Patent Grant
8186425
1. Technical Field
Embodiments described in the present application relate to stimulating tools and methods of using the same in downhole stimulation applications, and more particularly to methods for controlling pressure pulses to enhance stimulation of a subterranean formation.
2. Background Art
There are several techniques for stimulating subterranean formations. The most commonly used technique is “hydraulic fracturing,” in which a stimulation liquid (with an acid or proppants) is injected into a well under high pressure to fracture the formations. Alternatively, subterranean formations may be fractured by detonation of an explosive charge in the wellbore which fractures the formation by shattering the rock.
Another technique of well fracturing involves the use of a device incorporating a gas generating charge or propellant, which is typically lowered into a well on a wireline and ignited to generate a substantial quantity of gaseous combustion product at a pressure sufficient to break down the formation adjacent the perforations. This type of fracturing technique differs from explosive fracturing in a number of ways: (1) this type of fracturing is caused by high pressure gaseous combustion products moving through and splitting the formation rather than shock wave fracturing; and (2) the process is one of combustion rather than explosion. Solid propellant fracturing generates high pressure gases at a rate that creates fractures differently from high explosives or hydraulic fracturing.
Typically, gas generation stimulation tools include a propellant charge, generally in a perforated carrier, of a length that is easily handled. The propellants in these tools are generally ignited by an electrical signal transmitted through an insulated wireline to an assembly which contains a faster burning material which is more easily ignited.
After a fracture has been created, it is desirable that the fracture extend as deeply as possible in order to reach the producing region. In order to extend a fracture, there should be a source of energy applying pressure to the fluid driven by the initial detonation into the fracture. Therefore, solid propellants are typically selected for the production of pressures on the order of those required for propagating a fracture.
While these techniques have been useful in well stimulation, there exists a continuing need for stimulation techniques that can control the burn rate of a propellant and/or the peak pressures generated therefrom, in order to achieve a predetermined degree of stimulation.
One aspect of the present application relates to downhole propellant gas generators. A downhole propellant gas generator in accordance with one embodiment includes a propellant assembly that comprises a plurality of individual lengths of an energetic material packed in a selected configuration; and at least one initiator.
Another aspect relates to methods for creating a pressure pulse downhole. A method in accordance with one embodiment includes igniting one or more initiators, wherein the one or more initiators are packed with a plurality of lengths of an energetic material in a propellant assembly; and igniting the plurality of lengths of the energetic material subsequent to the igniting of the one or more initiators.
Another aspect relate to methods for stimulating a well. A method in accordance with one embodiment includes disposing in the well a propellant gas generator having a propellant assembly that comprises a plurality of lengths of an energetic material, wherein the propellant assembly comprises at least one initiator packed among the plurality of lengths of the energetic material; and igniting the at least one initiator, which in turn ignites the plurality of lengths of the energetic material.
Other aspects and advantages will be apparent from the following description and the appended claims.
Embodiments relate to methods and apparatus for controlling pressure pulses generated by high energy gas produced by combustion of energetic materials. Energetic materials, for example, may include HMX, RDX, HNS, TATB, or others. Other energetic materials, for example, may comprise a combination of a fuel and an oxidizer. Methods according to embodiments may be used to tailor the pressure pulses to achieve, for example, a predetermined degree of stimulation.
In accordance with some embodiments, the pressure pulses resulting from combustion of energetic materials (or propellants) may be controlled by varying the geometry of the arrangements of the energetic materials. For example, by using a plurality of individual lengths of energetic materials, one would be able to pack these individual sticks in a selected configuration to achieve the desired topology and exposed surfaces. Thus, methods permit control of the geometry of individual lengths of the energetic materials to allow for control of the pressure pulses. Some embodiments relate to methods for controlling the pressure pulses by varying the packing densities, shapes, and sizes of individual grains of the energetic materials to achieve different combustion patterns.
In accordance with some embodiments, based on the close packing concept, the ignition of energetic materials in a propellant assembly can be made to ignite sympathetically, igniting at one point or multiple points within the assembly. When initiating at multiple points, the initiation may be performed simultaneously or sequentially (with very short delays between them). By controlling different patterns of ignition and varying the geometry, density, and amounts of the energetic materials, embodiments can provide flexible control of the pressure pulses.
As noted above, propellants are often used in the oilfield industry for stimulation purposes. Such a propellant may be a single solid stick of an energetic material.
Typically, these propellants are loaded on a tool, which is then lowered into a wellbore.
Any gas generation tools known in the art may be adapted for use with various embodiments. For example,
The burn rate and the peak pressure produced by an energetic material during the combination are proportional to the total surface area exposed to the flame at any particular time. Applicants have found that the recession rate, r, of the exposed surface is proportional to the pressure produced. Furthermore, by experiments, the Applicants have found that a relationship between the recession rate, r, and the pressure may be approximated as in Equation 1.
r˜Pn Equation 1:
Where, P is the transient pressure of the combustion products (psi), and the burning index, n, may be experimentally determined. With energetic materials commonly used in oilfield operations, the burning index, n, is found to fall within the range of about 0.30 to about 1.25.
Based on these findings, embodiments are designed to provide means for controlling the rate of recession or the surface exposed on the energetic materials during combustion. For example, a method in accordance with embodiments for tailoring the rate of burning and/or the combustion pressures of a propellant assembly (e.g., a conglomerate of energetic material grains) may comprise varying the cross-sectional area, packing topology, and/or quantity of the grains in the conglomerate. These variations may be achieved with either homogeneous or heterogeneous stick dimensions (i.e., different sizes and/or shapes).
Therefore, in accordance with embodiments, a propellant assembly may comprise multiple propellant sticks (i.e., a plurality of individual lengths of an energetic material). The multiple energetic material lengths can be arranged in different packing configurations to vary the surface areas exposed to the flame during combustion to allow for control of the pressure pulses during combustion. Accordingly, embodiments include method for using different topology or geometries of individual lengths of energetic material arrangements to achieve control of burn rates and peak pressures during combustion.
Furthermore, some embodiments may include the use of one or more initiation cores (i.e., one or more initiation lengths) to achieve different patterns of initiation and burn. These initiation lengths may be arranged in any pattern within the closed packed configurations of energetic material lengths to allow for different patterns of initiation, and hence, different controls of the pressure pulses during the combustion of energetic materials.
For example,
Among the various individual lengths of an energetic material, one or more may function as one or more lengths of initiators, which may comprise a different energetic material from that of the remaining lengths of energetic materials, see for example initiation lengths 41 in
In accordance with some embodiments, a propellant assembly may comprise a plurality of individual lengths of an energetic material, wherein the individual lengths are of different dimensions (e.g., different sizes and/or shapes). For example, as shown in
The above examples shown in
Embodiments may include one or more of the following advantages. Methods according to embodiments provide flexible controls of pressure pulses during combustion of energetic materials, allowing the use of a solid propellant gas generator to achieve a predetermined degree of stimulation. In accordance with embodiments, the materials that form the solid propellant may comprise small propellant sticks to allow for packing of the energetic materials in the geometry and topology, to achieve different areas exposed to the flame during combustion. This allows for a fine control of the pressure pulses generated from the energetic materials. Furthermore, a propellant assembly may comprise one or more initiation grains to permit control of desired ignition patterns or to achieve sympathetic ignition. By using different packing of the individual grains of the solid propellant and different patterns of initiation grains, embodiments can achieve flexible control of the burn rates and peak pressures. Therefore, embodiments may be used to achieve the desired degree of stimulation of a well.
While various embodiments have been described herein with respect to a limited number of examples, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments and variations thereof can be devised which do not depart from the scope disclosed herein. Accordingly, the scope of the claims should not be unnecessarily limited by the present disclosure.
This claims the benefits, under 35 U.S.C. §109, of U.S. Provisional Application No. 61/033,997, filed on Mar. 5, 2008. This provisional application is incorporated by reference in its entirety.
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