Magnetic ranging systems and methods using random electric spark excitation

A downhole drilling system may include a pulse-generating circuit; a drill bit including a first and second electrode electrically coupled to the pulse generating circuit to receive an electrical pulse from the pulse-generating circuit and to form an electrical arc between the first and second electrodes during a pulsed drilling operation in a first wellbore; a sensor to record responses to a magnetic field generated by a current in a second wellbore, the current generated by the electrical arc; and a sensor analysis system communicatively coupled to the sensor, the sensor analysis system configured to obtain a measurement from the sensor, the measurement representing the recorded response to the magnetic field and determine a distance between the drill bit and the second wellbore based on the measurement.

RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/US2018/038398 filed Jun. 20, 2018, which designates the United States, and which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to downhole pulsed-power drilling and, more particularly, to magnetic ranging systems for downhole pulsed-power drilling.

BACKGROUND

Electrocrushing drilling uses pulsed-power technology to drill a wellbore in a rock formation. Pulsed-power technology repeatedly applies a high electric potential across the electrodes of a pulsed-power drill bit, which ultimately causes the surrounding rock to fracture. The fractured rock is carried away from the bit by drilling fluid and the bit advances downhole. Electrocrushing drilling operations may also be referred to as pulsed drilling operations.

DETAILED DESCRIPTION

Electrocrushing drilling may be used to form wellbores in subterranean rock formations for recovering hydrocarbons, such as oil and gas, from these formations. Electrocrushing drilling uses pulsed-power technology to fracture the rock formation by repeatedly delivering electrical arcs or high-energy shock waves to the rock formation. More specifically, a drill bit of a pulsed-power drilling system is excited by a train of high-energy electrical pulses that produce high power discharges through the formation at the downhole end of the drill. The high-energy electrical pulses, in turn, fracture part of the formation surrounding the drilling tool and produce electromagnetic and acoustic waves that carry information about properties of the formation. The azimuthal angles over which discharges take place between electrodes at the tip of the drill bit may occur randomly along those azimuthal angles for which the formation is still intact.

As described in detail herein, a pulsed-power drilling system with an associated sensor analysis system may implement logging-while-drilling techniques that include using electrical and/or electromagnetic sensors to record responses to the electromagnetic waves provided during a pulsed drilling operation. The sensors may be located on the surface and/or downhole to record responses to the electromagnetic waves produced by pulsed-power drilling operations as they travel through and reflect off various layers in the formation and are received by the sensors. Moreover, sensors may record responses to secondary electromagnetic waves produced by a current in other wellbores. The current may be created by the electromagnetic waves produced by pulsed-power drilling operations. The shape and magnitude of waves received by the sensors carry information that may be used to estimate properties of the formation layers through which the reflected waves have passed. The propagation time of a given reflection may be indicative of the distance traveled and may be used to generate a map of the formation properties as a function of distance.

The sensors may convert the recorded signals into one or more measurements in a form suitable for analysis by a sensor analysis system. The resulting measurements may represent voltages, currents, measurements of electric field strength, measurements of magnetic field strength, or any combinations thereof. The measurements may be provided by the sensors to a sensor analysis system, where they may be analyzed or stored for subsequent processing. The sensor analysis system may process the measurements received from the sensors. The results of an inversion process whose inputs include such measurements may be used to determine the distance and/or direction of another wellbore to avoid, intercept, or maintain the direction of pulsed-power drilling operations and/or for other purposes based on electrical or acoustic properties detected by the sensors.

There are numerous ways in which a ranging system may determine the distance and/or direction to a target wellbore based on responses recorded during a pulsed drilling operation. Thus, embodiments of the present disclosure and its advantages are best understood by referring toFIGS. 1 through 12, where like numbers are used to indicate like and corresponding parts.

FIG. 1is an elevation view of an exemplary pulsed-power drilling system used to form a wellbore in a subterranean formation. AlthoughFIG. 1shows land-based equipment, downhole tools incorporating teachings of the present disclosure may be satisfactorily used with equipment located on offshore platforms, drill ships, semi-submersibles, and drilling barges (not expressly shown). Additionally, while wellbore116is shown as being a generally vertical wellbore, wellbore116may be any orientation including generally horizontal, multilateral, or directional.

Drilling system100includes drilling platform102that supports derrick104having traveling block106for raising and lowering drill string108. Drilling system100may also include pump125, which circulates drilling fluid122through a feed pipe to kelly110, which in turn conveys drilling fluid122downhole through interior channels of drill string108and through one or more fluid flow ports in pulsed-power drill bit114. Drilling fluid122circulates back to the surface via annulus126formed between drill string108and the sidewalls of wellbore116. Fractured portions of the formation are carried to the surface by drilling fluid122to remove those fractured portions from wellbore116.

Pulsed-power drill bit114is attached to the distal end of drill string108and may be an electrocrushing drill bit or an electrohydraulic drill bit. Power may be supplied to drill bit114from components downhole, components at the surface and/or a combination of components downhole and at the surface. For example, generator140may generate electrical power and provide that power to power-conditioning unit142. Power-conditioning unit142may then transmit electrical energy downhole via surface cable143and a sub-surface cable (not expressly shown inFIG. 1) contained within drill string108or attached to the side of drill string108. A pulse-generating circuit within BHA128may receive the electrical energy from power-conditioning unit142, and may generate high-energy electrical pulses to drive drill bit114. The pulse-generating circuit may include a power source input, including two input terminals, and a first capacitor coupled between the input terminals. The pulse-generating circuit may also include a switch, a transformer, and a second capacitor whose terminals are coupled to respective electrodes of drill bit114. The switch may include a mechanical switch, a solid-state switch, a magnetic switch, a gas switch, or any other type of switch suitable to open and close the electrical path between the power source input and a first winding of the transformer. The transformer generates a current through a second winding when the switch is closed and current flows through first winding. The current through the second winding charges the second capacitor. As the voltage across the second capacitor increases, the voltage across the electrodes of the drill bit increases.

The pulse-generating circuit within BHA128may be utilized to repeatedly apply a large electric potential, for example up to or exceeding 150 kV, across the electrodes of drill bit114. Each application of electric potential is referred to as a pulse. When the electric potential across the electrodes of drill bit114is increased enough during a pulse to generate a sufficiently high electric field, an electrical arc forms through rock formation118at the bottom of wellbore116. The arc temporarily forms an electrical coupling between the electrodes of drill bit114, allowing electric current to flow through the arc inside a portion of the rock formation at the bottom of wellbore116. The arc greatly increases the temperature and pressure of the portion of the rock formation through which the arc flows and the surrounding formation and materials. The temperature and pressure are sufficiently high to break the rock itself into small bits or cuttings. This fractured rock is removed, typically by drilling fluid122, which moves the fractured rock away from the electrodes and uphole. The terms “uphole” and “downhole” may be used to describe the location of various components of drilling system100relative to drill bit114or relative to the bottom of wellbore116shown inFIG. 1, rather than to describe relative directions in terms of true up or true down. Therefore, if wellbore116is a horizontal wellbore or is otherwise angled away from vertical, the term “uphole” may refer to the direction away from drill bit114, regardless of whether that direction is to the right, to the left, up, or down relative to drill bit114. For example, a first component described as uphole from a second component may be further away from drill bit114and/or the bottom of wellbore116than the second component. Similarly, a first component described as being downhole from a second component may be located closer to drill bit114and/or the bottom of wellbore116than the second component. The electrical arc may also generate acoustic and/or electromagnetic waves that are transmitted within rock formation118and/or drilling fluid122. Sensors placed within wellbore116and/or on the surface may record responses to high-energy electrical pulses, electrical arcs, or the acoustic waves and/or electromagnetic waves. Sensor analysis system150may receive measurements representing the recorded responses and may analyze the measurements to determine characteristics of rock formation118or for other purposes.

Wellbore116, which penetrates various subterranean rock formations118, is created as drill bit114repeatedly fractures the rock formation and drilling fluid122moves the fractured rock uphole. Wellbore116may be any hole formed into a subterranean formation or series of subterranean formations for the purpose of exploration or extraction of natural resources such as, for example, hydrocarbons, or for the purpose of injection of fluids such as, for example, water, wastewater, brine, or water mixed with other fluids. Additionally, wellbore116may be any hole drilled into a subterranean formation or series of subterranean formations for the purpose of geothermal power generation.

Although pulsed-power drill bit114is described above as implementing electrocrushing drilling, pulsed-power drill bit114may also be used for electrohydraulic drilling, rather than generating an electrical arc within the rock, drill bit114applies a large electrical potential across one or more electrodes and a ground ring to form an arc across the drilling fluid proximate to the downhole end of wellbore116. The high temperature of the arc vaporizes the portion of the drilling fluid immediately surrounding the arc, which in turn generates a high-energy shock wave in the remaining fluid. The one or more electrodes of electrohydraulic drill bit may be oriented such that the shock wave generated by the arc is transmitted toward the bottom of wellbore116. When the shock wave contacts and bounces off of the rock at the bottom of wellbore116, the rock fractures. Accordingly, wellbore116may be formed in subterranean formation118using drill bit114that implements either electrocrushing or electrohydraulic drilling.

Distributed acoustic sensing (DAS) subsystem155may be positioned at the surface for use with pulsed-power drilling system100, or at any other suitable location. DAS subsystem155may be coupled to optical fiber160, which is positioned within a portion of the pulsed-power drilling system100. For example, optical fiber160may be positioned within wellbore116. Any suitable number of DAS subsystems (each coupled to an optical fiber160located downhole) may be placed inside or adjacent to wellbore116. With optical fiber160positioned inside a portion of wellbore116, DAS subsystem155may determine characteristics associated with formation118based on changes in strain caused by acoustic waves. DAS subsystem155may be configured to transmit optical pulses into optical fiber160, and to receive and analyze reflections of the optical pulse to detect changes in strain caused by acoustic waves.

Sensor analysis system150may be positioned at the surface for use with pulsed-power drilling system100as illustrated inFIG. 1, or at any other suitable location. Any suitable telemetry system may be used for communicating signals from various acoustic, electrical or electromagnetic sensors at the surface or downhole to sensor analysis system150during a pulsed drilling operation. For example, sensor analysis system150may be coupled to optical fiber160that extends downhole in wellbore116. More specifically, one or more input/output interfaces of sensor analysis system150may be coupled to optical fiber160for communication to and from acoustic, electrical or electromagnetic sensors positioned downhole. For example, the sensors may transmit measurements to sensor analysis systems. Any suitable number of sensor analysis systems150(each of which may be coupled to an optical fiber located downhole) may be placed inside or adjacent to wellbore116. An example sensor analysis system is illustrated inFIG. 5and described in more detail below.

Optical fiber160may be enclosed within a cable, rope, line, or wire. More specifically, optical fiber160may be enclosed within a slickline, a wireline, coiled tubing, or another suitable conveyance for suspending a downhole tool in wellbore116. Optical fiber cable160may be charged by a laser to provide power to DAS subsystem155, sensor analysis system150, or sensors located within wellbore116.

FIG. 2Ais a perspective view of exemplary components of bottom-hole assembly for a pulsed-power drilling system. BHA128may include pulsed-power tool230and drill bit114. For the purposes of the present disclosure, drill bit114may be integrated within BHA128, or may be a separate component coupled to BHA128. Pulsed-power tool230may provide pulsed electrical energy to drill bit114.

Pulsed-power tool230receives electrical power from a power source via cable220. For example, pulsed-power tool230may receive electrical power via cable220from a power source located on the surface as described above with reference toFIG. 1, or from a power source located downhole such as a generator powered by a mud turbine. Pulsed-power tool230may also receive electrical power via a combination of a power source located on the surface and a power source located downhole. Drill bit114may include ground ring250, shown in part inFIG. 2A. Ground ring250may function as an electrode. Pulsed-power tool230converts electrical power received from the power source into high-energy electrical pulses that are applied across electrodes208and ground ring250of drill bit114. Pulsed-power tool230may also apply high-energy electrical pulses across electrode210and ground ring250in a similar manner as described for electrode208and ground ring250. Pulsed-power tool230may include a pulse-generating circuit as described above in reference toFIG. 1.

Although illustrated as a contiguous ring inFIG. 2A, ground ring250may be non-contiguous discrete electrodes and/or implemented in different shapes. Each of electrodes208and210may be positioned at a minimum distance from ground ring250of approximately 0.4 inches and at a maximum distance from ground ring250of approximately 4 inches. The distance between electrodes208or210and ground ring250may be based on the parameters of the pulsed drilling operation and/or on the diameter of drill bit114. For example, the distance between electrodes208or210and ground ring250, at their closest spacing, may be at least 0.4 inches, at least 1 inch, at least 1.5 inches, or at least 2 inches.

Drilling fluid122is typically circulated through drilling system100at a flow rate sufficient to remove fractured rock from the vicinity of drill bit114. In addition, drilling fluid122may be under sufficient pressure at a location in wellbore116, particularly a location near a hydrocarbon, gas, water, or other deposit, to prevent a blowout. Drilling fluid122may exit drill string108via openings209surrounding each of electrodes208and210. The flow of drilling fluid122out of openings209allows electrodes208and210to be insulated by the drilling fluid. A solid insulator (not expressly shown) may surround electrodes208and210. Drill bit114may also include one or more fluid flow ports260on the face of drill bit114through which drilling fluid122exits drill string108, for example fluid flow ports260on ground ring250. Fluid flow ports260may be simple holes, or they may be nozzles or other shaped features. Because fines are not typically generated during pulsed-power drilling, as opposed to mechanical drilling, drilling fluid122may not need to exit the drill bit at as high a pressure as the drilling fluid in mechanical drilling. As a result, nozzles and other features used to increase drilling fluid pressure may not be needed on drill bit114. However, nozzles or other features to increase drilling fluid122pressure or to direct drilling fluid may be included for some uses. Additionally, the shape of a solid insulator, if present, may be selected to enhance the flow of drilling fluid122around the components of drill bit114.

If drilling system100experiences vaporization bubbles in drilling fluid122near drill bit114, the vaporization bubbles may have deleterious effects. For instance, vaporization bubbles near electrodes208or210may impede formation of the arc in the rock. Drilling fluid122may be circulated at a flow rate also sufficient to remove vaporization bubbles from the vicinity of drill bit114. Fluid flow ports260may permit the flow of drilling fluid122along with any fractured rock or vaporization bubbles away from electrodes208and210and uphole.

FIG. 2Bis a perspective view of exemplary components of bottom-hole assembly for a pulsed-power drilling system. BHA128may include pulsed-power tool230and drill bit115. For the purposes of the present disclosure, drill bit115may be integrated within BHA128, or may be a separate component that is coupled to BHA128. BHA128and pulsed-power tool230may include features and functionalities similar to those discussed above inFIG. 2A.

Drill bit115may include bit body255, electrode212, ground ring250, and solid insulator270. Electrode212may be placed approximately in the center of drill bit115. Electrode212may be positioned at a minimum distance from ground ring250of approximately 0.4 inches and at a maximum distance from ground ring250of approximately 4 inches. The distance between electrode212and ground ring250may be based on the parameters of the pulsed drilling operation and/or on the diameter of drill bit115. For example, the distance between electrode212and ground ring250, at their closest spacing, may be at least 0.4 inches, at least 1 inch, at least 1.5 inches, or at least 2 inches. The distance between electrode212and ground ring250may be generally symmetrical or may be asymmetrical such that the electric field surrounding the drill bit has a symmetrical or asymmetrical shape. The distance between electrode212and ground ring250allows drilling fluid122to flow between electrode212and ground ring250to remove vaporization bubbles from the drilling area. Electrode212may have any suitable diameter based on the pulsed drilling operation, on the distance between electrode212and ground ring250, and/or on the diameter of drill bit115. For example, electrode212may have a diameter between approximately 2 and approximately 10 inches (i.e., between approximately 51 and approximately 254 millimeters). Ground ring250may function as an electrode and provide a location on the drill bit where an electrical arc may initiate and/or terminate.

Drill bit115may include one or more fluid flow ports on the face of the drill bit through which drilling fluid exits the drill string108. For example, ground ring250of drill bit115may include one or more fluid flow ports260such that drilling fluid122flows through fluid flow ports260carrying fractured rock and vaporization bubbles away from the drilling area. Fluid flow ports260may be simple holes, or they may be nozzles or other shaped features. Drilling fluid122is typically circulated through drilling system100at a flow rate sufficient to remove fractured rock from the vicinity of drill bit115. In addition, drilling fluid122may be under sufficient pressure at a location in wellbore116, particularly a location near a hydrocarbon, gas, water, or other deposit, to prevent a blowout. Drilling fluid122may exit drill string108via opening213surrounding electrode212. The flow of drilling fluid122out of opening213allows electrode212to be insulated by the drilling fluid. Because fines are not typically generated during pulsed-power drilling, as opposed to mechanical drilling, drilling fluid122may not need to exit the drill bit at as high a pressure as the drilling fluid in mechanical drilling. As a result, nozzles and other features used to increase drilling fluid pressure may not be needed on drill bit115. However, nozzles or other features to increase drilling fluid122pressure or to direct drilling fluid may be included for some uses. Additionally, the shape of solid insulator270may be selected to enhance the flow of drilling fluid122around the components of drill bit115.

As described above with reference toFIGS. 1, 2A, and 2B, when the electric potential across electrodes of a pulsed-power drill bit becomes sufficiently large, an electrical arc forms through the rock formation and/or drilling fluid that is near the electrodes. The arc provides a temporary electrical short between the electrodes, and thus allows electric current to flow through the arc inside a portion of the rock formation and/or drilling fluid at the bottom of the wellbore. The arc increases the temperature of the portion of the rock formation through which the arc flows and the surrounding formation and materials. The temperature is sufficiently high to vaporize any water or other fluids that might be proximate to the arc and may also vaporize part of the rock itself. The vaporization process creates a high-pressure gas which expands and, in turn, fractures the surrounding rock.

Pulsed-power drilling systems and pulsed-power tools may utilize any suitable pulse-generating circuit topology to generate and apply high-energy electrical pulses across electrodes within the pulsed-power drill bit. Such pulse-generating circuit topologies may utilize electrical resonance to generate the high-energy electrical pulses required for pulsed-power drilling. The pulse-generating circuit may be shaped and sized to fit within the circular cross-section of pulsed-power tool230, which as described above with reference toFIGS. 2A and 2B, may form part of BHA128. The pulse-generating circuit may be enclosed within an encapsulant, such a thermally conductive material that protects the pulse-generating circuit from the wide range of temperatures (for example, from approximately 10 to approximately 200 degrees Centigrade) within the wellbore.

The pulsed-power drilling systems described herein may generate multiple electrical arcs per second using a specified excitation current profile that causes a transient electrical arc to form and arc through the most conducting portion of the wellbore floor. As described above, the arc causes that portion of the wellbore floor to disintegrate or fragment and be swept away by the flow of drilling fluid. As the most conductive portions of the wellbore floor are removed, subsequent electrical arcs may naturally seek the next most conductive portion. Therefore, obtaining measurements from which estimates of the excitation direction can be generated may provide information usable in determining characteristics of the formation.

FIG. 3is a flow chart illustrating an exemplary method300for performing a pulsed drilling operation using an electrocrushing drill bit or an electrohydraulic drill bit placed downhole in a wellbore. For example, drill bit114illustrated inFIG. 2Aor drill bit115illustrated inFIG. 2Bmay be placed downhole in wellbore116as shown inFIG. 1. Method300includes, at302, providing electrical power to a pulse-generating circuit coupled to the drill bit. For example, the pulse-generating circuit may be coupled to a first electrode and a second electrode of the drill bit. The first electrode may be electrode208,210, or212and the second electrode may be ground ring250discussed above with respect toFIGS. 2A and 2B. The pulse-generating circuit may be implemented within pulsed-power tool230shown inFIGS. 2A and 2B, and may receive electrical power from a power source on the surface, from a power source located downhole, or from a combination of a power source on the surface and a power source located downhole. Electrical power may be supplied downhole to a pulse-generating circuit by way of a cable, such as cable220described above with respect toFIGS. 2A and 2B. The power may be provided to the pulse-generating circuit within pulse-power tool230at a power source input.

At304, high-energy electrical pulses are generated by the pulse-generating circuit for the drill bit by converting the electrical power received from the power source into high-energy electrical pulses. For example, the pulse-generating circuit may use electrical resonance to convert a low-voltage power source (for example, approximately 1 kV to approximately 5 kV) into high-energy electrical pulses capable of applying at least 150 kV across electrodes of the drill bit.

At306, the pulse-generating circuit charges a capacitor between electrodes of the drill bit, causing an electrical arc. For example, a switch located downhole within the pulse-generating circuit may close to charge a capacitor that is electrically coupled between the first electrode and the second electrode. The switch may close to generate a high-energy electrical pulse and may be open between pulses. The switch may be a mechanical switch, a solid-state switch, a magnetic switch, a gas switch, or any other type of switch. Accordingly, as the voltage across the capacitor increases, the voltage across the first electrode and the second electrode increases. As described above with reference toFIGS. 1, 2A and 2B, when the voltage across the electrodes becomes sufficiently large, an electrical arc may form through the drilling fluid and/or a rock formation that is proximate to the electrodes. The arc may provide a temporary electrical short between the electrodes, and thus may discharge, at a high current level, the voltage built up across the capacitor.

At308, measurements associated with the electrical arc are obtained. For example, one or more acoustic, electrical and/or electromagnetic sensors may record responses to received signals including, but not limited to, high-energy electrical pulses, electrical arcs, or acoustic and/or electromagnetic waves produced by the electrical arc during a pulsed drilling operation, and may provide measurements representing the recorded responses to a sensor analysis system, such as sensor analysis system150illustrated inFIG. 1or sensor analysis system500illustrated inFIG. 5.

As described above with reference toFIGS. 1, 2A and 2B, the electrical arc greatly increases the temperature of the portion of the rock formation through which the arc flows as well as the surrounding formation and materials, such that the rock formation at the bottom of the wellbore may be fractured with the electrical arc. The temperature may be sufficiently high to vaporize any water or other fluids that may be touching or near the arc and may also vaporize part of the rock itself. The vaporization process creates a high-pressure gas which expands and, in turn, fractures the surrounding rock. At310, rock fractured by the electrical arc may be removed from the end of the wellbore. For example, as described above with reference toFIG. 1, drilling fluid122may move the fractured rock away from the electrodes and uphole from the drill bit. As described above with respect toFIGS. 2A and 2B, drilling fluid122and the fractured rock may flow away from electrodes through fluid flow ports260on the face of the drill bit or on a ground ring of the drill bit.

At312, the measurements obtained at308are analyzed to determine characteristics of the rock formation or for other purposes. For example, a sensor analysis system, such as sensor analysis system150inFIG. 1, may use measurements, such as a voltage, current, ratio of voltage to current, or magnetic field strength representing one or more responses recorded by one or more sensors, to determine the distance and/or direction to a target wellbore. The analysis may include one or more inversions, as described with respect toFIG. 6,FIGS. 10A-C, and/orFIG. 11.

Modifications, additions, or omissions may be made to method300without departing from the scope of the disclosure. For example, the order of the steps may be performed in a different manner than that described and some steps may be performed at the same time. Additionally, each individual step may include additional steps without departing from the scope of the present disclosure. The operations of method300illustrated inFIG. 3may be repeated, as needed, to perform a pulsed drilling operation.

FIG. 4is an elevation view of an exemplary measurement system associated with a pulsed drilling system. Measurement system400may include sensor analysis system422that receives data from one or more of sensors406,410,414and418via one or more of interfaces408,412,416, and420. A pulsed-power drilling system may include a pulsed-power drill bit402that is located at the distal end of wellbore424. During a pulsed drilling operation, electromagnetic waves404and acoustic waves426may be created by electric arcs formed proximate to drill bit402. Electromagnetic waves404may propagate through one or more of subterranean layers438,436,434before reaching surface432. Acoustic waves426may propagate through one or more subterranean layers and uphole along wellbore424from drill bit402to surface432and travel through one or more of subterranean layers438,436,434. One or more of sensors406,410,414and418may be located in wellbore424and/or on surface432. The sensors may be located a known distance from drill bit402. The sensors may record responses to received signals including, but not limited to, high-energy electrical pulses, electrical arcs, electromagnetic waves404and/or acoustic waves426created during a pulsed drilling operation. The sensors may send one or more measurements representing the recorded responses to sensor analysis system422, which analyzes the measurements. One or more components of sensor analysis system422may be located on surface432, in wellbore424, and/or at a remote location. For example, sensor analysis system422may include a measurement processing subsystem in wellbore424that processes measurements provided by one or more of the sensors and transmits the results of the processing uphole to another component of sensor analysis system422for storage and/or further processing.

During a pulsed drilling operation, high-energy electrical pulses are applied to the electrodes of drill bit402to build up electric charge at the electrodes. The rock in the surrounding formation fractures when an electrical arc forms at drill bit402. Electromagnetic waves404are created by the current associated with the electrical arc and/or the electric charge built up on the electrodes of drill bit402. In addition, acoustic waves404are created by the electrical arc and subsequent fracturing of rock in the formation proximate to the drill bit.

The duration of an electrical arc created during a pulsed drilling operation may be approximately 100 μs. The duration of the electrical arc may be shorter than the duration of the high-energy electrical pulses that are applied to the electrodes of drill bit402, which may repeat on the order of several to a few hundred hertz. Because the duration of the electrical arc is less than the repetition period of the pulses, electrical arcs that are generated at drill bit402may be represented by a series of impulses in which each impulse has a corresponding electromagnetic wave and acoustic wave. The time at which the impulse occurs may be used to measure, map, and/or image subterranean features. If the repetition period of the series of impulses is Ts, the Fourier transform of the impulses in the frequency domain consists of impulses occurring at multiples of a base frequency (f0) equal to 2nπ/Ts. If drill bit402provides pulses at a constant frequency, a range of corresponding discrete frequencies (e.g., f0, 2f0, 3f0) are generated in the frequency domain. The discrete frequencies may be used to measure, map, and/or image subterranean features.

Electromagnetic waves404and/or acoustic waves426originate from and/or in proximity to drill bit402at the distal end of wellbore424and propagate outward. For example, electromagnetic waves404and/or acoustic waves426may propagate through one or more of subterranean layers438,436, and/or434. AlthoughFIG. 4illustrates a formation having three layers, the subterranean region may include any number of layers and/or formations suitable for pulsed drilling. Electromagnetic waves404and/or acoustic waves426created at and/or in proximity to drill bit402may propagate from layer438to the surface432via layers434and/or436. Although electromagnetic waves404and acoustic waves426are illustrated inFIG. 4as propagating in certain directions, electromagnetic waves404and acoustic waves426may propagate in any direction.

Sensors406,410, and/or414may record responses to received signals including, but not limited to, high-energy electrical pulses, electrical arcs, or electromagnetic and/or acoustic waves. Each of the sensors may include an antenna. For example, sensors406and410may include linear dipole antennas and sensor414may include a loop antenna. Linear dipole antennas may be used to record responses to electric fields, including electric fields propagating from drill bit402. Linear dipole antennas may be oriented in various directions to record responses to electric fields with varying polarizations, while loop antennas may be used to record responses to magnetic fields. For example, the linear dipole antenna in sensor406may be oriented parallel to the propagation of electromagnetic waves404, while the linear dipole antenna in sensor410may be oriented perpendicular to the propagation of electromagnetic waves404. Although three electromagnetic sensors are illustrated, measurement system400may include any number of sensors of any suitable type to record responses to an electric and/or magnetic field. The sensors may be oriented in any suitable direction to record responses to an electric and/or magnetic field with any polarization. For example, a sensor may include a coaxial or tilted coil antenna to record responses to electromagnetic data. As another example, the sensor may be a magnetometer for recording responses to the magnetic field. As a further example, the sensor may be an electric sensor, such as a sensor with a monopole antenna, dipole antenna, or pair of electrodes that are spaced apart. The sensor may be rotated around the centerline of a bottom hole assembly (BHA) of a wellbore, such as wellbore424, to provide information about the formation at various azimuthal positions. Measurement system400may use more than one sensor simultaneously to provide polarization diversity with antennas oriented in different directions.

Sensors406,410and/or414may convert the recorded responses into measurements and send the measurements to sensor analysis system422. The measurements may be digital representations of the recorded responses. Sensor406may be communicatively coupled via interface408to sensor analysis system422, sensor410may be communicatively coupled via interface412to sensor analysis system422, and sensor414may be communicatively coupled via interface416to sensor analysis system422. Each sensor may provide differential or single-ended measurement data to sensor analysis system422via an interface. For example, sensor406is illustrated with interface408having two sub-interfaces to transmit differential measurement data to sensor analysis system422.

Sensor analysis system422may receive measurements from one or more of sensors406,410and414, and store the measurements as a function of pulse index and time or frequency. The pulse index may begin at one and be incremented each time a new pulse is generated at drill bit402during a pulsed drilling operation. The measurements may be represented in the time domain or the frequency domain. In the time-domain, sensors406,410and414may measure electromagnetic waves by determining a voltage or current and may measure acoustic waves by determining a pressure or displacement. In the frequency domain, a sensor may measure the amplitude and phase by recording responses to the received signal, such as a steady state monochromatic signal, or by performing a Fourier transform of the signal, such as a wide band signal.

Acoustic waves426originate at or near drill bit402and propagate uphole along wellbore424to surface432during a pulsed drilling operation. Sensor418may be located proximate to surface432and may record responses to the acoustic wave to provide measurements to sensor analysis system422via interface420such that sensor analysis system422may calculate the time at which the electrical arc is formed. Each acoustic wave may travel uphole to the surface along the casing of wellbore424and drill string440at a known velocity. For example, the acoustic wave travels at a velocity of approximately 5000 m/s if the casing and drill string440are formed of steel. Other materials suitable for pulsed drilling with known acoustic propagation velocities may be used for the casing and drill string440. For example, the acoustic propagation velocity is between 50 and 2000 m/s for rubber, on the order of 5000 m/s for titanium, and on the order of 4000 m/s for iron. The time of the formation of the electrical arc may be determined based on the known propagation velocity of the material used to form the casing and drill string440and the distance between surface432and drill bit402. The distance between drill bit402and surface432may be determined by depth and position information generated by known downhole survey techniques for vertical drilling, directional drilling, multilateral drilling, and/or horizontal drilling.

AlthoughFIG. 4illustrates one acoustic sensor at the surface, any number of acoustic sensors suitable to measure, map, and/or image subterranean features may be positioned at one or more locations on the surface or elsewhere. For example, an array of acoustic sensors may be used within the wellbore. The acoustic sensors in the array may be positioned at different locations within the wellbore, and may be oriented in different directions to record responses to propagating acoustic waves. The array may provide information about the surrounding formation at various depths sufficient for sensor analysis system422to form a three-dimensional image of the surrounding subterranean features.

The equipment shown inFIG. 4may be land-based or non-land based equipment or tools that incorporate teachings of the present disclosure. For example, some or all of the equipment may be located on offshore platforms, drill ships, semi-submersibles, or drilling barges (not expressly shown). Additionally, while the wellbore is shown as being a generally vertical wellbore, the wellbore may be any orientation including generally horizontal, multilateral, or directional.

Sensor analysis system422may process measurements received from sensors406,410,414and/or418to determine the distance and/or direction to a target wellbore based on responses recorded during a pulsed drilling operation. The distance and/or direction to another wellbore may be used to avoid, intercept, or maintain the direction of pulsed-power drilling operations. The distance may be determined by a sensor analysis system based on the measurements that are obtained. For example, the sensor analysis techniques described herein may be used to determine the distance to another wellbore based on the distance between two radially separated magnetometers (such as1042and1044inFIG. 10B) and the magnetic field measured by the magnetometers. In addition, the techniques may be used in an inversion as described forFIG. 6to determine the proper distance to the second wellbore.

FIG. 5is a block diagram illustrating an exemplary sensor analysis system associated with a pulsed-power drilling system. Sensor analysis system500may be positioned at the surface for use with pulsed-power drilling system100as illustrated inFIG. 1, or at any other suitable location. Sensor analysis system500may be configured to determine the range between wellbores including, but not limited to, the distance and/or direction to a wellbore.

In the illustrated embodiment, sensor analysis system500may include a processing unit510coupled to one or more input/output interfaces520and data storage518over an interconnect516. Interconnect516may be implemented using any suitable computing system interconnect mechanism or protocol. Processing unit510may be configured to determine the distance between wellbores based, at least in part, on inputs received by input/output interfaces520, some of which may include measurements representing responses recorded by various sensors within a wellbore, such as the recording of voltages, currents, ratios of voltages to current, or magnetic fields detected by one or more sensors. For example, processing unit510may be configured to perform one or more inversions to determine the distance and/or direction to a target wellbore based on measurements within a pulsed-power drilling wellbore or other wellbore.

Processing unit510may include processor512that is any system, device, or apparatus configured to interpret and/or execute program instructions and/or process data associated with sensor analysis system500. Processor512may be, without limitation, a microprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), or any other digital or analog circuitry configured to interpret and/or execute program instructions and/or process data. In some embodiments, processor512may interpret and/or execute program instructions and/or process data stored in one or more computer-readable media514included in processing unit510to perform any of the methods described herein.

Computer-readable media514may be communicatively coupled to processor512and may include any system, device, or apparatus configured to retain program instructions and/or data for a period of time (e.g., computer-readable media). Computer-readable media514may include random access memory (RAM), read-only memory (ROM), solid state memory, electrically erasable programmable read-only memory (EEPROM), disk-based memory, a PCMCIA card, flash memory, magnetic storage, opto-magnetic storage, or any suitable selection and/or array of volatile or non-volatile memory that retains data after power to processing unit510is turned off. In accordance with some embodiments of the present disclosure, computer-readable media514may include instructions for determining one or more characteristics of a formation, such as formation118inFIG. 1, or the distance and/or direction to a target wellbore based on signals received from various sensors by input/output interfaces520.

As described above, input/output interfaces520may be coupled to an optical fiber over which it may send and receive signals. Signals received by input/output interfaces520may include measurements representing responses recorded by various sensors at the surface or downhole during a pulsed drilling operation. For example, signals received by input/output interfaces520may include measurements representing responses recorded by electromagnetic, acoustic or electrical sensors. These measurements may include, without limitation, measurements of voltage, current, electric field strength, or magnetic field strength.

Data storage518may provide and/or store data and instructions used by processor512to perform any of the methods described herein for collecting and analyzing data from acoustic, electrical or electromagnetic sensors. In particular, data storage518may store data that may be loaded into computer-readable media514during operation of sensor analysis system500. Data storage518may be implemented in any suitable manner, such as by functions, instructions, logic, or code, and may be stored in, for example, a relational database, file, application programming interface, library, shared library, record, data structure, service, software-as-service, or any other suitable mechanism. Data storage518may store and/or specify any suitable parameters that may be used to perform the described methods. For example, data storage518may provide information used to direct components of sensor analysis system500to analyze measurements representing responses recorded by various acoustic, electrical or electromagnetic sensors during a pulsed drilling operation to determine one or more characteristics of a formation, such as formation118as shown inFIG. 1, or the distance and/or direction to a target wellbore. Information stored in data storage518may also include one or more models generated or accessed by processing unit510. For example, data storage518may store a statistical model for an electrical arc source or a model used in an inversion process, as described with respectFIG. 6.

The elements shown inFIG. 5are exemplary only and sensor analysis system500may include fewer or additional elements in other embodiments, Modifications, additions, or omissions may be made to sensor analysis system500without departing from the scope of the present disclosure. For example, sensor analysis system500illustrates one particular configuration of components, but any suitable configuration of components may be used. Components of sensor analysis system500may be implemented either as physical or logical components. Furthermore, in some embodiments, functionality associated with components of sensor analysis system500may be implemented with special and/or general purpose circuits or components. Components of sensor analysis system500may also be implemented by computer program instructions.

FIG. 6is a flow chart illustrating an exemplary inversion process. In this example, inputs to inversion process600include model generation inputs602, estimated signals606, and received signals604. Model generation inputs602may include initial estimates of the direction and/or distance to a target well based on a deterministic toroidal source model, as described with respect toFIG. 7. In addition, model generation inputs602may include initial estimates of an electric dipole source model at an arbitrary azimuth, as described inFIG. 10A. Model generation inputs602may be used to determine a model response, as shown in630, including various model parameters and estimated signals606. For example, the model response may include electrical and/or magnetic properties based on a toroidal source model and/or an electric dipole source model. Estimated signals606may include the estimated direction and/or estimated direction to a target well. Estimated signals606may be estimated with respect to the azimuth. Received signals604include any combination of unmodified measurements representing responses captured by various acoustic, electrical or electromagnetic sensors, and/or measurements derived from raw information recorded by the sensors, for example signals that have been normalized or otherwise modified as described herein. Received signals may indicate one or more measurements, such as a voltage, a current, ratio of a voltage to a current, or a magnetic field, of the electromagnetic waves created by a pulsed-power drilling operation. The measurement may represent a value in the time or frequency domain. In the frequency domain, for example, absolute values of received signals604may be used at discrete frequencies. As another example, the ratios of received signals604at different frequencies may be used in the inversion. The ratio of received signals604may reduce or filter out any undesirable factor in received signals604, such as the borehole effect or amplitude and/or phase fluctuations in the excitation of the electric pulse or electric arc. The inversion may consider the ratio of received signals604at different frequencies to be one received signal at one frequency.

As shown at610, received signals604may be compared with estimated signals606to determine whether there is a mismatch between received signals604and estimated signals606. If at620there is a mismatch between the signals, rather than a convergence, the model parameters may be updated, as shown in625, and an updated model response may be determined, as shown in630. When and if there is convergence between received signals604and estimated signals606, the results of the inversion process may be output, as shown in640. For example, if a match is found between a model response for the electrical and/or magnetic properties associated with an electric dipole source model or toroidal source model and received signals604, the distance and/or direction to the target well may be output as the result of the inversion process.

FIG. 7is a block diagram illustrating an exemplary model for a source of electrical arcs. The model may be used in an inversion, as described with respect toFIG. 6. In the pulsed-power drilling systems described herein, electrical arcs do not have rotational symmetry around a centerline of the bottom-hole assembly (BHA) and individual electric arcs may occur at random azimuthal locations between electrodes of a drill bit. A toroidal source model may be used to statistically estimate the excitation of the electrical arcs. More specifically, the toroidal source model may be used to generate a time-averaged signal that is the same as the measurements representing responses recorded by the electrical or electromagnetic sensors. The receiving sensors may record responses to received signals including, but not limited to high-energy electrical pulses, electrical arcs, or electromagnetic waves produced by the electrical arcs. The receiving sensors or arrays of such sensors may perform statistical averaging by recording responses to the received signals over a certain period of time. For example, hundreds of excitation pulses may be averaged at the receiving sensor. A moving average may be performed in real time, since recording all signals and averaging them in post-processing may be cost or space prohibitive in the BHA environment.

An equivalent excitation of the electrical arcs that generates the same signal as a time-averaged signal generated at the receiving sensors, modeled as a toroidal pulse source model700, is illustrated inFIG. 7. In the example embodiment illustrated inFIG. 7, it is assumed that individual electrical arcs may form at random locations on the pulsed-power drill bit. For example, electrical arcs may form between electrodes208and210as illustrated inFIG. 2Aor between electrode212and ground ring250as illustrated inFIG. 2B. Many such arcs may occur consecutively in time, with a frequency and intensity determined by the pulse scheme used for the drilling operation. The surrounding formation and the receiving sensors may behave linearly with the electrical arc excitation and the locations of the arcs and their random amplitude fluctuations may be time-averaged at the receiving sensors to arrive at the equivalent toroidal pulse source model700.

Model700, which may be referred to as an equivalent deterministic source model, includes voltage source720, and one or more electrodes as shown inFIGS. 2A and 2B. The voltage source may provide charge750on the end of electrodes715,740a, and740bvia high-energy electrical pulses. In this equivalent deterministic source model, constant-amplitude pulsed currents735flow downhole uniformly over the center electrode715, pass through the formation and; or drilling fluid745along a semi-toroidal surface represented by currents725aand725b, and flow uphole uniformly along the outer wall of the drill bit at710aand710bas currents730aand730b. The two opposite paths of the current flow are indicated by dashed loops740aand740bshown in the annular region between the center electrode715and the outer wall represented by710aand710b. In this equivalent deterministic source model, both the BHA and the current paths have rotational symmetry around the BHA axis. The toroidal source may behave like an electric dipole oriented parallel to the BHA.

FIG. 8Ais an elevation view of exemplary components of a drilling system including a sensor associated with the bottom-hole assembly (BHA) of the drilling system. Drilling system800may include pulsed-power drill bit806, bottom-hole assembly (BHA)804, and drill string810. Drill bit806may receive power via cable812to provide high-energy electrical pulses to the electrodes (not expressly shown) of drill bit806. The high-energy electrical pulses create electrical arcs through formation808surrounding wellbore802. The electrical arcs fracture rock in formation808, which is carried away by drilling fluid814. In addition to causing the rock to fracture, the electrical arcs generate electromagnetic waves that may be measured by sensor816. Sensor816may be communicatively coupled to a sensor analysis system, such as sensor analysis system150inFIG. 1, sensor analysis system422inFIG. 4, or sensor analysis system500inFIG. 5. Power may be delivered to sensor816by, for example, cable812. As another example, sensor816may be powered by a battery (not expressly shown). As a further example, sensor816may receive power by a laser (not expressly shown) through an optical fiber deployed downhole, such as optical fiber160inFIG. 1. A conversion unit, such as a photovoltaic cell, within bottom-hole assembly804may convert light from the laser into electrical energy to power sensor816.

Sensor816may include an antenna that is tilted as shown or that is coaxially oriented. Sensor816may receive a signal representing the electromagnetic wave created during a pulsed drilling operation and record responses at a particular orientation. The antenna of sensor816may be rotated along the centerline of BHA804in order for sensor816to record responses at different orientations. For example, the antenna in sensor816may be rotated to different azimuthal positions of approximately 0, 90, 180, and 270 degrees. Any number of responses at different azimuthal positions may be recorded to generate two-dimensional information about the surrounding formation including, but not limited to, the location of other wellbores. The antenna of sensor816may be rotated in any suitable manner for taking measurements. For example, if sensor816includes a tilted coil, the tilted coil may be rotated by rotating BHA804using drill string810. Although the rotation of BHA804may increase interference with recorded responses of low-frequency electromagnetic waves, such as electromagnetic waves having a frequency of approximately 100 Hz and below, the exemplary tilted coil may be azimuthally sensitive to electromagnetic waves having a frequency above approximately 100 Hz. As another example, a motor located proximate the antenna of sensor816may rotate the antenna at a rate independent of the rate at which BHA804may or may not rotate during a pulsed-power drilling operation. Sensor816may record responses to the electromagnetic waves and send measurements to a sensor analysis system to determine information about the surrounding formation, such as the resistivity of the formation, dielectric constant of the formation, magnetic permeability of the formation, resistivity anisotropy of the formation, layer positions, density of the formation, compressional velocity of the formation, shear velocity of the formation, or the bed boundaries around and ahead of drill bit806. Responses recorded by sensor816can also be used in ranging calculations to determine one or more parameters of a nearby wellbore, including but not limited to the distance, direction, and orientation as described in detail herein.

FIG. 8Bis an elevation view of exemplary components of a drilling system including multiple sensors associated with the bottom-hole assembly (BHA) of the drilling system. Drilling system820may include pulsed-power drill bit806, bottom-hole assembly (BHA)804, and drill string810. Similar to drilling system800inFIG. 8A, high-energy electrical pulses may be provided to the electrodes (not expressly shown) of drill bit806to create electrical arcs through the formation808surrounding the wellbore (not expressly shown). The electrical arcs fracture rock in the formation and generate electromagnetic and/or acoustic weaves that may be measured by sensor assembly822. Sensor assembly822may be communicatively coupled to a sensor analysis system, such as sensor analysis system422inFIG. 4. For example, sensors816a,816b, and816cmay be housed within one or more sensor assemblies. Each sensor may include an antenna, such as a tilted coil as shown. The antennas within sensor assembly822may be of the same or different types. The antennas of sensors816a,816b, and816cmay be oriented with different azimuthal directions to allow for azimuthal sensitivity to the electromagnetic waves emitted during a pulsed-power drilling operation. Responses may be recorded by each of the sensors, which may convert the responses into measurements that are sent to a sensor analysis system. Measurements representing these responses may subsequently be used by the sensor analysis system in ranging calculations to determine a parameter associated with a nearby wellbore, including but not limited to the distance, direction, and orientation.

FIG. 8Cis an elevation view of exemplary components of a drilling system including multiple magnetometers associated with a bottom-hole assembly (BHA) of the drilling system. Drilling system830may include a pulsed-power drill bit (not expressly shown), a bottom-hole assembly (BHA)804, and drill string812. Similar to drilling system800inFIG. 8A, high-energy electrical pulses may be provided to the electrodes the drill bit to create electrical arcs through the formation808surrounding the wellbore (not expressly shown). The electrical arcs fracture rock in the formation and generate electromagnetic and/or acoustic waves that may be measured by magnetometers832. A sensor subassembly822may house magnetometers832as shown. Magnetometers832may be any suitable type that measures a magnetic field. For example, the magnetometers may be flux gate magnetometers and/or rotating magnetometers that may measure a low-frequency magnetic field, such as the magnetic field corresponding to the electromagnetic waves created by the pulsed-power drill bit during a pulsed-power drilling operation. Magnetometers832may be individually placed at different positions around the perimeter of sensor assembly822or bottom-hole assembly (BHA)804. For example, magnetometers816a,816b, and816cmay be placed at different azimuthal locations to measure the magnetic field in different directions to provide two-dimensional information about the surrounding formation based on the propagation of electromagnetic waves from the pulsed-power drill bit. As another example, magnetometers832may be x-y magnetometers. Responses of magnetometers832may be used in ranging calculations to determine one or more parameters associated with a nearby wellbore, including but not limited to the distance, direction, and orientation. Magnetometers832may be co-located with one or more accelerometers (not expressly shown) to compensate for the direction of the wellbore relative to a gravitational reference, such as the magnetic north of the Earth's magnetic field.

FIG. 9Aillustrates a bottom up view of exemplary components of a pulsed-power drill bit with an associated sensor analysis system. Drill bit900may include one or more center electrodes, such as center electrode922, and one or more outer electrodes, such as a plurality of electrodes disposed proximate outer wall904. For example, the electrodes of drill bit900may have a configuration as shown inFIGS. 2A and/or 2B. Electrical arcs906may form during pulsed-power drilling operations between center electrode922and the electrodes proximate outer wall904.

Electrical arcs906may be detected by sensors908that are azimuthally distributed along outer wall904. Responses may be recorded by each of the sensors908. Sensors908may be magnetometers, buttons, current-meters, or any sensor suitable for detecting, measuring, and/or recording responses corresponding to electrical arcs906. Measurements representing these responses may be used to determine an excitation direction in terms of an azimuth angle, such as azimuthal angle910. For example, measurements representing raw recorded responses and/or modified measurements may be inputs to an inversion process, as described with respect toFIG. 6.

FIG. 9Bis a graph illustrating bins for measurements from multiple sensors azimuthally distributed around a centerline of the bottom-hole assembly (BHA) of the pulsed-power drilling system. Responses from the azimuth sensors, such as sensors908inFIG. 9A, may be organized into a series of bins that correspond to the azimuthal location or angle (ϕsrc) of the electrical arcs. For example, each of the azimuth sensors may be placed at different azimuthal locations. Each sensor may provide one or more measurements corresponding to an electrical arc to a sensor analysis system, such as sensor analysis system422inFIG. 4or sensor analysis system500inFIG. 5. For example, the sensor analysis system may compare measurements between the sensors to determine the measurement with the highest amplitude. The azimuthal direction for an electrical arc may be determined based on the azimuthal location of the sensor with the highest amplitude measurement. As another example, the azimuthal direction for an electrical arc may be determined based an average azimuthal direction. The sensor analysis system may average the measurements between adjacent sensors and then compare the measurements between the averaged values to determine the measurements with the highest average amplitude. The azimuthal direction for an electrical arc may be determined based on the average azimuthal location between two adjacent sensors with the highest average amplitude measurement. The measurements by the sensors may also be evaluated over many electrical arcs during pulsed-power drilling operations. The sensor analysis system may determine the azimuthal direction for an electrical arc and increment the count of a counter corresponding to the azimuthal direction. Over many electrical arcs, counters corresponding to different azimuthal directions may be incremented.

In the graph shown inFIG. 9B, the horizontal axis represents the azimuthal location (ϕsrc) and the vertical axis represents the number of electrical arcs binned at a particular azimuthal location. Each line in the graph represents a counter corresponding to an azimuthal direction. For example, line946represents a counter corresponding to an azimuthal direction of 30 degrees and line944represents a counter corresponding to an azimuthal direction of 270 degrees. Although the lines are shown representing counters corresponding to azimuthal directions 60 degrees apart (e.g., 30 degrees, 90 degrees, 150 degrees, etc.), any number of counters may be used.

Although the graph shows bins that correspond to the azimuthal location of the electrical arcs (ϕsrc), the sensor analysis system may make determinations regarding pulsed-power drilling operations based on bins that correspond to the azimuthal location of the received responses (ϕrev) as determined from measurements by one or more sensors located uphole from the drill bit, such as sensors816a.816b, and816cthat are shown oriented with different azimuthal directions inFIG. 8Bor sensor816as shown inFIG. 8A. Though the graph shows measurements organized into a series of bins, measurements from sensors may be organized into a two-dimensional array of bins, in which one dimension corresponds to the azimuthal direction of the electrical arcs (ϕsrc) measured by the azimuth sensors and the other dimension corresponds to the azimuthal direction of the received responses (ϕrev) as determined from measurements by one or more sensors.

As shown inFIG. 9B, the sensor analysis system may organize measurements into a series of bins, in which each bin corresponds to an azimuthal direction and contains a counter representing the number of electric arcs formed in the azimuthal direction of the bin. The sensor analysis system may make determinations regarding a pulsed drilling operation based on the azimuthal direction associated with the counter having the greatest number of counts. Responses recorded by azimuthally distributed sensors may be used in estimating the values of formation parameters around the drill bit, or in ranging calculations to determine one or more parameters associated with a nearby wellbore, including but not limited to the distance, direction, and orientation. These formation parameters may represent electrical and/or acoustic properties of the formation. For example, electrical parameters that may be estimated based on responses recorded by azimuthally distributed sensors include electrical conductivity σ, permeability c, and electrical resistivity, which is inverse of electrical conductivity. Acoustic parameters that may be estimated based on responses recorded by the azimuthally distributed sensors include density d, shear velocity Vs, compressed velocity Vc, and Young's modulus.

The sensor analysis system may also be configured to estimate a parameter of interest along the azimuthal direction at a particular azimuthal angle φ. Variations in the value of the parameter of interest at different azimuthal angles φ may indicate differences in the characteristics of a formation in different directions relative to the drill bit, which may be used to direct or modify a pulsed drilling operation. For example, the sensor analysis system may be configured to determine a more efficient drilling strategy or drilling direction based on differences in the electrical resistivity of the formation in different directions relative to the drill bit.

FIG. 10Ais a cross sectional view of exemplary components of a bottom hole assembly (BHA) for a ranging system. Pulsed-power drilling system1000may include a pulsed-power drilling wellbore1002and target wellbore1004. Drilling wellbore1002may avoid, intersect, or maintain a distance from target wellbore1004. A drilling system may be located in drilling wellbore1002within the walls of the borehole1012. The drilling system may include a drill bit1020, bottom-hole assembly1014, and drill string1010. Electricity may be provided to drill bit1020by power1016. A ranging system may include a sensor assembly1018, such as sensor assembly822inFIGS. 8B and/or 8C, and a sensor analysis system (not expressly shown). Sensor assembly1018may be coupled to bottom-hole assembly (BHA)1014of the drilling system. Sensors in assembly1018may record responses to electromagnetic fields and/or electromagnetic waves created by the pulsed-power drilling system. Sensor assembly1018may include one or more sensors, such as sensor816inFIG. 8Aand/or sensors816a,816b, and816cinFIGS. 8B and/or 8C. For example, the tilted coil antennas of the sensors may be oriented in different azimuthal positions as shown inFIG. 10A. During a pulsed-power drilling operation, drilling fluid1026may be transferred uphole to the surface to carry away fractured rock from drill bit1020. Although two wells are shown sensor assembly1018may be placed within a third wellbore to measure the electromagnetic fields and/or electromagnetic waves induced by drill bit1020.

During a pulsed-power drilling operation, electric arc1022may form between electrodes (not expressly shown) on drill bit1020. For example, the arc may form through a portion of the rock1024in the surrounding layer of the formation proximate the distal end of drill bit1020, which may cause a primary azimuthal magnetic field to form. Drilling wellbore1002may be oriented at an angle (θ)1006relative to target wellbore1004. As described below, angle (θ)1006between the two wellbores may be increased or decreased such that drilling wellbore1002may avoid, intersect, or maintain a distance from target wellbore1004. In addition, drilling wellbore1002may be a distance (d) from target wellbore1004. Target wellbore1004may include casing1028and collar1030. Casing1028may be metallic and may carry an axial target current1032, which may be caused by electric arc1022. Target current1032may travel uphole along casing1028and may create a secondary azimuthal magnetic field1034.

Secondary magnetic field1034may propagate from target wellbore1004toward drilling wellbore1002. One or more sensors within sensor assembly1018may be used to measure secondary magnetic field1034, such as the sensors shown inFIGS. 8A-C. As explained forFIG. 4, drill bit1020can generate a range of discrete frequencies in the frequency domain. Accordingly, an electric arc generated by drill bit1020may create secondary magnetic field1034with broad frequency content including, but not limited to, low-frequency magnetic fields and high-frequency magnetic fields.

The direction represented by angle1006and distance1008to target wellbore1004may be determined based on measurements sent from one or more sensors to a sensor analysis system. For example, the sensor analysis system may average electrical arcs and model the arcs as a toroidal source model, as explained forFIG. 7, to determine the direction and/or distance between drilling wellbore1002and target wellbore1004. A toroidal source model may be represented by an axial electric dipole. For the electric arc to generate secondary magnetic field1034suitable to be received and recorded by sensors in sensor assembly1018, a sufficient target current1032may be generated by orienting drilling wellbore1002at an angle of less than approximately 85 degrees (θ) relative to target wellbore1004. The direction (φsrc) of electric arcs1022generated by the drilling system in drilling wellbore1002may be determined by using azimuth sensors, as described inFIGS. 9A-B. A sensor analysis system may use the direction of electric arcs1022to determine a direction of target wellbore1004. Measurements associated with the electromagnetic waves that are received and recorded as responses by the sensors may be used in an inversion, such as inversion process600inFIG. 6, to determine the distance and/or direction from drilling wellbore1002to target wellbore1004. For example, the azimuthal direction may be based on the measurement as described forFIGS. 9A-B.

FIG. 10Bis a cross sectional view of exemplary components of a gradient ranging system. Pulsed-power drilling system1040may include similar components as drilling system1000. A gradient ranging system may include sensor assembly1018with two magnetometers1042and1044radially separated by a known distance1046. Magnetometers1042and1044may measure the gradient of secondary magnetic field1034to determine distance1008to target well1004. Although two magnetometers are shown, any number of magnetometers spaced apart by a known distance may be used.

If target current1032is constant, the secondary magnetic field1034may be determined by the following equation:

H→i⁡(r→)=It2⁢π⁢⁢d⁢ϕ^(1)
In equation (1), {right arrow over (H)}i({right arrow over (r)}) is the is the magnetic field at a point {right arrow over (r)}, Itis target current1032, d is the radial distance from target well1004to point {right arrow over (r)}, and {circumflex over (ϕ)} is the angle between the point and target well1004. At the same point, the gradient of the secondary magnetic field1034may be defined by the following equation:

∂H→i⁡(r→)∂d≈-It2⁢π⁢⁢d2⁢ϕ^(2)
Radial distance1008to target well1004, d, may be determined by taking the ratio of {right arrow over (H)}i({right arrow over (r)}) to

d=H→i⁡(r→)∂H→i⁡(r→)∂d(3)
Target current1032induced in target well1004may be assumed to be uniform if the sensors are close to target well1004and the distance between the sensors1046is small relative to the distance1008to target well1004. The gradient of magnetic field1034,

∂H→i⁡(r→)∂d,
may be approximated as

H→2i-H→1iδ
in which {right arrow over (H)}2iand {right arrow over (H)}1iare the magnetic fields in the radially-separated magnetometers and δ is the distance (1046) between them. Thus, the distance1008to target well1004may be determined based on the magnetic field1034at a point {right arrow over (r)} ({right arrow over (H)}i({right arrow over (r)})), the difference between the magnetic fields measured by the radially-separated magnetometers ({right arrow over (H)}2i−{right arrow over (H)}1i), the distance1046between the radially-separated magnetometers (δ).

FIG. 10Cis a cross sectional view of exemplary components of an intercept ranging system. Pulsed-power drilling system1060may include similar components as drilling system1000. An intercept ranging system may include one or more sensors (not expressly shown) to record responses as a voltage, current, and/or resistance and a sensor analysis system to determine whether drilling system1060has intercepted target well1004. Target well1004may include casing1028that may be electrically conductive. When pulsed-power drill bit1020advances down the drilling wellbore, it may come in contact with casing1028of target well1004. Because casing1028may be electrically conductive, pulsed-power drill bit1020may cause an electrical short1062with casing1028upon contact or when drill bit1120is in the vicinity of casing1028. Electrical short1062may be detected by one or more sensors located within bottom-hole assembly (BRA)1014or on the surface. Detection of electrical short1062may include checking for a sudden drop in the input impedance to drill bit1020, a sudden increase in input current to drill bit1020, or a sudden decrease in input voltage to drill bit1020. For example, the input current and voltage may be checked. If the voltage remains the same but the current suddenly increases beyond a threshold, electrical short1062may be detected. In a similar manner, if the current remains the same but the voltage suddenly decreases beyond a threshold, electrical short1062may be detected. As another example, the input impedance may be determined based on the voltage and/or current. The determined input impedance, or change in input impedance may be compared to a threshold. If the value is below the threshold, electrical short1062may be detected. In response to the detection of the electrical short, a pulsed-power drilling operation may be modified. For example, the drill string may be pulled out of the wellbore and pulsed-power drill bit1020may be replaced with a non-pulsed-power drill bit. As another example, pulsed-power drill bit1020may be begin rotation or increase the rate of rotation in order to breach the casing1028of target well1004.

FIG. 11is a flow chart of exemplary method for determining the range between wellbores. Method1100may begin if a first wellbore is to avoid or maintain a distance from a second wellbore. Method1100may begin and at1102one or more electrical arcs may be formed at the pulsed-power drill bit in a first wellbore. The electrical arcs may be formed between a first electrode and a second electrode of the drill bit. For example, the electrical arcs may form between electrode208and ground ring250inFIG. 2A, as shown by currents725aand725binFIG. 7.

At1106, it may be determined whether to record responses for an average measurement. If responses for an average measurement are to be recorded, method1100may proceed to1112. An average measurement may be generated by a sensor in real-time to reduce the number of measurements sent to a sensor analysis system. Otherwise if responses for an average measurement are not to be recorded, method1100may proceed to1108.

At1108, one or more sensors may record a response to a secondary magnetic field generated by a current in a second wellbore. Electrical arcs in the first wellbore create a primary magnetic field that induces the current in the second wellbore. For example, secondary magnetic field1034inFIG. 10Amay be generated by target current1032in target well1004. The responses may be recorded in the time-domain or frequency-domain as described inFIGS. 4 and 10A. The sensors may convert the recorded responses into one or more measurements and send the measurements to a sensor analysis system. The measurements may be a voltage, current, ratio of voltage to current, or magnetic field. The sensors may be located in the first wellbore or in a third wellbore such that the sensors can record responses to the secondary magnetic field. The sensors may include one sensor816as shown inFIG. 8Aor a plurality of sensors in a sensor assembly822as shown inFIG. 8B. The sensors may include coaxial coils, tilted coils, magnetometers, linear dipole antennas, and/or loop antennas as described inFIGS. 4 and 8A-C. The sensors may be powered by power cable812inFIG. 8A, a battery, or a charged fiber optic cable as described inFIG. 8A. If the sensor is located in the first wellbore, the first wellbore may be less than approximately 85 degrees from the second wellbore in order for the sensor to record responses to the secondary magnetic field.

At1110, one or more sensors may record responses to a first electrical arc and a second electrical arc. The sensors may be any of the sensors described with respect to1108or azimuth sensors908inFIG. 9A. The sensor may convert the response into a measurement, such as the measurements described in1108.

At1112, one or more sensors may record responses and convert them into an average measurement associated with the electrical arcs. The sensors described in1110may be used to record multiple responses over time to obtain an average measurement. For example, the recorded response to the first electrical arc and the second electrical arc may be averaged together to obtain an average measurement.

At1114, one or more measurements (such as those in1108,1110,1112and1118) may be obtained by a sensor analysis system, such as sensor analysis system500inFIG. 5.

At1116, the distance and/or direction to the second wellbore may be determined based on the measurements obtained in1114. For example, distance1008may be determined as described inFIGS. 10A-C. The distance may be determined based on the distance between two radially separated magnetometers (such as1042and1044inFIG. 10B) and the magnetic field received and recorded by the magnetometers. The measurement may be a ratio of measurements at different frequencies, such as the ratio of the measurement at a first frequency to the measurement at a second frequency. The ratio may filter or reduce the effects of undesirable information that is common to measurements at both the first and second frequency. In addition, the measurements may be used as received signals in an inversion as described forFIG. 6. For example, the inversion may compare the received signals to expected signals based on a model to determine the proper distance to the second wellbore.

As another example, the direction to the second wellbore may be determined, such as direction1006inFIGS. 10A-C. The determination of the direction to the second wellbore may be performed in parallel with the determination of the distance to the second wellbore. For example, the average measurement described in1112may be determined for each of a plurality of azimuth sensors. The azimuth sensor with the highest average measurement may be determined by comparing the average measurement of the azimuth sensors. The average measurement may be represented by an azimuthal direction as described inFIG. 9B. For example, if the second wellbore is in proximity to the first wellbore the electrical arcs may be oriented to form in the direction of the second wellbore due to the conductivity of the electrical path at or near the casing of the second wellbore. Although an average measurement is described, individual measurements may be used in the inversion to determine the direction. The measurements may be organized into bins as shown inFIG. 9B. The first wellbore may avoid or maintain a distance from the second wellbore based on the determined distance and/or direction. Method1100may end after1116, repeat, or continue to1118.

Modifications, additions, or omissions may be made to method1200without departing from the scope of the disclosure. The order of the steps may be performed in a different manner than that described and some steps may be performed at the same time. Additionally, each individual step may include additional steps without departing from the scope of the present disclosure. For example,1108may be performed even if1112is performed.

FIG. 12is a flow chart of exemplary method for intercepting a wellbore. As another example, method1200may begin if the distance between the first wellbore and second wellbore indicates that the two wellbores are in the vicinity of each other or if the first wellbore is intended to intercept the second wellbore. Method1200may begin and at1202one or more electrical arcs may be formed at the pulsed-power drill bit in a first wellbore. The electrical arcs may be formed between a first electrode and a second electrode of the drill bit. For example, the electrical arcs may form between electrode208and ground ring250inFIG. 2A, as shown by currents725aand725binFIG. 7.

At1204, one or more sensors may measure an input voltage and/or input current by recording responses to the electrical arcs or an electromagnetic wave generated by the electrical arcs. The input voltage may be derived from voltage source720inFIG. 7. The input current may be derived from current735inFIG. 7. The input voltage and/or input current may be measured by one or more sensors, such as the sensors described for1108and1110above. The input voltage and input current may be referred to herein as measurements.

At1206, one or more measurements (such as those in1204) may be obtained by a sensor analysis system, such as sensor analysis system500inFIG. 5.

At1208, the impedance may be determined by the sensor analysis system based on the input voltage and input current. The impedance may represent the input impedance of the pulsed-power drill bit. When the pulsed-power drill bit is in proximity to the casing of the secondary wellbore, the bit may electrically short if the casing is metallic (such as steel). If the bit shorts, the impedance that is determined may be significantly lower than the impedance associated with the pulsed-power drill bit being further away of the second wellbore.

At1210, the change in input voltage, input current, and/or impedance may be determined by the sensor analysis system. The system may store previous values of the input voltage, input current, and/or impedance and compare those values to the values obtained in1204and/or1208.

At1212, the second wellbore may be detected in proximity to the first wellbore by the sensor analysis system based on a comparison. For example, the comparison may involve the input voltage, input current, and/or impedance and a threshold. If the input voltage or impedance is below a threshold, the second wellbore may be detected in proximity to the first wellbore. If the input current is above a threshold, the second wellbore may be detected in proximity to the first wellbore. As another example, the comparison may involve the change in input voltage, input current, and/or impedance and a threshold. If the change in the value is above a threshold, the second wellbore may be detected in proximity to the first wellbore. In addition, if the drill bit in the first wellbore was previously shorted electrically to the second wellbore, a change in the impedance below a threshold may indicate that the second wellbore is in proximity to the first wellbore.

At1214, the formation of electrical arcs at the pulsed-power drill bit may be stopped to avoid crushing the casing of the second wellbore. For example, the sensor analysis system may send a notification to the pulse generating circuit to stop generate high-energy electrical pulses which cause the electrical arcs to form.

At1216, the drill bit may be rotated to breach the casing of the second wellbore to intercept the borehole. The drill bit may be the same drill bit as the pulsed-power drill bit or may be a separate non-pulsed-power drill bit that replaced the pulsed-power drill bit to remove rock or casing as the bit rotates. Subsequently, method1200may end or the operations in method1200may be repeated, as needed, to perform a pulsed drilling operation.

Modifications, additions, or omissions may be made to method1200without departing from the scope of the disclosure. The order of the steps may be performed in a different manner than that described and some steps may be performed at the same time. Additionally, each individual step may include additional steps without departing from the scope of the present disclosure.

Embodiments herein may include:

A. A downhole drilling system including a pulse-generating circuit; a drill bit including a first electrode and a second electrode, the first and second electrodes electrically coupled to the pulse-generating circuit to receive an electrical pulse from the pulse-generating circuit and to form an electrical between the first and second electrodes during a pulsed drilling operation in a first wellbore; a sensor to record responses to a magnetic field generated by a current in a second wellbore, the current generated by the electrical arc; and a sensor analysis system communicatively coupled to the sensor, the sensor analysis system configured to obtain a measurement from the sensor, the measurement representing the recorded response to the magnetic field and to determine a distance between the drill bit and the second wellbore based on the measurement.

B. A method including generating, by a drill bit, an electrical arc in a first wellbore during a pulsed drilling operation; recording a response to a magnetic field generated by a current in a second wellbore using a sensor in proximity to the drill bit, the current generated by the electrical arc; obtaining a measurement representing the recorded response to the magnetic field; and determining the distance to the second wellbore based on the measurement.

C. A sensor analysis system including a computer processor and a computer-readable medium for storing instructions, the instructions, when read and executed by the computer processor, to cause the processor to: receive a measurement from a sensor, the sensor communicatively coupled to the sensor analysis system to record responses during a pulsed drilling operation to a magnetic field generated by a current in a second wellbore, the current generated by an electrical arc between a first electrode and a second electrode of a drill bit in a first wellbore; and determine a distance between the drill bit and the second wellbore based on the measurement.

Each of embodiments A, B, and C may have one or more of the following additional elements in any combination: Element 1: the downhole drilling system further includes a plurality of azimuth sensors communicatively coupled to the sensor analysis system and positioned in proximity to the drill bit in the first wellbore, each of the plurality of azimuth sensors to record, during the pulsed drilling operation, responses to the electrical arc and a second electrical arc; and the sensor analysis system is further configured to obtain for each of the plurality of azimuth sensors a first measurement and a second measurement, the first measurement representing the recorded response to the electrical arc and the second measurement representing the recorded response to the second electrical arc; and determine a direction of the second wellbore based on the first measurement and the second measurement for at least one of the plurality of azimuth sensors. Element 2: wherein the sensor analysis system is further configured to: determine for each of the plurality of azimuth sensors an average measurement based on the on the first measurement and the second measurement; and determine an azimuth sensor among the plurality of azimuth sensors with a highest average measurement, the azimuth sensor with the highest average measurement being the azimuth sensor of the plurality of azimuth sensors used to determine a direction of the second wellbore. Element 3: wherein the first measurement and the second measurement are selected from the group consisting of voltages, currents, ratios between voltage and current and combination thereof. Element 4: wherein the plurality of azimuth sensors are battery powered. Element 5: wherein the downhole drilling system further includes a fiber optic cable coupled to the plurality of azimuth sensors, the fiber optic cable charged by a laser to provide power to the plurality of azimuth sensors. Element 6: wherein the sensor analysis system is further configured to transmit the measurement representing the recorded response to the magnetic field, the first measurement representing the recorded response to the electrical arc, and the second measurement representing the recorded response to the second electrical arc to a surface above the first wellbore using the fiber optic cable. Element 7: wherein the sensor analysis system placed on a surface above the first wellbore and is communicatively coupled to the sensor via a telemetry system, the telemetry system to transmit measurements from the sensor to the sensor analysis system. Element 8: the downhole drilling system further includes a plurality of azimuth sensors communicatively coupled to the sensor analysis system and positioned in proximity to the drill bit in the first wellbore, each of the plurality of azimuth sensors to record, during the pulsed drilling operation, responses to the electrical arc and a second electrical arc to generate an average measurement; and the sensor analysis system further configured to: obtain for each of the plurality of azimuth sensors the average measurement; and determine a direction of the second wellbore based on the average measurement for one of the plurality of the azimuth sensors. Element 9: the downhole drilling system further includes a sensor communicatively coupled to the sensor analysis system and positioned in proximity to the drill bit in the first wellbore to measure a voltage and a current for the electrical arc generated during the pulsed drilling operation; and the sensor analysis system further configured to: obtain the voltage and the current from the sensor; and determine an impedance at the drill bit in the first wellbore based on the voltage and current. Element 10: wherein the sensor analysis system is further configured to: determine whether the impedance is less than a threshold; and detect that the second wellbore is in proximity to the drill bit in the first wellbore based on a determination that the impedance is less than the threshold. Element 11: wherein the sensor analysis system is further configured to: detect a change in the impedance based on a comparison of the determined impedance and a second impedance, the determined impedance associated with the voltage and current measured at a first time and the second impedance associated with a measurement at a time prior to the first time; determine whether the impedance change is greater than a threshold; and detect that the second wellbore is in proximity to the drill bit in the first wellbore based on a determination that the impedance change is greater than the threshold. Element 12: wherein the pulse-generating circuit is configured to stop generation of electrical pulses based on the detection by the sensor analysis system of the second wellbore proximity to the drill bit. Element 13: wherein an angle between the first wellbore and the second wellbore is less than approximately 85 degrees. Element 14: wherein the sensor includes a first coil tilted in a first direction to record responses to the magnetic field. Element 15: wherein the downhole drilling system further includes a second sensor to record a response to the magnetic field generated by the current in the second wellbore, the second sensor comprising a second coil tilted in a second direction; and the sensor analysis system further configured to obtain a measurement from the second sensor, the measurement representing the response recorded by the second sensor. Element 16: wherein the sensor comprises a first magnetometer placed at a first azimuth around the first wellbore to record the response to the magnetic field. Element 17: the downhole drilling system further includes a second sensor to record a response to the magnetic field generated by the current in the second wellbore, the second sensor comprising a second magnetometer placed at a second azimuth around the first wellbore the sensor analysis system further configured to obtain a measurement from the second sensor, the measurement representing the response recorded by the second sensor. Element 18: wherein the response is recorded by the sensor is measured at a first frequency and a second frequency; and the sensor analysis system is further configured to determine a ratio of a measurement representing the response recorded by the sensor at the first frequency and another measurement representing the response recorded by the sensor at the second frequency, wherein the distance to the second wellbore is further based on the ratio. Element 19: wherein the sensor is placed in a third wellbore. Element 20: wherein the response is the magnitude of the magnetic field recorded by the sensor.

Although the present disclosure has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompasses such various changes and modifications as falling within the scope of the appended claims.