Pulsed fracturing method and apparatus

Abstract

The branching of fractures in shale formations surrounding a wellbore can be enhanced so that more rock surface is exposed in the formation and more hydrocarbon resources can be recovered using smaller quantities of fracturing fluids. The branching of the fractures can be enhanced by establishing a substantially static sub-threshold fluid pressure in a well in a geological formation, and generating a pressure pulse in the well such that the pressure pulse combined with the substantially static sub-threshold fluid pressure forms a plurality of fractures in the geological formation. An arc jet insertable into a passage in the geological formation and having an electric arc channel can be used to generate the pressure pulse.

Claims

1. A method for fracturing a subterranean geological formation, comprising: establishing a substantially static fluid pressure in a well in the geological formation, wherein the substantially static pressure is less than a threshold pressure that fractures the geological formation; and generating a pressure pulse in the well such that the pressure pulse combined with the substantially static pressure forms a plurality of fractures in the geological formation; wherein the pressure pulse is generated with an electric arc means, and further wherein the electric arc means forms an electric arc in an arc jet.

2. The method of claim 1, further comprising generating the pressure pulse in superposition with the static pressure more than once during a pre-selected time period within the well.

3. The method of claim 2, wherein the pressure pulses are generated in various treatment zones along the well.

4. The method of claim 1, wherein the geological formation is a hydrocarbon-bearing geological formation.

5. The method of claim 1, wherein the well is a geothermal well or an injection well.

6. The method of claim 1, wherein the electric arc means comprises an electrical transmission line for transmitting electrical energy generated outside the well to a selected location within the well.

7. The method of claim 1, wherein the electric arc means comprises a pulsed electrical power supply located outside of the well for supplying electrical energy to initiate and sustain the electric arc.

8. The method of claim 7, wherein the pulsed electrical power supply comprises a capacitor bank to store energy for sustaining the electric arc.

9. The method of claim 7, wherein the pulsed electrical power supply comprises a compulsator to generate pulsed energy for sustaining said electric arc.

10. The method of claim 7, wherein the pulsed electrical power supply comprises a flywheel-energy-storage-means for supplying energy to sustain the electric arc.

11. The method of claim 1, further comprising injecting a fluid into the electric arc of the arc jet to replenish material that has been heated and subsequently expelled from a channel in the arc jet.

12. The method of claim 11, wherein the fluid is ammonia.

13. The method of claim 11, wherein the arc jet includes means for directing the movement of the fluid that is heated by the electric arc radially outward from the well and into the geological formation.

14. The method of claim 1, wherein the pressure pulse is generated by an exothermic-chemical-reaction-means.

15. A method for fracturing a subterranean geological formation, comprising: establishing a sub-threshold substantially static fluid pressure in a well in the geological formation; generating a pressure pulse in the well such that the pressure pulse combined with the substantially static pressure forms a plurality of fractures in the geological formation, wherein the pressure pulse is generated by an arc jet.

16. An apparatus for fracturing a subterranean geological formation from a passage extending through the geological formation, comprising: an arc jet insertable into the passage and having an electric arc channel, wherein the arc channel contains a fluid; a pulsed electrical power supply in communication with the arc jet; and means for transmitting electrical energy from the pulsed electrical power supply to the arc jet, wherein the pulsed electrical power supply transmits electrical energy to the arc jet to generate an electric arc in the fluid in the arc channel thereby generating a pressure pulse for generating a plurality of fractures in the geological formation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. The left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.

(2) FIG. 1 shows the fracture extent in the vertical direction for hydraulic treatments in the Marcellus shale in the Appalachian Basin in Ohio, Pennsylvania, and West Virginia;

(3) FIG. 2 is a large granite bolder fractured into many sections using plasma blasting with an arc-source.1 Left: Granite rock before plasma blasting. Right: Shattered granite rock after plasma blasting;

(4) FIG. 3 is a circuit analog of small fractures extending from a larger principal fracture. Colored probes measure voltage differentials analogous to pressure differentials in FIG. 4 with corresponding colors;

(5) FIG. 4 is an electrical analog of pressure within fractures resulting from a transient pressure burst at the root of a principal fracture. Green: Voltage (pressure) burst at fracture root. Other colors are analogous to pressures within fracture branches at distances: Red—0.125 L, Blue—0.375 L, Yellow—0.625 L, Purple—end of fracture (1.0 L), where L is the length of the principal fracture;

(6) FIG. 5 is one embodiment of the arc-source for driving high-pressure impulses in hydrocarbon-bearing shale formations;

(7) FIG. 6 is a cross-sectional view of a 3-D rendering of a preferred embodiment of an arc-source for the invention that includes callouts for major subassemblies including a fluid injector subassembly, a pulsed arcjet subassembly, and a blast chamber subassembly, all shown within a perforated casing that lines the wellbore; and

(8) FIG. 7 is a cross-sectional view of a 3-D rendering of the preferred embodiment of the arc-source that includes further descriptive breakdown and callouts for specific components of the fluid injector subassembly and the pulsed arcjet subassembly referred to in FIG. 9.

(9) FIG. 8 is an overall circuit topology for the pulsed power system of one embodiment;

(10) FIG. 9 is a single-pulse waveform generated by the circuit in FIG. 6 using a moderately sized cap bank.

(11) FIG. 10 shows pulsed waveforms with cyclic discharge of a large capacitor bank to create extended pulse duration.

DETAILED DESCRIPTION

(12) The invention will use reduced fluid volumes for a given production level and will recover a larger fraction of hydrocarbon resources from shale formations. By reducing fluid volumes and associated chemical additive volumes, fracturing fluids may be better managed to help protect the environment. In particular the risk of accidental release of contaminated flowback fluids into the environment is greatly diminished. With lower fluid volumes, fewer trucks will be needed to haul fluids, reducing transport costs for producers, and minimizing disturbances and road damage in local communities. By containing stimulation fractures closer to the production zone, the chances of intersecting abandoned wells and vertical faults is decreased, easing public concerns that these pathways or other undetected pathways could provide conduits to potable water aquifers near the surface.

(13) Electric Discharge Apparatus

(14) Pulsed pressures can be applied in the shale formation by means of powerful electric discharges in the fracture fluid near the fracture site. Pulses of electric energy can be transmitted to a compact arc-source located in a perforated section of a wellbore, or the uncased portion of some wells. During the electric discharges, a plasma forms at the arc-source, sending a high pressure surge into the formation with sufficient force to fracture rock. The arc-source is submerged in a column of fluid within the well that provides back pressure to tamp the impulsive force of the pressure surge and channel the surge into the formation with considerable force. Electric pulse energy is supplied by a capacitor bank charged by a generator located at the surface of the well. Other pulsed energy sources may be used, including compulsators, flywheel systems, and other well-known pulsed power sources. The electric pulse of energy is transmitted to the arc-source at high-voltages using an electric cable connected between the pulsed power supply on the surface and the arc-source at depth in the well.

(15) Pulsed electric discharges have been utilized previously for generating very high pressures that can fracture rock at the surface or in shallow mines. For example, in FIG. 2, an electric discharge in water was used to fracture large granite boulders in a laboratory. Pulsed energies were less than about 30 kJ and pulse lengths were on the order of 100 microseconds. In one embodiment of the invention, at least sixty-seven times more energy can be used to fracture rock, yielding a far more powerful effect over a much larger volume of rock.

(16) Arc Source

(17) FIG. 5 shows one embodiment of an arc-source that can generate high pressure pulses in and around the wellbore. In this embodiment, the arc-source consists of a pair of electrodes (gold tori in FIG. 5) separated by a small gap where the electric discharge is initiated. The electrodes can be composed of tungsten in order to minimize electrode erosion and maximize heat tolerance. The electrodes can be backed by structural members that provide inertial cooling of the electrodes and help guide the tool down the wellbore. In the illustrated configuration, forces on the probe body will be mostly radial, minimizing the tendency for the probe to move along the wellbore during a shot. To limit mechanical shock to components attached to the arc-source, the system can be equipped with shock absorbing devices (not shown) that use spring tension, inertia, and water resistance to dampen impulses. With a short axial extent for the electric discharge, the arc-source can concentrate its discharge energy at an individual perforation in the wellbore. In doing so, conductance of the pressure surge into the formation will be maximized, enhancing the effectiveness of the electric discharge.

(18) FIG. 6 shows another embodiment of the arc-source, or arc-discharge device, for electro-fracturing around a wellbore. In this embodiment, the arc-source includes three subassemblies consisting of a fluid injector subassembly, a pulsed arcjet subassembly, and a blast chamber subassembly. For reference, the arc-source is shown in relation to a perforated casing that would line the wellbore and surround the arc-source in the wellbore. In addition to the arc-source shown in FIG. 6, the electro-fracturing system can include an electrical power supply located at the surface of the well and a power transmission line that transmits energy from the power supply at the surface to the arc-discharge device in the wellbore.

(19) Referring generally to FIG. 7, the pulsed arcjet subassembly can include an arcjet central electrode; an arcjet outer electrode; an arc channel; and an arcjet nozzle. Components of the fluid injector subassembly attached to the pulsed arcjet subassembly of the present embodiment include a fluid injection chamber; a fluid conduit; a sliding piston; a piston drive; and an injector anchor. Components of the blast chamber subassembly attached to the pulsed arcjet subassembly of the present embodiment include a channel plug. The channel plug can slide axially within the blast chamber subassembly.

(20) Before each pulse, the channel plug is retracted into the arcjet nozzle by springs or the like in order to block fluid flow into the pulsed arcjet subassembly from fluids present in the wellbore. With the channel plug in this position, the fluid injection chamber within the electro-fracturing device is isolated from wellbore fluids. In this initial configuration, the fluid injection chamber can be filled with a pressurized fluid flowing inside of the fluid conduit from an external source of fluid. An opening is provided from the fluid conduit into the fluid injection chamber to allow fluid filling of the fluid injection chamber. During the filling operation and prior to a pulse, the sliding piston is pushed back and the piston drive elements are compressed against the injector anchor in preparation for energizing the piston drive during a pulse.

(21) A fluid delivery means is included that may consist of a pressurized fluid storage tank containing fluid sufficient for one or more pulses. The tank may be connected to an inlet to the fluid conduit located behind the injector anchor. Alternatively, a tube from the surface may connected to the fluid conduit inlet to supply fluid from the surface. Means may also be provided for drawing in-situ fluids present in the wellbore into the fluid conduit inlet and from there into the fluid injection chamber. In-situ fluids may be filtered or processed before use to minimize possible degradation of the electro-fracturing device from particulates and undesirable chemical components.

(22) Fluids may be comprised of a variety of materials including in-situ wellbore fluids, purified water, ammonia, supercritical carbon dioxide, and the like. A preferred fluid is ammonia with chemical formula NH3. Ammonia has been used successfully in many experimental arcjets. Decomposition products of ammonia consist of nitrogen and hydrogen primarily, which will leave no residue in the arcjet or in the well. With no residue in the well, there will be minimal interference with hydrocarbon flow after treatment of the well. In a preferred embodiment, ammonia is fed to the fluid conduit inlet as a pressurized fluid either from a pressurized storage tank in the wellbore close to the arc-discharge device or via a tube from the surface.

(23) In one embodiment, means may be provided to heat and pressurize the ammonia so that the ammonia becomes a supercritical fluid. The critical point for ammonia occurs at a temperature of 132.4 deg. C. and a pressure of 11.28 MPa (1,636 psi). In-situ wellbore pressures can easily exceed the critical pressure for ammonia, and wellbore temperatures can exceed the critical temperature of ammonia in some wells, so that supercritical ammonia is easily formed and sustained in wellbore equipment. In some cases, a small amount of heat may need to be added to background heat in order to form supercritical ammonia. Above the critical point, ammonia begins to act like a gas in that it no longer wets surfaces as does a liquid, although it has a density similar to a liquid. Supercritical ammonia is then well-suited to high-speed flow through the pulsed arcjet subassembly and arc generation between electrodes during each pulse of energy.

(24) At the beginning of each pulse of energy, the fluid supplied to the fluid injection chamber by the fluid delivery means is driven through the gap between the arcjet central electrode and the arcjet outer electrode and into the arc channel by a forward motion of the sliding piston pushed by the piston drive once it is energized. High-voltage is simultaneously applied between the arcjet central electrode and the arcjet outer electrode to form an arc discharge through the fluid in the space between the arcjet central electrode and the arcjet outer electrode. The solid walls surrounding the fluid conduit serve as the main body of the arcjet central electrode.

(25) With a positive voltage polarity on the arcjet central electrode relative to the arcjet outer electrode, electrical current from a power supply at the surface flows along the fluid conduit to the arcjet central electrode and through the arc once the arc forms. Return current back to the surface flows through the arcjet outer electrode and the outer shell of the fluid injector. With this voltage polarity, ions in the arc impinge on the arcjet outer electrode and electrons impinge on the arcjet central electrode. Surface erosion from ion impingement occurs in the arcjet outer electrode while simple heating is produced in the arcjet central electrode from electron impingement.

(26) In a preferred embodiment, the polarity is reversed so that the arcjet central electrode is negative relative the arcjet outer electrode. In this case, the current flow reverses and material erosion occurs primarily in the arcjet central electrode and very little erosion occurs in the arcjet outer electrode. The more complex and expensive arcjet outer electrode is thereby preserved and its useful lifetime extended. Erosion is confined to the arcjet central electrode where it can be more easily corrected by employing a consumable electrode that is inserted into the arc region as the electrode material erodes in the arc.

(27) Typical erosion rates are 100-200 micrograms per Coulomb of charge transferred in common spark-gap switches. Using this erosion rate, and assuming 500 Coulombs of charge is transferred in each arc, approximately 0.05-0.1 grams of material will erode away on each pulse. An arcjet central electrode made of tungsten with a diameter of 2 cm, will then have to be fed into the arc at the rate of approximately 1.6 cm for every thousand shots.

(28) During the arc discharge, fluid is injected into the arc at a selected fluid velocity and a selected pressure using the fluid injector subassembly attached to the pulsed arcjet subassembly. The selected fluid velocity and pressure are sufficient to drive the arc into and through the arc channel. The restricted flow within the arc channel forces most of the fluid to pass into the arc region and helps to mix the fluid and heat it uniformly. Selected shapes for the arc channel and arcjet nozzle cause the arc to terminate on the surface of the arcjet nozzle, where heat flux to the surface can be spread out along the expanding surface of the arcjet nozzle. In the preferred embodiment, the shape of arcjet nozzle is tapered with a selected taper angle.

(29) A jet of hot fluid, much of which is in a plasma state, is emitted from the arcjet nozzle during an arc discharge. The channel plug is then driven to its fully extended position by the force of the initial jet of hot fluid ejected from the arcjet nozzle, allowing hot gases and plasma to enter the blast chamber. The jet of hot fluid is quickly thermalized outside of the pulsed arcjet subassembly, and pressure in the blast chamber rises rapidly. The blast chamber subassembly attached to the pulsed arcjet subassembly directs the hot fluids radially outward through any perforations in the wellbore casing and into the rock formation around the wellbore. The pressurized fluid then expands into the formation outside the wellbore causing the rock to fracture. The length of the blast chamber along the wellbore axis may be selected to provide blast pressure over a pre-determined length of the wellbore.

(30) Pulsed Power Supply

(31) The pulsed power supply can be located at the surface of the well. Since size and weight are not issues at the surface of the well, energy equivalent to GPF methods can easily be applied. In fact, energy far in excess of GPF methods can be applied if desired. Special engineering measures can be taken to protect the wellbore casing as the pulse energy exceeds typical GPF energies. Power is transmitted from the pulsed power supply to the arc-source deep within the well along a power transmission cable, typically a coaxial transmission cable. Based upon GPF results and previous plasma rock fracturing tests, pulse energies of two megajoules should produce extensive rock fracturing around the wellbore without damaging the wellbore casing. For reference, two megajoules equals the energy in one pound of TNT. This energy is about sixty-seven times the energy applied in the granite fracturing tests shown in FIG. 2. Assuming system efficiencies of 80%, approximately 2.5 MJ of energy would be required at the pulsed power source at the surface of the well.

(32) FIG. 8 shows a schematic diagram for the overall circuit, including the pulsed power supply at the surface of the well, the power transmission line along the length of the wellbore, and the arc-source at depth within the well. This circuit is well suited to electro-fracturing in which the impedance of the load changes abruptly when the arc forms in the discharge device. In addition, there is no reversal of the capacitor bank voltage, which will lead to substantially increased capacitor lifetimes.

(33) Waveforms produced at the arc source for a relatively small capacitor bank and short pulse duration are shown in FIG. 9. A coaxial cable length of 2000 meters is assumed for the transmission line modeled in FIG. 8 to accurately capture electrical effects that will occur in a deep well. In operation, the high voltage switch remains closed until the capacitor bank is completely discharged. At the zero-voltage crossing of the capacitor bank, diode Dl becomes forward biased so that current flow from the inductances is diverted to the loop around the diode and inductors, and damaging voltage transients are avoided. If a much larger capacitor bank is used, the pulsed power supply may be operated in a mode in which the high-voltage switch is cycled on and off repeatedly during a specified pulse period in order to achieve a variable pulse length from the superposition of multiple short pulses. FIG. 10 shows an example of this type of operation for the circuit topology in FIG. 8.

(34) The time required to form the initial fracture network over an entire well is ultimately determined by the power available for charging the capacitor bank and the length of the stimulation zone. Mobile generators up to about two megawatts are readily available commercially. Assuming 90% efficiency in the capacitor charging equipment at 2 MW, the capacitor bank could be charged from zero to 2.5 MJ in about 1.4 seconds. With this pulse repetition rate, and the assumption that five pulses are applied at each location in the wellbore on top of several thousand psi of bias pressure, a well could be pretreated at the rate of 8.6 ft. per minute if the treatment intervals are one foot apart. Initial pulsed fracturing for 5000 ft. of wellbore would then require less than ten hours. The completion rate would be faster if fewer pulses were effective at each perforation, or if more power was used in the operation. Total energy consumed in the initial fracture formation would be about 20,000 kW-hrs. This is roughly the energy contained in natural gas output from a typical gas well producing 2000 Mcf/day over a two day period. Energy consumed in practice of the invention is clearly negligible on the scale of the energy content produced by a single gas well.