CA2029203C

CA2029203C - Steam process with foam for recovering viscous oils through horizontal wells

Info

Publication number: CA2029203C
Application number: CA002029203A
Authority: CA
Inventors: Roland P. Leaute
Original assignee: Esso Resources Canada Ltd
Current assignee: Imperial Oil Resources Ltd
Priority date: 1990-11-02
Filing date: 1990-11-02
Publication date: 1994-04-19
Anticipated expiration: 2010-11-02
Also published as: CA2029203A1

Abstract

ABSTRACT

The effectiveness of vertical and horizontal wells in combination for recovery of viscous oil is increased by injecting a mobility control agent into the horizontal well after breakthrough of a thermal fluid from a vertical well down to the horizontal well. The area for gravity drainage of oil into the horizontal well is increased, resulting in greater recovery of oil from the horizontal well.

Description

2029?JQ3 - "

APPLICATION FOR PATENT

INVENTOR: ROLAND P. LEAUTE
TITLE: STEAM PROCESS WITH FOAM FOR RECOVERING
VISCOUS OILS THROUGH HORIZONTAL WELLS

BACKGROUND OF THE INVENTION
Field of the Invention This invention relates to a thermal process for recovering viscous oils from subterranaan formations.
In particular, it relates to the use o~ steam and foam in a formation to improve recovery of oil when horizontal and vertical wells are used in combination.

Description of Related Art Thermal fluids, such as steam and hot water, are widely employed to recover viscous or heavy crude oils from subterranean formations penetrated by one or more wells. StPam is normally preferred to hot water because steam injection puts heat into the formations at a higher rate. The reduction in viscosity of the oil with increasing temperatures brought about by injection of thermal fluids makes possible greater recovery of the oil, but the recovery is still severely limited by the fact that the thermal fluids tend to bypass or override a viscous oil. A number of methods have ~een proposad to achieve greater contact of viscous oil by thermal fluids and greater recovery of the oil ~rom subterranean formations.

2 ~ 3 One method to increase the contact between the oil and the thermal ~luid uses a plugging agent to decrease high permeability zones and force the thermal fluid to contact low permeability oil zones. For example, foam has been used to decrease the mobility or flow of steam through the volume of rock from which viscous oil has been displaced. It is advantageous to combine a non-condensible gas with the steam to decrease steam mobility more effectively. The use of such foams in a steamflood process is disclosed in U.S. Patent No.
4,570,711 and references cited therein.
The use of horizontal wells offers one approach to the problem of obtaining greater recovery of viscous oils. Horizontal wells can be used with the three common types of steam recovery processes: cyclic injection and production ("huff and puff"), flooding and gravity drainage. The cyclic injection and production process is often used to initiate a steam flooding process and to establish the ability to inject steam into a formation containing very viscous oil.
This process can be used in vertical or horizontal wells. Flooding between a horizontal well and vertical wells nearby has often been considered, and it requires careful placement of the relative positions of the horizontal and vertical wells, as described in the paper by R. R. MacDonald, "Drilling the Cold Lake Horizontal Well Pilot No. 2,'~ SPE Paper 14428, September, 1985. Gravity force can be utilized to drive the oil downward into horizontal w~lls after a significant amount of thermal fluid injection, either by cyclic injection or flooding. This method was discussed in the paper by R.M. Butler and D.J. Stephens entitled, "The Gravity Drainage of Steam-Heated Heavy Oil to Parallel Horizontal Wells," The Journal of Canadian Petroleum Technoloay, April-June, 1981.

~2~ 3 Gravity drainage of the viscous oil after it has been heated by a thermal fluid shows significant promise as a method to recover a relatively high percentage of the viscous oil in formations, especially in mature projects where a large heated zone has been established in the top of the formation.
Flooding processes using one or more horizontal wells have been studied in scaled models constructed in the laboratory, as described in the paper by P.F. Ahner and A.H. Sufi, "Physical Model Steamflood Studies Using Horizontal Wells," SPE/DOE 20247, April, 1~90.
Although horizontal wells can be utilized to improve the recovery of viscous oils after thermal fluids are injected, particularly by taking advantage of the force of gravity to drive the oil downward into horizontal wells, there is still the need to drive oil downward into the horizontal well at a more rapid rate than can be achieved by gravity drainage of cold oil into the horizontal well. If cyclic steam injection and production in the horizontal well is used to heat the rock around the horizontal well and improve gravity drainage rate when the well is put on production, only a small segment of the horizontal well is affected.
Vertical wells drilled and completed above the horizontal well can be used to initiate thermal fluid ~ommunication in the vertical direction down to small segments along the horizontal well, but there is a significant need ~or a process which increases the length of the horizontal well along which oil can drain into the well because the increase of this length will increase the rate of gravity drainage of oil into the horizontal well.

2!92~3 SUMMARY OF THE INVENTION
In a subterranean formation containing a viscous oil, vertical wells are drilled which partially penetrate khe oil-bearing formation. These wells are perforated near the bottom of the well and produced by known cyclic steam injection and production methods for a significant period of time, usually more than five years. When these productive methods become uneconomical, one or more horizontal wells are drilled through the same formation on a path near the bottom-hole location of the vertical wells and at a depth of a few meters below the bottom of the vertical wells.
Steam is injected into the vertical wells to establish ~luid communication between one or more of the vertical wells and the horizontal well, taking advantage of the heating which has occurred below the perforations in the vertical well. A foam made with steam or a mixture of steam and a non-condensible gas is injected into the horizontal well to reduce the permeability of the more permeable zones. The plugging action from the foam increases the volume of the fluid communication paths between the vertical wells and the horizontal well.
Finally, oil is produced from the horizontal well. The temperature changes in the formation can be monitored by instrument~ and foam injection through the horizontal well repeated to optimize oil recovery.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 illustrates the conventional configuration of vartical wells used in steam recovery operations.
Fig. 2 shows horizontal wells which have been drilled beneath the vertical wells.
Fig. 3 illustrates the distribution of the hot steam zone and the cold reservoir around a vertical ~2~

well after a significant time of cyclic steam injection and production from the well.
Fig. 4 illustrates the initiation of vertical communication of fluids between the vertical well and a segment of the horizontal well.
Fig. 5a illustrates the configuration of the steam zone and the cold reservoir after the vertical well has been steamed and produced for a significant time and before the horizontal well has been drilled or completed. Fig. 5b shows the enlargement in the heated zone along the horizontal well which is achieved by the practice of this invention. Fig. 5c illustrates the configuration of the steam zone and the cold reservoir as gravity drainage occurs into the enlarged vertical heated zone.
Fig. 6 illustrates cross-sectional views along the horizontal well. Fig. 6a shows conditions soon after the horizontal well is completed and fluid communication is established between the vertical well and the horizontal well. Fig. 6b illustrates the configuration soon after foam injection is initiated into the horizontal well. Fig. 6c illustrates the enlargement of the vertical heated zone which is accomplished by injection of steam and foam.

DETAXLED DESCRIPTION OF PREFERRED EMBODIMENTS
In Fig. 1, a plurality of vertical wells 10 are drilled into the oil-bearing formation 20, the bottom hole location of the wells being preferably aligned in a selected direction. The casings in the wells contain perforations 11 near the bottom of the wells. The wells are not drilled to the bottom of the formation 20, but are preferably drilled to a depth to place the perforations in the bottom half of the formation 20.
The wells are substantially vertical but may be -- 2 Q 2~

inclined from vertical by as much as 30 degrees when they enter the oil-containing formation, and may be drilled as directional wells from the surface.
Preferably, the wells are inclined from vertical less than 10 degrees in the producing formation. Wells in this configuration are used for steam stimulation by sequentially injecting steam and producing fluids for a plurality of cycles. As this steam stimulation process matures, however, the process thermal recovery efficiency declines to its economic limit after only 15 to 30 per cent of the original oil in place is recovered. At that stage, typically 5 to 15 years after initial introduction of steam into the formation, the remaining oil driving mechanisms only support marginal well productivity. These mechanisms may include steam drive between wells or gravity drainage of oil.
In Fig. 2, one or more substantially horizontal wells 30 have been drilled through the formation 20, the horizontal well being directed such that it passes approximately beneath one or more of the vertical wells, and preferably beneath three or more of the vertical wells. The well is substantially horizontal but may deviate by as much as 20 degrees from horizontal. Preferably, the deviation is less than 10 degrees. The horizontal well preferably contains a slotted liner (not shown) extending along its length in the formation 20. Alternately, the well 30 may be completed with cemented casing and perforations or other completion techniques well known in industry.
The horizontal well is drilled and completed at any convenient tima in the life of the field, either before or after the vertical wells are drilled and completed.
It is important that the location of the horizontal well with respect to the vertical wells be carefully 21~2~3 controlled. The horizontal well will normally be drilled from 3 meters to 20 meters below the vertical wells, and preferably will be targeted from 5 to 15 meters below the vertical wells.
Fig. 3 shows an enlarged vertical cross-section of the oil-bearing formation 20 surrounding a typical well in which the cyclic steam injection and production process has been applied for a period of years. The formation volume can be divided into three distinct regions. A first region 22 near the vertical well corresponds to the zone depleted in oil during the steaming operations. Significant increases in temperature as well as decreases in oil saturation have occurred. Gravity drainage of oil into the well continues to proceed most effectively at the periphery of the region 22.
Beyond the region 22 is the region 21, where negligible changes in temperature and saturation have taken place. The region particularly important in this invention is labelled 23 which is the region primarily below the perforated interval in the vertical wells.
This region can be considered a type of sump where high saturations of hot mobile oil exist. The higher temperatures result from leakoff of hot fluids and conduction of heat downward in the formation from the vertical well. It is expected that the temperature and fluid mobility will decrease rapidly away from the perforations. In some reservoirs, the permeability of the formation will also decrease at greater depths in the formation. Preferably, the horizontal well should intersect as many of the heated sumps as possible.
Therefore, the horizontal wells are oriented in line with each row or column of the vertical wellsO
The lateral and vertical extent of the sump region 23 at each vertical well is variable, depending on g~ 2 ~ 3 formation properties, the amount of fluids injected in each well and other factors. Upon resumption of steam injection in each vertical well, the viscous oil inflow from the vertical well to the horizontal well will be limited and confined to narrow isolated paths along the horizontal well. To help monitor the situation between the vertical wells and the horizontal well, it is preferable to have installed thermocouples or other temperature sensing devices along the horizontal well.
Similarly, it is preferable to have installed in the bottom of the vertical wells temperature sensing devices. In this way, interchange of hot fluids between the various sumps and the horizontal well can be monitored.
Fig. 4 depicts the injection of hot fluid into the vertical wells. A mixture of non-condensible gas, such as nitrogen or field fuel gas, and low quality steam is preferable as the injection fluid. The non-condensible gas will hslp increase pressure in the region 22 and region 23. Because of the low quality of the steam, the amount of heat transferred to the region 22 will be minimized in favor of the sump region 23, where hot condensate can be circulated towards the underlying horizontal well 30. As pressure differential builds and temperature increases, breaXthrough of hot fluids into the horizontal well 30 is initiated. Breakthrough is detected by temperature sensors in the horizontal well (not shown) or by use of chemical tracers which are added to the injected fluids in vertical wells and detected in fluids produced from the horizontal well 30. Suitable chemical tracers are inorganic tracers, such as thiocyanate, radioactive tracers such as tritium, or other tracers well-known in industry.
After multiple fluid links between the vertical wells and the horizontal well have been establishad, 2 ~

g the methods of this invention are used to greatly expand the utilization of the horizontal well by increasing the drainage of oil into the horizontal well. This improvement is accomplished by providing a reliable method of enlarging each of the previously established vertical communication paths.
Fig. 5a depicts the sump region 23 after fluid communication has been established betwesn the vertical and horizontal wells as explained above. Fig. 5b illustrates the larger or expanded vertical sump region 23 which is established by the methods of this invention. Fig. 5c illustrates how the gravity drainage mechanism proceeds to recover viscous oil by driving the oil along the interface between the cold zone 21 and the heated region 22 into the expanded sump region 23. While the exact mechanism is not entirely understood, it is believed that the higher rate of oil drainage into the horizontal well 30 is caused by several factors, e.g., less distance for the oil to drain, higher drainage angle at khe interface between regions 21 and 22, and lower resistance to vertical flow in the sump region 23.
Fig. 6 is a cross-sectional view illustrating the method of this invention. After vertical communication to fluids is established between the vertical well and horizontal well, as shown in Fig. 6a, a foam is injected into the formation from the horizontal well, as shown in Fig. 6b. The foam prsferably comprises a mixture of non-condensible gas, steam and surface active foaming agent at an appropriate concentration, as taught in U. S. Patent No. 4,570,71~l but other foaming agents for steam floods are known in industry and are suitable for the practice of this invention.
Fig. 6c illuskrates the enlargement o~ the sump region 23 by the action of the very viscous foam. The non-@ ~

condensible gas helps stabilize and propagate a plug of foam to fill the region 23. As foam is injected, steam is diverted to the cooler edges of the region 23, where it condenses and drives oil upward until the oil saturation decreases to a level sufficiently low to enable foam invasion. As foam injection continues, a progressive enlargement of the sump region 23 above the horizontal well is propagated. Typically, the effective gas/steam permeability is reduced by a factor of about 100 at the residual oil saturations in the foam bank. This reduction is sufficient to divert the injected steam into the cooler zones at the edge of the sump region 23, where higher oil saturations can act as foam breakers and the local temperature is increased by steam. The oil displaced by the foam is moved into a hot region of $he formation, either region 22 or region 23, where it can readily be recovered by drainage into the horizontal well 30 when production into the well is re-initiated. It is an important advantage of this invention that the foam is not required to remain stable for long distances of displacement through the formation 20. It is well-known that such stability is an important limitation to the utilization of foam in steam floods. Furthermore, instability of the foam is desirable after sufficient foam has been injected to enlarge the sump region 23 to the desired size.
Important considerations in sizing the volume of foam to inject in a horizontal well are related to cost and effectiveness, and the optimum size slug of foam must be determined for each formation. It is preferable that the foam be evenly distributed to all vertical wells in communication with the horizontal well. The volume of foam injected should be sufficient to provide a diameter of all the sump areas, assuming they are all equal in diameter, to total more than 10 ~ .

- 2 ~

per cent of the length of the liner in the horizontal well, and preferably more than 20 per cent. It is not necessary that the enlargement of the sump regions be achieved to maximum size by one injection of foam.
Rather, the injection of foam into the horizontal well can be repeated as needed to optimize the recovery rate from the horizontal well. It is preferable to monitor flow of foam from the horizontal well into each vertical well such that the process can be optimized more easily. Temperature sensors in the wells can be used for this purpose. Tracers can be added to the foam and detected in the vertical wells, using techniques well known in industry, to assist in this monitoring process. If uneven flow of foam is occurring into the different sump regions from the horizontal well, injection of water or foam can be initiated into the vertical wells receiving excess foam to restrict the penetration of the ~oam fluid injected through the horizontal well at these locations.
Having described the invention above, various modifications of the techniques, procedures, material and equipment will be apparent to those in the art. It is intended that all such variations within the scope and spirit of the appended claims be embraced thereby.

Claims

1. A method of increasing the recovery of viscous oil from a formation penetrated by at least one substantially vertical well and a substantially horizontal well drilled through the same formation beneath the vertical well comprising:
(1) establishing fluid communication between the vertical well and the horizontal well by injecting a thermal fluid through the vertical well;
(2) injecting foam and steam into the horizontal well;
(3) ceasing injection through the horizontal well; and (4) producing oil from the horizontal well.

2. The method of claim 1 wherein injection through the vertical well is ceased before step 2 is initiated.

3. The method of claim 1 wherein fluid communication between vertical and horizontal wells is detected by temperature sensors.

4. The method of claim 1 wherein fluid communication between the vertical and horizontal wells is detected by a tracer which is added to injected fluid and monitored in produced fluid.

5. The method of claim 1 further comprising injecting a non-condensible gas with the foam and steam in step (2).

6. The method of claim 1 wherein the steps of injecting foam and steam into the horizontal well, ceasing said injection and producing oil from the horizontal well are repeated.

7. The method of claim 1 wherein two or more vertical wells are in fluid communication with the horizontal well.

8. The method of claim 7 further comprising injecting fluid through at least one of the vertical wells while injecting foam and steam through the horizontal well.