U.S. patent application number 14/016134 was filed with the patent office on 2014-01-02 for method for gas zone detection using sonic wave attributes.
This patent application is currently assigned to Schlumberger Technology Corporation. The applicant listed for this patent is Schlumberger Technology Corporation. Invention is credited to Pierre Henri Campanac, Alain Dumont, David Linton Johnson, Peter T. Wu.
Application Number | 20140003190 14/016134 |
Document ID | / |
Family ID | 40551990 |
Filed Date | 2014-01-02 |
United States Patent
Application |
20140003190 |
Kind Code |
A1 |
Wu; Peter T. ; et
al. |
January 2, 2014 |
Method for Gas Zone Detection Using Sonic Wave Attributes
Abstract
A method for determining on a real time logging while drilling
(LWD) basis gas within earth formations traversed by a borehole.
Continuous LWD acoustic measurements are recorded and processed
including coherent energy and attenuation attributes to detect
downhole gas zones and kick during drilling operations.
Inventors: |
Wu; Peter T.; (Missouri
City, TX) ; Dumont; Alain; (Kawasaki-Shi, JP)
; Campanac; Pierre Henri; (Sugar Land, TX) ;
Johnson; David Linton; (Somerville, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schlumberger Technology Corporation |
Sugar Land |
TX |
US |
|
|
Assignee: |
Schlumberger Technology
Corporation
Sugar Land
TX
|
Family ID: |
40551990 |
Appl. No.: |
14/016134 |
Filed: |
September 2, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11964731 |
Dec 27, 2007 |
8547789 |
|
|
14016134 |
|
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Current U.S.
Class: |
367/26 ;
367/35 |
Current CPC
Class: |
G01V 1/40 20130101; G01V
1/48 20130101 |
Class at
Publication: |
367/26 ;
367/35 |
International
Class: |
G01V 1/40 20060101
G01V001/40 |
Claims
1. A method for determining the presence of a formation gas kick on
a real time basis by a logging while drilling process comprising:
emitting periodic sonic wave energy toward a borehole formation by
a logging while drilling tool; receiving Stoneley waveform signals
transmitted within the borehole; determining the lithology of
formations as drilling progresses; determining the Stoneley wave
slowness (DTst) within the well bore; and for an essentially
uniform formation lithology determining the presence of gas kick
within the borehole by a sudden change at different depths in
Stoneley slowness (DTst) over formations with essentially similar
lithology.
2. A method for determining the presence of a formation gas kick on
a real time basis by a logging while drilling process as defined in
claim 1 and further comprising: determining the Stoneley wave
attenuation (ATTst) within the well bore; and for an essentially
uniform formation lithology confirming the presence of a gas kick
within the borehole by a sudden increase in Stoneley attenuation
(ATTst).
3. A method for determining the presence of formation gas kick on a
real time basis by a logging while drilling process as defined in
claim 1 and further setting as gas kick flag comprising: detecting
a change in the Stoneley slowness (DTst); and setting a kick flag
to warn a driller of the drilling operation encountering a
formation gas kick in the event the detected change in (DTst)
exceeds a set value.
4. A method for determining the presence of a formation gas kick on
a real time basis by a logging while drilling process as defined in
claim 1 and further comprising: determining the Stoneley wave
coherent energy (CEst) within the well bore; and for an essentially
uniform formation lithology confirming the presence of a gas kick
within the borehole by a sudden decrease in Stoneley coherent
energy (CEst).
5. A method for determining the presence of a formation gas kick on
a real time basis by a logging while drilling process as defined in
claim 4 and further comprising: detecting a change in one or more
Stoneley attributes of (DTst, ATTst and CEst) and prior to setting
a kick detection flag assigning a weighting factor to one or more
of the Stoneley attributes (DTst, ATTst and CEst).
6. A method for determining the presence of a formation gas kick on
a real time basis by a logging while drilling process comprising:
emitting periodic sonic wave energy toward a borehole formation by
a logging while drilling tool; receiving Stoneley waveform signals
transmitted within the borehole; determining the lithology of
formations as drilling progresses; determining the Stoneley wave
attenuation (ATTst) within the well bore from the attributes of
received coherent Stoneley waveform signal peaks; and for an
essentially uniform formation lithology determining the presence of
gas kick within the borehole by a sudden change at different depths
in Stoneley wave attenuation (ATTst) over formations with
essentially similar lithology.
7. A method for determining the presence of a formation gas kick on
a real time basis by a logging while drilling process as defined in
claim 6 and further comprising: determining the Stoneley wave
coherent energy (CEst) within the well bore; and for an essentially
uniform formation lithology confirming the presence of a gas kick
within the borehole by a sudden decrease in Stoneley coherent
energy (CEst).
8. A method for determining the presence of formation gas kick on a
real time basis by a logging while drilling process as defined in
claim 6 and further setting as gas kick flag comprising: detecting
a change in the Stoneley attenuation (ATTst); and setting a kick
flag to warn a driller of the drilling operation encountering a
formation gas kick in the event the detected change in (ATTst)
exceeds a set value.
9. A method for determining the presence of a formation gas kick on
a real time basis by a logging while drilling process comprising:
emitting periodic sonic wave energy toward a borehole formation by
a logging while drilling tool; receiving Stoneley waveform signals
transmitted within the borehole; determining the lithology of
formations as drilling progresses; determining the Stoneley wave
coherent energy (CEst) within the well bore; and for an essentially
uniform formation lithology determining the presence of gas kick
within the borehole by a sudden change in Stoneley coherent energy
(CEst) over formations with essentially similar lithology.
10. A method for determining the presence of formation gas kick on
a real time basis by a logging while drilling process as defined in
claim 9 and further setting as gas kick flag comprising: detecting
a change in the Stoneley coherent energy (CEst); and setting a kick
flag to warn a driller of the drilling operation encountering a
formation gas kick in the event the detected change in (CEst)
exceeds a set value.
Description
TECHNICAL FIELD
[0001] This application is a continuation application of U.S.
patent application Ser. No. 11/964,731, filed Dec. 27, 2007.
[0002] This invention relates to wireline and
logging-while-drilling measurement of sonic wave component
attributes and use of that information for determining gas zones
within a formation and/or kick detection. More specifically, this
invention is directed to determining traditional compressional
slowness (DTc), shear slowness (DTs) and Stoneley slowness (DTst)
and in addition determining attributes of coherent energy (CE) and
attenuation (ATT) for use in detecting the real time presence of
gas in a formation and/or kick detection.
BACKGROUND OF THE INVENTION
[0003] In the oil and gas industry acoustic tools are used to
provide operationally significant information about borehole and
formation attributes adjacent the tools such as compressional,
shear and Stoneley slowness. These attributes are analyzed for
determining, inter alia, the rate of flow of a hydrocarbon (gas or
oil) out of a producing borehole in the hydrocarbon production
industry. This critical information fundamentally depends on
permeability of the formation, viscosity of the hydrocarbon and the
existence of fractures. Collecting and recording this information
on a delayed or real time basis is known as well logging.
[0004] Evaluation of physical properties such as pressure,
temperature and wellbore trajectory in three-dimensional space and
other borehole characteristics while extending a wellbore is known
as measurements-while-drilling (MWD) and is standard practice in
many drilling operations. MWD tools that measure formation
parameters such as resistivity, porosity, sonic velocity, gamma
ray, etc. of a formation are known as logging-while-drilling (LWD)
tools. An essential formation parameter for determination in a
drilling operation is the existence of gas deposits or zones in a
formation, on a real time basis. Similarly, early detection of kick
is essential information for conducting safe and efficient drilling
operations.
[0005] For the above and other reasons, the oil industry has
developed acoustic well logging techniques that involve placing an
acoustic tool within a well bore to make measurements indicative of
formation attributes such as compressional slowness (DTc), shear
slowness (DTs) and Stoneley slowness (DTst). Sonic logs can be used
as direct indications of subsurface properties and in combination
with other logs and knowledge of subsurface properties can be used
to determine subsurface parameters, such as those related to
borehole structure stability, that can not be measured directly.
Early efforts in this connection were reported by Rosenbaum in
"Synthetic Microseismograms: Logging in Porous Formations",
Geophysics, Vol. 39, No. 1, (February 1974) the disclosure of which
is incorporated by reference as though set forth at length.
[0006] Acoustic logging tools typically include a transmitter and
an array of axially spaced acoustic detectors or receivers. These
tools are operable to detect, as examples, formation compressional
waves (P), formation shear waves (S) and Stoneley waves. These
measurements can be performed following drilling or intermediate
drill string trips by wireline logging operations. In wireline
logging, sonic monopole tools can be used to measure compression
waves (P) and shear waves (S) in fast formations. In addition to
wireline logging, techniques have been developed where
piezoelectric transmitters and hydrophone receivers are imbedded
within the walls of drill string segments so that sonic LWD
operations can be performed.
[0007] Early wireline and LWD and sonic data processing techniques
developed by the Schlumberger Technology Corporation such as a
slowness-time-coherence (STC) method is disclosed in U.S. Pat. No.
4,594,691 to Kimball et al. entitled "Sonic Well Logging" as well
as in Kimball et al. "Semblance Processing of Borehole Acoustic
Array Data," Geophysics, Vol. 49, No. 3 (March 1984). This method
is most useful for non-dispersive waveforms (e.g. monopole
compressional and shear head waves). For processing dispersive
waveforms a dispersive slowness-time-coherence (DSTC) is preferred.
This process is disclosed in U.S. Pat. No. 5,278,805 to Kimball
entitled "Sonic Well Logging Methods and Apparatus Utilizing
Dispersive Wave Processing." The disclosures of these patents, of
common assignment with the subject application, as well as the
noted Geophysics publication authored by an employee of
Schlumberger are hereby also incorporated by reference.
[0008] Sonic wireline tools, such as a Dipole Shear Sonic Imager
(DSI--trademark of Schlumberger) and Schlumberger's Sonic Scanner
generally have a multi-pole source. A multi-pole source may include
monopole, dipole and quadrupole modes of excitation. The monopole
mode of excitation is used traditionally to generate compressional
and shear head waves in logging operations such that formation
compressional and shear slowness logs can be obtained by processing
the head wave components. The head wave components are
non-dispersive and are generally processed by
slowness-time-coherence (STC) methods as discussed in the
Schlumberger Kimball et al. '691 patent and Vol. 49 Geophysics
article noted above.
[0009] The slowness-time-coherence (STC) method is employed to
process the monopole wireline or LWD sonic waveform signals for
coherent arrivals, including the formation compressional, shear and
borehole Stoneley waves. This method systematically computes the
coherence (C) of the signals in time windows which start at a given
time (T) and have a given window move-out slowness (S) across the
array. The 2D plane C(S,T) is called the slowness-time-plane (STP).
All the coherent arrivals in the waveform will show up in the STP
as prominent coherent peaks. The compressional, shear and Stoneley
slowness (DTc, DTs, and DTst) will be derived from the attributes
of these coherent peaks.
[0010] Traditionally, the attributes associated with the wave
components found in the STP are the slowness, time and the peak
coherence values. These three attributes are used in a labeling
algorithm, discussed below, to determine the compressional, shear
and Stoneley slowness from all of the STP peak candidates. These
attributes can also be used for quality control purposes.
[0011] Although determining traditional attributes has been highly
effective in the past a need exists for enhancing information that
can be determined from traditional wave form attributes and
determining additional attributes such as coherent energy and
attenuation that can be used to determine the existence of a gas
zone and/or kick detection, on a real time basis, during LWD
operations.
SUMMARY OF THE INVENTION
[0012] The methods of the subject invention includes the slowness,
time, coherence attributes and in addition the attributes of
coherent energy and attenuation. The combination of these
attributes can be advantageously used for detecting with well
logging and logging while drilling operations formation gas zones
and kick detection on a real time basis.
THE DRAWINGS
[0013] Other aspects of the present invention will become apparent
from the following detailed description of embodiments taken in
conjunction with the accompanying drawings.
[0014] FIG. 1 is a schematic of a typical derrick and a
logging-while-drilling (LWD) system where a drill string is
positioned within a borehole and a well logging segment near a
drill bit is shown within a borehole;
[0015] FIG. 2a is an enlarged diagram of a logging tool within a
borehole taken at a location above a drill bit within a borehole of
FIG. 1;
[0016] FIG. 2b is a schematic cross-sectional view of a quadrupole
sonic transmitter taken from the LWD segment shown in FIG. 2a.
[0017] FIG. 2c is a schematic cross-sectional view of a quadrupole
receiver from a stack of receivers of the LWD tool shown in FIG.
2a;
[0018] FIG. 3 is a schematic diagram disclosing traditional sonic
wave technology including a representative transmitter, receiver
and compressional waves, shear waves and Stoneley sonic waves;
[0019] FIG. 4 is a synthetic waveform illustrating waveform
attribute computation;
[0020] FIG. 5a is a graph depicting a combination of an increase in
compressional slowness (DTc) along with a decline in compressional
to shear velocity (Vp/Vs) indicative of the presence of gas;
[0021] FIG. 5b is a graph depicting the pattern of compressional
and shear attenuation (ATTc and ATTs) when a gas zone is
encountered;
[0022] FIG. 5c is a graph showing a pattern of shear and
compressional energy (CEs and CEc) indicative of when a driller is
about entering into a gas bearing formation;
[0023] FIG. 6a is a graph illustrating the effect on Stoneley
slowness (DTst) of the influx of gas in a well bore;
[0024] FIG. 6b is a graphic illustration of the effect on a ratio
of Stoneley slowness (DTst) to shear slowness (DTs) due to an
influx of gas within a borehole;
[0025] FIG. 6c is a graph showing a baseline Stoneley coherent
energy (CEst) and a baseline Stoneley attenuation (ATTst) and the
effect due to an influx of gas useful for kick detection;
[0026] FIG. 7 is an illustrative flow diagram for gas zone
detection in accordance with one embodiment of the subject
invention;
[0027] FIG. 8 is an illustration of Gas Response Indicator (GRI)
and Gas Flag (GF) as a function of Depth or Time for gas zone
detection;
[0028] FIG. 9 is a flow diagram for sonic attributes kick detection
within a bore hole in accordance with another embodiment of the
invention; and
[0029] FIG. 10 is an illustration of Gas Response Indicator (GRI)
and Kick Flag (KF) as a function of depth or time to provide
warning of an imminent gas kick event.
DETAILED DESCRIPTION
[0030] Turning now to the drawings, the subject invention is
directed to the concept of sonic measurements and systematically
determining formation attributes of compressional, shear, and
Stoneley slowness (DT) coherent energy (CE) and attenuation (ATT)
and using the information on a real time basis to detect the
presence of a gas zone or kick within a borehole.
Context of the Invention
[0031] FIG. 1 discloses a drilling derrick 100 positioned over a
well hole 102 being drilled into an earth formation 104. The
drilling derrick has the usual accompaniment of drilling equipment
including a processor 106 and recorder 108 of the type used for
measurements-while-drilling (MWD) or logging-while-drilling (LWD)
operations. A more detailed disclosure of conventional drilling
equipment of the type envisioned here is described in
Schlumberger's Wu et al published application No. 2006/0120217 the
disclosure of which is incorporated by reference as though set
forth at length.
[0032] The borehole is formed by a drill string 110 carrying a
drill bit 112 at its distal end. The drill bit crushes its way
through earth formations as the drill string is rotated by drilling
equipment within the drilling derrick. The depth of a well will
vary but may be as much at 25,000 feet or more in depth.
[0033] Turning to FIGS. 2a-2c a quadrupole acoustic shear wave LWD
tool segment 114 is shown in a degree of schematic detail. A more
detailed discussion of a LWD tool of this type can be seen in Hsu
et al. Publication No. US 2003/0058739 of common assignment with
the subject application. The disclosure of this entire publication
is incorporated by reference here. Briefly, however, the quadrupole
LWD tool segment 114 includes at least one transmitter ring 200 and
an array of receivers 212.
[0034] FIG. 2b illustrates a transmitter 200 divided into four
quadrants 202, 204, 206 and 208. Each quadrant contains a
quarter-circle array of piezoelectric transducer elements 210. FIG.
2B shows six piezoelectric transducer elements in each quadrant
although in some embodiments nine elements may be preferred
uniformly spaced around the azimuth.
[0035] As noted above an array of quadrupole receivers 212 is shown
in FIG. 2a embedded within the side wall of drill pipe segment 114.
These receivers are equally spaced vertically and may be ten to
fifty or more in a vertical array. The receivers are similar to the
transmitter in that each receiver 214 of receiver array 212 has a
quarter circle of piezoelectric transducer elements in each of
quadrants 216, 218, 220 and 222 as shown in FIG. 2c. Each ring
transducer is capable of detecting a quadrupole shear wave
refracted through a formation as discussed more fully in the above
referenced Hsu et al publication US 2003/0058739.
[0036] While FIGS. 1-2 schematically disclose a LWD system where
sonic transmitters and receivers are embedded within the side walls
of a drill string near the drilling bit, FIG. 3 discloses a
wireline tool or sonde 300 which is lowered down a borehole
suspended by a wireline 302 following a drill string tripping
operation or subsequent logging following drilling operations. The
sonde carries a transmitter 304 and an array of receivers 306
similar to the LWD tool discussed in connection with FIGS. 1 and 2.
In this, the transmitting component 304 sends sonic waves 308 into
the surrounding earth formation 310 and compressional or "P" waves
312, shear or "S" waves 314 and Stoneley or tube waves 316 (that
are propagated along the interface between a formation and the
borehole fluid) are received by an array of the receiver components
306 as illustrated in FIGS. 2A-2C above.
[0037] Measurement of arrivals of these waveforms will show up in a
slowness-time plane (STP) as prominent coherent peaks. The
compressional, shear and Stoneley slowness (DTc, DTs and DTst) will
be derived from the attributes of these coherent peaks. The subject
invention expands the wave component attributes to include coherent
energy (CE) and attenuation (ATT) which are useful in detecting the
presence of formation gas and kick detection.
Waveform Attributes
[0038] FIG. 4 depicts a set of synthetic waveforms, as a function
of time, as they appear to an array of receivers placed at equal
intervals, 400 (RR), along the tool. The abscissa in FIG. 4 is the
arrival time of sonic waves in micro seconds and the ordinate
represents sonic receivers 1-12. (Equal spacing of receivers (RR)
is not a requirement, although this assumption is made here to
simplify calculations.) As FIG. 4 illustrates, the compressional
402, shear 404, and borehole (or Stoneley) 406 wave components
generally appear at different times and, because the components
differ in "slowness" (S), move out across the array at different
rates.
[0039] A waveform arriving at time (T) at the first receiver will
arrive at the nth receiver at time (T)+(n-1)(receiver spacing)(S).
The slowness-time-coherence (STC) method discussed in Kimball et
al. Geophysics, above, is used to process the monopole wireline or
LWD sonic waveform signals for coherent arrivals. This method
systematically computes the coherence (C) of signals that start at
the first receiver at time (T) and move out across the array at a
rate corresponding to slowness (S). All of the coherent arrivals
appear in the slowness-time plane (STP) as prominent coherent
peaks. Estimates of compressional, shear, and Stoneley slowness
(DTc, DTs, and DTst) are derived from the attributes of these
coherent peaks.
[0040] For each coherent peak in the S/T plane, the slowness (S)
and arrival time at the first receiver (T) are used to construct a
time window over the array. One such time window is shown in FIG.
4. The time window is of the same length for each receiver, but, to
account for the slowness (S) a time window that begins at time (T)
for the first receiver will begin at time (T)+(n-1)(receiver
spacing)(S) for the nth receiver. The length of the window is the
same as the (STC) computation window and consists of the number
equally spaced points within a time window (nptw) at which the
waveform is computed.
[0041] Let TR(k), k=1, 2, . . . , number of receivers (nrec) be the
transmitter-to-receiver distance for the k-th receiver. Under the
assumption of equal spacing, (RR), between adjacent receivers,
TR(k+1)-TR(k)=RR, k=1, 2, . . . , nrec-1.
[0042] Let w(j,k), j=1, 2, . . . , (nptw), k=1, 2, . . . , (nrec)
be the sampled waveform (at the "j"th sampling point and at the
"k"th receiver) within the selected time window--"j" represents the
time index, and "k" represents the receiver index.
[0043] Let hw(j,k) be the (discrete) Hilbert transform of w(j,k) in
the time domain. The analytic representation of the signal, w.sub.a
(j,k), is a complex signal defined in terms of w(j,k) and hw(j,k):
w.sub.a (j,k)=w(j,k)+(i)(hw(j,k)).
[0044] The proposed invention uses the framework described above of
defining several attributes of the wave components found in the
slowness-time plane (STP). In addition to slowness, time, and
coherence, however, the subject invention demonstrates the utility
of coherent energy and attenuation attributes to oil and gas
drilling and production operations.
Coherent Energy Attribute
[0045] The wave component coherent energy attribute (CE) is
computed for a given (S) and (T) in the (STP) by stacking the
analytic signals across the array for a given time index "j",
multiplying the result by its conjugate to get the square of the
magnitude for each "j", and finally averaging over the time index
"j". Specifically:
C E = 1 ntpw j = 1 nptw { [ 1 nrec k = 1 nrec w a ( j , k ) ]
.times. conj [ 1 nrec k = 1 nrec w a ( j , k ) ] } ##EQU00001##
Attenuation Attribute
[0046] The wave component attenuation attribute (ATT) is computed
for a given (S) and (T) in the (STP) by using a linear least square
fit algorithm to determine how TE(k), the total energy within the
time window for receiver "k", attenuates as a function of TR(k),
the distance from the transmitter to the kth receiver.
[0047] The total energy within the time window for receiver "k",
TE(k), is computed using the formula:
T E ( k ) = 10 .times. log 10 [ j = 1 nptw { [ w a ( j , k ) ]
.times. conj [ w a ( j , k ) ] } ] , k = 1 , 2 , nrec
##EQU00002##
Here, TE(k) is in a log scale with unit of dB referenced to 1
Pascal.
[0048] For the set of real data pairs (TR(k), TE(k)), k=1, nrec, an
nth order least squares fit polynomial Pn(x)=a.sub.0+a.sub.1x+ . .
. +a.sub.nx.sup.n can be constructed for n.ltoreq.nrec-1. This
polynomial will minimize:
k = 1 nrec ( P ( TR ( k ) ) - TE ( k ) ) 2 ##EQU00003##
over all polynomials (P) of degree.ltoreq.n.
[0049] The linear least square fit polynomial,
P.sub.1(x)=a.sub.0+a.sub.1x, is used here to determine the
attenuation attribute. In particular, the negative value of the
coefficient "a.sub.1" (which is normally negative) will be defined
as the attenuation (ATT).
[0050] In the general case of the nth order least square fit
polynomial for the data set {(x.sub.i, y.sub.i), i=1, 2, . . . , N}
where N.gtoreq.n-1, the coefficient matrix (A) for the polynomial
can be obtained from the data pairs by means of the matrix
formula:
A = ( X T X ) - 1 X T Y where : ##EQU00004## A = [ a 0 a 1 a n ]
##EQU00004.2## X = [ 1 x 1 x 1 2 x 1 n 1 x 2 x 2 2 x 2 n 1 x N x N
2 x N n ] ##EQU00004.3## Y = [ y 1 y 2 y N ] ##EQU00004.4##
For the linear least square fit polynomial where n=1, this formula
for the coefficients reduces to:
a 0 = i = 1 N x i 2 i = 1 N y i - i = 1 N x i i = 1 N x i y i N i =
1 N x i 2 - ( i = 1 N x i ) 2 ##EQU00005## and ##EQU00005.2## a 1 =
N i = 1 N x i y i - i = 1 N x i i = 1 N y i N i = 1 N x i 2 - ( i =
1 N x i ) 2 ##EQU00005.3##
[0051] For the particular application involving attenuation, as
described above, N=nrec; x.sub.i=TR(i), the distance between the
transmitter and the "i"th receiver; and y.sub.i=TE(i), the total
energy at the "i"th receiver.
Where: (ATT), the attenuation, =-a.sub.1
Gas Zone Detection
[0052] Gas, even in trace amounts, affects certain wave components
such as compressional waves and Stoneley waves. Accordingly, the
attributes of the sonic wave components discussed above can be used
to detect the presence of gas in the formation in real time.
[0053] When a drill bit penetrates a gas-bearing formation with
unexpected high pressure (higher than the mud pressure), gas may
seep into the well bore. The sonic tool, which may be 50 to 100
feet behind the drill bit, can provide the data needed to determine
the attributes of the sonic wave components and can, therefore,
provide a driller with near real-time detection of gas zones. This
information will, for example, help the driller choose an
appropriate mud weight so that the formation gas does not
continually seep into the borehole. Alternatively, if the mud
weight has to be lower for other drilling reasons, the driller
could set pipe to protect the borehole from a gas zone.
[0054] Since gas will travel with the circulating mud uphole
immediately, Stoneley (ST) waves, as detected by the sonic tool,
will be affected by the presence of gas in the borehole well before
the sonic tool reaches the gas zone. Thus, the attributes of the
Stoneley waves may be the first indicators of gas in the formation.
Gas, even a trace amount, is known to slow down and attenuate
tremendously the Stoneley wave in the borehole over the sonic
logging frequency range. The slowness (DT), coherent energy (CE),
and the attenuation (ATT) attributes of the Stoneley wave
components can be monitored as a function of depth to provide early
detection of the presence of gas in a borehole.
[0055] The attributes of compressional (C) and shear (S) waves can
also be used to detect the presence of gas. Like the Stoneley
waves, compressional waves are known to slow down and attenuate
tremendously in the presence of a small amount of gas. On the other
hand, the slowness and attenuation of the shear wave changes
relatively little in the presence of gas.
[0056] FIG. 5 illustrates how compressional and shear waveform
attributes can be used to detect a gas zone. The three graphs in
FIGS. 5a, 5b and 5c show how the different attributes (slowness,
attenuation, and coherent energy) are affected by the presence of
gas. The three patterns of variation can be used to corroborate
each other in identifying the presence of gas.
[0057] In the gas zone, DTc 500 increases rapidly, while the change
in DTs 502 is relatively minor (FIG. 5a). This results in a
significant increase in the ratio DTc/DTs (or as shown in FIG. 5a a
significant decrease in its reciprocal (Vp/Vs 504) decrease in the
compressional to shear velocity as the tool moves down the hole. If
DTc increases due to lithology changes, as opposed to the presence
of gas, Dts will also increase and the velocity ratio Vp/Vs is also
likely to increase.
[0058] Normally, in formations without gas, the attenuation
associated with shear waves, (ATTs) 506, is slightly higher than
the attenuation associated with compressional waves, (ATTc) 508
(FIG. 5b). Gas causes ATTc to increase more rapidly than ATTs,
resulting in a crossover of the two curves--note point 510 in FIG.
5b and the ratio ATTc/ATTs will increase.
[0059] The coherent energy attribute of compressional waves, (CEc)
512, will also show a much greater rate of decrease than the
coherent energy attribute of shear waves, (CEs) 514 (see FIG. 5c)
in the presence of gas. Thus the ration CEC/CES will decrease.
[0060] Data from the compressional and shear wave attributes as a
function of depth or time, as depicted in FIGS. 5a-5c, can be used
to set a trigger level or gas zone flag that is sent uphole to warn
the driller of entry into a gas bearing formation. As an example,
an alert could be triggered when the baseline Vp/Vs ratio decreases
below a certain value. Increases in (DTc) and behavior of
DTc/DT.sub.s, ATTc, ATTs, ATTc/ATTs, CEc, CEs and CEc/CEs,
consistent with FIGS. 5a-5c, could be used to substantiate the
presence of gas.
Kick Detection
[0061] A sudden infusion of fluid or gas within a borehole is known
as kick. Stoneley wave attributes (DTst, ATTst, and CEst) are
particularly sensitive indicators for the presence of gas in the
well bore. Accordingly they can be used on a real time basis as
incipient kick indicators to provide a driller with valuable
reaction time for safe drilling of a well. The added reaction time
provided by the Stoneley wave attributes, as opposed to
compressional and shear wave attributes, may significantly increase
drilling safety.
[0062] The Stoneley slowness (DTst), attenuation (ATTst), and
amplitude (CEst) are functions of the mud and formation properties.
When drilling through formations of the same lithology, the
variation in these Stoneley wave attributes are sensitive
indicators of kick of gas or formation fluid. Normally, these
attributes will be very slowly changing variables within a given
zone of the same lithology. Their baseline values, as a function of
time or well depth, can be established by other LWD measurement
techniques such as gamma ray (GR), sonic delta-t, resistivity and
nuclear tools. Any abrupt changes in the attributes may signify the
possible influx of gas or formation fluids and will, therefore,
trigger a warning flag.
[0063] FIGS. 6a-6c illustrate how Stoneley wave attributes can be
used to construct a kick warning flag. In FIG. 6a, the lithology of
the zone of interest is shown to be essentially uniform, as
verified by sonic delta-t logs (DTc 600 and DTs 602) and (GR 604)
logs. These logs are controlled primarily by properties of the
formation, while the Stoneley wave slowness, (DTst), is also
sensitive to the borehole and mud properties. A sudden change in
(DTst) in the uniform formation zone of FIG. 6a usually implies
significant influx of gas or formation fluid. An influx of gas will
cause (DTst) to increase drastically (note 606 in FIG. 6a) while an
influx of connate water will usually cause (DTst) to decrease
somewhat (note 608 in FIG. 6a).
[0064] In order to detect a sudden change in Stoneley slowness due
to an influx of gas or fluid, it may be advantageous to monitor the
ratio (DTst/DTs), which can normalize some variation in (DTst) due
to changes in the properties of the formation. FIG. 6b depicts an
increase of the ratio (DTst/DTs) 610 as compared with its baseline
ratio, which is due to an influx gas. FIG. 6b also depicts a
decrease of the ratio (DTst/DTs) 612 as compared with its baseline,
which is due an influx in formation fluid. It suggests that a
trigger level for this ratio would be typically a certain
fractional increase or decrease relative to the baseline of the
(DTst/DTs) ratio.
[0065] FIG. 6c shows that an influx of gas will result in a
significant increase in (ATTst) 614, while (CEst) 616 will
experience a rapid decrease. An influx of fluid will usually cause
(ATTst) to decrease slightly (note 618), while (CEst) may increase
somewhat (note 620). (ATTst) and (CEst) can, therefore, provide
corroboration to a warning triggered by changes in the (DTst/DTs)
ratio.
[0066] Change detection logic can be used to set change flags (CFs)
based on a given type of input that is continually generated as a
tool proceeds down the borehole. FIGS. 7 and 9 are illustrative
diagrams, based on the change detection logic described here, for
gas zone detection and kick detection, respectively. Input used in
the illustrative charts include:
[0067] Gamma Ray measurement (GR)
[0068] Coherent Energy ratio for compressional and shear waves
(R.sub.CE)
[0069] Attenuation ratio for compressional and shear waves
(R.sub.ATT)
[0070] Slowness ratio for compressional and shear waves
(R.sub.DT)
[0071] Coherent Energy for Stoneley waves (CE.sub.ST)
[0072] Attenuation for Stoneley waves (ATT.sub.ST)
[0073] Slowness for Stoneley Waves (DT.sub.ST)
[0074] The inputs involving compressional and shear waves are
primarily useful in formation gas zone detection while the inputs
involving Stoneley waves are primarily useful in kick detection.
Gamma ray measurements could be used in both gas zone and kick
detection, but are most useful in gas zone detection, since the
Stoneley waves reacts almost immediately to a small amount of gas
released to the borehole fluid at the bit. The gamma ray input will
be particularly helpful for kick detection if the gamma ray sonde
is very near or inside the bit.
[0075] A Change Flag for a given type of input can take on the
values 1, -1, or 0, corresponding, respectively, to the input
exhibiting a large increment, a large decrement, or no change,
relative to previous measurements. A driller needs to determine how
much past data is stored for comparison and how large an increment
or decrement over earlier data is required to assign the flag a
value of 1 or -1.
[0076] If (N) represents the (user-chosen) amount of previous input
data that is maintained for comparison. The most recent (N) inputs
are placed in a buffer that maintains a running average (M)--the
most recent (N) inputs are added and the result is divided by (N)
to determine (M) at any given time. In the start-up period when the
buffer is not full, (M) will be the average over the inputs that
have been recorded.
[0077] A driller chosen number (D) represents the number that will
be used to determine if a significant change has occurred. The most
recent input (X) is compared with (M), the running average in the
buffer. If (X-M>D), (CF) is set to 1 indicating a large
increment in the particular input data. If (X-M<-D), (CF) is set
to -1 indicating a large decrement in the particular input data. If
|X-M|.ltoreq.(D), (CF) is set to 0 indicating no significant change
in the input data.
[0078] The (N) and (D) will likely be different for the different
kinds of input and thus are subscripted as (N.sub.in) and
(D.sub.in) for generic index in.
Flow Diagram for Attributes Gas Zone Detection
[0079] In FIG. 7 gas zone detection flags are used to detect
gas-containing formations in the vicinity (.about.100 feet behind
the bit) of the sonic tool. The gas in the formation may or may not
seep out into the well bore depending on the mud weight and the
bottom hole pressure. Thus, Stoneley or borehole waves may not
provide instantaneous detection at the tool. Thus, this flow
diagram exploits the changes--particular the relative changes--in
coherent energy (CE), attenuation (ATT) and slowness (DT) of the
compressional and shear waves to the presence of gas in the
formation.
[0080] Unfortunately, the responses of the compressional and shear
waves also vary with lithology or rock type. Lithology change is
reflected independently using gamma ray measurement and this
measurement is used to minimize false alarms triggered by changes
in the compressional and shear waves due to lithology.
[0081] Change detection, as described in the "Change Detection
Logic" discussed above, relies on the following input data and
derived ratios: [0082] Gamma Ray measurement (GR)--box 702; [0083]
Coherent Energy ratio for compressional and shear waves
(R.sub.CE=CEc/CEs)--boxes 700 and 704; [0084] Attenuation ratio for
compressional and shear waves (R.sub.ATT=ATTc/ATTs) boxes 700 and
706; and [0085] Slowness ratio for compressional and shear waves
(R.sub.DT=DTc/DTs) boxes 700 and 708.
[0086] Each of the four types of input has its own selected (N)
(number of retained data points or the size of the data buffer) and
(D) (the difference between the buffer average (M) and the most
recent input data that will trigger a change flag for the type of
input). The change flags (CFs) for the four types of input (each
with value 1, -1, or 0) 710, 712, 714 and 716 are used to compute
the value of the Gas Zone Flag 718, which may then trigger a
response by the driller to suspected gas in the formation. The
computation of this value may also involve driller-supplied
weights. To get a correct result, the weights assigned to the flags
for the slowness and attenuation ratios (R.sub.DT and R.sub.ATT)
will be positive, and the weight assigned to the flag for the
coherent energy ratio (R.sub.CE) will be negative. The flag
associated with gamma ray measurement may be incorporated as a
separate term or a factor of the form: [1-abs (CF.sub.GR)] in order
to help eliminate setting a gas flag when changes are due to
lithology.
[0087] The Gas zone Flag (GF) in FIG. 7 behaves like a switch on
state change indicator. If the attributes respond to the onset of
gas, GF would be expected to increase from zero to a positive value
depending on the weighting factors and the number of corresponding
indicators. If the attributes remain unchanged, there will be no
change in GF, whether gas is present or not present. If the sonic
tool moves away from a gas zone, GF would take on a negative value
signifying the disappearance of gas.
[0088] The Gas Flag (GF) should be used in conjunction with a gas
response indicator (GRI) which may be a combination of the basic
attributes. The following are some examples of GRI that will have
higher values in a gas zone.
GRI=ATTc/ATTs*DTc/DTs*CEs/CEc
GRI=(DTc*ATTc)/CEc
GRI=Att*ATTc/ATTs+Wdt*DTc/DTs+Wce*CEs/CEc
[0089] where Watt, Wdt and Wce are nonnegative weighting
coefficients.
[0090] FIG. 8 illustrates the relationship between the Gas Flag 800
(GF) which is a state change indicator and Gas Response Indicator
(GRI) 802 which takes on larger values when gas is present than
when it is absent. The two indicators would be used in combination
to inform the decision maker of the presence of a gas zone.
Flow Diagram for Sonic Attributes Kick Detection
[0091] In FIG. 9 kick detection flags are used to detect a small
amount of gas released to the borehole fluid from the formation at
the bit and, therefore, provide early warning time to a driller.
Stoneley or borehole waves, which exhibit predictable changes in
coherent energy, attenuation and slowness in this situation,
provide the primary method of detection.
[0092] Change detection, as described in the "Change Detection
Logic" above, uses the following input data:
[0093] Gamma Ray measurement (GR)--box 900;
[0094] Coherent Energy for Stoneley waves (CE.sub.ST)--box 902;
[0095] Attenuation for Stoneley waves (ATT.sub.ST)--box 902;
and
[0096] Slowness for Stoneley Waves (DT.sub.ST)--box 902.
[0097] Each of the four types of input has its own selected (N)
(number of retained data points or the size of the data buffer) and
(D) (the difference between the buffer average (M) and the most
recent input data that will trigger a change flag for the type of
input). The change flags (CFs) for the four types of input (each
with value 1, -1, or 0) 904, 906, 908 and 910 are used to compute
the value of a Kick Detection Flag 912, which may then trigger a
response by the driller to suspected gas at the drill bit. The
computation of this value will probably involve driller-supplied
weights. To get the correct result, the weights assigned to the
flags for Stoneley slowness and attenuation (DTst and ATTst) will
be positive, and the weight assigned to the flag for the coherent
energy (CEst) will be negative. A flag associated with gamma ray
measurement may be incorporated as a separate term or a factor of
the form: [1-abs (CF.sub.GR)] if the gamma ray sonde is very near
the drill bit.
[0098] The Kick detection Flag (KF) in FIG. 9 behaves like a switch
or state change indicator. As with the Gas zone Flag (GF) in FIG.
7, this flag should be used in conjunction with a Gas Response
Indicator (GRI). In this case, some examples of a GRI based on the
attributes of Stoneley or borehole waves are:
GRI=(ATTst*DTst)/CEst
GRI=Watt*ATTst+Wdt*DTst+Wce/CEst
[0099] where Watt, Wdt and Wce are nonnegative weighting
coefficients.
[0100] FIG. 10 illustrates the relationship between the Kick
detection Flag (KF) 1000 and the Gas Response Indicator) GRI) 1002,
which takes on larger values when kick is imminent. The two
indicators are used in combination to inform a driller of the
possibility of a kick event.
[0101] The various aspects of the invention were chosen and
described in order to best explain principles of the invention and
its practical applications. The preceding description is intended
to enable those of skill in the art to best utilize the invention
in various embodiments and aspects and with modifications as are
suited to the particular use contemplated. It is intended that the
scope of the invention be defined by the following claims.
* * * * *