GB2394774A

GB2394774A - Microseismic monitoring of hydrocarbon production well including means to reduce fluid flow noise from production tubing

Info

Publication number: GB2394774A
Application number: GB0225048A
Authority: GB
Inventors: Robert Hughes Jones; Ian S Brown; Andrew John Jupe
Original assignee: ABB Offshore Systems Ltd
Current assignee: Baker Hughes International Treasury Services Ltd
Priority date: 2002-10-28
Filing date: 2002-10-28
Publication date: 2004-05-05
Also published as: GB0225048D0; NO20034808D0; BR0304775A; NO20034808L; US20040125696A1

Abstract

A method of monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing 1 and an outer casing 13, said method comprising providing one or more microseismic sensors 5 in contact with the well casing of the well, and taking steps to enhance the ability of said sensors to detect microseismic signals over the background noise generated by fluid flow inside said production tubing is enhanced. The enhancement is achieved either by processing data detected at the sensors 5 to reduce the effects of fluid flow noise, or by providing sound insulation between the sensors 5 and the production tubing 1.

Description

A METHOD AND INSTALLATION FOR

MONITORING MICROSEISMIC EVENTS

The present invention relates to a method and installation for monitoring microseismic 5 events. Microseismic events are of interest as they can provide information about fluid extraction fiom a hydrocarbon production reservoir or injection of fluid into the reservoir. The r emoval of oil or gas fiom the r eservoir leads to stress equalization processes, which can 10 cause rock failure in the reservoir itself or in other underground cavities in the area, which in turn leads to an elastic wave propagating away fiom the source. Depending on the source mechanism, different proportions of the acoustic energy are shared between compressional (Pwave) and shear (S-wave) waves. During the waves transit, the P and S waves travel through the interposing vibrational media, such as different rock strata.

15 Each rock type that the waves pass through has different P and S wave velocities and attenuation. By using a suitable arrangement of microseismic sensors (for example by using a biaxial arrangement of geophones or accelerometers and analysing the time lag between arrival of the P and S waves), it is possible via known techniques to locate the approximate location of the microseismic event.

While microseismic monitoring is well developed in some fields, for example that of

mining and similar rock engineering activities, most microseismic work in the petroleum industry has to date been of a temporary nature, ea. monitoring short-term operations such as fiacturings or cuttings, or experimental nature, ea. pilots for permanent systems In 25 most cases, where one or more production wells have already been constructed, measurements are conducted by locating one or more microseismic sensors inside one or more of the production wells.

In order to carry out a scan for microseismic events, it is important to identify a large 30 number of signals in order to ensure that the data collected is correctly interpreted and applied to the reservoir management. Thus, where the microseismic sensors are located

inside a production well, it normally becomes necessary to suspend production because, during operation, the production flow through the well tubing causes a relatively large amount of noise, which will swamp the microseismic signals which are, by comparison, inherently small. Without a good signal to noise ratio the number of microseisrnic signals 5 detected reduces and with this goes speed and confidence of interpretation of microseismic events. Furthermore, noise can affect the event localisation accuracy and hence result in an unclear understanding of the r esults being obtained.

If production is not suspended, only those signals large enough to stand out above the 10 background noise will be usable for the event localization. This presents a serious

problem, because, on the one hand, if production is not suspended it may take days or even weeks for sufficient numbers of signals to be obtained in order to obtain statistically relevant information, while on the other suspending production is a costly interruption for the oil company. Thus, it is in the interest of the petroleum industry to obtain a method of 15 readily obtaining information about the effect of the extraction process on the reservoir while extraction is in progress.

The only permanent production designed sensor array tool that is currently available is that produced by Createch Industrie, of 91882 Massy, France. The latter's effectiveness is 20 limited when in close proximity to the production tubing of a well because, as explained above, of the r eduction in the number of events detectable over and above the background

fluid flow noise. Likewise, determining the correct arrival time of P and S waves also becomes subject to errors.

25 US Patent No. 6,049,508 discloses a method of improving the chance of determining a significant microseismic event by avoiding spurious data from events directly connected with mechanical well operation, such as valve openings and closures. The method uses one or more sensors, such as geophones and hydrophores, and at least one reference pick up, placed in contact with the production casing. However, it does not consider the 30 difficulties posed by background flow noise.

According to the present invention from one aspect, there is provided a method of monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said method comprising: 5 a) providing two or more microseismic sensors adjacent the outer casing of a well; and b) processing the output of said sensors in order to provide said sensors with a directional response comprising a reduced sensitivity to sound coming fiom the direction of the production tubing, such that the ability of said sensors to detect 10 microseismic signals over the background noise generated by fluid flow inside said

production tubing is enhanced.

Preferably, step (b) comprises providing said sensors with a cardioid response 15 According to the present invention from another aspect, there is provided a method of monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said method comprising: a) providing one or more microseismic sensors adjacent the well casing of a well; 20 b) providing one or more microseismic sensors between the production tubing and the sensors located adjacent the casing; and c) processing the output of the sensors nearer the tubing in conjunction with the output of the sensors adjacent the casing such that the ability of the sensors adjacent the casing to detect microseismic signals over the background noise

25 generated by fluid flow inside said production tubing is enhanced.

According to the present invention from another aspect, there is provided a method of monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said method comprising a) providing one or more microseismic sensors adjacent the well casing of a well; and

b) providing increased sound insulation between said sensors and the production tubing such that the ability of said sensors to detect microseismic signals over the background noise generated by fluid flow inside said production tubing is

enhanced Optionally, some or all of the above methods of monitoring for microseismic events may be combined, in order to f rther improve the ability of the sensors to detect microseismic signals over the background fluid flow noise Where microseismic monitoring is to be

conducted using sensors installed in more than one well, one or more of the above methods 10 may be employed in each of said wells According to the present invention fi om another aspect, there is provided an installation for monitoring microseismic events in a hydrocarbon production r eservoir provided with a well comprising inner production tubing and an outer casing, said installation comprising 15 one or more microseismic sensors adjacent the well casing of the well and means for processing the output of said sensors in order to provide said sensors with a directional response comprising a reduced sensitivity to sound coming from the direction of the production tubing, such that the ability of said sensors to detect microseismic signals over the background noise generated by fluid flow inside said production tubing is enhanced

According to the present invention fi om another aspect, there is provide an installation for monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said installation comprising one or more microseismic sensors adjacent the casing of the well, one or more 25 microseismic sensors between the production tubing and the sensors located adjacent the casing of said well, and means for processing the output of the sensors nearer the tubing in conjunction with the output ofthe sensors adjacent the casing such that the ability ofthe sensors adjacent the casing to detect microseismic signals over the background noise

generated by fluid flow inside said production tubing is enhanced.

According to the present invention firom another aspect, there is provided an installation

for monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said installation comprising one or more microseismic sensors adjacent the casing of the well and increased sound insulation between said sensors and the production tubing such that the ability of said 5 sensors to detect microseismic signals over the background noise generated by fluid flow

inside said production tubing is enhanced.

Optionally, the features of one of the above installations for monitoring for rnicroseismic events may be combined with the features of one or both of the other installations, in order ] 0 to further enhance the sensors abilities to detect microseismic signals over the background

fluid flow noise As discussed above, where sensors are to be installed in production wells, sensor placement close to the flow-generated noise is inevitable. Thus a means of r educing the 15 flow-generated noise acting on these sensors, and thus enhancing their ability to detect a microseismic event, is required.

Generically speaking, a number of different methods are possible in order to reduce the amount of noise r eceived by a sensor. Noise reduction techniques can, broadly speaking, 20 be divided into "active" and "passive" techniques.

Passive techniques involve insulating the sensor against the potential source of noise, for example by changes in cross-sectional area/material property leading to an increase in r eflection/scattering, and/or adding an elastomeric inter-layer. In the context of attempting 25 to r educe the amount of fluid flow noise reaching one or more sensors located against a well casing, solutions such as placing sound-absorbent material on the sensor housing on the side facing the flow noise, or surrounding the sensors with acoustic foam filled airspace, all involve passive attenuation of the fluid flow noise 30 Active techniques consist of active noise control, beam-forming/null-steering. Both methods use signal processing to improve the signal to noise r atio, which in the context of

the present invention means increasing the microseismic signal to flow noise ratio, such that the ability of the sensors to pick out the desired signals over the background noise is

enhanced These techniques will be explained more fully below, with reference to the following drawings.

How active and passive techniques are applied differs fundamentally. In the present context, since the creation of r egions of quiet around the sensors is not of concern, the active techniques are applied to the signals only. In physical terms, all that is required is that the sensors be placed in the appropriate positions to ensure that the active techniques 10 can be applied effectively. By comparison, passive techniques cannot easily be applied once the sensors have been positioned in the completed production well and so must usually be engineered in the design of the installation, for example by making suitable modifications to the sensor array housing, production tubing, well casing or fluid surrounding the production tubing.

Since passive techniques have a tendency to be more effective at higher frequencies and active techniques more effective at lower fi equencies, a combination of both will in many cases be beneficial, in order to provide broadband attenuation ofthe noise signal. In some cases once the likely attenuation versus frequency for each type of active technique has 20 been determined, the precise form and location that of passive attenuation that will be beneficial will be apparent, and thus an appropriate combination can then be easily decided upon. In some cases a combination of several types of one active and passive techniques may prove helpful (ie. a combination of active noise control, beam- forming/null-steering, and more than one type of insulation).

Embodiments of the invention will now be described, by way of example, with r eference to accompanying drawings, in which Fig. 1 is a simplified vertical section through a length of production well illustrating an 30 installation for monitoring microseismic events;

Fig 2 is a simplified vertical section through a length of production well illustrating an alternative installation for monitoring microseismic events; Fig 3 is a graph charting the polar r espouse, at various discrete frequencies, of a sensor S having a cardioid response; and Fig 4 is a graph charting the r espouse versus fi equency, at various discrete angles, of the same sensor 10 Pig 1 shows, in simplified form, a vertical section of a length of production well, comprising a length of production tubing I, surrounded by a fluid filled annulus 2 and well casing 3 In active production, fluid extracted from the hydrocarbon reservoir flows through the production tubing in the direction of arrow 4 A first pair of microseismic sensors 5 is mounted on the inside of the well casing 3, and a second pair of microseismic I S sensors 6 is mounted on the outside of the production tubing 1 facing the casing mounted sensors S and at approximately the same height The signal outputs ofthe casing mounted sensors S and tubing mounted sensors 6 are connected to a data processing apparatus (not shown), which is preferably located topside The data processing apparatus is adapted to simultaneously process the signal outputs of the casing S and tubing 6 mounted sensors, 20 utilising active noise control (ANC) techniques in order to improve the rnicroseismic signal to fluid noise ratio ANC involves distinguishing a signal fiom the background noise at the frequency r ange

of interest It is most effective in simple cases, for example where the background noise

25 originates fiom a slowly varying, periodic, noise sources from reciprocating engines and at low fi equencies If the source is periodic then it is possible to measure the background

noise over one period, and generate the inverse and the appropriate transfer function The sample rate is synchronized with the engines' rotation The noise consists of the fundamental and a number of harmonics which are measured by a force transducer placed 30 in series with the engine mounting points and the cancelling source (vibrator)

Where the noise source is flow noise transmitted from the production tubing of an active well, the noise will not be so readily distinguishable from the microseismic signal.

However, the presence of sensors 6 mounted against the production tubing allows the noise signal up-stream, ie. closer to the noise source, from the casing mounted sensors 5 to be 5 measured. By estimating the noise at the tubing mounted sensors 6, the transfer function between the tubing mounted sensors 6 and casing mounted sensors 5 (based on the expected noise path between the sensors, as indicated on Fig. 1 by all ow 7), and the time for sound to travel between the tubing 6 and casing 5 mounted sensors, it is then possible for the data processing apparatus to subtract the estimated flow noise at the casing mounted 10 sensors 5 sensors fiom the output of the casing mounted sensors, thereby resulting in an improved ability to detect microseismic events during active production.

Fig.2 shows, again in simplified form, a vertical section of a length of production well, with the production tubing, fluid filled annulus and casing bearing the same reference 15 numbers as before. In the alternative installation shown, only casing mounted sensors 5 are required, with the topside data processing apparatus being programmed to process the signal outputs of the sensors 5 utilising beam forming/null steering techniques, in order to improve the microseismic signal to fluid noise r atio.

20 Beam forming involves processing the signal outputs of a minimum of two sensors and applying a phase shift or time delay of one relative to the other in order to provide each sensor with a directional response in which the sensitivity ofthe sensor to sound is r educed in one or more directions, the angle over which sensitivity is substantially maintained being referred to as the sensor's beam and the angle over which sensitivity is substantially 25 r educed being referred to as the null or beam minima. Null-steering involves then rotating the sensor's beam until the null is pointed in the direction in which sound is to ignored Thus, in the embodiment illustrated in Fig 2, the data processor operates to maximise the signal to noise ratio by forming an appropriate directional r espouse for each casing sensor 30 6, and then rotating the sensor's beam such that each sensor's null is pointed in the direction of the production tubing I. It should be noted that it is not necessary that the

casing mounted sensors 5 be directly adjacent each other, as sensor spacing will affect the final sensitivity of the sensors, with a tradeoff of noise reduction against signal reduction being necessary 5 Where omni-directional casing mounted sensors 5 are used, it is possible, using beam forming, to convert their response fiom omni-directional to cardioid (ie. to use beam forming to create a cardioid beam). Referring now to Figs.3 and 4, Fig.3 shows the polar r espouse of a sensor having a cardioid response, at various frequencies, while Pig.4 charts the change in response versus fi equency of the same sensor at various angles. As the 10 diagrams shown in Figs.3 and 4 represent in-air acoustics, the actual response obtainable by casing sensors in the production well environment may in some r espects be quantitatively different in some respects, but in qualitative terms the same type of response should be obtainable. As both figures clearly show, the response ofthe sensor remains flat between + and - 90 degrees except at high fi equencies (above 10 kHz), while the r espouse 15 of the sensor in the 180 degree direction is significantly reduced, particularly in the I to 2 kHz range.

Thus, if the casing sensors 6 are provided with such a cardioid response, orientated such that the production tubing dies at null position (180 degrees in the case ofthe response 20 illustrated in Pigs.3 and 4), then it is clear that significantly less flow noise will be picked up by the sensors (particularly in the fi equency r anges where attenuation at 180 degrees is significant) At the same time, while there will be some attenuation of microseismic signals originating fiom the 180 degree direction, the ability of the sensors to pick up microseismic signals originating from the + or - 90 degree region will be largely 25 unaffected, though at high frequencies some attenuation may occur as compared to what would be achieved if beam forming techniques had not been applied. Thus, overall the signal to noise ratio will be substantially improved.

Claims

CLAI1\IS:

]. A method of monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said 5 method comprising: a) providing two or more microseismic sensors adjacent the outer casing of a well; and b) processing the output of said sensors in order to provide said sensors with 10 a directional response comprising a reduced sensitivity to noise coming fi om the direction of the production tubing, such that the ability of said sensors to detect microseismic signals over the background noise generated

by fluid flow inside said production tubing is enhanced.

15
2. A method according to claim 1, wherein step (b) comprises providing said sensors with a cardioid response.
3. A method according to claim 1 or 2, wherein microseismic sensors are also provided between the production tubing and the sensors located adjacent the 20 casing, the output of the sensors nearer the tubing being processed in conjunction with the output of the sensors adjacent the casing in order to further enhance the ability of the sensors adjacent the casing to detect microseismic signals over the fluid flow noise.

25
4. A method according to any preceding claim, wherein increased sound insulation is provided between the casing sensors and the production tubing in order to further enhance the ability ofthe sensors adjacent the casing to detect microseismic signals over the fluid flow noise.

30
5 A method of monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said

method comprising: a) providing one or more microseismic sensors adjacent the well casing of a well; 5 b) providing one or more microseismic sensors between the production tubing and the sensors located adjacent the casing; and c) processing the output of the sensors nearer the tubing in conjunction with the output of the sensors adjacent the casing such that the ability of the sensors adjacent the casing to detect microseismic signals over the 10 background noise generated by fluid flow inside said production tubing is

enhanced.
6. A method according to claim 5, wherein increased sound insulation is provided between the casing sensors and the production tubing in order to further enhance 15 the ability ofthe sensors adjacent the casing to detect microseismic signals over the fluid flow noise.
7. A method of monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said 20 method comprising: a) providing one or more microseismic sensors adjacent the well casing of a well; and b) providing increased sound insulation between said sensors and the 25 production tubing such that the ability of said sensors to detect microseismic signals over the background noise generated by fluid flow

inside said production tubing is enhanced.
8. An installation for monitoring microseismic events in a hydrocarbon production 30 reservoir provided with a well comprising inner production tubing and an outer casing, said installation comprising one or more microseismic sensors adjacent the

well casing of the well and means for processing the output of said sensors in order to provide said sensors with a directional response comprising a reduced sensitivity to noise coming from the direction of the production tubing, such that the ability of said sensors to detect microseismic signals over the background noise generated

5 by fluid flow inside said production tubing is enhanced.
9. An installation for monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said installation comprising one or more microseismic sensors adjacent the 10 casing of the well, one or more microseismic sensors between the production tubing and the sensors located adjacent the casing of said well, and means for processing the output of the sensors nearer the tubing in conjunction with the output of the sensors adjacent the casing such that the ability of the sensors adjacent the casing to detect microseismic signals over the background noise

generated by fluid flow inside said production tubing is enhanced.
10 An installation for monitoring microseismic events in a hydrocarbon production reservoir provided with a well comprising inner production tubing and an outer casing, said installation comprising one or more microseismic sensors adjacent the 20 casing of the well and increased sound insulation between said sensors and the production tubing such that the ability of said sensors to detect microseismic signals over the background noise generated by fluid flow inside said production

tubing is enhanced.

25
1 l. A method of monitoring microseismic events in a hydrocarbon production r eservoir substantially as hereinbefore described with reference to Figures 1, 2, 3 and/or 4.
12. An installation for monitoring microseismic events substantially as hereinbefore described with reference to Figures 1, 2, 3 and/or 4.