WO2009146548A1

WO2009146548A1 - System and method for determining downhole positions

Info

Publication number: WO2009146548A1
Application number: PCT/CA2009/000779
Authority: WO
Inventors: Garry Holmen; Derek W. Logan
Original assignee: Schlumberger Technology Corporation
Priority date: 2008-06-03
Filing date: 2009-06-03
Publication date: 2009-12-10
Also published as: CA2746078A1

Abstract

Described herein are a method and system for tracking a position of a borehole for oil and gas drilling. The method includes locating an acoustic transmitter or an acoustic receiver downhole in the borehole; locating the other of the transmitter or the receiver in a known position relative to the surface; transmitting an acoustic signal from the transmitter to the receiver; recording when the signal arrives at the receiver; and determining the position of the transmitter or the receiver located downhole based on when the signal arrives at the receiver and the known position of the transmitter or the receiver. The system includes either an acoustic transmitter or an acoustic receiver located downhole in the borehole, the other of the transmitter or the receiver located in a known position relative to the surface; and a controller configured to determine the position of the transmitter or receiver located downhole based on when a signal transmitted from the transmitter to the receiver arrives at the receiver and based on the known position of the other of the transmitter or the receiver located in the known position.

Description

SYSTEM AND METHOD FOR DETERMINING DOWNHOLE POSITIONS

FIELD OF THE INVENTION

The present invention relates to a method and system for determining downhole positions. More particularly, the present invention relates to a method and system for tracking the position of boreholes in the context of oil and gas drilling.

BACKGROUND OF THE INVENTION

In the context of oil and gas drilling, accurately and precisely tracking the position of a borehole can be vitally important. In directional drilling, for example, when boreholes deviate from being substantially vertical, it can be necessary to orient a borehole such that it either avoids or intersects with one or more existing boreholes. Knowing the position of a borehole also allows the borehole to be drilled at an angle such that oil recovery from a reservoir can be increased by ensuring that the area of intersection between the borehole and the reservoir is relatively high. Furthermore, knowing the position of a borehole may be important to ensure that drilling does not occur in a prohibited area, such as an area in which only a competitor has rights to drill.

For example, accurately and precisely tracking the position of boreholes is important when conducting steam assisted gravity drainage ("SAGD"). SAGD refers to a thermal, in-situ method used to recover bitumen from tar sands. SAGD is described, for example, in Canadian Patent 1 ,304,287 to Edmunds et al. In SAGD, two parallel boreholes are drilled, the boreholes being substantially horizontal over a significant portion of their lengths. One borehole (the "production well") is located below the other (the "injector well"). The wells are approximately 3 - 5 meters apart, the substantially horizontal portions being roughly 400 - 500 meters below the surface of the Earth, and the wells can extend for up to a kilometer. In SAGD, steam is injected into the injector well, which heats the bitumen surrounding the wells and consequently allows gravity to pull the bitumen into the production well, following which it can be pumped to the surface. The relative positioning of the injector and production wells is critical. The wells have to track each other closely enough in depth such that they are both within the thermal envelope generated by the injected steam, but not so closely that the integrity of the surrounding rock is compromised. The wells also have to track each other closely enough in length such that bitumen that is heated as a result of steam injected into the injector well drains downward into the production well. Clearly, then, being able to accurately and precisely track the relative positions of the injector and production wells is critical.

Currently, multiple methods of tracking the positions of boreholes is known in the art. Using survey information in conjunction with a measurement-while-drilling ("MWD") device that periodically measures drill bit orientation, and knowing the rate at which the drill bit is descending, one can interpolate the position of the drill bit from a previously known position of the bit. By tracking the position of the drill bit, the position of the resulting borehole is determined. A problem with this process, however, is that it is dependent on previously known positions of the drill bit; consequently, errors in position are cumulative, resulting in less accurate and precise measurements as drilling progresses.

One example of a prior art method of tracking the position of a borehole is disclosed in US 6,026,913. In US 6,026,913, an acoustic transmitter located downhole in a known position relative to a drill bit that is drilling the borehole emits an acoustic signal that is received by a plurality of downhole acoustic receivers. The positions of the downhole acoustic receivers relative to the surface of the Earth are not known with any commercially useful precision. For example, it may be known that the downhole acoustic receivers used in US 6,026,913 are located underground, but the lateral position of the receivers relative to the surface may be unknown. Consequently, the method and system disclosed in US 6,026,913 can be used to determine the position of an acoustic transmitter in one borehole relative to the position of acoustic receivers in a second borehole, but cannot be used to determine the position of the acoustic transmitter relative to a known position on the surface. Furthermore, as both the acoustic transmitters and receivers in US 6,026,913 are located downhole, the method and system of US 6,026,913 require at least two boreholes; consequently, it is more expensive and cumbersome to implement than a method and system that can determine downhole positions using only one borehole.

Consequently, there is a need for a system and method for determining downhole positions, and more particularly for tracking the position of boreholes in the context of oil and gas drilling, that improves upon at least one of the deficiencies in the prior art.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method for tracking a position of a borehole for oil and gas drilling. The method includes locating an acoustic transmitter or an acoustic receiver downhole in the borehole; locating the other of the transmitter or the receiver in a known position relative to the surface; transmitting an acoustic signal from the transmitter to the receiver; recording when the signal arrives at the receiver; and determining the position of the transmitter or the receiver located downhole based on when the signal arrives at the receiver and the known position of the transmitter or the receiver.

The acoustic transmitter can be on the surface at the known position and can transmit to at least three acoustic receivers located downhole in the borehole. Alternatively, the acoustic receiver can be located downhole in the borehole and at least three acoustic transmitters can be located at known positions relative to the surface, the three acoustic transmitters transmitting acoustic signals of three different frequencies; the acoustic transmitter can be downhole in the borehole and the transmitter can transmit to at least three acoustic receivers, at least one of which is on the surface; or the acoustic receiver can be located on the surface and at least three acoustic transmitters are located downhole in the borehole, the three acoustic transmitters transmitting acoustic signals of three different frequencies.

When at least three receivers are used, determining the position of the transmitter located downhole can include, for the at least three receivers, determining a propagation time of the signal as it propagates from the transmitter to the receiver; determining an average velocity of the signal as it propagates from the transmitter to the receivers; for each of the receivers for which a propagation time is determined, determining a distance traveled by the signal as it propagates from the transmitter to the receiver by multiplying the average velocity of the signal by the determined propagation time; and determining the position of the transmitter from the position of each of the at least three receivers and the distance traveled by the signal from the transmitter to each of the receivers.

The transmitter can also transmit to at least four acoustic receivers, at least one of which is located in a known position relative to the surface. When four receivers are used, determining the position of the transmitter located downhole can include determining differences between when the signal arrives at each of three of the receivers and a fourth receiver; determining an average velocity of the signal as it propagates from the transmitter to the receivers; and determining the position of the transmitter from the average velocity of the signal and the differences between when the signal arrives at each of three of the receivers and the fourth receiver.

The method can also include determining a propagation time of the signal as it propagates from the transmitter to a supplemental receiver; and determining a distance between the transmitter and the supplemental receiver by multiplying the average velocity of the signal by the propagation time. The supplemental receiver can be located in a known position relative to the surface.

Determining the average velocity of the signal can include multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver. Alternatively, determining the average velocity of the signal can include iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

Determining the propagation time of the signal can include synchronizing the transmitter and receivers in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time; and for each of the receivers, determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the known start time. Alternatively, determining the propagation time of the signal can include sending a pilot signal from the transmitter to a controller simultaneously with transmitting the acoustic signal from the transmitter to the receivers, the controller in data communication with the transmitter and with the receivers; recording the time at which the pilot signal arrives at the controller; and for each of the receivers, determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the time at which the pilot signal arrives at the controller.

The signal may be transmitted using a spread pattern.

The method can also involve positioning the acoustic transmitter downhole in the borehole and positioning the acoustic receiver in a known position relative to the surface, and transmitting the acoustic signal from the transmitter to the receiver, when the receiver is located in the known position; determining a propagation time of the signal as it propagates from the transmitter to the receiver located in the known position; moving the receiver to a second known position relative to the surface; transmitting the acoustic signal from the transmitter to the receiver, when the receiver is located in the second known position; determining a propagation time of the signal as it propagates from the transmitter to the receiver located in the second known position; moving the receiver to a third known position relative to the surface; transmitting the acoustic signal from the transmitter to the receiver, when the receiver is located in the third known position; determining a propagation time of the signal as it propagates from the transmitter to the receiver located in the third known position; determining an average velocity of the signal as it propagates from the transmitter to the receiver at each of the known positions; determining the distance traveled by the signal from the transmitter to the receiver at each of the known positions by multiplying the average velocity of the signal by the determined propagation times; and determining the position of the transmitter from each of the known positions and the distance traveled by the signal from the transmitter to the receiver at each of the known positions.

Alternatively, the method can involve positioning the acoustic receiver downhole in the borehole and positioning the acoustic transmitter in a known position relative to the surface, transmitting the acoustic signal from the transmitter to the receiver, when the transmitter is located in the known position; determining a propagation time of the signal as it propagates to the receiver from the transmitter located in the known position; moving the transmitter to a second known position relative to the surface; transmitting the acoustic signal from the transmitter to the receiver, when the transmitter is located in the second known position; determining a propagation time of the signal as it propagates to the receiver from the transmitter located in the second known position; moving the transmitter to a third known position relative to the surface; transmitting the acoustic signal from the transmitter to the receiver, when the transmitter is located in the third known position; determining a propagation time of the signal as it propagates to the receiver from the transmitter located in the third known position; determining an average velocity of the signal as it propagates to the receiver from the transmitter located at each of the known positions; determining the distance traveled by the signal to the receiver from the transmitter located at each of the known positions by multiplying the average velocity of the signal by the determined propagation times; and determining the position of the transmitter from each of the known positions and the distance traveled by the signal from the transmitter to the receiver at each of the known positions.

The method can also involve locating both the transmitter and the receiver in a first borehole, and transmitting the acoustic signal from the transmitter to the receiver by reflecting the acoustic signal off a second borehole; determining an average velocity of the signal as it propagates from the transmitter to the receiver; determining the propagation time of the signal as it travels from the transmitter to the receiver; and determining the distance between the first borehole and the second borehole from the average velocity of the signal and the propagation time of the signal.

According to a further aspect of the invention, there is provided a system for tracking a position of a borehole for oil and gas drilling. The system includes either an acoustic transmitter or an acoustic receiver located downhole in the borehole, the transmitter configured to transmit an acoustic signal for receipt by the receiver; the other of the transmitter or the receiver located in a known position relative to the surface; and a controller, in communication with at least the receiver, configured to record a propagation time of the signal from the transmitter to the receiver; and to determine a position of the transmitter or receiver located downhole relative to the other of the transmitter or the receiver from the known position of the other of the transmitter or the receiver and from the propagation time.

The acoustic transmitter can be located at the known position and at least three acoustic receivers can be located downhole in the borehole. Alternatively, the acoustic receiver can be located downhole in the borehole and at least three acoustic transmitters can be located at known positions relative to the surface, and the three acoustic transmitters can transmit acoustic signals of three different frequencies; the acoustic transmitter can be located downhole in the borehole and the transmitter can transmit to at least three acoustic receivers located at known positions relative to the surface; the acoustic receiver can be located at the known position and at least three acoustic transmitters can be located downhole in the borehole, and the three acoustic transmitters can transmit acoustic signals of three difference frequencies; or the transmitter and the receiver can both be located in a first borehole, and the receiver can receive a reflected acoustic signal that is transmitted from the transmitter and reflected off a second borehole.

When at least three receivers are used, the controller can be configured to locate the position of the transmitter located downhole by for the at least three receivers, determining a propagation time of the signal as it propagates from the transmitter to the receiver; determining an average velocity of the signal as it propagates from the transmitter to the receivers; for each of the receivers for which a propagation time is determined, determining a distance traveled by the signal as it propagates from the transmitter to the receiver by multiplying the average velocity of the signal by the determined propagation time; and determining the position of the transmitter from the position of each of the at least three receivers and the distance traveled by the signal from the transmitter to each of the receivers.

The system can also include at least four receivers. The transmitter can transmit to the at least four acoustic receivers, at least one of which is located in a known position relative to the surface, and the controller can be configured to determine the position of the transmitter located downhole by determining differences between when the signal arrives at each of three of the receivers and a fourth receiver; determining an average velocity of the signal as it propagates from the transmitter to the receivers; and determining the position of the transmitter from the average velocity of the signal and the differences between when the signal arrives at each of three of the receivers and the fourth receiver.

The system can also include a supplemental receiver, and the controller can be configured to determine a propagation time of the signal as it propagates from the transmitter to the supplemental receiver; and to determine a distance between the transmitter and the supplemental receiver by multiplying the average velocity of the signal by the determined propagation time. The supplemental receiver can be located in a known position relative to the surface.

The controller can be configured to determine the average velocity of the signal by multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver. Alternatively, the controller can be configured to determine the average velocity of the signal by iteratively solving for the average velocity by using a starting velocity value of between 5 to 8 km/sec.

The controller can be configured to determine the propagation time of the signal by synchronizing the transmitter and receivers in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time; and for each of the receivers, determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the known start time.

Alternatively, the controller can be configured to determine the propagation time of the signal by sending a pilot signal from the transmitter to a controller simultaneously with transmitting the acoustic signal from the transmitter to the at least three receivers, the controller in data communication with the transmitter and with the receivers; recording the time at which the pilot signal arrives at the controller; and for each of the receivers, determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the time at which the pilot signal arrives at the controller.

The transmitter can transmit the signal using a spread pattern. The receiver may be a fiber optic receiver.

According to a further aspect of the invention, there is provided a method for drilling a well pair comprising a first borehole and a second borehole. The method includes drilling the first borehole; commencing drilling of the second borehole; locating both an acoustic transmitter and an acoustic receiver in the second borehole; transmitting an acoustic signal from the transmitter to the receiver by reflecting the acoustic signal off the first borehole; determining an average velocity of the signal as it propagates from the transmitter to the receiver; determining the propagation time of the signal as it travels from the transmitter to the receiver; determining the distance between the first borehole and the second borehole from the average velocity of the signal and the propagation time of the signal; and extending the second borehole while maintaining a suitable distance between the first borehole and the second borehole.

The transmitter can be located on a transmission sub and the receiver can be located on a receiver sub.

Determining the average velocity of the signal can involve multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver. Alternatively, determining the average velocity of the signal can involve iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

Determining the propagation time of the signal can include synchronizing the transmitter and receiver in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time; and determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the known start time.

When at least three receivers are utilized, determining the propagation time of the signal can include sending a pilot signal from the transmitter to a controller simultaneously with transmitting the acoustic signal from the transmitter to the at least three receivers, the controller in data communication with the transmitter and with the at least three receivers; recording the time at which the pilot signal arrives at the controller; and determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the time at which the pilot signal arrives at the controller.

The signal can be transmitted using a spread pattern.

According to a further aspect of the invention, there is provided a system for drilling a well pair comprising a first borehole and a second borehole. The system includes an acoustic transmitter disposed in one of the first and second boreholes; an acoustic receiver disposed in the same borehole as the acoustic transmitter and located to receive a reflected acoustic signal that is transmitted from the acoustic transmitter and reflected off of the other of the first and second boreholes; a controller, in communication with at least the receiver, configured to determine an average velocity of the signal as it propagates from the transmitter to the receiver; determine the propagation time of the signal as it travels from the transmitter to the receiver; and determine the distance between the first borehole and the second borehole from the average velocity of the signal and the propagation time of the signal.

The transmitter and the receiver can respectively be located on a transmission sub and a receiver sub. The controller can be configured to determine the average velocity of the signal by multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver. Alternatively, the controller can be configured to determine the average velocity of the signal by iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

The controller can be configured to determine the propagation time of the signal by synchronizing the transmitter and receiver in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time; and by determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the known start time.

When at least three receivers are utilized, the controller can be configured to determine the propagation time of the signal by sending a pilot signal from the transmitter to a controller simultaneously with transmitting the acoustic signal from the transmitter to the at least three receivers, the controller in data communication with the transmitter and with the at least three receivers; recording the time at which the pilot signal arrives at the controller; and determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the time at which the pilot signal arrives at the controller.

The transmitter can transmit the signal using a spread pattern.

According to a further aspect of the invention, there is provided a method for reducing uncertainty in the position of a drill bit in a first borehole. The method can include locating an acoustic transmitter or an acoustic receiver downhole in the first borehole in a known position relative to the drill bit; locating the other of the transmitter or the receiver in a second borehole; obtaining an approximate position of the drill bit using a conventional measurement while drilling (MWD) system, the approximate position of the drill bit having a known maximum uncertainty; transmitting an acoustic signal from the transmitter to the receiver; determining a propagation time of the signal as it propagates from the transmitter to the receiver; determining an average velocity of the signal as it propagates from the transmitter to the receiver; determining a distance travelled by the signal by multiplying the propagation time by the average velocity; and reducing the known maximum uncertainty by using the determined distance.

The drill bit can be constrained to move only substantially vertically. Additionally, the receiver in the second borehole can be in a known position relative to the surface.

According to a further aspect of the invention, there is provided a system for reducing uncertainty in the position of a drill bit in a first borehole. The system includes a drill bit located downhole in the first borehole; a measurement while drilling (MWD) system coupled to the drill bit, the MWD system generating an approximate position of the drill bit having a known maximum uncertainty; one of an acoustic transmitter or an acoustic receiver located in a known position relative to the drill bit, the transmitter configured to transmit an acoustic signal to the receiver; the other of the transmitter or the receiver located in a second borehole; and a controller, in communication with at least the receiver. The controller can be configured to determine a propagation time of the signal as it propagates from the transmitter to the receiver; to determine an average velocity of the signal as it propagates from the transmitter to the receiver; to determine a distance travelled by the signal by multiplying the propagation time by the average velocity; and to reduce the known maximum uncertainty by using the determined distance.

The drill bit can be constrained to move only substantially vertically. Additionally, the other of the transmitter or the receiver in the second borehole can be in a known position relative to the surface. BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate an exemplary embodiment of the present invention:

Figure 1 is a schematic of a drill bit assembly attached to other components in a drill string according to one embodiment of the invention, in use in a well site.

Figure 2 is a schematic representation of a first embodiment of the present invention wherein an acoustic transmitter is located downhole in a well in a known position relative to a drill bit, and acoustic receivers are located both on the surface of the Earth and in an existing, nearby well.

Figure 3 is a vector diagram showing the acoustic transmitter and three of the acoustic receivers of the embodiment depicted in Figure 2.

Figure 4 is a schematic representation of a second embodiment of the present invention wherein an acoustic transmitter is located downhole in a well in a known position relative to a drill bit, and all acoustic receivers are located in an existing, nearby well in the form of a measurement sub.

Figure 5 is a schematic representation of a third embodiment of the present invention wherein an acoustic transmitter is located downhole in a well in a known position relative to a drill bit, and an acoustic receiver, in the form of a fiber optic sensor, is located in an existing, nearby well.

Figure 6 is a schematic representation of a fourth embodiment of the present invention wherein both the acoustic transmitters and receivers are placed in the same well, and position is determined by reflecting an acoustic transmission off of a nearby well.

Figure 7 is a block diagram of a system according to the embodiment of the invention depicted in Figure 2. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Drill String

Figure 1 illustrates a wellsite system in which the present invention can be employed. The wellsite can be onshore or offshore. In this exemplary system, a borehole 11 is formed in subsurface formations by rotary drilling in a manner that is well known. Embodiments of the invention can also use directional drilling, as will be described hereinafter.

A drill string 12 is suspended within the borehole 11 and has a bottom hole assembly 100 which includes a drill bit 105 at its lower end. The surface system includes platform and derrick assembly 10 positioned over the borehole 11 , the assembly 10 including a rotary table 16, kelly 17, hook 18 and rotary swivel 19. The drill string 12 is rotated by the rotary table 16, energized by means not shown, which engages the kelly 17 at the upper end of the drill string. The drill string 12 is suspended from a hook 18, attached to a traveling block (not shown), through the kelly 17 and a rotary swivel 19 which permits rotation of the drill string relative to the hook. As is well known, a top drive system could alternatively be used.

In the example of this embodiment, the surface system further includes drilling fluid or mud 26 stored in a pit 27 formed at the well site. A pump 29 delivers the drilling fluid 26 to the interior of the drill string 12 via a port in the swivel 19, causing the drilling fluid to flow downwardly through the drill string 12 as indicated by the directional arrow 8. The drilling fluid exits the drill string 12 via ports in the drill bit 105, and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows 9. In this well known manner, the drilling fluid lubricates the drill bit 105 and carries formation cuttings up to the surface as it is returned to the pit 27 for recirculation. The bottom hole assembly 100 of the illustrated embodiment includes a logging- while-drilling (LWD) module 120, a measuring-while-drilling (MWD) module 130, a roto-steerable system and motor, and drill bit 105.

The LWD module 120 is housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at 120A. (References, throughout, to a module at the position of 120 can alternatively mean a module at the position of 120A as well.)

The LWD module includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. In the present embodiment, the LWD module includes a pressure measuring device.

The MWD module 130 is also housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. The MWD tool further includes an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. In the present embodiment, the MWD module includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and an inclination measuring device.

A particularly advantageous use of the system hereof is in conjunction with controlled steering or "directional drilling." In this embodiment, a roto-steerable subsystem 150 (Figure 1 ) is provided. Directional drilling is the intentional deviation of the wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction. Directional drilling is, for example, advantageous in offshore drilling because it enables many wells to be drilled from a single platform. Directional drilling also enables horizontal drilling through a reservoir. Horizontal drilling enables a longer length of the wellbore to traverse the reservoir, which increases the production rate from the well. A directional drilling system may also be used in vertical drilling operation. Often the drill bit will veer off of a planned drilling trajectory because of the unpredictable nature of the formations being penetrated or the varying forces that the drill bit experiences. When such a deviation occurs, a directional drilling system may be used to put the drill bit back on course.

A known method of directional drilling includes the use of a rotary steerable system ("RSS"). In an RSS, the drill string is rotated from the surface, and downhole devices cause the drill bit to drill in the desired direction. Rotating the drill string greatly reduces the occurrences of the drill string getting hung up or stuck during drilling. Rotary steerable drilling systems for drilling deviated boreholes into the earth may be generally classified as either "point-the-bit" systems or "push-the-bit" systems.

In the point-the-bit system, the axis of rotation of the drill bit is deviated from the local axis of the bottom hole assembly in the general direction of the new hole. The hole is propagated in accordance with the customary three point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and lower stabilizer results in the non-collinear condition required for a curve to be generated. There are many ways in which this may be achieved including a fixed bend at a point in the bottom hole assembly close to the lower stabilizer or a flexure of the drill bit drive shaft distributed between the upper and lower stabilizer. In its idealized form, the drill bit is not required to cut sideways because the bit axis is continually rotated in the direction of the curved hole. Examples of point-the-bit type rotary steerable systems, and how they operate are described in U.S. Patent Application Publication Nos. 2002/0011359; 2001/0052428 and U.S. Patent Nos. 6,394,193; 6,364,034; 6,244,361 ; 6,158,529; 6,092,610; and 5,113,953 all herein incorporated by reference.

In the push-the-bit rotary steerable system there is usually no specially identified mechanism to deviate the bit axis from the local bottom hole assembly axis; instead, the requisite non-collinear condition is achieved by causing either or both of the upper or lower stabilizers to apply an eccentric force or displacement in a direction that is preferentially orientated with respect to the direction of hole propagation. Again, there are many ways in which this may be achieved, including non-rotating (with respect to the hole) eccentric stabilizers (displacement based approaches) and eccentric actuators that apply force to the drill bit in the desired steering direction. Again, steering is achieved by creating non co-linearity between the drill bit and at least two other touch points. In its idealized form the drill bit is required to cut side ways in order to generate a curved hole. Examples of push-the-bit type rotary steerable systems, and how they operate are described in U.S. Patent Nos. 5,265,682; 5,553,678; 5,803,185; 6,089,332; 5,695,015; 5,685,379; 5,706,905; 5,553,679; 5,673,763; 5,520,255; 5,603,385; 5,582,259; 5,778,992; 5,971 ,085 all herein incorporated by reference.

Determining Downhole Positions Using Acoustic Signals

Figure 2 depicts a borehole 11 that has been drilled as described in respect of Figure 1. In Figure 2, the platform and derrick assembly 10 is coupled to the drill string 12. Disposed at one end of the drill string 12 is the drill bit 105. An acoustic transmitter 13 is located on the drill string 12 at a known position relative to the drill bit 105. The acoustic transmitter 13 may, for example, form an integral part of the LWD module 120 or the MWD module 130.

The acoustic transmitter 13 can be any type of transmitter that is capable of reliably and repeatedly generating acoustic signals, such as those used with known MWD or seismic while drilling applications. Exemplary acoustic transmitters 13 include piezo-electric stacks, magnetorestrictive transducers, hydraulic jars, mud hammers, drill bits, explosives, off-center motors and other suitable vibration sources. The drill bit 105 is used for boring and consequently is at the distal end, relative to the platform and derrick assembly 10, of the borehole 11. The acoustic transmitter 13 can be used to transmit acoustic signals to determine the position of the drill bit 105, as described with respect to various exemplary embodiments, below.

Determining Downhole Positions Using Trilateration

According to a first embodiment, the acoustic transmitter 13 transmits acoustic signals 14 to a plurality of acoustic receivers 3 located on the surface of the Earth. The acoustic signals 14 ideally have a very short rise time, and rise to their peak magnitude as soon as possible. The acoustic signals 14 can optionally be transmitted in a pattern, such as a spread pattern, which can be beneficial to overcome transmission-related problems, such as aliasing. Exemplary acoustic receivers 3 used include piezo-electric transducers, geophones, hydrophones, accelerometers, fiber optic sensors, microphones, magnetorestrictive transducers, and piezo-electric sheets. As with the exemplary acoustic transmitters 13, the exemplary acoustic receivers 3 can be those used in known MWD or seismic while drilling applications. The position of the acoustic receivers 3 on the surface can be determined via, for example, use of global positioning satellites.

As the position of the acoustic transmitter 13 is known relative to the drill bit 105, determining the position of the acoustic transmitter 13 is tantamount to knowing the position of the drill bit 105, which results in being able to track the position of the borehole 11 itself. In the depicted exemplary embodiment, in order to determine the position of the acoustic transmitter 13, three acoustic receivers 3a, 3b, 3c are used. The method by which the position of the acoustic transmitter 13 is determined is best explained with reference to Figure 3, which is a vector diagram of the acoustic transmitter 13 and the three acoustic receivers 3a, 3b, 3c.

The first step in determining the position of the transmitter 13 is to have the transmitter 13 transmit the acoustic signal 14. In order to measure the time required for the signal 14 to propagate from the transmitter 13 to the receivers 3a, 3b, 3c, the transmitter 13 and receivers 3a, 3b, 3c can be synchronized in time with reference to any external point of reference, such as a central controller 15 (as depicted in Figure 7) located in a logging and control unit 30 (see Figure 1) on the surface. With respect specifically to SAGD applications, either nanosecond or millisecond level synchronization can be used. Alternatively, at the time of transmission, a pilot signal can be sent from the transmitter 13 to the central controller 15 located that records the time at which the signal 14 is sent. The difference between this recorded time and the time at which the signal 14 arrives at the receiver 3 can be compared to determine the signal propagation time.

The transmitter 13 can be pre-programmed to periodically transmit the signal 14, or can act in response to instructions conveyed to it from the surface via typical methods of downhole telemetry, such as mud pulse, electromagnetic, and acoustic telemetry, and flow/rotation events. The signal 14 propagates through the Earth and eventually reaches each of the three receivers 3a, 3b, 3c. The times at which the signal 14 reach the three receivers 3a, 3b, 3c are recorded by the central controller 15, which is in communication with each receiver 3a, 3b, 3c. The time for the signal 14 to reach the receivers 3a, 3b, 3c from the transmitter 13 are T_A) T_B) and T_c, respectively.

Referring to Figure 3, elementary linear algebra reveals that the distances from each of the receivers 3a, 3b, 3c to the acoustic transmitter 13, D_AE, D_BE, and D_CE respectively, can be determined using the following equations for spheres: (D_AE)²=(x-x_a)²+(y-y_a)² +(z-z_a)² (1)

(D_BE)² = (x-x_b)²+(y-y_b)²+(z-z_b)² (2)

(D_CE)² =(χ-χ_c)² +(y-y_c)² +(z-z_c)² (3)

where the receiver 3a is located at (x_a, y_a, Z₃), the receiver 3b is located at (x_b, Yb, z_b), the receiver 3c is located at (x_c, y_c, Z₀), and the transmitter 13 is located at (x, y.z).

Finding D_AE using Equation (1) allows the position of the transmitter 13, (x,y,z), to be narrowed to a series of points that define the surface of a sphere with a radius of DAE and centered on the known position of receiver 3a. Subsequently finding DBE using Equation (2) further narrows the position of the transmitter 13 to a circle formed by the intersection of the sphere centered on receiver 3a and a second sphere of radius D_BE centered on receiver 3b. Subsequently finding D_CE using Equation (3) narrows the position of the transmitter 13 to a single point, as the intersection of a third sphere centered on receiver 3c of radius DCE only intersects the circle formed by the intersection of the spheres centered on receivers 3a, 3b at two positions, one of which is above the plane of the receivers 3a, 3b, 3c (i.e.: above the surface of the Earth) and consequently cannot represent the position of the transmitter 13. Thus, the one point of intersection that represents a point beneath the surface of the Earth represents the position of the transmitter 13.

DAE, DBE, and D_CE also equal

D_AE =V_AVG -T_A (4)

D_BE =V_AVG -T_B (5) D_ΓB = V_A T (6)

where V_AVG is the average velocity of the acoustic signal 14 as it travels through the rock.

Following the measurement of T_A, TB, and Tc, the velocity of the acoustic signal 14 at each receiver 3a, 3b, 3c can be calculated in one of two ways. First, assuming that the rock formation through which the signal 14 has travelled is homogenous, then

VAVG = f ^■ λ (7)

where f is the frequency of the signal 14 and λ is the wavelength of the signal 14, as measured at the receiver 3a, 3b, 3c.

Alternatively, VAVG ^■ T_A can be substituted for DAE in Equation (1 ), VAVG ^■ T_B can be substituted for D_BE in Equation (2), VAVG ^■ Tc can be substituted for D_CE in Equation (3), and VAVG can be solved iteratively using the modified Equations (1) - (3). As the velocity of the acoustic signal 14 in rock is typically between 5 - 8 km/sec, VAVG can be solved by beginning the iterative process using a value, for example, of VAVG = 8 km/sec.

Following the determination of VAVG, DAE, DBE, and DCE can be determined using Equations (4) - (6), and the values of DAE, DBE, and DCE can be inserted into Equations (1 ) - (3). Equations (1 ) - (3) then represent a series of three equations having three unknowns, x, y, and z, and consequently x, y, and z can be solved.

One method in which to solve Equations (1 ) - (3) for x, y, and z is through trilateration, which involves transforming Equations (1 ) - (3) such that they represent three spheres: a first sphere centred at (0,0,0); a second sphere centred at (a,0,0); and a third sphere centred at (b,c,0). Transforming Equations (1 ) - (3) in this fashion result in the following equations: D² =x²+y²+z² (8)

D₂ ² =(x-a)²+y²+z² (9)

D² =(x-b)²+(y-c)²+z² (10)

Algebraic manipulation of Equations (8) - (10) result in

Df -Di+ a

X = (11)

2a

Df -D, ²+b²+c² bx y = (12)

2c

If no real solution for z exists, there is no downhole solution for z. Such a result indicates measurement error, noisy data, or time drift in the data collected.

Where there are two, real solutions for z, only one of the solutions will result in a source that is located underground, and it is this solution that represents the downhole position of the emitter.

Once x, y, and z are solved using Equations (11) - (13), they must be translated back into the original coordinate system in which Equations (1) - (3) are expressed so that the position of the emitter can be expressed in the original coordinate system. Measurement errors can be addressed using a least squares approach or an adaptive filter, such as a Kalman filter.

Equations (1) - (13), and instructions relating to their method of application, may be encoded on a computer readable medium that forms part of or is coupled to the controller 15. Suitable controllers 15 for this application include a personal computer located on the surface, an ASIC, an FPGA, a programmable logic controller, a microcontroller and a microprocessor. Suitable computer readable media include disc media, RAM, ROM, magnetic storage drives such as hard drives, and various types of rewritable non-volatile memory such as EEPROM and flash memory. In operation, the controller 15 reads and executes the instructions encoded on the computer readable medium.

While the above embodiment has been described as utilizing three receivers 3a, 3b, 3c, more than three receivers can be used, with each additional receiver resulting in an additional equation that provides the distance between such additional receiver and the transmitter 13. Consequently, each additional receiver allows an additional sphere to be plotted, which reduces experimental error and increases the accuracy and precision of the determined position of the transmitter 13.

When an additional receiver, such as fourth receiver 3d is added, Equation (14), below, is added to the set of equations formed by Equations (1 ) - (3):

(D_DE)² = (x -x_d)² + (y - y_d)² + (z - z_d)² (14)

and Equation (15) is added to the set of equations formed by Equations (4) - (6):

D_DE = V_AVG -T_D (15)

Consequently, adding the fourth receiver 3d allows wave velocity V_AVG to be solved directly from Equations (1) - (3) and (14), as these four equations represent a system of four equations and four unknowns.

As the borehole 11 is being drilled, simultaneous readings can be received by the controller 15 from several receivers 3a - 3f. During operation, the most accurate determinations of the position of the transmitter 13 can be calculated using a least squares approach, for example. Consequently, if the controller 15 receives readings from six receivers 3a - 3f, then the least squares method could be used to solve the overdetermined system that would result from Equations (1 ) - (3), thereby reducing error in the determined position of the transmitter 13.

Referring now to Figure 7, there is depicted a block diagram of an exemplary embodiment of the present invention wherein an electronics package 23 is combined with the acoustic transmitter 13. The electronics package 23 may form part of either the LWD module 120 or the MWD module 130. The electronics package 23 can communicate with the controller 15 using, for example, mud pulses, acoustic or electromagnetic signals in the manner as is known in the art. As in the embodiment depicted in Figure 2, present are the transmitter 13, drill bit 105, controller 15, and receivers 3a, 3b, 3c. In the embodiment of Figure 7, in order to determine the position of the transmitter 13, the controller 15 first communicates a "start" signal to the electronics package 23. The electronics package 23 typically contains a processor, real time clock, batteries, memory, and driver circuitry, and is coupled to the acoustic transmitter 13, which transmits the signal 14 to the receivers 3a, 3b, 3c. Simultaneous with the transmission of the signal 14, the electronics package 23 returns a signal to the controller 15 indicating that the signal 14 has been sent, thus providing a start time from which T_A, T_BI and T_c can be measured. The position of the transmitter 13 can then be determined according to Equations (1 ) - (13), as outlined above with respect to Figure 1.

Determining Downhole Positions Using Multilateration

According to a further embodiment, the transmitter 13 can transmit the acoustic signal 14 to each of the four receivers 3a, 3b, 3c, and 3d. Instead of determining the position of the transmitter 13 by measuring the time it takes for the signal 14 to travel from the transmitter 13 to the receivers 3a, 3b, 3c as is done in the first embodiment, the position of the transmitter 13 is determined from the difference in the arrival times of the signal 14 to the receivers 3a, 3b, 3c, and 3d. Again, referring to Figure 3, the equations for T_A, T_B, T_c, and T₀ are as follows:

J(^χ-^χ _cΫ +(y-y_c)²+(z-z_c)²

Tc = (19)

V AVG

where receiver 3d is presumed to be located at (0,0,0) and receivers 3a, 3b, and 3c are presumed to be located at (x_a,ya,Za), (Xb.Vb.Zb), and (x_c,yc,Zc), respectively. VAVG can be determined as it is in the first embodiment.

Manipulating Equations (17) - (20) results in

J(^χ -^χι y + (y-y_b)² + (z- z_b Ϋ -J_x ² + y² + z²

V AVG

J(^χ -Xc y + (y-y_c)² + (z- z_c Ϋ -Jx² + y² + z²

¹C ¹D - 71 \^zo^

V AVG

As the receivers 3a, 3b, 3c, and 3d are all coupled to the controller 15, T_A - TD, TB - T_D, and Tc - T₀ can all be measured and substituted into Equations (21) - (23). What is left is a system of three equations with three unknowns, (x,y,z), which can be solved using known algebraic methods.

Another method in which multilateration can be used to determine the position of the transmitter 13 (x,y,z) is using the Bucher algorithm. As above, the acoustic transmitter 13 is located at (x,y,z). Four receivers are located at fa.yi.z,), (Xj.yj.Zj), (x_k,y_k,z_k) and (xι,yι,zι), and R₁, R_j, R_k, and Ri represent the distance from the transmitter 13 position (x,y,z) to the receiver in question. Then

-X)² +Cv, -y)² + (Z₁ -Z)² (24)

= ^^XJ -X)² +Cv j -y) ² + (*, -z)² (25)

R_k -X)² +(y t -y) ² Hz_k-z)² (26)

For any variable X, Xg_b = Xa - X_b- Then, it can be shown that

B = (29)

V_*. -Vo,

_C

D = ^Rkl^Xjk ^Rkj^Xlk

(31)

_^Rkjyik -^Rkiy_Jk_ E = ^Rkl^Zjk ~^Rkj^Zlk (32)

^Rkjyik -^Ruy_Jk

F = ^RA^Rl +^χl -^χ] +^yl -^y) ^+zl -^z)Y^RA^Rl +^χl -^χf ^+yl -^yi ^+zl (33)

2[^R _kJy,_k-^R _klyJ

E-B

G = (34) A-D

F-C

H = (35) A-D

I=AG+B (36)

J= AH+ C (37)

N = SRl[G(X₁ -H) + I{y,-J) + z]+2LK (41)

O = 4Rl [(X₁ - H)² +(y,-J)²+ z² ]+-K² (42)

and the position of the transmitter 13 at (x,y,z) can be determined using the following three equations:

N z = ^■ ^'JL) _<L (43) ~ΪM 2MJ M y = Iz + J (44)

x = Gz + H (45)

When utilizing multilateration according to any of the above embodiments, more than four receivers 3 can be used in order to increase the precision of the determined downhole position of the transmitter 13. For example, if more than four receivers 3 are used, the position of the transmitter 13 can be determined multiple times using different combinations of any four of the receivers 3. Each of the positions that is determined using the different combinations of the receivers 3 can then be averaged to result in a more precise overall determination of the position of the transmitter 13.

In all of the trilateration and multilateration embodiments described above, the positions of the receivers 3 on the surface are known using, for example, GPS. Since the position of the downhole transmitter 13 is determined relative to the known position of the receivers 3, the determined position of the transmitter 13 is also known relative to the surface. For example, when using Cartesian coordinates (x,y,z) in which (x,y,0) corresponds to positions on the surface and negative values of z correspond to positions below the surface, the determined position (x,y,z) of the transmitter 13 in each of the aforedescribed embodiments can be solved to a commercially useful precision (e.g.: to a precision of +/- 1 meter). In contrast, in systems wherein the position of the receivers is not known relative to the surface, the position of the transmitter can only be determined relative to the position of the receivers, and consequently the transmitter position cannot be expressed in terms of Cartesian coordinates relative to the surface.

Additionally, the use of surface receivers 3 allows the aforedescribed embodiments to be employed with one or more boreholes. In contrast, in systems wherein only downhole receivers are used, at least two boreholes are required: one borehole to contain the acoustic transmitter and one borehole to contain the acoustic receivers.

Variations on the Above Embodiments

Also illustrated in Figure 2 is a supplemental receiver in the form of a downhole acoustic receiver 4 placed in an existing borehole 24. The downhole receiver 4 can take the place of one of or be used in addition to the surface acoustic receivers 3, regardless of whether trilateration or multilateration is employed. While only one additional downhole receiver 4 is illustrated, a plurality of downhole receivers 4 may be used. If the position of the downhole receiver 4 is known, and if the downhole receiver 4 is used to supplement the three surface receivers 3a, 3b, 3c, then the downhole receiver 4 acts to increase the accuracy and precision of the determined position of the transmitter 13. The position of the downhole receiver 4 can be determined using a high resolution gyroscope, for example, or can be determined using trilateration or multilateration, as described above, if the downhole receiver 4 is a source as well as a receiver (e.g.: if the downhole receiver 4 is a piezoelectric sensor).

Alternatively, even if the position of the downhole receiver 4 is not known, it can still serve a useful function. By using the downhole receiver 4, it is possible to directly calculate the distance between the acoustic transmitter 13 and the existing borehole 24 by using Equation (46):

D_FE = V_AVG - T_F (46)

where D_FE is the distance between the transmitter 13 and the downhole receiver

4 and TF is the time it takes for the signal 14 to reach the downhole receiver 4 from the transmitter 13. Such information is useful if a goal of the drilling is either specifically to intersect with or to avoid the existing borehole 24. An advantage of using the downhole receiver 4 is a more accurate determination of the position of the transmitter 13, especially of the transmitter's depth.

Referring now to Figure 4 and according to another embodiment of the present invention, there is illustrated an injector well 31 and a production well 32 used for SAGD. The injector well 31 has already been drilled, and the production well 9 is in the process of being drilled. As with the borehole 11 of Figure 2, present in the production well 32 is a drill string 12 having a drill bit 105 whose position is known relative to an acoustic transmitter 13. In contrast with the embodiment depicted in Figure 2, however, the embodiment depicted in Figure 4 contains spaced acoustic receivers 3 in the form of a measurement sub 7 positioned downhole the injector well 31. Using trilateration or multilateration as described above, the position of the acoustic transmitter 13 relative to the measurement sub 7 can be analogously determined. This allows the position of the production well 32 to be tracked to the degree necessary for SAGD. Typically, in SAGD, the injector and production wells 31 , 32 are within 3 - 5 meters of each other for up to a kilometer in length.

In lieu of the measurement sub 7, a fiber optic acoustic sensor 33 coupled to a fiber optic collection system 34 may be used, as illustrated in Figure 5. An additional advantage of using the measurement sub 7 or the fiber optic acoustic sensor 33 instead of the receivers 3 located on the surface of the Earth is that the distance between the transmitter 13 and the sub 7 is less than the distance from the transmitter 13 to the surface, thus resulting in a higher signal-to-noise ratio and fewer transmission errors. This decrease in distance also allows higher frequency acoustic signals 14 to be used. While higher frequency signals are not capable of travelling as far as lower frequency signals, they are less subject to the filtering effects of rock formations than low frequency signals, and their use consequently results in greater measurement accuracy. A relatively high frequency signal may be one that is 600 Hz or higher, while a relatively low frequency signal may be one that is below 600 Hz.

Although the above-described embodiments all describe a system wherein the transmitter 13 is located downhole in the borehole 11 whose position is being tracked and receivers 3, 4, 7, 33 that are located either on the surface or downhole in nearby wells, equally operable alternative embodiments utilize a single receiver (not shown) located downhole and multiple transmitters (not shown) located either on the surface or downhole in nearby wells. In contrast with the aforedescribed trilateration embodiment wherein a single transmission 14 is received by all three surface receivers 3a, 3b, 3c, in the trilateration embodiment wherein there is only one receiver and multiple spaced transmitters, each transmitter can send a signal to the receiver in turn. That is, a first transmitter transmits a signal to the receiver which allows T_A to be recorded; then, a second transmitter transmits a signal to the receiver which allows TB to be recorded; and then, a third transmitter transmits a signal to the receiver which allows Tc to be recorded. If a fourth transmitter is employed, T₀ can be recorded. Either the trilateration or multilateration algorithms as described above can then be used to determine the position of the receiver and, consequently, track the well.

Alternatively, in lieu of sending one signal after the other, multiple signals can be transmitted at different frequencies simultaneously from the multiple transmitters; multiple signals can be transmitted simultaneously using different signalling patterns; or both. In both such alternative cases, either the trilateration or multilateration algorithms as described above can be employed.

According to a still further embodiment of the present invention, there is provided a single acoustic receiver located downhole in a well, and a single acoustic transmitter located on the surface. In such an embodiment, the acoustic transmitter can send a first signal from a known position from which TA can be measured; it can then be moved to known second and third positions from which it can send second and third signals and from which T₆ and T_c can be measured, respectively. As discussed above, the position of the transmitter can be determined using, for example, GPS readings. The trilateration algorithm can then be applied to determine the position of the downhole receiver. The transmitter can also be moved to further positions from which it can emit signals, thus increasing the precision and accuracy of the determined position of the receiver. According to a related embodiment, a single transmitter can be located downhole and a single receiver located on the surface, the receiver being moved to different positions and receiving a transmission at each position. These embodiments are especially useful in cost conscious applications, as costs can be reduced by using only a single receiver and transmitter with the trade-off being the greater time required to obtain the necessary data for use in trilateration. If a fourth position is utilized and a corresponding T_D measured, then the multilateration algorithm, as described above, can be used. Also as described above, more than three positions can be used to increase the accuracy of the trilateration and multilateration algorithms.

According to a still further embodiment of the present invention particularly useful in SAGD, both the acoustic transmitters and receivers can be placed in a single borehole and the acoustic signal can be reflected off a neighbouring borehole. In the SAGD context, both the acoustic transmitters and receivers can be placed in the second of either the production or injector wells being drilled. In the embodiment depicted in Figure 6, for example, a single transmitter 13 located on a transmission sub 35 in a production well (not shown in Figure 6), transmits a signal that reflects off the exterior of the injector well 31. The reflected signal is reflected back at receivers 21 located at known positions within the production well on a receiver sub 20, and the acoustic transmission can be used to determine the distance between the injector and production wells. The advantages of this further embodiment include that it is a self-contained solution that does not require surface transmitters or receivers to collect data at the surface, or service rigs to move a measurement sub in one of the wells. Higher acoustic frequencies can also be used due to the reduced distance and filtering effects encountered during transmission from the production well to the injector well 31 as opposed to transmission from a well to the surface, for example. This allows for greater resolution and precision when calculating distance. The transmitter 13 and receivers 21 do not need to be located on subs, but can instead be standalone components disposed on a drill string.

The time T_travei for the acoustic signal to travel from the transmitter 13 to the receivers 21 can be measured. Given the known speed of the acoustic signal Vsignai and the known distance D between the transmitter and any one of the receivers 21 , the distance between the two parallel wells D_weιι_s (injector and production wells in a SAGD context) can be calculated using Pythagorean formulas as follows:

D_wells = ^al^^{' ι} γ -^ (47)

The Combination of the Above Embodiments with a Typical MWD Tool

Any of the aforedescribed embodiments can be combined with known methods of locating a downhole position in order to increase the accuracy of or confirm the readings of such known methods.

The use of a single acoustic transmitter and a single acoustic receiver could be used to increase the accuracy of known methods of locating downhole positions. For example, when locating a downhole position using interpolation based on readings of drill bit orientation from a MWD device and the known rate of descent of the drill bit, the single transmitter could transmit a signal to the single receiver, and the velocity (which can be determined as with the trilateration and multilateration embodiments) and the time (which can be measured using the controller 15) can be used to determine the distance the acoustic signal has travelled. If one of the transmitter or receiver positions is known, then the distance from the known transmitter or receiver position to the drill bit can be determined. This distance can be used to reduce the uncertainty that results from the cumulative errors that are a consequence of estimating drill bit position from drill bit orientation and the rate of drill bit descent.

In the case of a well that is being drilled only vertically, for example, the uncertainty of position calculated by MWD measurements can be represented by a circle in a horizontal plane that is centred on the drill bit. For example, when using Cartesian coordinates (x,y,z), this uncertainty is represented by uncertainty in x and y, where (x,y,z) represents the position of the drill bit. By measuring the distance of the drill bit from one acoustic transmitter or receiver, the uncertainty in drill bit position can be reduced, as follows.

The uncertainty in drill bit position as determined using traditional MWD techniques can be modelled as the area of a circle of uncertainty that forms a bottom end of a cone, where the cone's apex is the last known position of the drill bit. When using Cartesian coordinates, this circle of uncertainty is a circle in the (x,y), or horizontal, plane. When the drill bit is constrained to move substantially vertically, to a very good approximation the depth (z in Cartesian coordinates) of the drill bit is equal to the amount of pipe that has been used for drilling, which can be measured using known instrumentation on the drilling rig. The radius Di of the circle (the "maximum uncertainty") can be calculated from known directional module inaccuracies and depth drilled pursuant to Equation (48), where x and y are obtained from conventional MWD measurements:

By using an acoustic signal to determine a distance between the drill bit and a known transmitter or receiver, the radius Di of the circle of uncertainty can be reduced. For example, an acoustic transmitter can be placed along the drill string in a known position relative to the drill bit, and an acoustic receiver can be placed in a neighbouring second borehole. A controller, in communication with the acoustic receiver, can record the propagation time of the signal as it travels from the transmitter to the receiver and can determine the average velocity of the signal, as discussed above. From the propagation time and average velocity, the controller can determine the distance that the acoustic signal travels. Subsequently, as is done in respect of Equations (1) - (10) and trilateration, the acoustic signal again generates a spherical equation that can be linearly transformed to Equation (49):

D² = (x-a)² +y² +z² (49)

With z being known, Equation (49) transforms into:

D_γ ² = (x-a)² +y² (50)

Solving these two equations results in Equations (51 ) and (52):

_{X ≤} ^D' ^~ (51 )

which corresponds to a lower level of maximum uncertainty in the position of the drill bit than Equation (48).

Given a non-vertical well where the MWD inaccuracies also gives an unknown in the z direction, uncertainty can be reduced using two transmitters or receivers. The resulting equations can be solved, for example, using the trilateration methods described above, with one distance being calculated using the MWD measurements and the remaining two distances being extracted from the acoustic transmitter/receiver calculations.

While particular embodiments of the present invention has been described in the foregoing, it is to be understood that other embodiments are possible within the scope of the invention and are intended to be included herein. It will be clear to any person skilled in the art that modifications of and adjustments to this invention, not shown, are possible without departing from the spirit of the invention as demonstrated through the exemplary embodiment. The invention is therefore to be considered limited solely by the scope of the appended claims.

Claims

CLAIMS:

1. A method for tracking a position of a borehole for oil and gas drilling, the method comprising:

(a) locating an acoustic transmitter or an acoustic receiver downhole in the borehole;

(b) locating the other of the transmitter or the receiver in a known position relative to the surface;

(c) transmitting an acoustic signal from the transmitter to the receiver;

(d) recording when the signal arrives at the receiver; and

(e) determining the position of the transmitter or the receiver located downhole based on when the signal arrives at the receiver and the known position of the transmitter or the receiver.

2. A method as claimed in claim 1 wherein the acoustic transmitter is on the surface at the known position and transmits to at least three acoustic receivers located downhole in the borehole.

3. A method as claimed in claim 1 wherein the acoustic receiver is located downhole in the borehole and at least three acoustic transmitters are located at known positions relative to the surface, the three acoustic transmitters transmitting acoustic signals of three different frequencies.

4. A method as claimed in claim 1 wherein the acoustic transmitter is downhole in the borehole and the transmitter transmits to at least three acoustic receivers, at least one of which is on the surface.

5. A method as claimed in claim 1 wherein the acoustic receiver is located on the surface and at least three acoustic transmitters are located downhole in the borehole, the three acoustic transmitters transmitting acoustic signals of three different frequencies.

6. A method as claimed in claim 4 wherein determining the position of the transmitter located downhole comprises:

(a) for the at least three receivers, determining a propagation time of the signal as it propagates from the transmitter to the receiver;

(b) determining an average velocity of the signal as it propagates from the transmitter to the receivers;

(c) for each of the receivers for which a propagation time is determined in (a), determining a distance traveled by the signal as it propagates from the transmitter to the receiver by multiplying the average velocity of the signal by the propagation time determined in (a); and

(d) determining the position of the transmitter from the position of each of the at least three receivers and the distance traveled by the signal from the transmitter to each of the receivers.

7. A method as claimed in claim 6 further comprising

(a) determining a propagation time of the signal as it propagates from the transmitter to a supplemental receiver; and

(b) determining a distance between the transmitter and the supplemental receiver by multiplying the average velocity of the signal by the propagation time determined in (a).

8. A method as claimed in claim 7 wherein the supplemental receiver is located in a known position relative to the surface.

. A method as claimed in claim 6 wherein determining the average velocity of the signal comprises multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver.

10. A method as claimed in claim 6 wherein determining the average velocity of the signal comprises iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

11. A method as claimed in claim 6 wherein determining the propagation time of the signal comprises:

(a) synchronizing the transmitter and receivers in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time;

(b) for each of the at least three receivers, determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the known start time.

12. A method as claimed in claim 6 wherein determining the propagation time of the signal comprises:

(a) sending a pilot signal from the transmitter to a controller simultaneously with transmitting the acoustic signal from the transmitter to the at least three receivers, the controller in data communication with the transmitter and with the at least three receivers;

(b) recording the time at which the pilot signal arrives at the controller; and

(c) for each of the at least three receivers, determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the time at which the pilot signal arrives at the controller.

13. A method as claimed in claim 6 wherein the signal is transmitted using a spread pattern.

14. A method as claimed in claim 1 wherein the transmitter transmits to at least four acoustic receivers, at least one of which is located in a known position relative to the surface, and wherein determining the position of the transmitter located downhole comprises:

(a) determining differences between when the signal arrives at each of three of the receivers and a fourth receiver;

(b) determining an average velocity of the signal as it propagates from the transmitter to the receivers; and

(c) determining the position of the transmitter from the average velocity of the signal and the differences between when the signal arrives at each of three of the receivers and the fourth receiver.

15. A method as claimed in claim 14 further comprising

16. A method as claimed in claim 14 wherein determining the average velocity of the signal comprises multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver.

17. A method as claimed in claim 14 wherein determining the average velocity of the signal comprises iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

18. A method as claimed in claim 14 wherein determining the propagation time of the signal comprises:

19. A method as claimed in claim 14 wherein determining the propagation time of the signal comprises:

(b) recording the time at which the pilot signal arrives at the controller; and

20. A method as claimed in claim 14 wherein the signal is transmitted using a spread pattern.

21. A method as claimed in claim 1 wherein the acoustic transmitter is downhole in the borehole and the acoustic receiver is in a known position relative to the surface, and wherein (c) to (e) comprise:

(i) transmitting the acoustic signal from the transmitter to the receiver, when the receiver is located in the known position;

(ii) determining a propagation time of the signal as it propagates from the transmitter to the receiver located in the known position;

(iii) moving the receiver to a second known position relative to the surface;

(iv) transmitting the acoustic signal from the transmitter to the receiver, when the receiver is located in the second known position;

(v) determining a propagation time of the signal as it propagates from the transmitter to the receiver located in the second known position;

(vi) moving the receiver to a third known position relative to the surface;

(vii) transmitting the acoustic signal from the transmitter to the receiver, when the receiver is located in the third known position;

(viii) determining a propagation time of the signal as it propagates from the transmitter to the receiver located in the third known position;

(ix) determining an average velocity of the signal as it propagates from the transmitter to the receiver at each of the known positions; (x) determining the distance traveled by the signal from the transmitter to the receiver at each of the known positions by multiplying the average velocity of the signal by the propagation times determined in (ii), (v) and (viii), respectively; and

(xi) determining the position of the transmitter from each of the known positions and the distance traveled by the signal from the transmitter to the receiver at each of the known positions.

22. A method as claimed in claim 1 wherein the acoustic receiver is downhole in the borehole and the acoustic transmitter is in a known position relative to the surface, and wherein (c) to (e) comprise:

(i) transmitting the acoustic signal from the transmitter to the receiver, when the transmitter is located in the known position;

(ii) determining a propagation time of the signal as it propagates to the receiver from the transmitter located in the known position;

(iii) moving the transmitter to a second known position relative to the surface;

(iv) transmitting the acoustic signal from the transmitter to the receiver, when the transmitter is located in the second known position;

(v) determining a propagation time of the signal as it propagates to the receiver from the transmitter located in the second known position;

(vi) moving the transmitter to a third known position relative to the surface;

(vii) transmitting the acoustic signal from the transmitter to the receiver, when the transmitter is located in the third known position; (viii) determining a propagation time of the signal as it propagates to the receiver from the transmitter located in the third known position;

(ix) determining an average velocity of the signal as it propagates to the receiver from the transmitter located at each of the known positions;

(x) determining the distance traveled by the signal to the receiver from the transmitter located at each of the known positions by multiplying the average velocity of the signal by the propagation times determined in (ii), (v) and (viii); and

23. A method as claimed in claim 1 wherein the transmitter and the receiver are both located in a first borehole, and wherein (c) - (e) comprise:

(i) transmitting the acoustic signal from the transmitter to the receiver by reflecting the acoustic signal off a second borehole;

(ii) determining an average velocity of the signal as it propagates from the transmitter to the receiver;

(iii) determining the propagation time of the signal as it travels from the transmitter to the receiver; and

(iv) determining the distance between the first borehole and the second borehole from the average velocity of the signal and the propagation time of the signal.

24. A system for tracking a position of a borehole for oil and gas drilling, the system comprising

(a) either an acoustic transmitter or an acoustic receiver located downhole in the borehole, the transmitter configured to transmit an acoustic signal for receipt by the receiver;

(b) the other of the transmitter or the receiver located in a known position relative to the surface; and

(c) a controller, in communication with at least the receiver, configured to:

(i) record a propagation time of the signal from the transmitter to the receiver; and

(ii) determine a position of the transmitter or receiver located downhole relative to the other of the transmitter or the receiver from the known position of the other of the transmitter or the receiver and from the propagation time.

25. A system as claimed in claim 24 wherein the acoustic transmitter is located at the known position and at least three acoustic receivers are located downhole in the borehole.

26. A system as claimed in claim 24 wherein the acoustic receiver is located downhole in the borehole and wherein at least three acoustic transmitters are located at known positions relative to the surface, the three acoustic transmitters transmitting acoustic signals of three different frequencies.

27. A system as claimed in claim 24 wherein the acoustic transmitter is located downhole in the borehole and the transmitter transmits to at least three acoustic receivers located at known positions relative to the surface.

28. A system as claimed in claim 24 wherein the acoustic receiver is located at the known position and at least three acoustic transmitters are located downhole in the borehole, the three acoustic transmitters transmitting acoustic signals of three difference frequencies.

29. A system as claimed in claim 24 wherein the transmitter and the receiver are both located in a first borehole, and wherein the receiver receives a reflected acoustic signal that is transmitted from the transmitter and reflected off a second borehole.

30. A system as claimed in claim 27 wherein the controller is configured to locate the position of the transmitter located downhole by:

31. A system as claimed in claim 30 further comprising a supplemental receiver and wherein the controller is configured to: (a) determine a propagation time of the signal as it propagates from the transmitter to the supplemental receiver; and

(b) determine a distance between the transmitter and the supplemental receiver by multiplying the average velocity of the signal by the propagation time determined in (a).

32. A system as claimed in claim 31 wherein the supplemental receiver is located in a known position relative to the surface.

33. A system as claimed in claim 30 wherein the controller is configured to determine the average velocity of the signal by multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver.

34. A system as claimed in claim 30 wherein the controller is configured to determine the average velocity of the signal by iteratively solving for the average velocity by using a starting velocity value of between 5 to 8 km/sec.

35. A system as claimed in claim 30 wherein the controller is configured to determine the propagation time of the signal by:

36. A system as claimed in claim 30 wherein the controller is configured to determine the propagation time of the signal by: (a) sending a pilot signal from the transmitter to a controller simultaneously with transmitting the acoustic signal from the transmitter to the at least three receivers, the controller in data communication with the transmitter and with the at least three receivers;

(b) recording the time at which the pilot signal arrives at the controller; and

37. A system as claimed in claim 30 wherein the transmitter transmits the signal using a spread pattern.

38. A system as claimed in claim 30 wherein the receiver is a fiber optic receiver.

39. A system as claimed in claim 24 wherein the transmitter transmits to at least four acoustic receivers, at least one of which is located in a known position relative to the surface, and wherein the controller is configured to determine the position of the transmitter located downhole by:

(b) determining an average velocity of the signal as it propagates from the transmitter to the receivers; and (c) determining the position of the transmitter from the average velocity of the signal and the differences between when the signal arrives at each of three of the receivers and the fourth receiver.

40. A system as claimed in claim 39 wherein the controller is further configured to:

(a) determine a propagation time of the signal as it propagates from the transmitter to a supplemental receiver; and

41. A system as claimed in claim 39 wherein the controller is further configured to determine the average velocity of the signal by multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver.

42. A system as claimed in claim 39 wherein the controller is further configured to determine the average velocity of the signal by iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

43. A system as claimed in claim 39 wherein the controller is configured to determine the propagation time of the signal by:

(a) synchronizing the transmitter and receivers in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time; (b) for each of the at least three receivers, determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the known start time.

44. A system as claimed in claim 39 wherein the controller is configured to determine the propagation time of the signal by:

(b) recording the time at which the pilot signal arrives at the controller; and

45. A system as claimed in claim 39 wherein the transmitter transmits the signal using a spread pattern.

46. A system as claimed in claim 39 wherein the receiver is a fiber optic receiver.

47. A method for drilling a well pair comprising a first borehole and a second borehole, the method comprising:

(a) drilling the first borehole;

(b) commencing drilling of the second borehole; (c) locating both an acoustic transmitter and an acoustic receiver in the second borehole;

(d) transmitting an acoustic signal from the transmitter to the receiver by reflecting the acoustic signal off the first borehole;

(e) determining an average velocity of the signal as it propagates from the transmitter to the receiver;

(f) determining the propagation time of the signal as it travels from the transmitter to the receiver;

(g) determining the distance between the first borehole and the second borehole from the average velocity of the signal and the propagation time of the signal; and

(h) extending the second borehole while maintaining a suitable distance between the first borehole and the second borehole.

48. A method as claimed in claim 47 wherein the transmitter is located on a transmission sub and wherein the receiver is located on a receiver sub.

49. A method as claimed in claim 47 wherein determining the average velocity of the signal comprises multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver.

50. A method as claimed in claim 47 wherein determining the average velocity of the signal comprises iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

51. A method as claimed in claim 47 wherein determining the propagation time of the signal comprises: (a) synchronizing the transmitter and receiver in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time; and

(b) determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the known start time.

52. A method as claimed in claim 47 wherein at least three receivers are utilized, and wherein determining the propagation time of the signal comprises:

(b) recording the time at which the pilot signal arrives at the controller; and

(c) determining the propagation time of the signal by subtracting the time at which the signal arrives at the receiver from the time at which the pilot signal arrives at the controller.

53. A method as claimed in claim 52 wherein the signal is transmitted using a spread pattern.

54. A system for drilling a well pair comprising a first borehole and a second borehole, the system comprising:

(a) an acoustic transmitter disposed in one of the first and second boreholes; (b) an acoustic receiver disposed in the same borehole as the acoustic transmitter and located to receive a reflected acoustic signal that is transmitted from the acoustic transmitter and reflected off of the other of the first and second boreholes;

(c) a controller, in communication with at least the receiver, configured to:

(i) determine an average velocity of the signal as it propagates from the transmitter to the receiver;

(ϋ) determine the propagation time of the signal as it travels from the transmitter to the receiver; and

(iϋ) determine the distance between the first borehole and the second borehole from the average velocity of the signal and the propagation time of the signal.

55. A system as claimed in claim 54 wherein the transmitter and the receiver are respectively located on a transmission sub and a receiver sub.

56. A system as claimed in claim 54 wherein the controller is configured to determine the average velocity of the signal by multiplying the frequency of the signal by the wavelength of the signal as measured at the receiver.

57. A system as claimed in claim 54 wherein the controller is configured to determine the average velocity of the signal by iteratively solving for the average velocity using a starting velocity value of between 5 to 8 km/sec.

58. A system as claimed in claim 54 wherein the controller is configured to determine the propagation time of the signal by: (a) synchronizing the transmitter and receiver in time prior to transmission of the acoustic signal such that the acoustic signal is transmitted at a known start time; and

59. A system as claimed in claim 54 wherein at least three receivers are utilized, and wherein the controller is configured to determine the propagation time of the signal by:

(b) recording the time at which the pilot signal arrives at the controller; and

60. A system as claimed in claim 54 wherein the transmitter transmits the signal using a spread pattern.

61. A method for reducing uncertainty in the position of a drill bit in a first borehole, the method comprising:

(a) locating an acoustic transmitter or an acoustic receiver downhole in the first borehole in a known position relative to the drill bit;

"" JJ "" (b) locating the other of the transmitter or the receiver in a second borehole;

(c) obtaining an approximate position of the drill bit using a conventional measurement while drilling (MWD) system, the approximate position of the drill bit having a known maximum uncertainty;

(d) transmitting an acoustic signal from the transmitter to the receiver;

(e) determining a propagation time of the signal as it propagates from the transmitter to the receiver;

(f) determining an average velocity of the signal as it propagates from the transmitter to the receiver;

(g) determining a distance travelled by the signal by multiplying the propagation time by the average velocity; and

(h) reducing the known maximum uncertainty by using the distance calculated in (g).

62. A method as claimed in claim 61 wherein the drill bit is constrained to move only substantially vertically.

63. A method as claimed in claim 61 wherein the other of the transmitter or the receiver in the second borehole is in a known position relative to the surface.

64. A system for reducing uncertainty in the position of a drill bit in a first borehole, the system comprising:

(a) a drill bit located downhole in the first borehole; (b) a measurement while drilling (MWD) system coupled to the drill bit, the MWD system generating an approximate position of the drill bit having a known maximum uncertainty;

(c) one of an acoustic transmitter or an acoustic receiver located in a known position relative to the drill bit, the transmitter configured to transmit an acoustic signal to the receiver;

(d) the other of the transmitter or the receiver located in a second borehole; and

(e) a controller, in communication with at least the receiver, configured to:

(i) determine a propagation time of the signal as it propagates from the transmitter to the receiver;

(ii) determine an average velocity of the signal as it propagates from the transmitter to the receiver;

(iii) determine a distance travelled by the signal by multiplying the propagation time by the average velocity; and

(iv) reduce the known maximum uncertainty by using the distance calculated in (iii).

65. A system as claimed in claim 64 wherein the drill bit is constrained to move only substantially vertically.

66. A system as claimed in claim 64 wherein the other of the transmitter or the receiver in the second borehole is in a known position relative to the surface.