WO2002075363A1

WO2002075363A1 - Time-lapse seismic surveying at sea

Info

Publication number: WO2002075363A1
Application number: PCT/EP2002/003074
Authority: WO
Inventors: Rodney William Calvert
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Canada Limited
Priority date: 2001-03-15
Filing date: 2002-03-14
Publication date: 2002-09-26

Abstract

Method of carrying out at sea a time-lapse survey of an underground target (1, 2) comprising arranging a seismic sensor system at a predetermined position (20, 21, 22, 23, 24, 25); positioning a seismic source at at least one location, wherein each location has a predetermined position (33, 34), and recording for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source; positioning after a predetermined period of time the seismic source at the location(s) (33, 34) of the previous step, and recording, when the sea properties are substantially repeating, for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source; and subtracting the obtained signal from a signal previously obtained to get a difference that is used to detect changes in the target layer as a function of time.

Description

TIME-LAPSE SEISMIC SURVEYING AT SEA

The present invention relates to time-lapse seismic surveying of a target layer in an underground formation. Time-lapse seismic surveying or monitoring involves obtaining seismic data of the same underground formation at different times. It allows studying the changes in seismic properties of the target layer as a function of time due to for example fluid flow through the underground formation. Seismic data can be combined to generate images that show for example spatial and temporal variation in fluid saturation, pressure and temperature. The time-lapse seismic surveying technique has applications such as mapping bypassed oil, monitoring injected reservoir fluids such as water, steam and CO2, and estimating fluid-flow heterogeneity related to pressure corαpartmentalization, and the hydraulic properties of faults and fractures.

In particular the present invention relates to a method of marine time-lapse seismic surveying of a target layer in an underground formation, which method comprises the steps of:

(a) arranging a seismic sensor system at a predetermined position;

(b) positioning a seismic source at a number of locations, each having a predetermined position, and recording for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source;

(c) positioning after a predetermined period of time the seismic source at the locations of step (b) , and recording for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source; and

(d) subtracting the obtained signal from a signal previously obtained to get a difference that is used to detect changes in the target layer as a function of time. In the specification and in the claims, the word 'signal' is used to refer to raw, unprocessed data as well as to processed data. The target layer is a layer or a number of layers in which one is interested. In the latter case the layers can be separated by layers that are not of interest.

It is well known that repeatability is the key to the success of time-lapse seismic surveying, and that major concerns are source and receiver positions (see for example the article Time-lapse seismic monitoring: Some shortcomings in nonuniform processing, C P Ross and M S Altan, The Leading Edge, June 1997) . Other concerns are source and receiver waveform responses and coupling, and various forms of undesired noise. Applicant had found that there is another factor that adversely affects the success of marine time-lapse seismic surveying and that is the sea, and more in particular the properties of the sea.

In the following paragraphs the effects of the properties of the sea will be discussed for each of the two types of waves that can be propagated in the earth formations, the longitudinal wave or P-wave and the transverse wave or S-wave.

For a P-wave the recorded signal not only contains the primary signal P(t) that comes from the underground formation and more in particular from the target layer, but also surface multiples M (t), M2(t) and so on of order 1, 2 and so on, which surface multiples are caused by reflections from the sea surface. In time-lapse surveying the difference ΔP between the primary signal at t=t2 and the primary signal at t=t]_ is evaluated. The difference is obtained by subtracting the seismic signals. The difference does not only contain ΔP, but also ΔM^, ΔM2 and so on. If the surface multiples do not repeat, that is to say if ΔM]_, ΔM2 and so on are not very small, their contribution cannot be ignored.

A reason why the surface multiples do not repeat is the existence of tidal differences, or water velocity changes due to changes in temperature or salinity, and a numerical example will illustrate the effect. Let us assume a tidal range of 1 m (meter) and a sonic velocity of 1 500 m/s (meter per second) . Assume the seismic survey at t=t is carried out at low tide, then the two- way water travel time is twice the depth d divided by the sonic velocity. Assume now the seismic survey at t=t2 is carried out at high tide, then the two-way water travel time is twice the tidal range (1 m) divided by the sonic velocity plus twice the depth d divided by the sonic velocity. The difference Δt is 2/1500 s or 1.3 msec (millisecond). This will cause the primary signals P(t) to be shifted by time shift Δt between surveys, but the surface multiples M]_(t), M2(t) and so on will have additional shifts of Δt, 2Δt and so on, because they have an additional passage through the layer of sea water. In processing we can time-shift the signal to align the primaries for subtracting in step (d) , but this will not align the surface multiples. Not aligning the surface multiples has a considerable effect on the multiples. Let us look at a frequency component f and assume the first order multiple is M]_=m]_. cos (2πft) . The difference Δ ]_ is approximately equal to (dMl/dt) .Δt, or ΔM_]_=-m_]_.2πf. sin (2πft) .Δt . For a frequency f of 50 Hz (Hertz) and a time shift Δt the difference ΔM]_ is -0.41.mi, or 41% of the amplitude of the surface multiple Mχ(t) .

Because the surface multiples can be as strong as the primary signal, and because in time-lapse surveying one is interested in small differences in the primary signals, a multiple difference of 41% may completely dominate the ability to determine accurately ΔP. For this reason the industry has developed techniques for multiple suppression involving various assumptions, processing algorithms and survey data requirements.

For an S-wave, tidal differences have an effect on the stress state in the overburden that is also non- repeatable. A change in the stress state in the overburden will cause a change in the shear properties of the overburden and in the measurable variables, such as S-wave velocity, birefringence or S-wave splitting and S-wave reflectivity. Changes of the overburden stress state may ask the shear differences that one wishes to determine. Sea tidal differences are a key cause of changes in the overburden stress state.

Not only tidal changes have an effect on the two-way water travel time and on the stress state in the overburden, but also other sea properties, such as water temperature and salinity affect the travel time and the stress state.

It is therefore an object of the present invention to provide a method of carrying out at sea a time-lapse survey of a target layer in an underground formation in which non-repeatable effects caused by the sea can be suppressed in a simple manner.

To this end the method of carrying out at sea a time- lapse survey of a target layer in an underground formation according to the invention comprises the steps of: (a) arranging a seismic sensor system at a predetermined position;

(b) positioning a seismic source at at least one location, wherein each location has a predetermined position, and recording for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source;

(c) positioning after a predetermined period of time the seismic source at the location (s) of step (b) , and recording, when the appropriate sea properties are substantially repeating, for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source; and

(d) subtracting the obtained signal from a signal previously obtained to get a difference that is used to detect changes in the target layer as a function of time.

For some purposes it is sufficient that the time- lapse seismic survey is repeated at the same tide to ensure that the sea properties are substantially repeating. However, in many cases this will not be sufficient .

In order to suppress the adverse effect of surface multiples that do not repeat, the P-wave recording is suitably made when the two-way water time between seabed and sea surface is substantially the same as for the previous recording. The two-way water time is substantially the same as for the previous recording when the difference of the dominating multiple is less then the difference in the primary signals that one wants to detect. Thus if one wants to detect changes in the primary signal of 4%, then the difference in the dominant surface multiple should be about 4% or less. In the above numerical example, it would mean that the time shift has to be reduced to 0.13 msec, which implies that the recording has to be made when the change in tide is less than 0.10 m compared to the previous recording.

The S-wave recording is suitably made, when the seabed pressure is substantially the same as for the previous recording. The seabed pressure is substantially the same as for the previous recording when the absolute value of the difference is less than subsurface stress changes caused by effects we wish to determine by the time-lapse surveys. An advantage of using the seabed pressure is that the stress due to the atmospheric pressure and earth tide is taken into account as well.

By repeating the recordings according to step (c) the multiple reflections off the sea surface are better subtracted and the resulting difference is better for determining changes in the underground formation as a function of time.

The present invention is particularly suitable for applications where the seabed is a hard reflector so that the multiples are pronounced. The present invention relates to collecting the seismic data in such a way that the effect of multiples is suppressed.

In contrast thereto, the method as disclosed in International patent application publication No. 98/11455 relates to comparing seismic survey data sets in order to remove effects of conditions that change over time, other than the changes in the state of the reservoir. To this end the data sets are matched by crossequalization from one survey to the other. After crossequalization, the difference between two data sets should be zero except where there are changes in the reservoir. This is a way of aligning the seismic signals, which results in aligning the primary signals. However, this aligning does not take into account the multiples. Therefore this publication is not relevant to the present invention. The invention will now be described by way of example with reference to the accompanying Figure showing schematically a perspective view of the time-lapse surveying according to the present invention. The Figure shows a perspective view of target layers 1 and 2 in a layered underground formation, which layered formation further comprises a layer 5 below the target layer 1 and an overburden 10 that extends to a seabed 12. The seabed 12 is covered with a layer of seawater 13 that has a surface 14.

The method of carrying out at sea a time-lapse survey of the target layers 1 and 2 comprises first arranging a seismic sensor system at a predetermined position. In this case, the seismic sensor system comprises a set of seismic sensors 20, 21, 22, 23, 24 and 25 located on the seabed 12. Such a set is also called an ocean seismic bottom cable (OBC) . The seismic sensors are conventional underwater sensors that include hydrophones and multi- component geophones . Then a seismic source is positioned at at least one location, wherein each location has a predetermined position. In this case, the seismic source is towed behind a vessel 30 and the predetermined locations are indicated by crosses referred to with reference numerals 33 and 34. For each location 33 and 34 a recording is made of the signal from the seismic sensor system 20, 21, 22, 23, 24 and 25 in response to a sound wave emitted by the seismic source at the location 33 and 34. Thus the first recording (20-33) is the signal from the sensor 20 in response to a sound wave emitted at the location 33, the second recording (21-33) is the signal from the sensor 21 in response to a sound wave emitted at the location 33, and so on until the sixth recording. Which sixth recording (25-33) is the signal from the sensor 25 in response to a sound wave emitted at the location 33. Then the vessel 30 is moved so that the source is at the location 34, and there are six recordings (20-34 through 25-34) from the sensors in response to a sound wave emitted at the location 34. In addition, in order to enable carrying out the next surveys under substantially the same conditions, the appropriate sea properties are also determined at the locations 33 and 34. In case one is interested in the P-waves, the two-way water travel time is measured at the locations 33 and 34, whereas, in case one is interested in the S-waves, the seabed pressure, which is the water pressure directly under the locations 33 and 34 is measured.

So far, we created the reference recordings. Now the time-lapse aspect is introduced. After a predetermined period of time, for example one week, the seismic source at the location 33 and 34. A next set of recordings is made when, at the locations 33 and 34 the appropriate sea properties are substantially repeating, that is to say, the two-way travel times or the water pressures are substantially the same. If this condition is satisfied the signal from the seismic sensor system 20, 21, 22, 23, 24 and 25 in response to a sound wave emitted by the seismic source at the locations 33 and 34. This results in 12 recordings of the signals.

In order to detect changes in the target layers 1 and 2, the obtained signals are subtracted from the previously obtained signals. The differences between the corresponding signals are then used to detect changes in the target layers 1 and 2 as a function of time.

Subtracting corresponding signals means that signal 20-33 is subtracted from signal 20-33 as previously obtained. The survey can be repeated many more times; and the obtained signals can be subtracted from any of the previously obtained signals. By carrying out the subsequent surveys under conditions such that the appropriate sea properties are substantially the same, it is ensured that surface multiples repeat for P-waves, or that the stress conditions in the formation repeat for the S-waves .

Moreover because the recorded signals are subtracted, the repeating contributions to the signals do not appear in the difference. And thus the effects of surface multiples or stress conditions do not appear in the difference. Thus the present invention provides a simple method of time-lapse surveying in which non-repeatable effects caused by the sea can be suppressed.

A further advantage of the method according to the present invention is that there is no need to carry out each survey such that a high data stacking fold is obtained; thus there is no need to record in each survey the data a number of times with varied source and sensor locations and then combining the data.

In the embodiment of the invention as discussed with reference to the Figure, the appropriate sea properties, two-way water travel time or sea bed pressure, were determined at the locations 33 and 34 of the source. Alternatively, the appropriate sea properties can be determined at a fixed location near the area where the marine survey is carried out. This is a more simple way of determining the sea properties.

In the embodiment of the invention as discussed with reference to the Figure, the vessel 30 had moved so as to bring the seismic source from position 33 to position 34. Alternatively two or more sensors can be attached to the vessel to form a seismic streamer.

In the embodiment as discussed with reference to the Figure, the seismic sensor or sensors were located on the seabed. However, it is not necessary to do so. Alternatively, the seismic sensor or sensors are located in a bore hole traversing through the underground formation.

In a suitable alternative embodiment, the seismic sensor is a single seismic sensor or a set of seismic sensors arranged in a borehole, and for every survey the seismic source is positioned at a plurality of locations arranged in a closed loop surrounding the projection of the position (s) of the sensor (s) to the sea surface along a normal to the target formation layer. The signal that is recorded consists of a set of signals, each signal belonging to the combination of a position of the seismic source and a sensor.

Claims

C L A I M S

1. Method of carrying out at sea a time-lapse survey of a target layer in an underground formation, which method comprises the steps of:

(a) arranging a seismic sensor system at a predetermined position;

2. The method according to claim 1, wherein step (c) comprises positioning after a predetermined period of time the seismic source at the location(s) of step (b) , and recording, when the two-way water time from sea surface to seabed is substantially the same as for the previous recording, for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source.

3. The method according to claim 1, wherein step (c) comprises positioning after a predetermined period of time the seismic source at the location (s) of step (b) , and recording, when the seabed pressure is substantially the same as for the previous recording, for each location the signal from the seismic sensor system in response to a sound wave emitted by the seismic source.

4. The method according to any one of the claims 1-3, wherein the seismic sensor system comprises a set of seismic sensors located on the seabed.

5. The method according to any one of the claims 1-3, wherein the seismic sensor system comprises a set of seismic sensors located in a wellbore arranged in the underground formation.

6. The method according to claim 4 or 5, wherein the seismic source is a single seismic source, wherein the seismic source is positioned in steps (b) and (c) at a plurality of locations, each having a predetermined position, and wherein the signal that is recorded consists of a set of signals, each signal belonging to a position of the seismic source and a seismic sensor.

7. The method according to claim 6, wherein the seismic source is a single seismic source, wherein the seismic source is positioned in steps (b) and (c) at a plurality of locations arranged in a closed loop surrounding the projection of the position (s) of the sensor (s) to the sea surface along a normal to the target formation layer, and wherein the signal that is recorded consists of a set of signals, each signal belonging to the combination of a position of the seismic source and a sensor.