Situ Combustion Initiation Process

Abstract

The initiation of in situ combustion in formations penetrated by a wellbore is accomplished by extracting a portion of the air stream being injected into the formation, removing oxygen from the extracted air portion, and adding the oxygen to the air stream being injected into the formation.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method for initiating in situ combustion. In situ combustion is a process which involves burning hydrocarbon fluids contained in a formation. This process is employed for several purposes. The primary purpose of in situ combustion is to stimulate the recovery of hydrocarbon fluids. Another purpose of in situ combustion is to consolidate the formation adjacent the wellbore so that particles of the formation do not clog the well or damage well equipment.

Ordinarily in situ combustion involves several steps. Usually air is injected into the formation through an injection well. The injection pressure is maintained at a level to cause air to flow through the formation from the injection well to one or more producing wells, and it is injected at a sufficient rate to support a combustion reaction of a fraction of the oil in the formation. In order to initiate the combustion reaction in most oil bearing formations, it is necessary to inject heat along with the air. The heat is carried by the air into the reservoir, where it contacts the formation fluids. By flowing a sufficient volume of hot air into the reservoir, the crude oil in the vicinity of the injection well is heated to its ignition temperature and commences to burn. As the flame front of the ignited formation fluids moves away from the injection well, the heat and pressure created aids movement of unburned hydrocarbon fluids to production wells. The process just described is the most common process of reservoir stimulation by in situ combustion.

In a process of sand consolidation, the in situ combustion process usually operates quite differently. In such a process, air is usually injected in wells adjacent the well having a sand consolidation problem, and heat is provided in the well having such a problem. Upon achieving the ignition of formation fluids, the flame front will move outwardly from the wellbore having the sand consolidation problem toward the source of air injected in the adjacent wells. This burning of reservoir fluids is continued until the area to be consolidated has been traversed. This process is ordinarily termed a "reverse burn" in situ combustion process. A characteristic of such a reverse burn is that a residue of coke is left on the particles making up the formation. This coke residue effectively bonds together the elements of the formation, thereby eliminating the deficiency in formation consolidation.

The most common methods of initiating in situ combustion in the formation is to apply heat by using downhole heaters. These heaters usually consist of downhole electrical heaters, gas burners, and catalytic reactors. Even with a high injection rate of a heated gas in the formation, several days are often necessary before combustion is initiated. Several days are needed in order to supply a sufficient amount of heat to initiate an oxygen-hydrocarbon burn in the formation. Because of the substantial cost of wellbore heating services, it is desirable to initiate in situ combustion as quickly as possible. Another reason for the lengthy time required for initiating the in situ combustion relates to a temperature limitation. In order to prevent well damage due to excessive heat, heater temperatures should be maintained below approximately 800.degree. F. It is therefore an object of the present invention to provide an improved method of initiating in situ combustion.

SUMMARY OF THE INVENTION

With these and other objects in view, the present invention includes injecting air into the formation, which together with hydrocarbon fluids contained in the formation comprises a fuel mixture. Oxygen is added to the air stream being injected into the formation to make a more combustible fuel mixture. This oxygen is supplied by extracting a portion of the air being supplied to the wellbore, liquefying such air, vaporizing the nitrogen, and subsequently injecting the remaining oxygen into the air stream entering the formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a wellbore with a heater contained therein and a schematic illustration of an oxygen enriching system connected therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the FIGURE, a wellbore 10 is shown penetrating a formation 50. The wellbore 10 comprises casing 40 extending from the surface 54 through the formation 50. Located within casing 40 and extending from the surface to a point adjacent and above the formation 50 is tubing 42. At the lower end of tubing 42 is seating nipple 46 which is an annular flange for seating wellbore tools. Suspended in tubing 42 by line 52 is heater 48. Line 52 may be an armored electrical cable if the heater 48 is electrical or an armored thermocouple cable if the heater is catalytic, or a combination of the two. Heater 48 is suspended in the tubing by resting on seating nipple 46.

Wellhead 44 atop wellbore 10 has an air line 66 connected thereto through valve 38. Air compressor 14 having air inlet 12 is connected with air line 66 for providing air to the wellbore through the annular space between casing 40 and tubing 42. An air tap 16 located in air line 66 allows a portion of air leaving air compressor 14 to enter heat exchanger 18. The heat exchanger 18 may consist of two helical conduit coils 56 and 58, immersed in a heat transfer fluid occupying the chamber housing the helical coils 56 and 58. Located in series with heat exchanger 18 by line 19 is expansion valve 20. The expansion valve 20 comprises a valve which allows for quick expansion of a gas flowed through the valve. Connected with expansion valve 20 by line 21 is liquefier 22. Liquefier 22 may comprise a series of compressors and expansion valves located in area 24 of such liquefier 22.

Liquids removal tap 32 is located on the lower side of liquefier 22, and may be float controlled. In such a system, a float may be connected to a gravity seated valve so that as the liquid level rises, it raises the float and connected valve to drain certain liquids from the liquefier 22. The liquefier may be one of the well known types such as the Hampson-Linde Regenerative Process or the Claude System of liquefying air both of which are described on Pages 4-58, 59, of MARK'S MECHANICAL ENGINEERS HANDBOOK, Sixth Edition. Both of these liquefaction processes utilize successive steps of compression and expansion.

Area 26 of liquefier 22 contains a gas port 28 located on its upper side for removal of gaseous material. Such gas port is a one way pressure operated valve. Located on the lower end of area 26 is liquid tap 30. Such liquid tap 30 may also be float valve operated in the same manner as liquid removal tap 32. The gas port 28 is connected to helical coil 56 of heat exchanger 18 by line 29. Helical coil 56 extends through the heat exchanger 18 and connects with exit port 34. Liquid tap 30 which also may be float valve operated as are liquids removal tap 32 and liquid tap 30 is located at the lower end of liquefier section 26 and connects with gasifier 60 through line 31. Gasifier 60 comprises a chamber which is constructed to provide for a controlled elevation in temperature. It may be a heavily insulated chamber having a liquid circulating therein in enclosed pipes whose temperature may be accurately controlled by an air conditioning system. Heat for such a system may be derived from the compressor or cold may be derived from gases being vaporized in area 26.

In the operation of the apparatus previously described, air is extracted from the atmosphere through air inlet 12 whereupon it is compressed in air compressor 14. The air exiting air compressor 14 flows through air line 66 which connects with wellhead 44. A substantial portion of the air stream leaving air compressor 14 flows through air tap 16. Such flow is caused by a lower pressure in the initial portion of the system connected with air tap 16. A sufficient amount of air is extracted at air tap 16 so that a portion of the compressed air can be utilized to operate the air liquefaction equipment located downstream from air tap 16. This extracted air portion then flows through the heat exchanger 18, where the air is cooled by a cold nitrogen stream coming from liquefier 22 through line 29. After the air leaves heat exchanger 18 through line 19 by the force supplied by compressor 14, it passes through expansion valve 20 where the air is quickly expanded in order to further reduce its temperature.

The air which is now cold enters liquefier 22 where in area 24 it is subjected to the successive steps of compressing, cooling, and expansion until the air is liquefied. The liquefier 22 is operated by utilizing the expansive powers of the extracted air portion. The compressed air is allowed to expand to drive pistons which supply energy for operation of the compressor in liquefier 22. During liquefaction of the air, water vapor and carbon dioxide are the first to liquefy. They may be separated from the liquefier through the float operated valve of liquids removal tap 32. The water vapor and carbon dioxide may also be allowed to solidify before separation.

The liquefied air is then pumped to area 26 of liquefier 22, whereupon its temperature is raised above the boiling point of nitrogen, but is kept below the boiling point of oxygen. The nitrogen boils from the liquid air in area 26 and exits through gas port 28. The gasified nitrogen under its own pressure then passes through line 29 to heat exchanger 18. In the heat exchanger, the cold nitrogen is used to cool the air passing through the heat exchanger which has been extracted from line 66. The nitrogen in passing through helical coil 56 cools the heat transfer fluid which surrounds coils 56 and 58, and such fluid in turn cools the air in helical coil 58. The nitrogen is then exhausted through port 34 to the atmosphere.

The liquid oxygen remaining after the nitrogen has been boiled off, is removed by gravity from liquid tap 30 which is a float operated valve located on the underside of area 26 of the liquefier 22. This liquid stream of oxygen then enters gasifier 60 through line 31. The gasifier allows controlled temperature elevation to permit the temperature to exceed the boiling point of oxygen. As the oxygen is boiled from the liquid in gasifier 60, it is removed through overhead outlet 33, and proceeds through valve 68 into line 66 through oxygen inlet port 36, where it joins the main air stream entering wellbore 10.

This oxygen enriched air stream is then pumped down the annulus between casing 40 and tubing 42, whereupon it flows past heater 48 and enters formation 50 through perforations 49 in casing 40. Upon entering the formation, the oxygen enriched air starts to oxidize hydrocarbon fluids contained in the formation 50. Once sufficient oxidation has occurred, the formation fluids will commence to burn and the burning front will move away from the wellbore 10. When gas analysis from adjacent wells indicate that in situ combustion has commenced, oxygen enrichment of the air stream entering wellbore 10 may be terminated, so that the full capacity of the air compressor 14 can be utilized to provide sufficient air to support the in situ combustion.

The downhole heater 48 is not always essential in this process of initiating in situ combustion. Some reservoirs will ignite using an oxygen enriched air stream which has not been heated. Other reservoirs may require that such oxygen enriched stream be heated before in situ combustion will commence. Heaters which may be employed can be of any conventional type, i.e., downhole gas burners or electrical and catalytic heaters.

An advantage of the method of supplying oxygen to the formation described herein is the availability of excess compressor capacity for use in providing energy for the liquefaction of the air. A large compressor is required to supply air to support a flame front when the front covers a large area because of its distance from the wellbore. Accordingly, since only a small amount of air is needed for initiation of in situ combustion adjacent the wellbore, there is excess compressor capacity available for other purposes. The above described process makes use of this capacity by using it to provide energy for liquefaction of air. It is not essential to the operation of this process to have a pure oxygen stream being injected into the air line 66. This lack of a purity requirement will result in a savings in equipment and operational costs.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

Claims

What is claimed is:

1. In an in situ combustion process wherein air is supplied to a formation being ignited, the improvement comprising: separating a portion of the air stream being supplied to the formation; removing the oxygen from the separated air stream; and injecting the oxygen removed from the separated air stream into the air stream being supplied to the formation.

2. The process of claim 1 wherein the oxygen is removed from the separated air stream by successive stages of compression and expansion of the air until the air is liquefied, and then boiling off the nitrogen component of the air.

3. The process of claim 2 including cooling the separated portion of the air stream by heat exchanging such separated air portion with the nitrogen boiled from the liquefied air.

4. The process of claim 1 including supplying heat to the formation being ignited.

5. In an in situ combustion initiation process where compressed air is supplied to the formation being ignited, the improvement comprising: separating a portion of the compressed air being supplied to the formation; removing the oxygen from the separated air portion; and injecting the oxygen removed from the separated air portion into the air stream being supplied to the formation.

6. The process of claim 5 wherein the oxygen is removed from the separated air portion by liquefying the air portion and boiling off the nitrogen component of the air.

7. The process of claim 5 wherein the expansive power of the compressed air portion is used to supply energy for the machinery to liquefy the air portion.

8. The process of claim 6 wherein the nitrogen boiled from the liquefied air is used to cool the separated air portion prior to liquefaction.

9. Apparatus for initiating in situ combustion in an earth formation penetrated by a wellbore compris gas conduit means connected to the wellbore for passing air into the wellbore and into the formation; means attached to the condui means for extracting a portion of the air stream from the gas conduit means; air liquefaction means connected with the extracti means; means for boiling nitrogen from the liquid air which is in communication with the air liquefaction means; and means in communication with the air liquefaction means and attached to the conduit means for injecting into the gas conduit means the oxygen portion remaining after boiling the nitrogen from the air.

10. The apparatus of claim 9 including means positioned in the wellbore for heating the oxygen enriched air entering the formation.

11. The apparatus of claim 9 including heat exchanger means adjacent the air stream portion extraction means, and connected with the nitrogen boiling means.