Pulse Pressure Fracking

Abstract

A method and apparatus for providing pulse fracturing of a wellbore using a combination of timing and/or explosive properties to provide a pressure pulse, resulting in the necessary pressure to open artificial fractures.

Description

BACKGROUND

Generally, when completing a subterranean well for the production of fluids, minerals, or gases from underground reservoirs, several types of tubulars are placed downhole as part of the drilling, exploration, and completions process. These tubulars can include casing, tubing, pipes, liners, and devices conveyed downhole by tubulars of various types. Each well is unique, so combinations of different tubulars may be lowered into a well for a multitude of purposes.

A subsurface or subterranean well transits one or more formations. The formation is a body of rock or strata that contains one or more compositions. The formation is treated as a continuous body. Within the formation hydrocarbon deposits may exist. Typically, a wellbore will be drilled from a surface location, placing a hole into a formation of interest. Completion equipment will be put into place, including casing, tubing, and other downhole equipment as needed. Perforating the casing and the formation with a perforating gun is a well-known method in the art for accessing hydrocarbon deposits within a formation from a wellbore.

Explosively perforating the formation using a shaped charge is a widely known method for completing an oil well. A shaped charge is a term of art for a device that when detonated generates a focused output, high energy output, and/or high velocity jet. This is achieved in part by the geometry of the explosive in conjunction with an adjacent liner. Generally, a shaped charge includes a metal case that contains an explosive material with a concave shape, which has a thin metal liner on the inner surface. Many materials are used for the liner; some of the more common metals include brass, copper, tungsten, and lead. When the explosive detonates, the liner metal is compressed into a super-heated, super pressurized jet that can penetrate metal, concrete, and rock. Perforating charges are typically used in groups. These groups of perforating charges are typically held together in an assembly called a perforating gun. Perforating guns come in many styles, such as strip guns, capsule guns, port plug guns, and expendable hollow carrier guns.

Perforating charges are typically detonated by a detonating cord in proximity to a priming hole at the apex of each charge case. Typically, the detonating cord terminates proximate to the ends of the perforating gun. In this arrangement, an initiator at one end of the perforating gun can detonate all of the perforating charges in the gun and continue a ballistic transfer to the opposite end of the gun. In this fashion, numerous perforating guns can be connected end to end with a single initiator detonating all of them.

The detonating cord is typically detonated by an initiator triggered by a firing head. The firing head can be actuated in many ways, including but not limited to electronically, hydraulically, and mechanically.

Expendable hollow carrier perforating guns are typically manufactured from standard sizes of steel pipe with a box end having internal/female threads at each end. Pin ended adapters, or subs, having male/external threads are threaded one or both ends of the gun. These subs can connect perforating guns together, connect perforating guns to other tools such as setting tools and collar locators, and connect firing heads to perforating guns. Subs often house electronic, mechanical, or ballistic components used to activate or otherwise control perforating guns and other components.

Perforating guns typically have a cylindrical gun body and a charge tube or loading tube that holds the perforating charges. The gun body typically is composed of metal and is cylindrical in shape. Charge tubes can be formed as tubes, strips, or chains. The charge tubes will contain cutouts called charge holes to house the shaped charges.

It is generally preferable to reduce the total length of any tools to be introduced into a wellbore. Among other potential benefits, reduced tool length reduces the length of the lubricator necessary to introduce the tools into a wellbore under pressure. Additionally, reduced tool length is also desirable to accommodate turns in a highly deviated or horizontal well. It is also generally preferable to reduce the tool assembly that must be performed at the well site because the well site is often a harsh environment with numerous distractions and demands on the workers on site.

Electric initiators are commonly used in the oil and gas industry for initiating different energetic devices down hole. Most commonly, 50-ohm resistor initiators are used. Other initiators and electronic switch configurations are common.

Conventional perforating in vertical wells or unconventional perforating in horizontal wells conveyed by electrical line during which one or more of the perforating guns in the downhole tool string are oriented by either one or more of the following orientating methods: motorized orientation tool, eccentric weight bars and self-orienting charge tube assemblies.

SUMMARY OF EXAMPLE EMBODIMENTS

An example embodiment may include a method of fracking a wellbore, comprising lowering at least one module with a plurality of initiators disposed within at least one propellant, locating the at least one module at a desired location in the wellbore, firing a first initiator at a first predetermined time, wherein the resulting ignition produces a first pressure in the wellbore, firing a second initiator at a second predetermined time, wherein the resulting ignition produces a second pressure in the wellbore higher than the first pressure, and firing a third initiator at a third predetermined time, wherein the resulting ignition produces a third pressure in the wellbore higher than the third pressure, wherein the first pressure, second pressure, and third pressure are initiated in a series to form a pressure pulse.

A variation of the example embodiment may include the interval between the first predetermined time, second predetermined time determining the resulting second pressure. The interval between the first predetermined time, second predetermined time, and third predetermined time may determine the resulting third pressure. The propellant may be a first propellant, a second propellant, and a third propellant. The first propellant properties may determine the first pressure. The second propellant properties may determine the second pressure. The third propellant properties may determine the third pressure. It may include setting a plug. The plurality of initiators may include an electrode with a spark gap. The propellant may be a liquid propellant.

An example embodiment may include an apparatus for fracking a wellbore, comprising a housing containing at least one propellant, and a plurality of initiators embedded in the propellant, wherein the initiators are detonated in a sequence timed to cause a plurality of pressure spikes in at least one wellbore fracture, where the plurality of initiators are adapted to detonate a series of pressure spikes, with each subsequent pressure spike producing a higher pressure than the previous pressure spike, resulting in a pressure pulse.

A variation of the example embodiment may include the propellant being a liquid propellant. The propellant may be a plurality of propellant portions. The propellant may be a plurality of propellant portions. Each of the plurality of propellant portions may have a dedicated at least one of the plurality of initiators. At least one of the plurality of propellant portions may be a black powder based propellant. At least one of the plurality of propellant portions may be a nitrate based propellant. It may include a plug setting tool coupled to the housing. It may include a wireline electrically connecting the housing to a controller, wherein the housing is disposed within a wellbore and the controller is located at a surface. Each of the plurality of initiators may includes an electrode with a spark gap.

BRIEF DESCRIPTION OF THE DRAWINGS

For a thorough understanding of the example embodiments, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings in which reference numbers designate like or similar elements throughout the several figures of the drawing. Briefly:

FIG. 1 shows an example embodiment of an electrically initiated liquid propellant.

FIG. 2 shows an example embodiment of a pulse pressure fracking device deployed in a wellbore.

FIG. 3 shows an example embodiment the effect of pulse pressure fracking in terms of pressure with respect to time.

FIG. 4 shows an example embodiment of a mixed energetic pulse pressure fracking device.

FIG. 5 shows an example embodiment of the mixed pulse pressure fracking in terms of pressure with respect to time.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

In the following description, certain terms have been used for brevity, clarity, and examples. No unnecessary limitations are to be implied therefrom and such terms are used for descriptive purposes only and are intended to be broadly construed. The different apparatus, systems and method steps described herein may be used alone or in combination with other apparatus, systems and method steps. It is to be expected that various equivalents, alternatives, and modifications are possible within the scope of the appended claims.

Terms such as booster may include a small metal tube containing secondary high explosives that are crimped onto the end of detonating cord. The explosive component is designed to provide reliable detonation transfer between perforating guns or other explosive devices, and often serves as an auxiliary explosive charge to ensure detonation.

Detonating cord is a cord containing high-explosive material sheathed in a flexible outer case, which is used to connect the detonator to the main high explosive, such as a shaped charge. This provides an extremely rapid initiation sequence that can be used to fire several shaped charges simultaneously.

A detonator, initiator, or initiation device may include a device containing primary high-explosive material that is used to initiate an explosive sequence, including one or more shaped charges. Two common types may include electrical detonators and percussion detonators. Detonators may be referred to as initiators. Electrical detonators have a fuse material that burns when high voltage is applied to initiate the primary high explosive. Percussion detonators contain abrasive grit and primary high explosive in a sealed container that is activated by a firing pin. The impact of the firing pin is sufficient to initiate the ballistic sequence that is then transmitted to the detonating cord.

Initiators may be used to initiate a perforating gun, a cutter, a setting tool, or other downhole energetic device. For example, a cutter is used to cut tubulars with focused energy. A setting tool uses a pyrotechnic to develop gases to perform work in downhole tools. Any downhole device that uses an initiator may be adapted to use the modular initiator assembly disclosed herein.

The low permeability of formations common to unconventional wells makes it difficult for hydrocarbons to be extracted as they cannot freely flow through the formation. Therefore, hydraulic fracking methods are commonly used to complete these unconventional wells. The purpose of hydraulic fracking is to artificially break or fracture the formation to create new pathways for hydrocarbons to flow more easily into the well bore. Hydraulic tracking is most effective when the artificial fractures connect with natural fractures in the rock formation. Once the artificial fractures initiate, proppant is pumped into the fractures to hold them open as they grow further.

Breakdown pressure is the pressure required to fracture the formation. It is closely tied to the compressive strength of the formation. Tortuosity is a measure of the difficulty in connecting artificial fractures with natural fractures. Pumping pressures must overcome tortuosity in order to breakdown the formation, resulting in higher treating pressures. Hydraulic tracking operations are one of the most expensive steps in the completion process. During hydraulic fracturing, 20 pump trucks may be deployed. Additionally, the operation requires the support of large amounts of water and sand. There are high operational costs associated with mobilization, fuel, maintenance, labor, etc. which drive the cost of the operation and impact the well production economics.

The hydraulic tracking process can be simplified with the following steps: (1) break the formation to create artificial fractures, (2) grow the fractures to connect with natural fractures in the formation, and (3) hold the fractures open by filling them with a media.

Propellants are comprised of energetic materials which generate pressure during combustion. Pressure propellants could be used as an alternative to the pressure created from the hydraulic fracking trucks on the surface. In general, the propellant is burned to create a single pressure surge or hold sustained pressure.

A propellant device can provide a pulsing pressure output. The premise of a Pulsed Pressure Fracking Device (PPFD) is that the cyclic loading on the formation will more effectively break the formation and grow the subsequent artificial fractures. The method is analogous to a jackhammer type effect used in construction to break rock or concrete. While conventional tracking may still be required with the use of a PPFD, the horsepower requirements should be significantly diminished, breakdown pressure should be diminished or eliminated, and tortuosity should be reduced or eliminated. In addition, the well productivity may be improved with a more effective fracture network in the formation.

An example embodiment, as shown in FIG. 1, may include an electrically initiated liquid propellant PPFD system 10 where the propellant 14 is contained in a PPFD module 17. The propellant 14 may be a liquid propellant. The propellant 14 is electrically ignitable using igniters 15, an example igniter may include a plurality of electrodes 15 having spark gap 16. The plurality of electrodes 15 are placed within the liquid propellant 14. The electrodes are individually controlled using signal wires 12 such that when electrical power is applied from a controller 11 to an electrode 15 the liquid propellant 14 is initiated locally near the electrode 15. As a result, there is a local pressure surge. By applying power to individual or sets of electrodes in sequence, a series of pressure pulses can be created.

An example embodiment of a method of using a PPFD may include achieving the PPFD effect during a plug and perf operation followed by conventional hydraulic fracking. As shown in FIG. 2, a PPFD module 17 with igniters 13 may be deployed by a wireline truck at a well 25. The PPFD module 17 is located within the wellbore 27 using wireline 26. The formation 28 has natural fractures 24. At least one PPFD module 17 is deployed on a plug and perf tool string. First, a plug 22 is set and then at least one perforating gun is fired to create perforations 29. Next, a PPFD module 17 is activated to create artificial fractures 23. Multiple PPFD modules may be used. The PPFD(s) may be activated after all the perforating guns have been fired. The tool string is then recovered to surface and conventional hydraulic tracking may occur.

As shown in FIG. 3, the pulse pressure fracking device uses a plurality of initiators or igniters to cause a plurality of pressure spikes 31, 33, and 35. The pressure function with respect to time 30 rises to an initial pressure spike 31, then the pressure falls to a first trough 32, then either on command or pre-planned, the second initiator activates, causing pressure spike 33. This pressure pulse shown, resulting from the back and forth between pressure spikes 31, 33, and 35 with pressure troughs 32, 34, and 36, shock the formation and continue to boost the overall pressure function 30 higher.

An example embodiment, shown in FIG. 4, may include a Mixed Energetic PPFD 40 where multiple energetic materials 45 are used in combination within the PPFD 40. The distinguishing feature of each energetic material is the burn rate, or deflagration rate, or detonation velocity. As a result of varying rates of combustion, varying rates of pressure output could be generated, thereby creating a pulsing action. A method to create a Mixed Energetic PPFD may include laminating layers of different energetic materials. For instance, a layered device could be crafted wherein the first layer, layer n, is a slow burning nitrate-based propellant 41 and the subsequent layer, layer n+1, is a fast-burning black powder composition 42, followed by another slow burning nitrate-based propellant 43 and a subsequent black powder 44, thus creating a combustion reaction that is Slow-Fast-Slow-Fast. This is reflected in FIG. 5 where the pressure function 50 increases at different rates. The nitrate-based propellant, 51 and 53, builds pressure slower than the black powder-based propellant 52 and 54. In this embodiment, the pressure amplitude can be controlled by the mass of each layer and the frequency of the pulser is controlled by the length of each layer. While in this description, the combustion reaction is self-propagating through the layers, an alternate method could use electric igniters to initiate each layer on demand.

In these embodiments, an electrical controller may be used to control the initiation sequence and thereby control the frequency and amplitude of the applied pressure. The control system may be a downhole device or on the surface.

In addition, a feedback system may be incorporated into the control system to adapt the pulse frequency and amplitude in conjunction with the well response. The feedback system could include pressure, flow rate of stimulating fluids being pumped into the well, temperature, etc. The feedback system may be a down hole device or surface device.

A method of using a PPFD may include achieving the PPFD effect during a plug and perf operation followed by conventional hydraulic fracking. At least one PPFD is deployed on a plug and perf tool string. First, a plug is set and then at least one perforating gun is fired. Next, a PPFD is activated. Multiple PPFD's may be used. The PPFD(s) may be activated after all the perforating guns have been fired. The tool string is then recovered to surface and conventional hydraulic tracking may occur.

A method of using a PPFD may include achieving the PPFD effect after the plug and perf operation, before conventional hydraulic fracturing. PPFD(s) is (are) deployed into a well stage and activated. The devices may or may not be recovered. Next, conventional hydraulic fracking may occur.

A method of using a PPFD may include achieving the PPFD effect during hydraulic tracking. The PPFD is deployed down hole and is activated while hydraulic frack trucks are in operation and pressurizing the well. Additionally, hydraulic fracturing may not be necessary if the effectiveness of the PPFD is high. In this case, proppant and stimulating fluids would be pumped into the well following formation breakdown.

Although the example embodiments have been described in terms of embodiments which are set forth in detail, it should be understood that this is by illustration only and that the example embodiments are not necessarily limited thereto. For example, terms such as upper and lower or top and bottom can be substituted with uphole and downhole, respectfully. Top and bottom could be left and right, respectively. Uphole and downhole could be shown in figures as left and right, respectively, or top and bottom, respectively. Generally downhole tools initially enter the borehole in a vertical orientation, but since some boreholes end up horizontal, the orientation of the tool may change. In that case downhole, lower, or bottom is generally a component in the tool string that enters the borehole before a component referred to as uphole, upper, or top, relatively speaking. The first housing and second housing may be top housing and bottom housing, respectfully. In a gun string such as described herein, the first gun may be the uphole gun or the downhole gun, same for the second gun, and the uphole or downhole references can be swapped as they are merely used to describe the location relationship of the various components. Terms like wellbore, borehole, well, bore, oil well, and other alternatives may be used synonymously. Terms like tool string, tool, perforating gun string, gun string, or downhole tools, and other alternatives may be used synonymously. The alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the example embodiments are contemplated which may be made without departing from the spirit of the claimed example embodiments.

Claims

1. A method of fracking a wellbore, comprising: lowering at least one module with a plurality of initiators disposed within at least one propellant;locating the at least one module at a desired location in the wellbore;firing a first initiator at a first predetermined time, wherein the resulting ignition produces a first pressure in the wellbore;firing a second initiator at a second predetermined time, wherein the resulting ignition produces a second pressure in the wellbore higher than the first pressure; andfiring a third initiator at a third predetermined time, wherein the resulting ignition produces a third pressure in the wellbore higher than the third pressure, wherein the first pressure, second pressure, and third pressure are initiated in a series to form a pressure pulse.

2. The method of fracking a wellbore of claim 1, wherein the interval between the first predetermined time, second predetermined time determines resulting second pressure.

3. The method of fracking a wellbore of claim 2, wherein the interval between the first predetermined time, second predetermined time, and third predetermined time determines resulting third pressure.

4. The method of fracking a wellbore of claim 1, wherein the propellant is a first propellant, a second propellant, and a third propellant.

5. The method of fracking a wellbore of claim 4, wherein the first propellant properties determine the first pressure.

6. The method of fracking a wellbore of claim 4, wherein the second propellant properties determine the second pressure.

7. The method of fracking a wellbore of claim 4, wherein the third propellant properties determine the third pressure.

8. The method of fracking a wellbore of claim 1, further comprising setting a plug.

9. The method of fracking a wellbore of claim 1, wherein the plurality of initiators include an electrode with a spark gap.

10. The method of fracking a wellbore of claim 1, wherein the propellant is a liquid propellant.

11. An apparatus for fracking a wellbore, comprising: a housing containing at least one propellant; anda plurality of initiators embedded in the propellant, wherein the initiators are detonated in a sequence timed to cause a plurality of pressure spikes in at least one wellbore fracture, where the plurality of initiators are adapted to detonate a series of pressure spikes, with each subsequent pressure spike producing a higher pressure than the previous pressure spike, resulting in a pressure pulse.

12. The apparatus for fracking a wellbore of claim 11, wherein the propellant is a liquid propellant.

13. The apparatus for fracking a wellbore of claim 11, wherein the propellant is a plurality of propellant portions.

14. The apparatus for fracking a wellbore of claim 13, wherein the propellant is a plurality of propellant portions.

15. The apparatus for fracking a wellbore of claim 14, wherein the each of the plurality of propellant portions has a dedicated at least one of the plurality of initiators.

16. The apparatus for fracking a wellbore of claim 15, wherein at least one of the plurality of propellant portions is a black powder based propellant.

17. The apparatus for fracking a wellbore of claim 16, wherein at least one of the plurality of propellant portions is a nitrate based propellant.

18. The apparatus for fracking a wellbore of claim 11, further comprising a plug setting tool coupled to the housing.

19. The apparatus for fracking a wellbore of claim 11, further a wireline electrically connecting the housing to a controller, wherein the housing is disposed within a wellbore and the controller is located at a surface.

20. The apparatus for fracking a wellbore of claim 11, wherein each of the plurality of initiators includes an electrode with a spark gap.