CN116398120A

CN116398120A - Downhole casing quality monitoring system and method based on optical fiber sensing technology

Info

Publication number: CN116398120A
Application number: CN202310438955.4A
Authority: CN
Inventors: 余刚; 梁兴; 张少华; 苟量; 夏淑君; 王熙明; 安树杰; 蔡志东; 吴俊军; 陈沅忠
Original assignee: Optical Science and Technology Chengdu Ltd of CNPC
Current assignee: Optical Science and Technology Chengdu Ltd of CNPC
Priority date: 2023-04-23
Filing date: 2023-04-23
Publication date: 2023-07-07

Abstract

The invention discloses an underground casing quality monitoring system and a monitoring method based on an optical fiber sensing technology, relates to the technical field of drilling and technology and oil and gas reservoir development, and can at least partially solve the problem of real-time quality detection of underground casings.

Description

Downhole casing quality monitoring system and method based on optical fiber sensing technology

Technical Field

The invention belongs to the technical field of drilling and technology and oil and gas reservoir development, and particularly relates to a downhole casing quality monitoring system and a downhole casing quality monitoring method based on an optical fiber sensing technology.

Background

Cementing, setting a casing into the well, and injecting cement into an annular space between the well bore and the casing. Is an essential link in the drilling operation process, and comprises casing running and cementing. The well cementation technology is a multidisciplinary comprehensive application technology and has the characteristics of systematicness, disposability and short time. The main purpose of well cementation is to protect and support the casing in the oil and gas well and seal and isolate the stratum such as oil, gas and water.

The well cementation operation is one of the most important links in oil and gas well drilling engineering, and the main purpose of the well cementation operation is to seal an oil layer, a gas layer and a water layer in a well bore, protect an oil and gas well casing, prolong the service life of the oil and gas well and improve the oil and gas yield. There are typically at least two well cementing operations (production wells), up to 4-5 well cementing operations (deep well). The uppermost well cementation is surface casing well cementation, which plays the role of a mud path and an oil gas portal. Before the next drill-out, a blowout preventer is arranged on the surface casing to prevent blowout. A mud conduit is arranged above the blowout preventer, and is a passage for drilling fluid to return to a mud pit. In the well drilling process, a technical sleeve is often used for well cementation, and the well cementation plays a role in consolidating the rear and safely exploring a road. As well as tunnels of highways and tunnels in coal mines, well collapse, high pressure and unstable stratum can be encountered in the well drilling process, and meanwhile, the well drilling device also plays a role in rescue in order to get a way in danger in the forward road exploration.

The bushings are of different sizes and steel grades. Surface cementing typically uses 20-13/8 inch casing, most often with "J" casing of low steel grade. Technical bushings typically use 13/8-7 inch bushings, with higher steel grades. Casing cementing typically uses 7-5 inch casing with the same steel grade strength as technical casing. The strength of the casing is designed according to the application, the formation predicted pressure and the casing running depth, and the application wall thickness, the steel grade and the screw thread type of the casing are determined.

The casing is different from the drill rod, is a one-time-to-put pipe, has no thickened part and has no strict length. In order to ensure the well cementation quality and smoothly putting into the casing, the structural design of the casing string is required.

Downhole fiber optic sensing systems may be used downhole to make measurements of pressure, temperature, noise, vibration, acoustic waves, seismic waves, flow rates, component analysis, electric and magnetic fields. The system is based on a fully armored optical cable structure, and the sensor, the connection cable and the data transmission cable are made of optical fibers. There are various methods for laying the armored optical cable on the ground and underground, such as laying the armored optical cable in a underground control pipeline, putting the armored optical cable into a coiled tubing, directly integrating the armored optical cable into the wall of the coiled tubing made of composite materials, binding and fixing the armored optical cable on the outer side of the coiled tubing, putting the armored optical cable into a sleeve, binding the armored optical cable on the outer side of the sleeve, and permanently fixing the armored optical cable with cement.

A well operation is a construction operation in which a casing is run into a well and cement is injected into the annular space between the wellbore and the casing. Is an essential link in the process of drilling and completing well, and comprises casing running and cementing. The poor quality of the underground casing and well cementation is easy to cause the oil-water mixing in the underground, which not only affects the subsequent operation, but also damages the ground stress balance of one area, thereby causing the damage of the casing with large area. Thus, real-time quality inspection of casing running downhole is highly desirable.

Disclosure of Invention

The invention aims to disclose an underground casing quality monitoring system and method based on an optical fiber sensing technology, wherein an armored optical cable connected with a composite modem is fixed on the outer side of a casing, DAS, DTS, DSS and DPS data of the measured armored optical cable outside the casing are acquired in real time, and real-time quality detection of the casing which is arranged underground is realized.

In order to achieve the above purpose, the specific technical scheme of the invention is as follows:

the application provides a down-hole casing quality monitoring system based on optical fiber sensing technology, including the compound modem that has distributed optical fiber sensing signal port, be used for laying the sleeve pipe in the bore hole of bore hole, at least one is fixed in the armor optical cable outside the sleeve pipe along sleeve pipe extending direction to and be used for transmitting the optical signal of different passageway from the fixed axle to the optical fiber rotary connector or the smooth ring of rotating end, armor optical cable passes through optical fiber rotary connector or light sliding ring and links to each other with compound modem.

In some alternative embodiments, the armored fiber optic cable includes a composite sensing light unit for measuring vibration noise and temperature of the downhole casing, a strain sensitive light unit for measuring strain of the downhole casing, and a fluid pressure sensing light unit for measuring fluid pressure within the subterranean rock or rock pore;

the composite modem is used for demodulating the received DAS/DTS/DSS/DPS signals in real time during operation and is provided with a DAS/DTS/DSS/DPS sensing signal port;

the composite sensing optical unit of the armored optical cable is connected with the DAS sensing signal port and the DTS sensing signal port of the composite modem, the strain sensitive optical unit is connected with the DSS sensing signal port, and the fluid pressure sensing optical unit is connected with the DPS sensing signal port.

In some optional embodiments, the composite sensing optical unit includes a single mode fiber, two multimode fibers and a stainless steel tubule, the single mode fiber and the multimode fibers are respectively connected with a DAS sensing signal port and a DTS sensing signal port of the composite modem, the single mode fiber and the multimode fibers are both arranged in the stainless steel tubule, and the stainless steel tubule is also filled with high temperature resistant optical fiber paste.

In some alternative embodiments, the tail end of the single mode fiber remote from the DAS sensing signal port is provided with a delustrer; and the tail ends of the two multimode optical fibers far away from the DTS sensing signal port are welded into a U shape and are used for carrying out high-precision DTS temperature measurement under the double-end input state.

In some alternative embodiments, the strain sensitive optical unit includes a single mode fiber, a protective sleeve for wrapping the single mode fiber, and a stainless steel tubule, where the single mode fiber and the protective sleeve are tightly wrapped in the stainless steel tubule.

In some optional embodiments, the fluid pressure sensing optical unit includes a microstructure pressure sensing optical fiber and a stainless steel tubule, the pressure sensing optical fiber is disposed in the stainless steel tubule, and a plurality of windows uniformly distributed along the axial direction of the stainless steel tubule are formed on the stainless steel tubule.

In some optional embodiments, the fluid pressure sensing optical unit includes a continuous grating optical fiber and a stainless steel tubule, the continuous grating optical fiber is disposed in the stainless steel tubule, and the stainless steel tubule is provided with a plurality of windows uniformly distributed along the axial direction thereof.

In some alternative embodiments, the stainless steel capillary tube has an inner diameter in the range of 1mm to 2mm.

In some alternative embodiments, the armored optical cable has a rectangular cross-sectional shape, and at least one armored optical cable is provided; in the case that the number of the armored optical cables is more than one, two or more armored optical cables are uniformly distributed along the circumferential direction of the sleeve;

the device also comprises an annular clamp used for fixing the armored optical cable outside the sleeve, and the annular clamp is uniformly arranged along the axial direction of the sleeve.

Another aspect of the present application provides a method for monitoring quality of an underground casing based on an optical fiber sensing technology, which is applicable to any one of the above underground casing quality monitoring systems based on an optical fiber sensing technology, and specifically includes the following steps:

s1: the armored optical cables which are uniformly distributed along the circumferential direction of the casing pipe suspended above the wellhead are arranged outside the casing pipe, and the armored optical cables are connected with the composite modem through a fiber rotating connector or an optical slip ring;

s2: before the sleeve-running operation, starting a composite modem to test and judge whether a composite sensing light-sensing unit, a strain-sensitive light-sensing unit and a fluid pressure sensing light unit in the armored optical cable are intact;

s3: if the armored optical cable is intact, the sleeve pipe with the armored optical cable fixed is lowered into the open hole drilling hole, and real-time measurement is carried out by utilizing the armored optical cable in the lowering process;

s4: DAS, DTS, DSS and DPS data measured in real time by an armored optical cable are processed in real time, and DSS and DPS data are corrected for temperature drift by using DTS temperature data measured along the sleeve;

s5: the state of the casing in the well, the integrity of the casing and whether the casing is disjointed, deformed or damaged in the process of casing running operation are monitored, judged and evaluated in real time according to the data corrected by temperature drift; if abnormality occurs, alarming in time.

The invention has the beneficial effects that:

the invention discloses an underground casing quality monitoring system based on an optical fiber sensing technology, which comprises a composite modem with distributed optical fiber sensing signal ports, a casing arranged in an open hole borehole, at least one armored optical cable fixed outside the casing along the extending direction of the casing, and an optical fiber rotary connector or a smooth ring for transmitting optical signals of different channels from a fixed shaft to a rotary end, wherein the armored optical cable is connected with the composite modem through the optical fiber rotary connector or an optical slip ring.

The effect is as follows: when the casing is placed underground, DAS, DTS, DSS and DPS data of the measured armored optical cable outside the casing are acquired in real time, and the state of the casing in the underground and the integrity of the casing are monitored, judged and evaluated in real time during the casing operation according to the real-time change of the intensity and the frequency of underground vibration noise, the real-time change of the stratum stress acting on the outer side of the casing and the real-time change of the pressure of fluid in stratum pores, and the deformation or damage of the casing is monitored in real time during the casing operation.

After the sleeve operation is finished, vibration noise, abnormal stress change and abnormal pressure change well sections outside the sleeve can be continuously monitored in real time, long-term real-time monitoring can be carried out on underground interlayer series flow, deformation or sleeve damage generated by the sleeve under the action of underground abnormal stress strain or abnormal fluid pressure, the integrity, stability and reliability of an oil gas production well or a water injection steam injection drive well can be ensured, and long-term stable production and high production are maintained.

Drawings

FIG. 1 is a layout diagram of embodiment 1 of the present invention;

FIG. 2 is a schematic cross-sectional view of an armored fiber optic cable in accordance with embodiment 1 of the present invention;

FIG. 3 is a schematic diagram of a pressure sensing unit in embodiment 1 of the present invention;

FIG. 4 is a schematic cross-sectional view of the embodiment 1 of the present invention, in which 2 armored cables are arranged outside the casing;

FIG. 5 is a schematic view showing a cross-sectional structure of the 3 armored cables laid out of the jacket in embodiment 1 of the present invention;

fig. 6 is a schematic view showing a cross-sectional structure of the 4 armored cables laid out outside the jacket in embodiment 1 of the present invention.

Reference numerals:

the device comprises a 1-naked-eye drilling hole, a 2-sleeve, a 3-armored optical cable, a 4-annular clamp, a 5-composite modem, a 6-optical fiber rotary connector, a 7-single mode optical fiber, an 8-multimode optical fiber, a 9-strain sensitive optical unit, a 10-fluid pressure sensing optical unit, an 11-matting device, a 12-U-shaped structure, a 13-protective sleeve, a 14-stainless steel thin pipe, a 15-pressure sensing optical fiber, a 16-composite sensing optical unit, a 17-high-temperature-resistant optical fiber paste and an 18-window.

Detailed Description

The present invention will be described in further detail with reference to examples and drawings, but embodiments of the present invention are not limited thereto.

In the description of the present invention, it should be noted that, directions or positional relationships indicated by terms such as "center", "upper", "lower", "left", "right", "vertical", "longitudinal", "lateral", "horizontal", "inner", "outer", "front", "rear", "top", "bottom", etc., are directions or positional relationships based on those shown in the drawings, or are directions or positional relationships conventionally put in use of the inventive product, are merely for convenience of describing the present invention and for simplifying the description, and are not indicative or implying that the apparatus or element to be referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be construed as limiting the present invention.

In the description of the present invention, it should also be noted that, unless explicitly specified and limited otherwise, the terms "disposed," "configured," "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the above terms in the present invention will be understood in specific cases by those of ordinary skill in the art.

The invention is described in detail below by reference to the attached drawings and in connection with the embodiments:

example 1:

as shown in fig. 1 to 6, the present embodiment provides a downhole casing 2 quality monitoring system based on an optical fiber sensing technology, which includes a composite modem 5 having distributed optical fiber sensing signal ports, a casing 2 for being deployed in an open hole borehole 1, at least one armored optical cable 3 fixed outside the casing 2 along an extending direction of the casing 2, and an optical fiber rotary connector 6 or a smooth ring for transmitting optical signals of different channels from a fixed shaft to a rotary end, wherein the armored optical cable 3 is connected with the composite modem 5 through the optical fiber rotary connector 6 or an optical slip ring. The composite modem 5 and the fiber optic rotary connector 6 in this embodiment are both located on the surface near the wellhead.

In this embodiment, when the casing 2 is placed downhole for operation, DAS, DTS, DSS and DPS data of the measured armored optical cable 3 outside the casing 2 are collected in real time, and according to the real-time change of the intensity and frequency of downhole vibration noise, the real-time change of the formation stress acting on the outer side of the casing 2 and the real-time change of the pressure of fluid in the formation pore, the state of the casing 2 in the downhole during the casing 2 operation is monitored, judged and evaluated in real time, the integrity of the casing 2 is monitored in real time, and the deformation or damage of the casing 2 during the casing 2 operation is monitored in real time.

After the operation of the casing 2 is finished, the vibration noise, the abnormal stress change and the abnormal pressure change well section outside the casing 2 can be continuously monitored in real time, the deformation or sleeve damage generated by underground interlayer series flow, the casing 2 under the action of underground abnormal stress strain or abnormal fluid pressure can be monitored in real time for a long time, the integrity, stability and reliability of an oil gas production well or a water injection steam injection drive well can be ensured, and long-term stable production and high production can be maintained.

In some alternative embodiments, as shown in the schematic cross-sectional structure of the armored fiber optic cable 3 of fig. 2, the armored fiber optic cable 3 includes a composite sensing light unit 16 for measuring vibration noise and temperature of the downhole casing 2, a strain sensitive light unit 9 for measuring strain of the downhole casing 2, and a fluid pressure sensing light unit 10 for measuring fluid pressure in the underground rock or rock pore;

the composite modem 5 is used for demodulating the received DAS/DTS/DSS/DPS signals in real time during operation and is provided with a DAS/DTS/DSS/DPS sensing signal port;

the composite sensing light unit 16 of the armored optical cable 3 is connected with a DAS sensing signal port and a DTS sensing signal port of the composite modem 5, the strain sensitive light unit 9 is connected with a DSS sensing signal port, and the fluid pressure sensing light unit 10 is connected with a DPS sensing signal port. In this embodiment, the DTS sensing signal port of the composite modem 5 is a double-ended sensing signal port, and the two multimode fibers 8 of the composite sensing optical unit 16 are respectively connected to two ends of the DTS sensing signal port.

In some alternative embodiments, the composite sensing optical unit 16 includes one single-mode fiber 7, two multimode fibers 8, and a stainless steel tubule 14, where the single-mode fiber 7 and the multimode fibers 8 are respectively connected to the DAS sensing signal port and the DTS sensing signal port of the composite modem 5, the single-mode fiber 7 and the multimode fibers 8 are both disposed in the stainless steel tubule 14, and the stainless steel tubule 14 is further filled with a high-temperature resistant optical fiber paste 17.

The single-mode fiber 7 in the embodiment is a single-mode fiber 7 with high temperature resistance, hydrogen loss resistance, insensitive bending, high reflection coefficient and high sensitivity, and the multimode fiber 8 is a multimode fiber 8 with high temperature resistance, hydrogen loss resistance, insensitive bending, high reflection coefficient and high sensitivity. The sleeve 2 in this embodiment is a metal sleeve 2, but in some embodiments may also be a non-metal sleeve 2.

In some alternative embodiments, the tail end of the single-mode fiber 7 far away from the DAS sensing signal port is provided with an extinction device 11; the tail ends of the two multimode optical fibers 8 far away from the DTS sensing signal port are welded into a U-shaped 12 for high-precision DTS temperature measurement under the double-end input state.

In some alternative embodiments, the strain sensitive optical unit 9 includes a single mode fiber 7, a high temperature resistant composite protective sleeve 13 for wrapping the single mode fiber 7, and a stainless steel tubule 14, where the single mode fiber 7 and the protective sleeve 13 are tightly wrapped in the stainless steel tubule 14. The protective sheath 13 in this embodiment is made of a high-temperature and high-strength composite material. In this embodiment, the single-mode fiber 7 of the strain sensitive optical unit 9 is a single-mode fiber 7 with high temperature resistance, hydrogen loss resistance, bending insensitivity, high reflection coefficient and high sensitivity.

In some alternative embodiments, as shown in fig. 2 and 3, the fluid pressure sensing optical unit 10 includes a special microstructure pressure sensing optical fiber 15 and a stainless steel tubule 14, where the pressure sensing optical fiber 15 is disposed in the stainless steel tubule 14, and the stainless steel tubule 14 is provided with a plurality of windows 18 uniformly distributed along the axial direction thereof. The pressure sensing optical fiber 15 is susceptible to sensing the fluid pressure of various fluids within the subsurface rock or rock aperture acting on the fluid pressure sensing optical unit 10. In this embodiment, the special microstructure pressure sensing optical fiber 15 is a special microstructure pressure sensing optical fiber 15 with characteristics of high temperature resistance, hydrogen loss resistance, bending insensitivity, high reflection coefficient and high sensitivity.

In some alternative embodiments, the fluid pressure sensing optical unit 10 includes a continuous grating fiber and a stainless steel tubule 14, where the continuous grating fiber is disposed in the stainless steel tubule 14, and the stainless steel tubule 14 is provided with a plurality of windows 18 uniformly distributed along the axial direction thereof. In this embodiment, the continuous grating fiber is a continuous grating fiber with characteristics of high temperature resistance, hydrogen loss resistance, insensitive bending, high reflection coefficient and high sensitivity.

In some alternative embodiments, the stainless steel tubule 14 may have an inner diameter in the range of 1mm to 2mm. The stainless steel tubule 14 in this embodiment has an inner diameter of 1mm or 2mm.

In some alternative embodiments, the cross-sectional shape of the armored optical cable 3 is rectangular, and at least one armored optical cable 3 is arranged; in the case where the number of the armored fiber optic cables 3 is more than one, two or more armored fiber optic cables 3 are uniformly distributed along the circumferential direction of the sleeve 2;

and the device also comprises an annular clamp 4 for fixing the armored optical cable 3 outside the sleeve 2, wherein the annular clamp 4 is uniformly arranged along the axial direction of the sleeve 2. In this embodiment, the cross-sectional shape of the armored optical cable 3 is a rounded rectangle, and when the number of armored optical cables 3 is 2, the layout schematic diagram of the armored optical cable 3 when laid on the sleeve 2 is shown in fig. 4; when the number of the armored optical cables 3 is 3, a layout schematic diagram of the armored optical cables 3 when being laid on the sleeve 2 is shown in fig. 5; when the number of the armored optical cables 3 is 4, a layout diagram of the armored optical cables 3 when being laid on the sleeve 2 is shown in fig. 6; in this embodiment, the ring clips 4 are made of metal, so that high temperature resistance and durability are ensured, and the distance between the ring clips 4 is about 10 meters.

Example 2

Another aspect of the present application provides a method for monitoring quality of an underground casing 2 based on an optical fiber sensing technology, which is applicable to any one of the above underground casing 2 quality monitoring systems based on an optical fiber sensing technology, and specifically includes the following steps:

s0, completing the drilling operation of the open hole drilling 1 according to the drilling operation deployment;

s1: the armored optical cable 3 which is uniformly distributed along the circumferential direction of the sleeve 2 suspended above the wellhead is arranged outside the sleeve 2, and the armored optical cable 3 is connected with the composite modem 5 through a fiber rotating connector or an optical slip ring; when the armored optical cable 3 is laid, the armored optical cable 3 is tightly fixed on the peripheral wall of the sleeve 2 by the annular clip 4;

s2: before the operation of casing 2, starting the compound modem 5 for test measurement, and judging whether the compound sensing light unit 16, the strain sensitive light unit 9 and the fluid pressure sensing light unit 10 in the armored optical cable 3 are intact;

s3: if the armored optical cable 3 is intact, slowly lowering the sleeve 2 fixed with the armored optical cable 3 into the open hole drilling hole 1, and measuring in real time by utilizing the armored optical cable 3 in the lowering process;

s4: DAS, DTS, DSS and DPS data measured in real time by the armored optical cable 3 are processed in real time, and DSS and DPS data are corrected for temperature drift by using DTS temperature data measured along the sleeve 2;

s5: the state of the casing 2 under the well, the integrity of the casing 2 and whether the casing 2 is disjointed, deformed or damaged in the process of the casing 2 lowering operation are monitored, judged and evaluated in real time according to the temperature drift corrected data; if abnormality occurs, alarming in time;

the alarm can remind a casing 2 engineer of timely adjusting the operation flow of the casing 2, so that the casing 2 can be completely and smoothly laid at the bottom of the well, the whole well Duan Taoguan 2 is free from dislocation, deformation or damage, and the casing 2 lowering operation is finished with high quality;

after the operation of the casing 2 is finished, if each armored optical cable 3 outside the casing 2 and the ground distributed optical fiber sensing composite modem 5 device are utilized, vibration noise, abnormal stress change and abnormal pressure change well sections outside the casing 2 are continuously monitored in real time, long-term real-time monitoring can be carried out on deformation or sleeve loss of underground interlayer series flow and the casing 2 under the action of underground abnormal stress strain or abnormal fluid pressure, the integrity, stability and reliability of an oil gas production well or a water injection and steam injection drive well can be ensured, and long-term stable and high yield can be maintained.

It is to be understood that the above embodiments are merely illustrative of the application of the principles of the present invention, but not in limitation thereof. Various modifications and improvements may be made by those skilled in the art without departing from the spirit and substance of the invention, and are also considered to be within the scope of the invention.

Claims

1. The underground casing quality monitoring system based on the optical fiber sensing technology is characterized by comprising a composite modem (5) with distributed optical fiber sensing signal ports, a casing (2) arranged in an open hole drilling hole (1), at least one armored optical cable (3) fixed outside the casing (2) along the extending direction of the casing (2), and an optical fiber rotary connector (6) or a smooth ring used for transmitting optical signals of different channels from a fixed shaft to a rotary end, wherein the armored optical cable (3) is connected with the composite modem (5) through the optical fiber rotary connector (6) or the smooth ring.

2. The downhole casing quality monitoring system based on optical fiber sensing technology according to claim 1, wherein the armored fiber optic cable (3) comprises a composite sensing light unit (16) for measuring vibration noise and temperature of the downhole casing (2), a strain sensitive light unit (9) for measuring strain of the downhole casing (2), and a fluid pressure sensing light unit (10) for measuring fluid pressure in underground rock or rock pores;

the composite modem (5) is used for demodulating the received DAS/DTS/DSS/DPS signals in real time during operation and is provided with a DAS/DTS/DSS/DPS sensing signal port;

the composite sensing optical unit (16) of the armored optical cable (3) is connected with the DAS sensing signal port and the DTS sensing signal port of the composite modem (5), the strain sensitive optical unit (9) is connected with the DSS sensing signal port, and the fluid pressure sensing optical unit (10) is connected with the DPS sensing signal port.

3. The downhole casing quality monitoring system based on the optical fiber sensing technology according to claim 2, wherein the composite sensing optical unit (16) comprises a single mode fiber (7), two multimode fibers (8) and a stainless steel thin tube (14), the single mode fiber (7) and the multimode fibers (8) are respectively connected with a DAS sensing signal port and a DTS sensing signal port of the composite modem (5), the single mode fiber (7) and the multimode fibers (8) are both arranged in the stainless steel thin tube (14), and the stainless steel thin tube (14) is also filled with high-temperature-resistant optical fiber paste (17).

4. A downhole casing quality monitoring system based on optical fiber sensing technology according to claim 3, wherein the tail end of the single mode optical fiber (7) remote from the DAS sensing signal port is provided with a delustrer (11); the tail ends of the two multimode optical fibers (8) far away from the DTS sensing signal port are welded into a U-shaped (12) for high-precision DTS temperature measurement under a double-end input state.

5. The downhole casing quality monitoring system based on the optical fiber sensing technology according to claim 2, wherein the strain sensitive photosensitive unit (9) comprises a single mode fiber (7), a protective sleeve (13) for wrapping the single mode fiber (7), and a stainless steel tubule (14), and the single mode fiber (7) and the protective sleeve (13) are tightly wrapped in the stainless steel tubule (14).

6. The downhole casing quality monitoring system based on the optical fiber sensing technology according to claim 2, wherein the fluid pressure sensing optical unit (10) comprises a microstructure pressure sensing optical fiber (15) and a stainless steel tubule (14), the pressure sensing optical fiber (15) is arranged in the stainless steel tubule (14), and a plurality of windows (18) uniformly distributed along the axial direction of the stainless steel tubule (14) are formed on the stainless steel tubule.

7. The downhole casing quality monitoring system based on the optical fiber sensing technology according to claim 2, wherein the fluid pressure sensing optical unit (10) comprises a continuous grating optical fiber and a stainless steel tubule (14), the continuous grating optical fiber is arranged in the stainless steel tubule (14), and a plurality of windows (18) uniformly distributed along the axial direction of the stainless steel tubule (14) are formed on the stainless steel tubule.

8. A downhole casing quality monitoring system based on optical fiber sensing technology according to any of claims 3-7, wherein the stainless steel tubule (14) has an inner diameter in the range of 1-2 mm.

9. The downhole casing quality monitoring system based on optical fiber sensing technology according to claim 1, wherein the armored fiber optic cable (3) has a rectangular cross-sectional shape, and the number of armored fiber optic cables (3) is at least one; in the case that the number of the armored optical cables (3) is more than one, two or more armored optical cables (3) are uniformly distributed along the circumferential direction of the sleeve (2);

the novel armored optical cable comprises a sleeve (2), and is characterized by further comprising annular clamps (4) used for fixing the armored optical cable (3) outside the sleeve (2), wherein the annular clamps (4) are uniformly arranged along the extending direction of the sleeve (2).

10. The method for monitoring the quality of the underground casing based on the optical fiber sensing technology is suitable for the system for monitoring the quality of the underground casing based on the optical fiber sensing technology as claimed in any one of claims 1 to 9, and is characterized by comprising the following steps:

s1: an armored optical cable (3) which is uniformly distributed along the circumferential direction of the casing (2) suspended above the wellhead is arranged outside the casing, and the armored optical cable (3) is connected with a composite modem (5) through a fiber rotating connector or an optical slip ring;

s2: before the operation of casing (2), starting a compound modem (5) for test and judging whether a compound sensing light unit (16), a strain sensitive light unit (9) and a fluid pressure sensing light unit (10) in the armored optical cable (3) are intact;

s3: if the armored optical cable (3) is intact, the sleeve (2) fixed with the armored optical cable (3) is lowered into the open hole drilling hole (1), and real-time measurement is carried out by utilizing the armored optical cable (3) in the lowering process;

s4: DAS, DTS, DSS and DPS data measured in real time by an armored optical cable (3) are processed in real time, and DSS and DPS data are corrected for temperature drift by using DTS temperature data measured along the line of a sleeve (2);

s5: judging and evaluating the state of the casing (2) in the pit, the integrity of the casing (2) and whether the casing (2) is disjointed, deformed or damaged in the process of the casing (2) lowering operation when the casing (2) is subjected to the lowering completion operation according to the temperature drift corrected data in real time; if abnormality occurs, alarming in time.