WO2016106104A1

WO2016106104A1 - Seismic sensing and depth estimation of a target reflector

Info

Publication number: WO2016106104A1
Application number: PCT/US2015/066518
Authority: WO
Inventors: Masahiro Kamata
Original assignee: Schlumberger Canada Limited; Services Petroliers Schlumberger; Schlumberger Technology B.V.; Schlumberger Technology Corporation
Priority date: 2014-12-25
Filing date: 2015-12-17
Publication date: 2016-06-30

Abstract

A method and system for facilitating geophysical exploration is provided including deploying a seismic tool comprising a seismic receiver with a corresponding receiver clock, downhole into a borehole drilled in a formation. The method also includes initiating a seismic signal into the formation from a seismic source and recording a first down-going break time for a down-going wave and a first up-going break time for an up-going wave at a first deployment depth. In addition, the method includes initiating a seismic signal into the formation and recording a second down-going break time and a second up-going break time for the respective down-going wave and up-going wave at a second deployment depth. The method includes calculating a parameter of the formation using the first and second down-going and up-going break times and the first and second depth. The system includes a seismic tool comprising a seismic receiver with a receiver clock.

Description

PATENT APPLICATION

SEISMIC SENSING AND DEPTH ESTIMATION OF A TARGET REFLECTOR

RELATED APPLICATIONS

This application (claims the benefit of a related U.S. Provisional Application Serial No. 62/096,892) filed December 25, 2015, entitled "Seismic Sensing and Depth Estimation of a Target Reflector," to Masahiro Kamata, the disclosure of which is incorporated by reference herein in its entirety.

BACKGROUND

The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.

Hydrocarbon fluids such as oil and natural gas are obtained from a subterranean geologic formation, referred to as a reservoir. Various forms of geophysical exploration are employed to better understand the location, size, and characteristics of the reservoir. For example, seismic exploration techniques have been employed to facilitate an improved understanding of the reservoir. In some applications, logging-while-drilling (LWD) seismic tools have been used downhole to determine time-depth information for various reflectors and bit depth relative to the seismic reflectors and provide look ahead information to show the important reflectors which are to be drilled.

For example, the LWD seismic tools may be employed during drilling operations to predict the depth of a target reflector, for example such as a reflector of an over pressure zone. Drilling can be stopped just above the over pressure zone based on the information from LWD seismic tool. At this point, casing can be set and the mud weight increased in order to safely drill through the over pressure zone. Increasing mud weigh without setting casing may result in fracturing of the formation above the bit. . SUMMARY

In general, a method for facilitating geophysical exploration is provided including deploying a seismic tool comprising a seismic receiver with a corresponding receiver clock, downhole into a borehole drilled in a formation. The method also includes initiating a seismic signal into the formation from a seismic source and recording a first down-going break time for a down-going wave and a first up-going break time for an up-going wave at a first deployment depth. In addition, the method includes initiating a seismic signal into the formation from a seismic source and recording a second down-going break time and a second up-going break time for the respective down-going wave and up-going wave at a second deployment depth. The method includes calculating a parameter of the formation using the first and second down-going and up-going break times and the first and second depth.

A seismic tool for geophysical exploration is also provided and includes a seismic receiver with a corresponding receiver clock. The seismic receiver is configured to receive a seismic signal via a formation from a seismic source. In addition, the seismic tool records a first down-going break time for a down-going wave and a first up-going break time for an up-going wave at a first deployment depth and further records a second down-going break time and a second up-going break time for the respective down-going wave and up-going wave at a second deployment depth. The seismic tool also calculates a parameter of the formation using the first and second down- going and up-going break times and the first and second depths.

However, many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein, and: FIG. 1 is a schematic illustration of an example of a seismic system having a downhole seismic tool located in a borehole and a surface system working in cooperation with a seismic source to facilitate geophysical exploration, according to an embodiment of the disclosure; FIG. 2 is a graphical representation of an example of vertical seismic profiling, according to an embodiment of the disclosure;

FIG. 3 is a graphical representation of an example of downhole clock errors occurring in the vertical seismic profile, according to an embodiment of the disclosure;

FIG. 4 is a graphical representation of an example of determining transit time to various depths, according to an embodiment of the disclosure;

FIG. 5 is a graphical representation of an example of data obtained during a vertical seismic profiling operation One Way Time plot, according to an embodiment of the disclosure;

FIG. 6 is a graphical representation of an example of a vertical seismic profiling wave field with clock errors, according to an embodiment of the disclosure; FIGS. 7 A and 7B are graphical representations of an example of a vertical seismic profiling wave field with clock errors before and after shifting traces to an assumed velocity, according to an embodiment of the disclosure;

FIG. 8 is a graphical representation of an example of data resulting from shifting the vertical seismic profiling wave field to illustrate break time alignment, according to an embodiment of the disclosure;

FIG. 9 is a graphical representation of an example of data reflecting break time alignment with down going wave estimation, according to an embodiment of the disclosure;

FIG. 10 is a graphical representation of an example of data resulting from subtraction of down going waves, according to an embodiment of the disclosure; FIG. 11 is a graphical representation of an example of Two Way Time Alignment data plot with clock errors, according to an embodiment of the disclosure;

FIG. 12 is a graphical representation of an example of data resulting from up going wave alignment, according to an embodiment of the disclosure;

FIG. 13 is a graphical representation of an example of ray paths of an up going wave reflected from a dipped reflector, according to an embodiment of the disclosure; FIG. 14 is a graphical representation of an example of vertical seismic profiling in two way time, according to an embodiment of the disclosure; and

FIG. 15 is a graphical representation of an example of data representing a corridor stack in a graph of two way time versus depth, according to an embodiment of the disclosure.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.

The disclosure herein generally involves a system and methodology which are provided for facilitating geophysical exploration. A technique comprises deploying a seismic tool downhole with a high precision clock. The seismic tool is used in cooperation with a surface system and a seismic source in a manner that overcomes clock errors to obtain improved data. The surface system fires the source to generate seismic signals that propagate in the earth. The signal may be sensed by appropriate geophones or other types of seismic receivers in the seismic tool located at appropriate positions along a borehole and/or surface. The seismic tool digitizes the seismic signals and transmits the data to the surface system with the time stamps given by the high precision clock. In some applications, the technique enables improved seismic data processing for determination of a target depth for drilling. The technique also may be employed to improve translation of the seismic reflector and the driller's depth. According to an embodiment, the system and methodology are employed in an LWD application. The LWD application utilizes a downhole seismic tool which may be employed along a well string used in, for example, a drilling application. The downhole seismic tool is employed in cooperation with a surface system to collect and analyze seismic data. The system and methodology enable determination of, for example, a target depth for drilling by recording both down going waves and up going waves that are reflected from a target reflector located below the current depth of drilling. Additionally, the technique enables accurate determination of the target depth even when clock errors are present. The technique also facilitates additional or other applications, including those involving translation of bit depth or the surface seismic image.

Referring generally to FIG. 1, an example of a seismic system 20 is illustrated as a drilling rig 18 deploying a downhole seismic tool 22 in a borehole 24. The seismic tool 22 has a high precision clock 26 which is synchronized with a high precision clock 28 of a surface control system 30 before the seismic tool 22 is run downhole into borehole 24 via a well string 32. The well string 32 may be in the form of a drill string, conveyance, or other type of well string. In the example illustrated, the seismic system 20 further comprises at least one seismic source 34 operatively coupled with surface control 30 to initiate seismic signals which are transmitted through a formation 36 into which borehole 24 is drilled. Depending on the application, a single downhole seismic tool 22 may be utilized and measurements may be taken at different depths and at different times, or a plurality of downhole seismic tools 22 may be employed so that measurements may be taken simultaneously at a plurality of different depths.

According to an embodiment, well string 32 is in the form of a drill string which is formed by assembling drill pipe. As each drill pipe is added to the well string 32, the drilling stops and the environment becomes quiet because very little external noise is created by the drill string. During these low noise periods, the surface control system 30 is operated to fire seismic source 34 which may be located near a drilling rig 18 or at another suitable location. A reference sensor 38 is located near the seismic source 34 to monitor creation of the seismic waves and to provide timing data to clock 28.

As the seismic source 34 is fired, the surface break time tb when the seismic wave is created is determined by the surface clock 28. The seismic source 34 generates seismic energy which propagates through the earth and through formation 36 in the form of seismic waves (such as a direct wave 62 and a reflected wave 64). The seismic waves 62, 64 eventually reach the seismic tool 22, e.g. an LWD seismic tool, and the tool 22 acquires the seismic signals via seismic signal receivers 40, e.g. geophones. In the case of a reflected seismic wave 64, the reflected seismic wave 64 reaches the tool 22 after reflecting off of a reflector 44. The seismic tool 22 timestamps the acquired data by using downhole clock 26, determines the downhole break time from the acquired data, and sends time data to the surface control system 30. The surface control system 30 receives data indicating the downhole break time and depth of the drilling operation. The surface system 30 is then used to calculate the travel time for the seismic waves from the surface to the downhole seismic tool 22. By combining the travel time and depth, the drill bit of the drilling system may be located on surface seismic section displayed in time scale. It is also possible to convert the time scale of the surface seismic section to depth scale. As described in greater detail below, seismic system 20 is constructed and operated to overcome clock errors which can occur between the surface and downhole clock locations. Referring generally to FIG. 2, an example of vertical seismic recording is illustrated graphically. In this example, the seismic source 34 is fired at time zero. The creation of seismic waves is monitored by the surface system 30 by using sensor 38 located near the seismic source 34. The break time of the seismic waves is tb, and the seismic waves propagate down (see down going wave 42). The seismic waves reach the seismic tool 22 located at depth xl at time tl 1 and reach another seismic tool 22 located at depth x2 at t21. If a single seismic tool 22 is employed, the seismic waves are measured at different depths at different times by repeating the measurements. The drift of the clock may become larger as time passes by. However, the clock is still relatively accurate while measuring the down going wave 42 and up going wave 46 in one time acquisition, which typically is within the time span of a few second time period. This means that the time measurements between the down going wave arrival and the up going arrival are accurate, even if the long term absolute time is inaccurate.

The seismic waves propagate downwardly and are reflected from a reflector 44 at depth x3. The reflected seismic waves (see up going wave 46) reach the seismic tool 22 located at depth x2 at time t22. Then the seismic waves reach the seismic tool 22 at depth xl at time tl2. From tb, xl, ti l, tl2, x2, t21, and t22, the reflector depth x3 is calculated. The down going wave 42 and up going wave 46 appear in the vertical seismic profiling (VSP) section as a mirror image if the reflector 44 is horizontal. When the reflector 44 is horizontal, both down going wave 42 and up going wave 46 travel in the same path. Since the velocity of the media is the same, a depth and time relationship may be described by the equation:

Equation 1

The reflector depth may be found by solving Equation 1 for the reflector depth x3 as: Equation 2

If the downhole clock 26 has errors at depth xl and x2, then the down going wave 42 and the up going wave 46 are not symmetric, as reflected graphically in FIG. 3. In an embodiment in which two different downhole seismic tools 22 are located at xl and x2, the clock 1 and clock 2 of the two different seismic tools 22 are different and time measurement is not the same. If the same seismic tool 22 is used to acquire seismic signals at different times, the clock drifts when the measurement is repeated at depth x2. As a result, the arrival times may contain errors. The errors in tl 1 and tl2 in the acquisition at depth xl are the same, because tl 1 and tl2 are obtained in the same acquisition with the same clock 1. Also the errors in t21 and t22 at depth x2 are the same. If one lets the time error at depth xl be Atl and the time error at depth x2 be At2, then Equation 1 may be rewritten as:

Equation 3

{t_n + At, ) - {t_n +Δ {t₂₂ + At₂ ) - {t_2l + At₂ )

Because the errors are cancelled in Equation 3, the target depth estimation is the same. As long as tl 1, tl2, t21 and t22 can be identified in just two traces of the VSP traces, the target depth can be found without impact from clock errors. By way of example, the data may be processed via the processor(s) of the surface control system 30.

Since both down going wave and up going wave propagate in the same formation in oppsite directions, the interval velocity is the same. The interval velocity c between depths xl and x2 for down going wave and up going wave are; 2^{~ Χχ} = c Equation 4

^21 ^~*^{~ ~} \ - ¹— = c Equation 5

^12 ^_ ^22 ^_ where te is the clock difference between Clock 1 and Clock2.

Then from Equation 3 and Equation 4, the interval velocity is found to be x — x

c = 2 - Equation 6

The clock error is cancelled out.

If a transit time at a shallow depth is known, the transit time below is calculated as

TT_n = 77 + V ^{f ~ Xi~l} Equation 7

Equation 7 is graphically shown in FIG. 4.

Because the amplitude of up going wave 46 is small, it can be difficult to determine an up going wave 46 in just a trace. If the target depth is close to the tool depth, the wave train of the first arrival is overlaid with the up going wave 46 and it can be difficult to distinguish which data indicate the up going wave 46. In FIG. 5, a graphical example is provided which shows a VSP One Way Time plot without clock errors. In FIG. 5, the One Way Time plot, the downhole break time is the time for seismic waves to propagate from the surface to downhole. The reflection coefficient is 0.2 for visuality, although often it is smaller. In this illustrated example, various surface multiples are super imposed to show a realistic wavefield. Surface multiples occur at near surface reflections, such as sea surface, sea bottom, weathered zone. Although the down going wave 42 interferes with the up going wave 46, the up going wave 46 is visible because of continuity in many of the VSP traces.

Referring generally to FIG. 6, a graphical example is provided which illustrates the same VSP wavefield with clock errors. The errors are generated as random numbers within +/-10ms. It can be difficult to identify the arrival times of up going waves 46, however the break times for down going waves 42 remain clear.

As shown in exemplary FIGS. 7A and 7B, FIG. 7A illustrates the VSP wavefield with clock errors with an approximate line drawn along the break times on the VSP plot. FIG. 7B shifts the VSP traces to align on the approximate break time line. The up going wave is now aligned. Draw a line to extrapolate the up going wave. The intersection/cross point of the break time line and up going wave line indicates the depth of the target reflector as expressed by Equation 2. Equation 2 says that target depth is a correct approximation despite the clock errors. Then set tl 1 and t21 to zero and the target depth is still correct. The break time for each VSP trace can be determined and the VSP traces can be shifted according to the break times so that the down going waves 42 are aligned. FIG. 8 illustrates the result of shifting the VSP wave field, which may be referred to as Break Time Alignment. In this graphical plot, the up going wave 46 demonstrates constant moveout across the wavefield. As illustrated, a straight line 48 may be drawn across the arrival times of up going wave 46. The intercept of this line 48 with a line 50 at t=0 indicates the target depth.

In this break time alignment, tl 1 and t21 are zeros. Then Equation 3.3 becomes c = 2 ^{2 ~ Xl} Equation 8

^12 ^~~ ^22

Equation 8 means that the interval velocity is two times as the slope of the up going wave line.

As further illustrated in FIG. 9, the down going wave 42 which appears in the Break Time Alignment can be summed as illustrated at the top of FIG. 9 and shown as down going wave estimate summed trace 58.

Referring generally to FIG. 10, the summed trace 58 (see FIG. 9) may be subtracted from each trace in the Break Time Alignment. By subtracting the down going wave 42, the up going wave 46 is clearly illustrated and a better line can be drawn along the up going arrivals to estimate the target depth. In some embodiments, a slight deviation of each receiver response may artificially be added to each trace of the down going wave 42 to help represent a realistic wave field. In conventional VPS processing, the Break Time aligned traces after subtracting the down going waves are shifted by two times the break times so that up going waves are aligned. FIG. 11 shows such a Two Way Time aligned traces with clock errors. Two Way Time is the time for seismic waves to come back to surface after reflecting off of a reflector in the earth. This plot is the same scale as in the surface seismic section. Instead of taking two way time alignment, up going waves are arbitrary aligned and plotted in Up Going Wave Alignment. As further illustrated in FIG. 12, the up going wave 46 is clear in each trace and the traces are aligned.

If the reflector 44 is horizontal, the up going wave alignment is the same as the two-way time plot with a time shift that corresponds to the error in the downhole clock 26. If the reflector 44 is dipped, the appearance of the reflector 44 is deviated. The left portion of FIG. 13 illustrates ray paths of up going waves from a dipped reflector. If the seismic tool 22 is relatively far from the dipped reflector 44, the up going wave 46 is reflected by the dipped reflector 44 far from the borehole 24 but arrives at the seismic tool 22. If, on the other hand, the seismic tool 22 is close to the reflector 44, the reflection is generated close to the borehole 24. When the seismic tool 22 is relatively far from the reflector 44, the reflection arrives earlier than expected. Furthermore, the dip information does not appear in the up going wave alignment.

In FIG. 14, an example of a vertical seismic profile is illustrated in two-way time. In this example, a stack of traces is selected in a window near the break time as indicated by a solid line 52 in the two-way time plot. The result of the stack of traces is illustrated by the corridor stack 54 shown at the bottom of FIG. 14. Because the corridor stack 54 is limited to reflections which occurred near the borehole 24, the up going wave alignment can be used for corridor stack and the relative timing of reflections is still accurate even though the clock 26 has errors. In the corridor stack 54, the stack traces are repeated several times to compare the reflectors with a seismic section. The intercept of break time line 52 and the bottom of the VSP trace is the total depth, TD. The right side of TD is the look ahead image to show reflectors which will be drilled. As illustrated in FIG. 15, the corridor stack 54 may be positioned in a seismic section 56 recorded near the same borehole 24. In this example, the corridor stack 54 may be adjusted to match the seismic section 56 because the corridor stack 54 has a time shift due to the clock error. By matching the seismic section 56, the depth of a reflector 44 can be identified by the depth scale of the corridor stack 54. Furthermore, by using the methodologies and analyses described herein, the effects of the clock errors can be overcome by appropriately processing the data on control system 30 or on other suitable processor systems. The method facilitates the determination of a variety of parameters, including current depth of drilling, translation of seismic time and bit depth and look ahead image of reflectors which will be drilled.

Depending on the specifics of a given application and/or environment, the procedures for obtaining seismic data via seismic system 20 may vary. Additionally, the configuration of the overall seismic system 20, as well as the components of the overall system, may be adjusted to accommodate the parameters of a given procedure and/or environment. For example, various types of well strings, e.g. drill strings, may be employed and various types of seismic tools, e.g. LWD seismic tools, may be utilized in a given application.

Additionally, the surface control system 30 may comprise a variety of processing system, e.g. computer-based processing systems. For example, the surface control 30 may utilize a variety of individual or plural processors and may include a single processing unit or a plurality of processing units, e.g. a surface processing unit located on-site and/or remotely. The collected seismic data may be subjected to various available software, models, algorithms, and other processing techniques to obtain the desired seismic data, e.g. seismic wave and time data, from single or plural seismic tools 22 located downhole.

Although a few embodiments of the disclosure have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.

Claims

What is claimed is: 1. A method for facilitating geophysical exploration, comprising: deploying a seismic tool comprising two or more seismic receivers located at different depths, in which each of the seismic receivers has a corresponding receiver clock, downhole into a borehole drilled in a formation;

initiating a seismic signal into the formation from a seismic source; recording seismic traces received by the two or more seismic receivers according to depth vs time;

aligning down-going break time peaks in down-going waves with one another along a down-going line in the seismic traces;

determining a reflector boundary depth corresponding to an intersection between the down-going line and an up-going line corresponding to up-going break time peaks of up-going waves.

2. The method according to claim 1, wherein the down- going line is oriented according to an assumed velocity for the formation.

3. The method according to claim 1, wherein the intersection between the down-going line and the up-going line is determined by an equation:

^X {^22 ^21 ) ¾ (^12 ^l l )

(^22 ^_ ^21 ) ^_ (^12 ^_ ^l l ) wherein Λ¾ is the reflector boundary depth, and X\ is a depth of one of the two or more seismic receivers with tn the down-going break time and tn the up-going break time at the depth X\, and wherein Xi is a depth of another of the two or more seismic receivers with ti\ the down-going break time and tn the up-going break time at the depth Xi.

4. The method according to claim 1, wherein the down-going line is oriented at a zero reference time, and an internal velocity of the formation is equal to two times a slope of the up- going line.

5. The method according to claim 1, wherein down-going line is oriented at a zero reference time, a mean seismic trace is determined from adding the seismic traces; and

wherein the mean seismic trace is subtracted from each of the seismic traces in order to more readily determine the up-going line.

6. The method according to claim 1, further including:

aligning the up-going waves in an up-going wave alignment;

stacking the up-going waves in a selected window near the up-going break time to make a corridor stack for comparison with a surface seismic section.

7. The method according to claim 1, wherein separate internal velocities are estimated between each of the different depths.

8. A method for facilitating geophysical exploration, comprising: deploying a seismic tool comprising a seismic receiver with a corresponding receiver clock, downhole into a borehole drilled in a formation;

initiating a seismic signal into the formation from a seismic source; recording a first down-going break time for a down-going wave and a first up-going break time for an up-going wave at a first deployment depth;

initiating a seismic signal into the formation from a seismic source; recording a second down-going break time and a second up-going break time for the respective down-going wave and up-going wave at a second deployment depth;

calculating a parameter of the formation using the first and second down-going and up-going break times and the first and second depth.

9. The method of claim 8 wherein the parameter is a reflector boundary depth.

10. The method of claim 9 wherein the reflector boundary depth is calculated using a formula: _ ^l (^22 ^2l ) ¾ (^12 ^l l )

X₃—

(^22 ^_ ^21 ) ^_ (^12 ^_ i ) wherein X3 is the reflector boundary depth, and X\ is the first depth with tn the first down-going break time and tn the first up-going break time, and wherein X2 is the second depth with tn the second down-going break time and tn the second up-going break time.

11. The method of claim 8 wherein the parameter is interval velocity.

12. The method of claim 11 wherein the interval velocity is calculated using a formula:

C = 2-

wherein c is the interval velocity between the first depth and the second depth, and X\ is the first depth with tn the first down-going break time and tn the first up-going break time, and wherein X2 is the second depth with ¾i the second down-going break time and tn the second up-going break time.

13. The method of claim 8 wherein the seismic tool further comprises additional seismic receivers with corresponding receiver clocks provided along the seismic tool at different deployed depth locations.

14. A seismic tool for geophysical exploration comprising: a seismic receiver with a corresponding receiver clock;

wherein the seismic receiver is configured to receive a seismic signal via a formation from a seismic source;

wherein the seismic tool records a first down-going break time for a down-going wave and a first up-going break time for an up-going wave at a first deployment depth and further records a second down-going break time and a second up-going break time for the respective down-going wave and up-going wave at a second deployment depth; and wherein the seismic tool calculates a parameter of the formation using the first and second down-going and up-going break times and the first and second depths. seismic tool of claim 14 wherein the parameter is a reflector boundary depth.

16. The seismic tool of claim 15 wherein the reflector boundary depth is calculated using a formula:

wherein Λ¾ is the reflector boundary depth, and X\ is the first depth with tn the first down-going break time and tn the first up-going break time, and wherein Xi is the second depth with ti\ the second down-going break time and tn the second up-going break time.

17. The seismic tool of claim 14 wherein the parameter is interval velocity.

18. The seismic tool of claim 14 wherein the interval velocity is calculated using a formula:

c = 2-

wherein c is the interval velocity between the first depth and the second depth, and X\ is the first depth with tn the first down-going break time and tn the first up-going break time, and wherein X2 is the second depth with ti\ the second down-going break time and tn the second up-going break time.

19. The seismic tool of claim 14, wherein the seismic tool is deployed via a drill string.

20. The seismic tool of claim 14, wherein the seismic tool further comprises a second seismic receiver with a second seismic receiver clock located along the seismic tool so as to provide measurements from another depth location.