Toe-to-heel oil recovery process

Details Toe-to-heel oil recovery process Toe-to-heel oil recovery process

1998-09-04

2001-01-02

Bagnell, David (Department: 3672)

Wells

Processes

Distinct, separate injection and producing wells

C166S050000

Utility Patent

active

06167966

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to guiding the advance of a liquid displacement front by means of a production well having an open horizontal leg oriented toward the injection well, which acts as a linear pressure sink to which the front is attracted and by which it is guided, and to an oil recovery process utilizing this mechanism.
BACKGROUND OF THE INVENTION
Waterflooding of underground oil-bearing reservoirs is the most common secondary oil recovery process utilized. In some cases, for example in off-shore reservoirs drilled from platforms, water is injected continuously from the start of oil production, for the purposes of maintaining the high production rates necessary to off-set the high production costs and to extend field life.
In general, the usual technique involves providing spaced-apart vertical injection and production wells completed in a reservoir. Typically, an injection well will be located within a pattern and a displacement front is advanced outwardly toward the surrounding production wells. Or alternatively, a row of injection wells may feed the injection fluid to a laterally extending displacement front which advances as a line drive toward a parallel row of production wells. The arrangement of wells can be either in a direct or staggered line drive, but the staggered line drive is preferred. In both cases, the operator seeks to establish an upright displacement front which provides good vertical sweep and advances generally horizontally through the reservoir with good lateral sweep.
However, the process is characterized by major difficulties. Even in a perfectly homogeneous reservoir rock, the phenomenon of gravity segregation causes the advancing water displacement front to slump because water typically has a higher specific gravity than the oil it is displacing. The result is a poor volumetric sweep efficiency and a low oil recovery as oil in the upper part of the reservoir is left behind. This “under-riding” problem is compounded when the reservoir permeability is greatest in the lower reaches. The geological reservoir sand property of “fining upwards” is quite common and is a form of heterogeneity that aggravates to the described water underflow.
A partial solution to this problem can be achieved by adding to the injected fluid a viscosifying agent, such as Xanthan
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or polyacrylamide polymer, in order to reduce the mobility of the water. These agents tend to retard the advance of the injected fluid front through the more permeable streaks and/or lower layer sections. However, this solution, while it can be somewhat effective, seems to be relatively expensive and demanding from the operational point of view, and is not extensively practised. The problem of poor injectivity of viscous solutions of polymer, shear instability, and bacterial degradation have been barriers to polymer use.
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trade mark
One approach employed for waterflooding or chemical flooding oil reservoirs is the use of parallel horizontal wells alternating as injectors and oil producers. This approach provides high rates of fluid injectivity and oil production, but it is expensive since each horizontal well can cost millions of dollars. Problematically, once water breaks through in the production wells, the water rates rise quickly beyond the economical limit. The problems of water under-riding and poor volumetric sweep are not overcome by this well configuration.
It would therefore be desirable to re-engineer the flood technique so as to better control the way in which the injected liquid fluids such as water, brine, or chemical flooding fluids flow in the reservoir as the displacement front advances, so as to increase the volumetric sweep efficiency. The work underlying the present invention was undertaken to reach this objective.
A prior art process relevant to the present invention is disclosed in U.S. Pat. No. 5,626,191, issued to the same assignee as the present case. This process involves providing an injection well, completed high in the reservoir, and a production well having a horizontal leg completed low in the reservoir. The horizontal leg is oriented toward the injection well with its toe close to the injection well. In a preferred embodiment, in-situ combustion is initiated at the injection well and a laterally extending, upright combustion front is advanced toward the horizontal production well. The latter is kept open, to create a linear low pressure sink. The sink acts to attract and guide the advance of the laterally extending front along its length. It has been found that the combustion front will stay generally upright and its direction of advance is controlled to yield good vertical and lateral sweep.
The processes of the patent and the present case share the notion of using an open horizontal well to create a linear low pressure sink for guiding an oil displacement front. However they differ in other important respects which lead to different results, It will be noted that the patent is concerned with gaseous injectants and depends on the phenomenon of gravity drainage and a special special viscosity distribution, which is induced by a differential heating of the oil across the formation pay. The present invention differs in being based on a liquid injectant, different well completions and different procedures to achieve different results.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention it has been determined that:
if an injection well (usually vertical) is completed low in an oil-containing reservoir and a production well, having a horizontal leg, is completed relatively high in the reservoir, the horizontal leg being oriented toward the injection well so that the leg lies in the path of a displacement front emanating from the injection well; and
if a generally linear, laterally extending and upright water displacement front is established and propagated in the reservoir;
then the horizontal leg, which is at low pressure (normally achieved by keeping the production well open), provides a low pressure sink and outlet that functions to induce the front to advance in a guided manner, first toward the “toe” and then along the length of the leg to the “heel”;
under these circumstances, the front has been found to remain generally stable and relatively upright and is characterized by good sweep efficiency; and
additionally, as newly injected water flows through the water-invaded zone of the reservoir and through the relatively upright displacement front, the streamlines bend upwards toward the horizontal leg due to the upward flow gradient created by the action of the low pressure sink. Displaced oil tends to keep the bore of the horizontal leg full of oil, so that little water is produced through the production well until the water front has advanced along a substantial portion of the length of the horizontal leg.
This embodiment is referred to as the single-stage version of the invention. When tested, it demonstrated that a liquid fluid injectant that is heavier than the oil in place (such as water, brine or heavy brine containing a high content of dissolved salts or the like) will rise or advance upwardly in the form of a displacement front from a starting point low in the reservoir, if a linear, low pressure sink and outlet is provided high in the reservoir. In doing so, the front displaces oil ahead of it with a desirable degree of sweep and efficiency.
A second embodiment, referred to as the two-stage process, has also been developed and demonstrated. This process provides additional benefits of greater reservoir sweep, mainly in reservoirs with highest permeability in the lower reaches of the reservoir and which contain high viscosity oil.
According to the two-stage embodiment:
a well configuration is provided comprising an injection well completed low in the reservoir and production well means comprising a vertical leg completed low in the reservoir and a horizontal leg completed relatively high in the reservoir. The production well means may be a single well having two legs or two wells, one vertical and one

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