Geological Formation Heating

Abstract

A petroliferous geological formation in which or just below which a borehole terminates is heated by generating a series of explosion waves at the bottom of the borehole. In a preferred embodiment fuel and oxygen (air) are continuously passed into the borehole, the oxygen and fuel continuously mixed in the vicinity of the end of the borehole so as to form an explosive mixture, and the explosive mixture intermittently ignited so as to generate heat and shock waves which pass to the surrounding formation.

Description

This invention relates to geological formation heating, that is to the heating of porous geological formations which contain crude petroleum in the pores.

According to the invention a geological formation in which or just below which a borehole terminates is heated by:

A. separately and continuously introducing oxygen and fuel, preferably a gaseous fuel such as hydrogen or methane, into the borehole,

B. continuously mixing the oxygen and fuel in the vicinity of the end of the borehole so as to form an explosive mixture, and

C. intermittantly igniting the explosive mixture so as to generate heat and shock waves which pass to the surrounding formation.

The oxygen may be introduced as the pure gas or as a gaseous mixture, e.g. air.

(If it is desired to carry out the invention at a zone part way down a borehole, the borehole should be temporarily or permanently sealed so that the borehole effectively terminates just below the zone to be heated.)

The method described above may be carried out by operating a burner as described in the Provisional Specification accompanying British patent application 43938/68 filed Sept. 16, 1968, for "Burners Having a Pulsating Mode of Operation" (hereinafter called the copending specification) and corresponding to U.S. application Ser. No. 853,043, filed Aug. 26, 1969, at the bottom of a borehole with its open end directed downwards. Such a burner comprises:

A. an elongated, e.g. cylindrical, combustion chamber which has grossly rough walls, and

B. an oxygen/fuel inlet system which has a low resistance to gaseous flow and which is arranged to mix the oxygen and fuel at one end of the combustion chamber,

Whereby, during the use of the burner, a series of explosive wares is produced by repeated ignition of an explosive mixture formed in the combustion chamber.

The invention includes an apparatus for carrying out the method described above which comprises:

A. a burner as described in the last preceding paragraph,

B. means for suspending said burner near the bottom of a borehole,

C. oxygen and fuel supply lines for conveying oxygen and fuel down a borehole to the burner, and

D. a sealant device for shutting off the borehole above the burner to allow the bottom of the borehole to fill with gas against the pressure of formation fluids.

It is necessary to prevent the burners being lifted up the borehole by the recoil generated by the explosion waves. In many cases the burner is suspended on the end of a rigid system whose weight is sufficient to counteract the recoil. If desired a locking member may be positioned next to the burner so that the recoil is not transmitted to the suspension system.

During use the burner comprises a spark plug which requires a high voltage to generate a spark. The apparatus conveniently includes a power pack adapted to receive a low voltage from the surface, convert it to a high voltage and provide switching when a spark is required.

Thus when assembled for use the apparatus comprises, in the sequence specified beginning at the lowest member:

i. the burner,

ii. the locking member, if any,

iii. the power pack, and

iv. the sealant device.

As an alternative to a single burner an array of burners may be used.

A preferred embodiment uses a single burner and the wall of the borehole serves as the wall of the combustion chamber.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which:

FIG. 1 shows a burner positioned in a borehole,

FIG. 2 shows an arrangement in which the wall of the borehole is used as the wall of the combustion chamber, and

FIG. 3 is a fragmentary diagrammatic view on a somewhat enlarged scale showing a form of pawl-actuating mechanism of the embodiment shown in FIG. 1.

FIG. 1 shows a borehole 10 which terminates in a porous geological formation 11 with crude petroleum in its pores. The borehole 10 contains an apparatus according to the invention.

The apparatus comprises a burner, generally indicated by the numeral 12, attached to a locking member 14 having pawls 15 which can be swung outwards to the position shown in the FIG. 1 where they engage with wedges 16 formed on the lining of the borehole. As is depicted in Figure 3, each pawl 15 is normally yieldably held in a retracted position, the dotted line position of the pawl in Figure 3, by means of a helical spring 17. A pneumatic cylinder 20 connected to the oxygen pipe 33 has a piston 23 provided with a pawl-actuating rod 25 slidably engaging the pawl. Upon the application of fluid pressure to the piston from the pipe 33, the piston is moved from its retracted position, the dotted line position of the piston in FIG. 3, to an extended position, the solid line position in FIG. 3. The corresponding movement of the rod 25 causes the pawl to be swung out against the restraining action of the spring 17, into an extended position, the solid line position in FIG. 3, for engagement with the wedge 16. This arrangement holds the burner down against the recoil from the explosions.

The locking member 14 is attached by the supply pipes to a power pack 18 which contains circuitry 19 which receives low-voltage electric power from the cable 23 and converts it into suitably timed high-voltage pulses which are passed to the burner 12 via the high-voltage cable 24.

Finally the power pack 18 is attached by the supply pipes to a telescopic sealant device 21 which includes a flexible sleeve 22 which can be expanded to make fluidtight contact with the wall of the borehole 10 by shortening the telescopic sealant device 21.

The burner 12 has a rough-walled, cylindrical combustion chamber 30 with a sparking plug 31 situated centrally at its upper end. The sparking plug 31 has annular electrodes and it is connected to the high voltage cable 24.

The inlet system of the burner 12 comprises a fuel pipe 32 and an oxygen pipe 33 which are aligned parallel to the axis of the burner 12. These pipes extend to the surface and they pass via suitable channels contained in the locking member 14, the power pack 18 and the sealant device 21. (Suitable connections, not shown, are provided between the units.)

The fuel and oxygen pipes 32 and 33 have radial terminal sections 34 and 35 which open tangentially into an annular antechamber 36. During use this arrangement produces a swirling motion which mixes oxygen and fuel so that an explosive mixture passes into the combustion chamber 30 where it is ignited by sparks from the plug 31.

If desired the walls of the burner 12 may be water cooled via connections not shown in the drawing.

To set up the apparatus for use the units are connected together as shown in the drawing and lowered into the borehole 10 (which is full of liquid, e.g. water and formation fluids). The low-voltage armored cable 23 has sufficient mechanical strength to support the weight of the apparatus and it is used for lowering and support during use as well as supplying low-voltage electric power. The fuel and oxygen pipes 32 and 33 (and water pipes if any) are paged out as the apparatus is lowered.

During lowering the pawls 15 are retracted so that they can pass below the wedges 16 when the application of low oxygen (or air) pressure, via the pipe 33, opens out the pawls 15. At this stage the cable 23 is hauled in so that the pawls 15 engage with the wedges 16 to secure the burner 12 in the borehole. The hauling is continued to close the telescopic sealant device 21 so that the flexible sleeve 22 opens out to seal off the bottom of the borehole 12. Increasing the oxygen (or air) pressure in the pipe 33 blows the formation fluid back into the formation 11 leaving the space below the seal full of oxygen (or air).

At this stage heating can begin by starting a continuous flow of oxygen (or air) and fuel into the burner 12. Repeated sparking, at suitable time intervals, initiates a series of explosion waves which produce heat and shock waves which pass to the formation 11.

Each spark takes place when the burner 12 contains explosive mixture. After the spark the mixture burns as a relatively slow combustion wave which has a spherical front until it meets the sides of the combustion chamber 30 and separates into two waves, one traveling against the gas flow towards the inlet system the other travelling towards the end of the borehole 10. After contact with the sides the wave fronts accelerate and become explosion waves so that such waves pass to the surrounding formation.

Since oxygen and fuel are continuously supplied the initial slow moving combustion waves move downstream with the gas flow. When the combustion becomes explosive one wave travels upstream to the point with the fuel and oxygen systems separate and then its goes out. At this stage replacement of burnt gas with new explosive mixture begins. The next spark can take place as soon s there is enough explosive mixture in the borehole to propagate the next wave. (This may take into account mixture supplied after sparking.)

The burner described above was operated on hydrogen and air at an average power of 400 kw. at an explosion rate of 5 per second against a back pressure of 3 atmospheres.

In a preferred embodiment, shown in FIG. 2 as arranged for use, the wall of the borehole, e.g. the lining, is used as the wall of the combustion chamber. In this embodiment the burner components are attached to a conventional well production packer which acts as the sealant device and locking member.

As shown in FIG. 2, a conventional well production packer 40 comprised of sealant 40' and a locking member 40", is provided; with ducts 41, 42, and 43 for fuel, air and water respectively, and is situated near the bottom of a borehole 10. The borehole terminates in a porous geological formation 11. The space below the packer 40 is formed into a burner as described hereinafter.

The burner comprises a cylindrical air chamber 44 into which the air duct 42 opens. The fuel duct 41 terminates in the upper surface of an antechamber 45 which receives fuel from the duct 41 and receives air via a suitable opening in its vertical wall 45' from the air chamber 44. Thus an explosive mixture is formed in the antechamber and this passes into the combustion chamber 46, i.e. the part of the borehole below the air chamber 44. The combustion chamber 46 contains a spiral tube 47 which acts a macroscopic roughness so that combustion waves accelerate into explosion waves.

(The tube 47 is water cooled via the water duct 43. The cooling water is converted into steam which passes into the combustion products of the burner.)

Ignition is provided by a spark plug 48 which receives its power via a lead 49 which passes through the air duct 41.

During use water (admitted to the top of the borehole 10) collects above the packer 40 so that it can enter the open end of the water duct 43 to provide cooling as described above. Similarly the air duct 42 terminates the water so that air from the upper part of the borehole can pass to the air chamber 44. The fuel duct 41 continues to the surface and the lead 49 passes to an ignition control unit 50 which receives its power from the surface via the cable 51.

(It should be noted that the packer 40 provides a seal for the gases in the combustion chamber and the water and air in the borehole as well as providing locking against the reaction of the burner.)

To set up the apparatus as shown in FIG. 2 the units are connected together as shown in the drawing and lowered into the borehole (which is full of liquid, e.g. water and formation fluids). The apparatus is lowered on the fuel duct which is of suitably strong construction and the electrical cable is payed out as the apparatus is lowered. When at the required level the packer is locked and sealed. The top of the borehole is sealed (allowing passage of fuel duct and electric power cable and permitting access of air and water) and the borehole fluids blown into the formation by applying air pressure through the air duct 42. This leaves the combustion chamber full of air.

At this stage the fuel gas is passed into the burner antichamber 45 to produce an explosive mixture which is continuously fed into the combustion chamber 46 where it is periodically sparked 2-4 -minute intervals).

This initiates a series of explosion waves which apply heat and shock waves to the formation. In addition to the burnt gas passes into the formation.

An arrangement as shown in FIG. 2 was operated at a mixture in flow rate of 16 standard cubic meters/minute at 10 atmospheres pressure with a 70 meter .times. 0.2 m. diameter combustion volume in a test well. The thermal output was 110 millijoules/pulse with pulses at 3 minutes interval. Instantaneous pressures rose to 95 atmospheres.

(Note: The method will usually be carried out in lined boreholes and these should be perforated before carrying out the method.)

Claims

We claim

1. A method of heating a porous geological formation in which or just below which a borehole has a terminus by:

a. sealing the borehole in the vicinity of said terminus to provide a combustion chamber portion and to prevent the escape of fluid from said portion up the borehole,

b. separately and continuously introducing oxygen and fuel into said combustion chamber portion of the borehole,

c. continuously mixing the oxygen and fuel in said combustion chamber portion so as to form an explosive mixture, and

d. periodically igniting said explosive mixture at regular timed intervals so as to generate in said combustion chamber portion a series of explosion waves which apply regularly pulsating heat and shock waves to said surrounding porous formation while the burnt gas passes into said formation.

2. A method according to claim 1, in which the fuel is a gaseous fuel.

3. A method according to claim 1, in which the oxygen is supplied as air.