of the 
production well. Second, the method of recovery is changed from a 
horizontal drive to a vertical drive which is much more efficient for 
displacing fluids of significantly different densities, such as steam 
displacing oil. 
The method is first practiced by drilling a production well below the oil 
producing zone into an underlying interval. The well perforations in the 
oil producing zone are closed off to prevent oil and steam from entering 
the production well from the oil producing zone. The underlying interval 
is fractured in a way well known to those skilled in the art to establish 
fluid communication between the underlying interval and the oil producing 
zone. The production well is then completed for production through the 
fractured portion of the underlying interval. 
The steamflood is controlled to increase the pressure in the oil producing 
zone to drive the oil and steam downward to the new perforations in the 
underlying interval. It will normally be necessary to increase the steam 
injection rate to accomplish this goal. However, other steps may be taken 
to drive the oil and steam downward to the underlying interval. Such steps 
include controlling production rates, shutting in of selected production 
wells, altering steam flow paths through other methods such as the use of 
steam foaming agents, and other methods known to those skilled in the art. 
By increasing the pressure in the oil and steam zone to drive the oil 
downward, displacement is accomplished chiefly by a dynamic pressure 
gradient instead of buoyant forces. Thus, the oil can be made to move 
downward to the lower perforations even against a high water saturation in 
the underlying interval. If the lower zone is gas filled, then the 
replacement of gas by oil will also be affected by buoyant forces along 
with displacement by the dynamic pressure gradient. By closing the 
original perforations where steam was being lost and continuing steam 
injection, the dynamic pressure gradient in the steam zone increases. This 
changes the method of recovery from a horizontal drive to a more efficient 
vertical drive, at least in the proximity of the production well. 
The location and angle of fractures to the upper oil producing zone cannot 
be completely controlled. Fractures will vary depending on the method of 
fracturing employed, and geology factors such as the kind of rock, rock 
orientation and method of deposition. It is desirable to fracture with a 
maximum angle of 45.degree. with the interface between the oil producing 
zone and the underlying interval and have the fracture extend a 
substantial distance away from the production well, preferably up to half 
the distance to the injection well. Such a fracture location will pull 
steam down to sweep through the middle of the oil bypassed by the steam 
override zone. 
The new perforations in the underlying interval are preferably made about 
10 feet to about 50 feet below the oil producing zone. A minimum of 10 
feet is necessary to prevent the new completion from pulling steam down 
from around the closed perforations if the steam phase has already reached 
the base of the oil zone. Such a coning down of steam immediately around 
the production well will defeat the purpose of the invention in that 
instance. Otherwise, the distance at which to begin new perforations below 
the oil zone will depend upon the individual case. For example, in 
numerical simulation studies, the invention method with new perforations 
10 to 50 feet below the oil producing zone recovered more oil than new 
perforations made from 0 to 50 feet below the oil zone. However, both of 
these cases produced more oil than a base case without the invention 
method and a standard completion throughout the oil producing zone. 
The 50 foot guideline is quite arbitrary and could be changed depending on 
many factors, such as well spacing, type of rock, and rock orientation. 
Assuming a maximum 45.degree. fracture angle, a close well spacing of 165 
feet, and fractures extending about one-third of the distance between the 
injection and production wells, the new perforations should probably be no 
more than 50 feet below the upper zone. 
An additional improvement can be achieved by performing a standard steam 
stimulation in the newly perforated zone. This additional influx of 
thermal energy will preferentially move through the fractures toward the 
oil zone, and provide a reservoir of heat that will reduce the viscosity 
of the approaching oil. Response time will be shortened and oil flow 
increased by this procedure. 
The production well is completed in the underlying interval by the use of 
any of several methods well known in the art. Most production wells 
employed in steamfloods are completed with a gravel pack, slotted liner 
and a pump placed at the bottom of the production well to aid in 
production. Preferably, the production well will be completed with an 
underreamed gravel pack and slotted liner extending through the fractured 
portion of the underlying interval and the bottom one-half of the oil 
producing zone. 
Certain reservoir conditions may decrease the oil recovery benefit obtained 
by the instant invention. A high water saturation in the underlying 
interval may make it more difficult for the oil and steam to be driven 
downward to the new perforation interval in the the underlying interval. 
Water or gas in the underlying interval may tend to dominate initial 
production from the newly completed well. Such a problem may be diminished 
by the use of increased pressure in the steam and oil zone to drive the 
fluids down through the fracture into the underlying interval and into the 
newly completed production well. 
A relatively high permeability in the underlying interval will also 
decrease or perhaps eliminate the added recovery that may be obtained with 
the invention method. In such a case, the oil from the oil producing zone 
must first saturate the portion of the underlying interval down to the 
perforations to at least a residual oil saturation to steam before any oil 
can be produced from the perforations in the underlying interval. 
As a result, it is preferable to have an underlying interval having a 
relatively low permeability, most preferably a permeability of about 0 md 
to about 50 md. With low permeability, it is only be necessary to saturate 
the fracture flow channels. The loss in oil production due to the trapping 
of oil in and around the fractured channels will not be sufficient to 
prevent a successful application of the invention method. An example of an 
oil producing zone having an underlying interval with a relatively low 
permeability is the Aurignac zone of the San Ardo Field in Central 
California. Granite underlies a large portion of the Aurignac zone. 
Many other variations and modifications may be made in the concepts 
described above by those skilled in the art without departing from the 
concepts of the present invention. For example, the distance drilled into 
the underlying interval, the portion of the well completed, and the size 
of the fracturing job can all be varied within the scope of the invention. 
Different types of wells including vertical wells, horizontal wells or 
slanted wells, may all be employed in the invention method. Accordingly, 
it should be clearly understood that the concepts disclosed in the 
description are illustrative only and are not intended as limitations on 
the scope of the invention.