Method for producing viscous hydrocarbon using incremental fracturing

Abstract

A method for producing viscous hydrocarbon formations involves the use of a downhole burner. The well undergoes a mild hydraulic fracturing process that limits the fractured zone to a relatively small dimension so as to avoid intersecting any drainage zones of adjacent wells. The operator pumps fuel, steam and oxygen to the burner, which bums the fuel, causing the flow of hot, gaseous fluids into the fractured zone. The steam delivered from the surface cools the burner and becomes superheated as it enters the fractured zone. The operator allows the fractured zone to soak, then produces the oil. After the production declines, the operator may repeat the fracturing to incrementally increase the fractured zone, then repeat the injection and soak cycles.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional application Ser. No. 60/646,790 filed Jan. 25, 2005.

FIELD OF THE INVENTION

This invention relates in general to methods for producing highly viscous hydrocarbons, and in particular to injecting steam from a downhole burner into a fractured zone.

BACKGROUND OF THE INVENTION

There are extensive viscous hydrocarbon reservoirs throughout the world. These reservoirs contain a very viscous hydrocarbon, often called “tar”, “heavy oil”, or “ultraheavy oil”, which typically has viscosities in the range from 3,000 to 1,000,000 centipoise when measured at 100 degrees F. The high viscosity makes is difficult and expensive to recover the hydrocarbon. Strip mining is employed for shallow tar sands. For deeper reservoirs, heating the heavy oil in situ to lower the viscosity has been employed.

In one technique, partially saturated steam is injected into a well from a steam generator at the surface. The heavy oil can be produced from the same well that the steam is injected by allowing the reservoir to soak a selected time after the steam injection, then producing the well. The heavy oil can also be produced by means of a second well spaced apart from the injector well. When production declines, the operator repeats the process. A downhole pump may be required to pump the heated heavy oil to the surface. If so, the pump has to be pulled from the well each time before the steam is injected, then re-run after the injection.

Another techniques uses two horizontal wells, one a few feet above and parallel to the other. Each well has a slotted liner. Steam is injected continuously into the upper well bore to heat the heavy oil and cause it to flow into the lower well bore. Other proposals involve injecting steam continuously into vertical injection wells surrounded by vertical producing wells.

U.S. Pat. No. 6,016,867 discloses the use of one or more injection and production boreholes. A mixture of reducing gases, oxidizing gases, and steam is fed to downhole combustion devices located in the injection boreholes. Combustion of the reducing gas oxidizing gas mixture is carried out to produce superheated steam and hot gases for injection into the formation to convert and upgrade the heavy crude or bitumen into lighter hydrocarbons. The temperature of the superheated steam is sufficiently high to cause pyrolysis and/or hydrovisbreaking, which increases the gravity and lowers the viscosity of the hydrocarbon in situ. The '867 patent also discloses fracturing the formation prior to injection of the steam. The '867 patent discloses both a cyclic process, wherein the injection and production occur in the same well, and a continuous drive process involving pumping steam down boreholes of wells surrounding the producing wells. In the continuous drive process, the '867 patent teaches to extend the fractured zones to adjacent wells.

SUMMARY OF THE INVENTION

The well is fractured to create a fractured zone of limited diameter. The fractured zone extends from the well and preferably does not intersect any drainage or fractured zones of adjacent wells. A downhole burner is secured in the well. The operator pumps a fuel, which may be hydrogen, and oxygen in separate conduits down the well to the burner, and burns the fuel in the burner. The operator also pumps partially saturated steam from the surface into the well The steam flows into and cools the burner. The heat exchange creates superheated steam, which then flows into the fractured zone along with residual unburned fuel and other products of combustion.

The unfractured formation surrounding the fractured zone impedes leakage of these gaseous products from the fractured zone. After injecting the steam and other gaseous products for a selected time, the operator allows the fractured zone to soak for a selected time. During the soak interval, the operator may intermittingly pump fuel and steam to the burner to maintain a desired amount of pressure in the fractured zone. After the soak interval, the operator opens valves at the wellhead to cause the hydrocarbon to flow into the borehole and up the well. The viscous hydrocarbon, having undergone pyrolysis and/or hydrovisbreaking during this process, flows to the surface for further processing. Preferably, the flow occurs as a result of solution gas created in the fractured zone from the steam and residual hydrogen. A downhole pump could also be employed,

When production declines sufficiently, the operator may repeat the procedure of injecting steam and combustion products from the burner into the fractured zone. The operator may also fracture the formation again to enlarge the fracturing zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a well and a process for producing heavy oil in accordance with this invention.

FIG. 2 is a schematic illustrating the well of FIG. 1 next to an adjacent well, which may also be produced in accordance with this invention.

FIG. 3 is a schematic illustration of a combustion device employed with the process of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, well 11 extends substantially vertically through a number of earth formations, at least one of which includes a heavy oil or tar formation 15. An overburden earth formation 13 is located above the oil formation 15. Heavy oil formation 15 is located over an underburden earth formation 17. The heavy oil formation 15 is typically a tar sand containing a very viscous hydrocarbon, which may have a viscosity from 3,000 cp to 1,000,000 cp, for example. The overburden formation 13 may be various geologic formations, for example, a thick, dense limestone that seals and imparts a relatively high fracture pressure to the heavy oil formation 15. The underburden formation 17 may also be a thick, dense limestone or some other type of earth formation.

As shown in FIG. 1, the well is cased, and the casing has perforations or slots 19 in at least part of the heavy oil formation 15. Also, the well is fractured to create a fractured zone 21. During fracturing, the operator pumps a fluid down the casing, which flows through perforations 19 and imparts a pressure against heavy oil formation 15 that is greater than the parting pressure of the formation. The pressure creates cracks within formation 15 that extend generally radially from well 11, allowing flow of the fluid into fractured zone 21. The injected fluid to cause the fracturing may be conventional, typically including water, various additives, and proppant materials such as sand or ceramic beads.

The operator controls the rate of injection of the fracturing fluids and the duration of the hydraulic fracturing process to limit the extent or dimension of fractured zone 21 surrounding well 11. Fractured zone 21 has a relatively small initial diameter or perimeter 21a. The perimeter 21a of fractured zone 21 is limited such that it will not intersect any existing or planned fractured or drainage zones 25 (FIG. 2) of adjacent wells 23 that extend into the same heavy oil formation 15. Further, in the preferred method, the operator will later enlarge fractured zone 21 well 11, thus the initial perimeter 21a should leave room for a later expansion of fractured zone 21 without intersecting drainage zone 25 of adjacent well 23. Adjacent well 23 optionally may previously have undergone one or more of the same fracturing processes as well 11, or the operator may plan to fracture adjacent well 23 in the same manner as well 11 in the future. Consequently, fractured zone perimeter 21a does not intersect fractured zone 25. Preferably, fractured zone perimeter 21a extends to less than half the distance between wells 11, 23. Fractured zone 21 is bound by unfractured portions of heavy oil formation 15 outside perimeter 21a and both above and below fractured zone 21.

A production tree or wellhead 27 is located at the surface of well 11. Production tree 27 is connected to a conduit for directing a mixture of fuel and steam down well 11, as indicated by the numeral 37 The fuel may be hydrogen, methane, syngas, or some other fuel. The fuel may be a gas or liquid. Preferably, the steam is partially saturated steam, having a water vapor content up to about 20 percent. The water vapor content could be higher, and even water could be pumped down well 11 in lieu of steam, although it would be less efficient. A wellhead 27 is also connected to a conduit for delivering oxygen down well 11, as indicated by the numeral 39. Preferably the fuel and steam 37 is delivered separate from the conduit that delivers oxygen 39. The conduits for fuel and steam 37 and oxygen 39 may comprise coiled tubing or threaded joints of production tubing. One of the conduits could comprise the annulus in the casing of well 11.

A combustion device or burner 29 is secured in well 11 for receiving the flow of fuel and steam 37 and oxygen 39. As illustrated in FIG. 3, a packer and anchor device 31 seals and secures burner 29 to the casing of well 11. Burner 29 has a combustion chamber 33 surrounded by a jacket 35. Fuel and steam 37 enter combustion chamber 33 for burning the fuel. In this embodiment, at least some of the steam flows through jacket 35, as indicated by the arrows 41. If the fuel is hydrogen, some of the hydrogen can flow through jacket 35 along with steam.

Burner 29 ignites and burns at least part of the fuel, which creates a high temperature in burner 29. Without steam or water as a coolant, the temperature would likely be too high for burner 29 to withstand over a long period. The steam flowing into combustion chamber 33 reduces that temperature. Also, preferably there is an excess of fuel flowing into combustion chamber 33. The excess fuel does not burn, thus also lowers the temperature in combustion chamber 33. Further, the steam and fuel 41 flowing through jacket 35 cools combustion chamber 33. A downhole burner for burning fuel and injecting steam and combustion products into an earth formation is shown in U.S. Pat. No. 5,163,511.

The steam and excess fuel lower the temperature within combustion chamber 33, for example, to around 1600 degrees F., which increases the temperature of the partially saturated steam flowing through jacket 35 and through combustion chamber 33 to a superheated level. The gaseous product 43, which comprises superheated steam, excess fuel and other products of combustion, exits burner 29 preferably from about 550 to 700 degrees F. The hot, gaseous product 43 flows into fractured zone 21. The fractures within fractured zone 21 increase the surface contact area for these fluids to heat the formation and dissolve into the heavy oil to lower the viscosity of the oil and create solution gas to help drive the produced oil. The unfractured formation 15 is substantially impenetrable by the gaseous product 43 because the heavy oil or tar is not hot enough to be displaced. The surrounding portions of heavy oil formation 15 thus create a container around fractured zone 21 to impede leakage of hot gaseous product 43.

In the preferred method, the delivery of fuel, steam and oxygen into burner 29 and the injection of hot gaseous product 43 into fractured zone 21 occur simultaneously over a selected period, such as seven days. While gaseous product 43 is injected into fractured zone 21, the temperature and pressure of fractured zone 21 increases. At the end of the injection period, fractured zone 21 is allowed to soak for a selected period, such as 21 days. During the soak interval, the operator may intermittingly pump fuel, steam and oxygen to burner 29 where it burns and the hot combustion gases are injected into formation 15 to maintain a desired pressure level in fractured zone 21. Other than pressure maintenance, no further injection of hot gaseous fluid 43 occurs during the soak period.

Then, the operator begins to produce the oil, which is driven by reservoir pressure and preferably additional solution gas pressure. The oil is preferably produced up the production tubing, which could also be the same tubing through which the fuel and steam or oxygen is pumped. Preferably, burner 29 remains and place, and the oil flows through burner 29. Alternately, well 11 could comprise two boreholes a few feet apart, preferably no more than about 50 feet, with the oil flowing up a separate borehole from the one containing burner 29.

The oil production will continue as long as the operator deems it feasible, which could be up to 35 days or more. When production declines sufficiently, the operator may optionally repeat the injection and production cycle either with or without additional fracturing. It may be feasible to fracture again after one or more injection and production cycles to increase the perimeter 21a of fractured zone 21, then repeat the injection and production cycle described above. Preferably, this subsequent fracturing operation can take place without removing burner 29. The process may be repeated as long as fractured zone 21 does not intersect fractured zones or drainage areas 25 of adjacent wells 23 (FIG. 2). By incrementally increasing the fractured zone 21 diameter from a relatively small perimeter up to half the distance to adjacent well 23 (FIG. 2), the operator can effectively produce the viscous hydrocarbon formation 15. With each new fracturing operation, the previously fractured portion would provide flow paths for the injection of hot gaseous product 43 and the flow of the hydrocarbon into the well. Also, the previously fractured portion retains heat from the previous injection of hot combustion gases 43. The numeral 21b in FIGS. 1 and 2 indicates the perimeter of fractured zone 21 after a second fracturing process. The operator could be performing similar fracturing, injection, soaking and production cycles on well 23 at the same time, if desired.

Before or after reaching the maximum limit of fractured zone 21, which would be greater than perimeter 21b, the operator may wish to convert well 11 to a continuously driven system. This conversion might occur after well 11 has been fractured several different times, each increasing the dimension of the perimeter. In a continuously driven system, well 11 would be either a continuous producer or a continuous injector. If well 11 is a continuous injector, downhole burner 29 would be continuously supplied with fuel and steam 37 and oxygen 39, which burns the fuel and injects hot gaseous product 43 into fractured zone 21. The hot gaseous product 43 would force the oil to surrounding production wells, such as in an inverted five or seven-spot well pattern. Each of the surrounding production wells would have fractured zones that intersected the fractured zone 21 of the injection well. If well 11 is a continuous producer, fuel and steam 37 and oxygen 39 would be pumped to downhole burners 29 in surrounding injection wells, as in a normal five or seven-spot pattern. The downhole burners 29 in the surrounding injection wells would burn the fuel and inject hot gaseous product 43 into the fractured zones, each of which joined the fractured zone of the producing well so as to force the oil to the producing well.

The invention has significant advantages. The unfractured heavy oil formation surrounding the fractured zone serves as a container to impede leakage of excess fuel, steam and other combustion products into adjacent formations or to the surface. The container maximizes the effects of the excess fuel and other hot gases flowing into the fractured zone. By reducing leakage from the fractured zone, the expense of the fuel, oxygen, and steam is reduced. Also, containing the excess fuel increases the safety of the well treatment.

While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, although the well is shown to be a vertical well, it could have a horizontal component extending through the heavy oil formation The fractured zone could be one or more vertical fractures in that instance.

Claims

1. A method for producing a viscous hydrocarbon from a well, comprising:

(a) hydraulically fracturing a viscous hydrocarbon formation surrounding the well, creating a limited fractured zone surrounded by an unfractured portion of the formation;

(b) securing a downhole burner in the well;

(c) pumping a fuel into the burner and burning the fuel in the burner;

(d) creating superheated steam in the burner and injecting the superheated steam into the fractured zone to heat the hydrocarbon therein, and impeding the escape of the superheated steam into the surrounding unfractured portion of the formation; then

(e) flowing hydrocarbon from the fractured zone up the well; then

(f) at a selected time, hydraulically fracturing the formation again to increase the extent of the fractured zone, and repeating steps (c), (d) and (e).

2. The method according to claim 1, wherein the extent of the fractured zone created in step (a) is less than one-half a distance to any adjacent wells.

3. The method according to claim 1, wherein the extent of the fractured zone created in step (a) is limited so as to avoid intersecting any drainage zones of any adjacent wells.

4. The method according to claim 1, further comprising:

allowing the fractured zone to soak for a selected time after step (d) and before step (e) by stopping steps (c) and (d) other than to maintain the formation pressure at a desired level until beginning step (e).

5. The method according to claim 1, wherein:

steps (c) and (d) create a solution gas and causes a formation pressure within the fractured zone to increase; and wherein step (e) comprises using the solution gas as a source to force the hydrocarbon into and up the well in step (e).

6. The method according to claim 1, wherein step (d) comprises pumping partially saturated steam to the burner and flowing a portion of the partially saturated steam through a jacket around the burner to cool the burner and convert the partially saturated steam to superheated steam.

7. The method according to claim 1, wherein step (f) is performed without removing the burner.

8. A method for producing a viscous hydrocarbon from two adjacent wells, both of which extend into the same viscous hydrocarbon formation, comprising:

(a) hydraulically fracturing the hydrocarbon formation surrounding each of the wells, creating a separate fractured zone around each of the wells, and limiting the extent of the fractured zones so that they do not intersect each other, leaving unfractured portions of the formation surrounding each of the fractured zones;

(b) injecting steam into each of the fractured zones and impeding the escape of the steam from the fractured zones into the unfractured portions of the formation surrounding each of the fractured zones; then

(c) flowing hydrocarbon from each of the fractured zones up the well; and

(d) when the flow of hydrocarbon drops below a selected minimum in each of the wells, hydraulically fracturing the formation in each of the wells again to create enlarged fractured zones around each well, and limiting the extents of the enlarged fractured zones so as to avoid them intersecting each other; then repeating steps (b) and (c).

9. The method according to claim 8, further comprising allowing the fractured zones to soak for a selected time after step (b) and before starting step (c), and while soaking, ceasing step (b) other than to maintain a desired pressure in each of the fractured zones.

10. A method for producing a viscous hydrocarbon from a well, comprising:

(a) hydraulically fracturing a viscous hydrocarbon formation to create a fractured zone, but limiting an extent of the fractured zone so that the fractured zone is surrounded by an unfractured portion of the formation;

(b) securing a downhole burner in the well;

(c) pumping hydrogen, oxygen and partially saturated steam down the well to the burner, burning a portion of the hydrogen, cooling the burner with the partially saturated steam, and heating the partially saturated steam to create superheated steam;

(d) injecting the steam and unburned portions of the hydrogen from the burner into the fractured zone to heat the hydrocarbon and create a solution gas in the fractured zone, the unfractured portion of the formation impeding the escape of the heated steam and unburned portions of the hydrogen; then

(e) ceasing step (c) during a selected soak interval except for at least one repetition of step (c) to maintain a desired pressure in the fractured zone; then

(f) opening a valve at a wellhead and allowing the heated hydrocarbon to flow up the well driven at least in part by the solution gas; then

(g) repeating steps (a), (c), (d), (e) and (f).

11. The method according to claim 10, wherein the hydraulic fracturing of step (a) is limited to have an outer periphery separated from a drainage zone of any adjacent wells.