GB2276723A

GB2276723A - Monitoring subterranean formations

Info

Publication number: GB2276723A
Application number: GB9404549A
Authority: GB
Inventors: Laurence Nicoletis; Philippe Staron; Jean Laurent
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1993-03-29
Filing date: 1994-03-09
Publication date: 1994-10-05
Anticipated expiration: 2014-03-09
Also published as: NO941125L; CA2120190C; FR2703470B1; FR2703470A1; NO941125D0; GB2276723B; GB9404549D0; CA2120190A1

Abstract

A permanently installed acoustic or seismic transmission-reception device for monitoring changes in a subterranean formation over a period of time, comprising in combination an acoustic or seismic wave transmission and reception set permanently installed in at least one well, having several transmission and reception units (U1, U2... Up) located at different depths so that they may be coupled with the terrain surrounding the well whilst leaving the well free for another activity, a control and recording station comprising a control unit (Uc) to run the acoustic or seismic wave transmission by the transmission units in the transmission-reception sets and a transmission system (4) to link the station to the transmission and reception set. <IMAGE>

Description

A PERMANENT TRANSMISSION DEVICE FOR MONITORING A SUBTERRANEAN FORMATION AND A METHOD FOR IMPLEMENTING THE DEVICE The present invention relates to a transmissionreception device permanently installed in one or several wells to monitor changes in a subterranean formation over a period of time, as well as a method for implementing the device.

The device in accordance with the invention is well suited to monitoring changes in a formation containing effluent, for example, particularly petroleum effluent, since the changes that occur are connected with production operations, for example.

The present invention may be specifically applied to monitoring an oil or gas field where the operators need to follow very closely any changes in fluid saturation in a subsurface production zone over a period of time.

A known method for monitoring changes in a formation during production consists in, for example, lowering neutron logging tools at regular intervals into wells distributed over the field under observation and moving the tools through the depth of the reservoir beds to produce logs for analysing gas saturation which can then be correlated with the quantities of fluid produced or injected. This process has advantages in that the signal transmitters and receivers used offer mobility but on the other hand operations carried out in the well through which the measuring sonde is moved have to be halted. It is therefore desirable to be able to carry out logging operations in a subterranean zone of a greater or lesser thickness without having to place the well crossing this zone out of action.

A technique is known from the applicant's patents FR 2,600,172, 2,642,849, 2,674,029 and 2,681,373 which avoids having to place a well out of action, essentially consisting in installing a large number of sensors on the exterior of pipes used to line one or several wells crossing through a zone being monitored. These sensors linked to local electronic housings are connected by transmission lines to a control and recording station at the surface. Once a lining pipe (or casing) has been installed, the sensors located in the annular space between the casing and the well are coupled with the formations surrounding the well by means of sealing cement.This installation and coupling technique allows passive monitoring of the noises emitted spontaneously by a formation during production or enables seismic prospecting operations to be carried out by recording the signals received by the various sensors in response to acoustic or seismic waves transmitted into the formation from a seismic source located at the surface. A conventional well source, such as that described in the applicant's patent FR-A-2 629 216, may also be lowered into another well or possibly into the same well but this option, which places the well out of action, is not suitable for monitoring over a long period.

The permanently installed acoustic or seismic transmission-reception device in accordance with the invention allows changes in a subterranean formation to be monitored over a period of time. It is characterised in that it has in combination a transmission and reception set for acoustic and seismic waves permanently installed in at least one well, having several transmission and reception units located at different depths so that they may be linked to the terrain surrounding the well whilst leaving the well free for other activities (particularly production operations in the formation), a control and recording station having a control unit to operate the transmission units in the transmission and reception sets sending acoustic or seismic waves and a transmission system for linking the station to the transmission-reception set.

The transmission-reception set comprises, for example, several transmission and reception units distributed over a section of the length of at least one well and arranged alternately.

The transmission-reception set may also have units distributed in several wells.

In accordance with one embodiment, each transmission unit is linked to a local control module, in which case the transmission-reception set may have several transmissionreception groups located at different depths in the well, each of them being designed to transmit and receive acoustic or seismic waves and being linked to a local interface module.

Each transmission-reception group has, for example, at least one transmission-reception transducer.

In accordance with one embodiment, each transmissionreception transducer is a geophone or possibly an accelerometer and the device preferably incorporates means for applying signals to the transmission units, varying in frequency over a period of time.

In accordance with one possible configuration, a number at least of the transmission-reception sets are installed in at least one well fitted with a lining tube located in the annular space between the lining and the well and embedded in the sealing cement.

In accordance with another configuration, a number at least of the transmission-reception sets are located on the outside of a pipe installed in at least one well and connected to it by connecting means that permit acoustic uncoupling from the pipe.

The monitoring method in accordance with the invention allows changes in a subterranean formation to be monitored over a period of time by means of acoustic or seismic waves.

It comprises in combination: 1) installing permanently in at least one well drilled through the subterranean formation an acoustic or seismic wave transmission-reception set having at least one transmission unit and at least one reception unit located at different depths, this set being positioned so that it may be coupled with the terrain surrounding the well whilst leaving the well accessible (for an operation connected with putting the formation under production, for example).

2) running transmission-reception cycles, each of which consists in: a) transmitting waves from the transmission unit successively through several of the transmission units located at different depths, and b) receiving and recording waves emitted by the formation in response to the successive transmissions; and 3) comparing the recordings made during the successive cycles so as to pinpoint changes occurring in the formation during the periods of time following effluent production.

With certain configurations of transmission-reception sets, a log may be made in some cases of the formations surrounding the well. With other configurations, seismic prospecting operations may be carried out from well to well.

Depending on the configuration used, the device in accordance with the invention may be used to carry out various types of seismic prospecting or logging operation without inasmuch impeding working operations in the zone being monitored.

Other characteristics and advantages of the device in accordance with the invention will be more clearly shown from the following description of embodiments described by way of example and in no way limiting the scope of the invention, and with reference to the attached drawings in which: - Fig. 1 shows schematically a first embodiment of the device with an acoustic or seismic wave transmission reception set having at least one transmission unit and at least one receiving unit permanently installed at different depths behind a lining tube (casing) and coupled with the formations by means of the sealing cement;; Fig. 2 shows schematically a second embodiment in which the transmission-reception set has, for example, two transmission units and two reception units which are spaced apart from each other at intervals along the length of a tube and positioned in a well at different depths by installing the tube in the well; Fig. 3 shows schematically a variant of the embodiment illustrated in Fig. 1 in which a set of transmission units and reception units is positioned in the zone being monitored, behind a lining tube, spaced alternately at a distance from each other, a transmission unit being inserted between two successive reception units; Fig. 4 shows schematically a configuration similar to the previous one applied to the embodiment illustrated in Fig. 2;; Fig. 5 shows a variant of the embodiment illustrated in Fig. 1 in which the transmission and reception units used are transducers capable of operating both as wave transmitters and receivers, this embodiment also being applicable to the embodiment shown in Fig. 2; and Fig. 6 shows schematically a housing for three transducers whose axes are oriented along three orthogonal directions that may be used to transmit waves and pick up the induced wave components received.

The method in accordance with the invention is operated by means of a transmission-reception device located permanently in at least one well crossing a subterranean formation containing, for example, petroleum effluent, and coupled with the formations surrounding the well, so as to leave access to the well free for any other activity. This device has a given number p of acoustic or seismic wave transmission and reception sets and is connected to a central control and recording station by means of a transmission system comprising one or several transmission cables.

The transmission and reception sets U1, U2, ... Uk ...

Up may be permanently installed in a well 1 using a technique described in the French patents 2,600,172, 2,642,849 and 2,681,373, already mentioned. The p units U1 to Up are positioned behind a lining tube or casing 2 which is then lowered into the well. Preferably, the units are placed in purpose-built housings (not shown in the drawing). The cement that is then injected into the space 3 between the tube and the well walls provides acoustic coupling of the transmission and reception sets with the surrounding formations. The transmission and reception sets (U-Up) are permanently connected to a station 5 at the surface having a control unit (Uc) and a recording unit SR, by means of a transmission system having at least one cable 4. In all instances, the fact that the transmission-reception device is located on the outside of the tube means that the well is left free for other uses.

In accordance with the embodiment shown in Fig. 2, a transmission-reception set with a given number p of units U1, U2, ... Uk, ... Up is permanently installed using a technique described, for example, in patents FR 2,656,034 and 2,674,029. These units are linked to a tubular column 6 lowered into a lined well. During operation they are held against the lining tube so as to improve their coupling with the formations surrounding the well. Connecting means 7 connect the different units to the surface station 5. Here too, the fact that the transmission-reception set is located on the outside of the tubular column 6 means that the well is left free for other uses.

Preferably, the transmission system has electronic modules (not shown) connected to the reception units. They are supplied from the surface station 5 and are designed to digitise the signals received and transmit them in encoded form for recording.

The units U1 to Up may be installed on either side of the well section crossing through the zone Z and/or along this section of well. One or several of these units are acoustic or seismic wave transmitters. One or several of them are receivers of these same waves.

In its simplest configuration, the device in accordance with the invention has an acoustic or seismic wave transmission-reception set made up of two units U1 and U2.

One of the units U1 is a transmission unit located in the well above the zone being monitored, for example, and the other unit U2 is a reception unit located below this same zone.

By actuating the transmission unit U1, if it is a single unit, or each of the units if the set comprises several, the induced waves P or S received by the reception unit U2 or each of the reception units maybe recorded at the station 5 and by making comparisons between the successive sets of recordings, the variations in gas saturation in the formations crossed over a period of time, for example, may be ascertained.

In accordance with the different embodiments shown in Figs. 1 and 2 respectively, illustrated schematically in Figs. 3 and 4, the transmission-reception set may have, for example, p transmission and reception sets R1, El, R2, E2, ... Rn, En located alternately at regular intervals along the well sections crossing the zone Z being monitored.

With this type of configuration, an acoustic log of the zone Z may be made in waves P or S. Each transmitter El to En is actuated successively and the induced waves sent back from the formation are received by the receivers adjacent to each of them. Variations in the gas saturation of rocks over a period of time may thus be determined.

A practical embodiment of the transmission-reception set involves, for example, installing p transducers EK1, ERkl, ERk, ERk+l ... ERp, (Fig. 5) capable of operating both as acoustic or seismic wave transmitters or receivers. Each transducer ERk issues a signal in succession and the induced signals are received by the two adjacent transducers ERk.1 and ERk+1, for example.

Geophones, for example, designed to operate as vibrators, may be used as transducers. As they radiate low energy, they are preferably supplied with a vibratory signal, varying in frequency over a period of time. In order to obtain workable recordings, the transmission duration of the variable frequency signal is determined depending on the power the selected transducer can radiate.

Although their electrical impedance is generally greater than that of geophones, accelerometers of the piezoelectric type, for example, may also be used.

One method of controlling each transmission unit and in particular each transducer used as a transmitter is to generate the transmission signals in situ.

In this instance, the transmission system has local signal generators (not shown) powered by the surface station 5, these generators being designed to generate the transmission signal depending on the encoded signal received via the transmission lines in the cable 4 (Figs. 1, 2). As is the case, the signal applied to each transducer may be pulsed or continuous.

A connection system enabling bi-directional communication between the various transmission-reception sets and the surface station 5 and transmission of electrical energy to the transmission units is described, for example, in French patent applications FR 91/15.691 and 92/03.575 filed by the applicant.

Lines running in hydraulic channels may also be used to inter-connect the different transmission and reception sets as described in patent FR 91/15.483.

A transmission-reception set permanently connected to the surface station 5 has been described. However, the units in the transmission-reception set may be intermittently connected to the surface station 5 to coincide with periods when the zone Z is being seismically monitored, as per the technique described in the applicant's patent FR 92/02.201.

Embodiments have been described in which the transmission-reception set is located in a single well.

Clearly, however, the transmission and reception units may be distributed in several wells crossing the monitored zone so as to carry out seismic prospecting from well to well.

Claims

1) A permanently installed acoustic or seismic transmission-reception device for monitoring changes in a subterranean formation over a period of time, comprising in combination an acoustic or seismic wave transmission and reception set permanently installed in at least one well, having several transmission and reception units located at different depths so that they may be coupled with the terrain surrounding the well whilst leaving the well free for another activity, a control and recording station comprising a control unit (Uc) to run the acoustic or seismic wave transmission by the transmission units in the transmissionreception sets and a transmission system to link the station to the transmission and reception set.

2) A device as claimed in claim 1, wherein the transmission-reception set comprises several transmission and reception units distributed over a section of the length of at least one well and positioned alternately.

3) A device as claimed in claim 1 or 2, wherein the transmission-reception set has units distributed in several wells.

4) A device as claimed in claim 1, wherein each transmission unit (U) is linked to a local control module.

5) A device as claimed in claim 4, wherein the transmission and reception set comprises several transmission-reception groups positioned at different depths in the well, each of them being designed to transmit and receive acoustic or seismic waves and being linked to a local interface module.

6) A device as claimed in claim 5, wherein each transmission-reception group comprises at least one transmitting-receiving transducer.

7) A device as claimed in claim 6, wherein each transmitting-receiving transducer is a geophone.

8) A device as claimed in claim 6, wherein each transmitting-receiving transducer is an accelerometer.

9) A device as claimed in claim 1, comprising means for applying signals of varying frequency to the transmission units over a period of time.

10) A device as claimed in any one of claims 1 to 9, wherein at least a number of the transmission and reception units is located in at least one well fitted with a lining tube in the annular space between it and the well and embedded in the sealing cement.

11) A device as claimed in any one of claims 1 to 9, wherein at least a number of the transmission and reception units is located on the outside of a tubular column inserted in at least one well and fixed to this column by connecting means that allow acoustic uncoupling from the column.

12) An investigating method for following changes in a subterranean formation over a period of time by means of acoustic or seismic waves, comprising: 1) permanently installing in at least one well drilled through the subterranean formation an acoustic or seismic transmission-reception set having at least one transmission unit and at least one reception unit located at different depths, this set being arranged so that it may be coupled with the terrain surrounding the well whilst leaving access to the well free;

2) running transmission-reception cycles each comprising: a) the transmission of waves by each transmission unit and b) the reception and recording of waves emitted from the formation in response to the successive transmissions; and 3) comparing the recordings made during the successive cycles so as to pinpoint changes occurring in the formation during the period following effluent production.

13) A method as claimed in claim 12, wherein the transmission-reception set comprises several transmissionreception groups positioned at different depths along at least one well in a section of the well crossing a subsurface zone, and in that the transmission-reception cycles involve taking a log of the formations in the said section and actuating in succession the different transmissionreception groups to transmit waves at different points along the well.

14) A method as claimed in claim 13, wherein each transmission-reception group is a transducer that can operate alternately both as a transmitter and a receiver, so that a log is taken of the formations crossed by the well section by actuating all the transducers in succession by means of variable frequency signals and the induced signals are received by the transducers.

15) A method as claimed in claim 12, wherein since the transmission-reception set has several transmission units positioned at different depths along at least a first well in the section of the well crossing through a sub-surface zone, and several reception units distributed at different depths along at least a second well, the transmission-reception cycles involve actuating different transmission groups in one well in succession to transmit waves at different points along the first well and receive waves by means of the transmission units along the second well.

16) A device substantially as hereinbefore described with reference to Fig. 1 of the drawings.

17) A device substantially as hereinbefore described with reference to Fig. 2 of the drawings.

18) A device substantially as hereinbefore described with reference to Fig. 3 of the drawings.

19) A device substantially as hereinbefore described with reference to Fig. 4 of the drawings.

20) A device substantially as hereinbefore described with reference to Fig. 5 of the drawings.

21) A method substantially as hereinbefore described with reference to Figs. 1 to 6 of the drawings.