IMPROVED CASING DETECTION TOOLS AND METHODS
BACKGROUND
The world depends on hydrocarbons to solve many of its energy needs. Consequently, oil
field operators strive to produce and sell hydrocarbons as efficiently as possible. Much of the
easily obtainable oil has already been produced, so new techniques are being developed to
extract less accessible hydrocarbons. These techniques often involve drilling a borehole in close
proximity to one or more existing wells. One such technique is steam-assisted gravity drainage
("SAGD") as described in U.S. Patent 6,257,334, "Steam-Assisted Gravity Drainage Heavy Oil
Recovery Process". SAGD uses a pair of vertically-spaced, horizontal wells less than 10 meters
apart, and careful control of the spacing is important to the technique's effectiveness. Other
examples of directed drilling near an existing well include intersection for blowout control,
multiple wells drilled from an offshore platform, and closely spaced wells for geothermal energy
recovery.
One way to direct a borehole in close proximity to a cased well is through the use of
electromagnetic (EM) logging tools. EM logging tools are capable of measuring a variety of
formation parameters including resistivity, bed boundaries, formation anisotropy, and dip angle.
Because such tools are typically designed for measuring such parameters, their application to
casing detection may be adversely impacted by their sensitivity to such environmental
parameters. Specifically, the tool's response to nearby casing can be hidden by the tool's
response to various environmental parameters, making it impossible to detect and track a cased
well, or conversely making the tool produce false detection signals that could deceive the drilling
team into believing they are tracking a nearby cased well when such is not the case. Such
difficulties do not appear to have been previously recognized or adequately addressed.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the various disclosed system and method embodiments can be
obtained when the following detailed description is considered in conjunction with the drawings,
in which:
Fig. 1 shows an illustrative drilling environment in which electromagnetically-guided
drilling may be employed;
Fig. 2 is an illustrative tilted antenna system with parallel and perpendicular transmitterreceiver
pairs;
Fig. 3 is an illustrative two-layered formation model;
Figs. 4A and 4B are modeled tool responses to formation anisotropy as a function of
frequency and dip angle;
Figs. 5A and 5B are modeled tool responses to a nearby boundary as a function of
boundary distance and dip angle;
Figs. 6A and 6B are modeled tool responses to a nearby boundary as a function of
frequency and dip angle;
Figs. 7A and 7B are experimental 44" tool responses to a nearby casing as a function of
casing distance and frequency;
Figs. 8A and 8B are experimental 52" tool responses to a nearby casing as a function of
casing distance and frequency;
Figs. 9A and 9B are experimental tool responses to a nearby casing as a function of
casing distance and antenna spacing;
Fig. 10 shows a tool model that serves as a basis for a casing sensitivity calculation;
Fig. 11A shows tool sensitivity as a function of antenna spacing and frequency;
Fig. 1IB shows tool signal levels as a function of antenna spacing and frequency;
Figs. 12A and 12B are signal responses of a parallel and perpendicular transmitterreceiver
pair, respectively, as a function of antenna spacing and frequency; and
Figs. 13A and 13B are modeled 50' tool responses as a function of casing distance and
dip angle; and
Fig. 14 is a flow diagram of an illustrative casing detection method.
While the invention is susceptible to various alternative forms, equivalents, and
modifications, specific embodiments thereof are shown by way of example in the drawings and
will herein be described in detail. It should be understood, however, that the drawings and
detailed description thereto do not limit the disclosure, but on the contrary, they provide the
foundation for supporting all alternative forms, equivalents, and modifications falling within the
scope of the appended claims.
DETAILED DESCRIPTION
The issues identified in the background are at least in part addressed by the disclosed
casing detection tools and methods. At least one disclosed method embodiment includes
obtaining formation resistivity measurements from a first borehole. Based at least in part on
these measurements, an expected environmental signal level is determined for a second borehole
at a specified position relative to the first borehole. At least one of a transmitter-receiver spacing
and an operating frequency is then selected to provide a desired detection signal level for the first
borehole from the second borehole, such that the desired detection signal level will be greater
than the expected environmental signal level, and a bottomhole assembly (BHA) is constructed
with a tilted antenna logging tool having the selected spacing and/or operating frequency for use
in the second borehole.
At least one disclosed tool embodiment includes a tilted transmit antenna and two or
more tilted receive antennas at least a selected spacing distance from the transmit antenna to
detect components of a response to the transmit signal. The transmit signal has a frequency at or
below a selected operating frequency, the frequency being selected in conjunction with the
spacing to ensure that the expected casing detection signal level is greater than an expected
environmental signal level.
To further assist the reader's understanding of the disclosed systems and methods, we
describe an environment suitable for their use and operation. Accordingly, Fig. 1 shows an
illustrative geosteering environment. A drilling platform 2 supports a derrick 4 having a traveling
block 6 for raising and lowering a drill string 8. A top drive 10 supports and rotates the drill
string 8 as it is lowered through the wellhead 12. A drill bit 14 is driven by a downhole motor
and/or rotation of the drill string 8. As bit 14 rotates, it creates a borehole 16 that passes through
various formations. A pump 20 circulates drilling fluid through a feed pipe 22 to top drive 10,
downhole through the interior of drill string 8, through orifices in drill bit 14, back to the surface
via the annulus around drill string 8, and into a retention pit 24. The drilling fluid transports
cuttings from the borehole into the pit 24 and aids in maintaining the borehole integrity.
The drill bit 14 is just one piece of a bottom-hole assembly that includes one or more
drill collars (thick-walled steel pipe) to provide weight and rigidity to aid the drilling process.
Some of these drill collars include logging instruments to gather measurements of various
drilling parameters such as position, orientation, weight-on-bit, borehole diameter, etc. The tool
orientation may be specified in terms of a tool face angle (a.k.a. rotational or azimuthal
orientation), an inclination angle (the slope), and a compass direction, each of which can be
derived from measurements by magnetometers, inclinometers, and/or accelerometers, though
other sensor types such as gyroscopes may alternatively be used. In one specific embodiment, the
tool includes a 3-axis fluxgate magnetometer and a 3-axis accelerometer. As is known in the art,
the combination of those two sensor systems enables the measurement of the tool face angle,
inclination angle, and compass direction. In some embodiments, the tool face and hole
inclination angles are calculated from the accelerometer sensor output. The magnetometer sensor
outputs are used to calculate the compass direction.
The bottom-hole assembly further includes a ranging tool 26 to induce a current in
nearby conductors such as pipes, casing strings, and conductive formations and to collect
measurements of the resulting field to determine distance and direction. Using these
measurements in combination with the tool orientation measurements, the driller can, for
example, steer the drill bit 14 along a desired path 18 relative to the existing well 19 in formation
46 using any one of various suitable directional drilling systems, including steering vanes, a
"bent sub", and a rotary steerable system. For precision steering, the steering vanes may be the
most desirable steering mechanism. The steering mechanism can be alternatively controlled
downhole, with a downhole controller programmed to follow the existing borehole 19 at a
predetermined distance 48 and position (e.g., directly above or below the existing borehole).
A telemetry sub 28 coupled to the downhole tools (including ranging tool 26) can
transmit telemetry data to the surface via mud pulse telemetry. A transmitter in the telemetry sub
28 modulates a resistance to drilling fluid flow to generate pressure pulses that propagate along
the fluid stream at the speed of sound to the surface. One or more pressure transducers 30, 32
convert the pressure signal into electrical signal(s) for a signal digitizer 34. Note that other forms
of telemetry exist and may be used to communicate signals from downhole to the digitizer. Such
telemetry may employ acoustic telemetry, electromagnetic telemetry, or telemetry via wired
drillpipe.
The digitizer 34 supplies a digital form of the telemetry signals via a communications
link 36 to a computer 38 or some other form of a data processing device. Computer 38 operates
in accordance with software (which may be stored on information storage media 40) and user
input via an input device 42 to process and decode the received signals. The resulting telemetry
data may be further analyzed and processed by computer 38 to generate a display of useful
information on a computer monitor 44 or some other form of a display device. For example, a
driller could employ this system to obtain and monitor drilling parameters, formation properties,
and the path of the borehole relative to the existing borehole 19 and any detected formation
boundaries. A downlink channel can then be used to transmit steering commands from the
surface to the bottom-hole assembly.
Fig. 2 shows an illustrative antenna configuration for ranging tool 26. This particular
antenna configuration is used below as a specific example for explaining the relative effects of
environmental parameters as contrasted with a nearby casing string, but the conclusions are
applicable to nearly all electromagnetic logging tools having at least one tilted antenna.
Accordingly, the following discussion is not limiting on the scope of the disclosure. The
illustrated configuration includes two transmit antennas (labeled Tup and Tdn) and a receive
antenna (labeled Rx) midway between the two. Each of the antennas is tilted at 45° from the
longitudinal axis of the tool, such that the receive antenna is parallel to one transmit antenna and
perpendicular to the other. The centers of the antennas are equally spaced, with d being the
distance between the receiver and each transmit antenna. As the tool rotates, the transmitters fire
alternately and the receive signals detected by the receiver in response the transmitters Tup and
Tdn are v b ) a n d n b ), respectively, where is tool's azimuthal angle. The tool's
responses to a nearby casing string, a nearby fluid interface or bed boundary, or to an anisotropic
dipping formation, is expected to take the following form:
V b ) = cos(2 )+ B cos( )+
b ) = cos(2^)+ B cos( )+ C2
where Ai, Bi, and are complex coefficients representing the voltage amplitude of azimuthaldependent
double-period sine wave, a single-period sine wave, and a constant value for the
receiver's response to the upper transmitter (z=l) or lower transmitter (i=2). Using a curve fitting
function, the three complex voltage amplitudes for each response can be derived from the raw
measured signal voltages in a straightforward manner. Experiments indicate that when the
coefficients for the tool's response to a nearby casing string are compared to coefficients for the
tool's response to environmental parameters, the Ai coefficient for the casing string response has
a larger magnitude than the Bi coefficient, while for responses to environmental parameters the
reverse is generally true. Indeed, the Bi coefficient for the casing string response has been found
to be relatively small compared to the Ai coefficient. Accordingly, the proposed casing detection
tool preferably employs the Ai coefficient for detection and ranging measurements. Temperature
compensation and voltage normalization can be accomplished by using the ratio and it has
been found useful to employ a logarithm of this ratio, e.g., Iog10 ( C , when modeling the
tool's operation.
Three representative models will be employed to analyze the tool's response to (1)
formation anisotropy; (2) a nearby boundary; and (3) a casing string. Fig. 3A shows a first model
in which a tool is positioned in a relatively thick dipping formation having resistive anisotropy.
The horizontal resistivity (Rx and Ry) is taken as 1 Wh , while the vertical resistivity (Rz) is
taken as 2 Wh . Fig. 3B shows a second model in which the tool is in a resistive formation
(Rt=200 Qm) and is approaching a boundary with a more conductive formation (Rt=l ). The
tool's distance to the bed boundary (DTBB) is measured from the receive antenna to the closest
point on the boundary. Fig. 3C shows a third model in which the tool is positioned at a distance d
from a casing string in an otherwise homogeneous formation.
The tool's responses to each of these three models are compared, beginning with the
anisotropy model. Fig. 4A shows the measurements by the parallel transmit-receive antenna pair
(hereafter the "parallel response") with a 52 inch spacing between the antennas, while Fig. 4B
shows the measurements by the perpendicular transmit-receive antenna pair with the same
spacing. In both cases, the measurements are shown as a function of dip angle and transmit
signal frequency. The measurements are shown in terms of the logarithm of the coefficient ratio,
i.e., Iogl 0( C . Generally speaking, a stronger anisotropy response is observed at higher
signal frequencies. Moreover, the tool measurements are fairly steady at dips of greater than 10
degrees, but they fall off sharply at smaller dip angles as the model becomes more symmetric
about the tool axis.
Figs. 5A and 5B show the tool's parallel and perpendicular responses to a nearby bed
boundary as a function of dip angle and boundary distance. For these graphs, the tool is assumed
to have an antenna spacing of 52 inches and a signal frequency of 125 kHz. The tool's response
grows stronger as the distance to bed boundary shrinks, and the signal remains fairly steady so
long as the dip angles are greater than about 10 degrees. Below this, the model symmetry
increases and the measurements drop sharply. The nearby bed boundary measurements are also
shown in Figs. 6A and 6B as a function of signal frequency, confirming again that the tool
response increases as a function of frequency, though less dramatically than in the first model.
Figs. 7A and 7B show the tool's parallel and perpendicular responses to a nearby well
casing as a function of casing distance and signal frequency, assuming a 44 inch antenna
spacing. Figs. 8A and 8B show the expected responses for a tool having a 52 inch antenna
spacing. These responses represent actual measurements obtained via a water tank experiment in
which the tank was filled with 1 W-m water to represent a homogeneous isotropic formation. The
tool was positioned in the center of the tank and a casing tubular was positioned parallel to the
tool at a distance that could be varied as desired from 0.85 feet to 6 foot. These figures suggest
that signal strength increases as signal frequency decreases. Even though this trend is not
monotonic and it reverses slightly at lower signal frequencies (see Figs. 12A-12B), the
discrimination between the tool's response to casing and the tool's response to other
environmental factors is expected to improve as the signal frequency is reduced. Significantly,
the use of lower signal frequencies also enables feasible tool operation at increased antenna
spacings.
Figs. 9A and 9B show the parallel and perpendicular responses of the tool as a function
of casing distance for different antenna spacings, assuming a signal frequency of 500 kHz. From
this graph it can be observed that the tool's response to signal strength increases with antenna
spacing. A comparison of the tool's responses to each of the models reveals that a casing
detection tool would benefit from using a lower tool operating frequency and/or longer spacing
between tool's transmitter and receiver, as this increases the tool's sensitivity to nearby casing
and simultaneously decreasing the tool's response to formation anisotropy and nearby shoulder
beds.
On the other hand, reducing frequency also raises a couple of issues. First of all, lower
frequency reduces the signal amplitude received at tool's receiver when other specifications of
the tool are consistent (same spacing, same antenna design, etc.). Noise level or signal-to noise
ratio will be a challenging issue for very weak signal amplitude. Secondly, the majority of
received signal at a receiver is the direct signal transmitted directly from the transmitter to the
receiver if operated at low frequency. Processing schemes to determine a casing nearby the tool
may fail if direct signal is much stronger than signal from casing. In summary, it would be
beneficial to reduce operating frequency for a nearby casing detection, but different formation
resistivity and different casing distance to the tool define the optimized operating frequency as
well as the optimized spacing between transmitter and receiver.
To better quantify considerations that may go into an optimization analysis, we take as
an example an electromagnetic logging tool located in a homogeneous isotropic formation with
resistivity of 50W·h with a parallel casing string at a distance of 10 feet, as indicated in Fig. 10.
The tool's sensitivity to the casing can be characterized by measuring the relative strength of the
signal attributable to the casing. The casing signal is maximized when the antennas are oriented
along the y-axis as shown in Fig. 10, as this orientation induces the maximum current flow in the
casing and provides the maximum sensitivity to the fields induced by this current flow. The
complex amplitude of the signal component measured by this trasmitter and receiver orientation
is herein referred to as Vy
y . The tool sensitivity can then be expressed by comparing the relative
strength of the modeled signal (Vy
y) in the presence and absence of the casing:
Casing Sig l Casin Sensitivity = 100 (%)
Signal^ Casing
(2)
Fig. 11A shows this sensitivity as a function of antenna spacing and signal frequency. The
unsealed signal amplitude with casing is shown in Fig. 11B, again as a function of
antenna spacing and signal frequency. The tool designer may employ these figures in
conjunction with Figs. 12A and 12B, which show modeled responses of loglO(A/C) for the
parallel Tx-Rx antenna pair and perpendicular Tx-Rx antenna pair shown in Fig. 2, for the same
range of signal frequencies and antenna spacings of Figs. 11A and 11B. Collectively, these
figures can be used by the tool designers to select an optimized frequency and antenna spacing
to implement an EM tool customized for a nearby casing detection range of 10 feet in a
formation having 50 W m resistivity.
For example, Fig. 11A shows that a sensitivity of 100% can be obtained with, e.g., a
transmit signal frequency of 100kHz and an antenna spacing on the order of 35 feet; a transmit
signal frequency of 10kHz and an antenna spacing on the order of 40 feet; and a transmit signal
frequency of 1kHz with an antenna spacing on the order of 50 feet. Fig. 11B shows that the
amplitude of the signal component attributable to the casing is about -4.2, -5.5, and -6.8,
respectively, for these values, which are all acceptably strong enough. Transporting these values
(100kHz with 35 feet, 10kHz with 40 feet, and 1kHz with 50 feet) to Figs. 12A and 12B, the
designer observes that the scaled tool responses are expected to be in excess of -0.5.
Since the formation resistivity is assumed to be relatively high (50 W-m), formation
anisotropy effects will be negligible compared to shoulder bed effects. The designer estimates
the shoulder bed response with selected tool parameters. Figs. 13A and 13B show modeled
shoulder bed responses where a tool having a 50 foot antenna spacing and a transmit signal
frequency of 1kHz is positioned in a 50 W m at some distance from the boundary with a 1 W m
formation. The response is shown as a function of bed boundary distance and dip. Figs. 13A and
13B indicate that the highest bed boundary signal of loglO(A/C) is less than -1, which confirms
the tool is able to accurately determine a parallel casing 10 feet away from the tool in 50 W-m
formation without considerations of other formation effects, such as anisotropy and/or shoulder
beds.
Fig. 14 is a flow diagram of an illustrative casing detection method. The illustrative
method begins by obtaining resistivity measurements from a first borehole, as shown in block
1002. This first borehole is then cased or otherwise made conductive (e.g., by filling it with a
conductive fluid). In situations where a cased well already exists and its resistivity logs are
unavailable, the resistivity of the formation around the cased well may be estimated based on
other information such as remote wells, seismic surveys, and reservoir models. The resistivity
data for the formation containing the first borehole may then be employed in block 1004 to
predict environmental signals levels that would be encountered by a second borehole drilled near
the first. Based on the resistivity measurements, a modeled tool response to environmental
effects such as resistive anisotropy and nearby formation bed boundaries or fluid interfaces can
be determined along the length of a second borehole path as a function of antenna spacing and
transmit signal frequency.
The resistivity data may be further employed in block 1006 to model the tool's response
signal level to casing as a function of antenna spacing and operating frequency. An upper limit
on the desired casing detection range may be used as part of the modeling process. In block
1008, the casing response may be compared to the environmental signal levels to determine a
range of acceptable antenna spacings and a range of suitable operating frequencies. The range
may be determined to be a combination of spacing and frequency that provides a casing signal
greater than the anticipated environmental signal response, and in some cases at least an order of
magnitude greater. Such significant disparity would enable casing ranging measurements to be
made while neglecting environmental signal responses. In block 1010 a tilted antenna tool is
provided with an antenna spacing and operating frequency from the range of suitable values. The
selected values may be based upon available tools or feasible tool configurations. For example,
the available tool hardware may require some minimum required receive signal strength to
assure adequate receiver response, and this factor may prevent certain combinations of antenna
spacing and signal frequency from being chosen. As another example, some tilted antenna tools
may have a modular construction in which the transmit module can be spaced at a variable
distance from the receive module, thereby providing for a reconfigurable antenna spacing within
certain limits. Or the available tilted antenna tools may have a programmable operating
frequency range or they may employ multiple operating frequencies including at least one in the
designated operating range.
These and other variations and modifications will become apparent to those skilled in
the art once the above disclosure is fully appreciated. It is intended that the following claims be
interpreted to embrace all such variations and modifications.
CLAIMS
WHAT IS CLAIMED IS:
1. A downhole logging method that comprises:
obtaining formation resistivity measurements from a first borehole;
determining an expected environmental signal level for a second borehole at a specified
position relative to the first borehole, based at least in part on the formation resistivity
measurements;
selecting at least one of a transmitter-receiver spacing and an operating frequency to provide a
desired detection signal level for the first borehole from the second borehole, the desired
detection signal level being greater than the expected environmental signal level; and
providing a tilted antenna logging tool having the selected spacing and/or operating frequency
in a bottomhole assembly for the second borehole.
2. The method of claim 1, wherein said desired detection level is less than ten times said
expected environmental signal level.
3. The method of claim 1, wherein said first borehole is cased before the drilling of said second
borehole.
4. The method of claim 1, wherein said tilted antenna logging tool comprises antenna modules
that can be separated by a variable number of intervening subs.
5. The method of claim 1, wherein said tilted antenna logging tool has a programmable operating
frequency.
6. The method of claim 1, wherein said expected environmental signal level includes an
azimuthal signal dependence attributable to formation anisotropy.
7. The method of claim 1, wherein said expected environmental signal level includes an
azimuthal signal dependence attributable to a formation fluid interface or a bed boundary.
8. The method of claim 1, wherein said expected environmental signal level includes an
azimuthal signal dependence attributable to a borehole effect.
9. The method of claim 1, wherein said determining an expected environmental signal level
includes generating a model response based on a tentative transmitter-receiver spacing and
operating frequency.
10. The method of claim 9, wherein said selecting includes:
finding a model response for a casing detection signal based on the tentative transmitterreceiver
spacing and operating frequency; and
systematically varying the tentative transmitter-receiver spacing and operating frequency until
the modeled casing detection signal exceeds the modeled environmental signal level.
11.A casing detection tool designed for use in a high resistivity formation, the tool having:
at least a tilted transmitter antenna that emits a transmit signal; and
at least a two or more tilted receiver antennas that detect components of an induced magnetic
field,
wherein the receiver antennas are at least a selected spacing distance from said transmitter
antenna, and
wherein said transmit signal has at least one frequency component at or below a selected
operating frequency, said selected spacing distance and operating frequency providing an
expected casing detection signal level greater than an expected environmental signal level.
12. The tool of claim 11, wherein said expected environmental signal level includes at least one
of a dependence on formation anisotropy, a dependence on a formation fluid interface, a
dependence on a bed boundary, and a dependence on a borehole effect.
13. The tool of claim 11, wherein said expected casing detection signal level is based on a
specified detection range and a formation resistivity.
14. The tool of claim 11, wherein said selected spacing distance is greater than about 35 feet and
the selected operating frequency is below about 100 kHz.
15. The tool of claim 14, wherein said selected spacing distance is greater than about 40 feet and
the selected operating frequency is below about 10 kHz.
16. The tool of claim 15, wherein said selected spacing distance is greater than about 50 feet and
the selected operating frequency is below about 1 kHz.
17. The tool of claim 11, wherein said transmit signal has a programmable operating frequency.
18. The tool of claim 17, wherein said casing detection tool has a number of intermediate subs
between the transmitter antenna and at least one receiver antenna, wherein the number is variable
to provide at least the selected spacing distance.
19. The tool of claim 11, further comprising a processor that collects measurements at multiple
transmitter-receiver spacings.