Underwater wellhead and near wellbore area stability monitoring and graded early warning method suitable for deep water shallow gas reservoir exploitation

Abstract

This invention provides a method for monitoring and classifying early warning of subsea wellhead and near-wellbore stability in deep-water shallow gas reservoir development. The method includes: dividing the formation changes and instability evolution process during deep-water shallow gas reservoir development into three stages: a first stage (formation elastic deformation), a second stage (seabed deformation and subsidence), and a third stage (wellhead instability and subsidence). Based on these three stages, multi-source data and corresponding monitoring methods are used to conduct phased and classified early warning of the formation changes and instability evolution process during deep-water shallow gas reservoir development. Different monitoring methods are employed to monitor the formation changes and instability evolution process in stages based on these three stages. Therefore, this invention improves the accuracy of identifying instability risks in the wellhead and near-wellbore area and enhances the timeliness of early warning by establishing phased monitoring methods and a classified early warning mechanism.

Description

Technical Field

[0001] This invention relates to the field of oil and gas well safety monitoring and early warning technology in offshore oil and gas development, specifically to a method and system for stability monitoring and graded early warning of subsea wellhead and near-wellbore areas suitable for deep-water shallow gas layer exploitation. Background Technology

[0002] Shallow gas reservoirs in deep water are typically shallowly buried, weakly cemented, have low formation bearing capacity, and require stringent formation monitoring. Considering formation anisotropy, the creep and subsidence patterns during production are complex, potentially inducing secondary engineering geological hazards and leading to stability risks at the subsea wellhead and near-wellbore area. These risks manifest primarily as follows: prolonged development may cause pressure decay leading to hydrate decomposition and weakening of the overlying sedimentary layer; deformation and subsidence of the overlying sedimentary layer can cause shallow gas to migrate within the formation, affecting development efficiency and potentially even breaching the overlying layer and overflowing into the marine environment. Formation deformation alters the bearing capacity of shallow soils, affecting the stability of the wellhead tree structure. Long-term accumulation of formation deformation can induce seabed subsidence and subsea wellhead tilting instability. Once these problems develop, they can lead to serious consequences such as casing damage, wellhead seal failure, increased well control risks, and instability of subsea equipment connections.

[0003] Current methods for monitoring land subsidence include GPS, radar monitoring, and leveling monitoring. However, all of these methods have significant limitations in underwater scenarios. GPS is not suitable for monitoring seabed subsidence, as GPS signals cannot be received underwater. Radar monitoring is limited in its detection capability due to signal attenuation by seawater, making it unable to monitor seabed subsidence. Leveling monitoring suffers from difficulties in underwater deployment, high costs of implementation at sea, and challenges in data acquisition. Furthermore, none of these methods can directly monitor underground strata deformation processes.

[0004] In summary, existing technologies have the following shortcomings: 1) Lack of integrated monitoring solutions for deep-water shallow gas reservoir scenarios. Existing technologies mostly monitor single objects such as wellhead attitude, seabed deformation, wellbore strain, or local pressure, lacking an overall system design covering multiple monitoring objects such as formation, seabed, wellhead, and wellhead. This makes it difficult to form a comprehensive monitoring system for the stability of subsea wellhead and near-wellbore areas suitable for deep-water shallow gas reservoir development. 2) Lack of phased monitoring methods for the instability evolution process. Existing technologies mostly focus on monitoring a single stage or single parameter, with insufficient research on the continuous evolution process from formation elastic deformation, seabed deformation and subsidence to wellhead instability and subsidence during deep-water shallow gas reservoir development. It is difficult to select matching monitoring methods according to the characteristics of different stages. 3) The early warning mechanism is relatively crude. Existing technologies typically employ single-parameter over-limit alarm methods, lacking a graded early warning method corresponding to the instability evolution stage. This makes it difficult to identify risks in advance and intervene in a timely manner during the formation deformation and seabed subsidence stages, thereby preventing the risks from further developing into the wellhead instability stage.

[0005] Therefore, there is an urgent need for a method for monitoring and classifying early warning of stability in the underwater wellhead and near-wellbore area for deep-water shallow gas layer extraction scenarios. Summary of the Invention

[0006] This invention aims to address at least one of the technical problems existing in the prior art. Specifically, it addresses issues such as the dispersed nature of monitoring targets, inconsistent evaluation methods, delayed early warnings, and insufficient ability to identify wellhead instability in existing technologies. The purpose of this invention is to provide a method and system for monitoring and providing graded early warning of stability in subsea wellhead and near-wellbore areas suitable for deep-water shallow gas reservoir development. By constructing an integrated monitoring system, establishing phased monitoring methods, and implementing a graded early warning mechanism, this invention improves the accuracy of identifying instability risks in the wellhead and near-wellbore areas and enhances the timeliness of early warnings.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: In a first aspect, the present invention provides a method for monitoring and classifying early warning of stability in the subsea wellhead and near-wellbore area suitable for deep-water shallow gas reservoir exploitation, comprising: The formation changes and instability evolution process during the development of deep-water shallow gas reservoirs are divided into three stages: the first stage is the formation elastic deformation stage, the second stage is the seabed deformation and subsidence stage, and the third stage is the wellhead instability and subsidence stage. Based on the three different stages obtained, combined with multi-source data and corresponding monitoring methods, a phased and graded early warning system is implemented for the formation changes and instability evolution process during the development of deep-water shallow gas reservoirs. Based on the three different stages identified, different monitoring methods were used to conduct phased monitoring of stratigraphic changes and instability evolution during the development of deep-water shallow gas reservoirs.

[0008] In some possible implementations, the three different stages are as follows: Elastic deformation stage of formation: Deformation occurs in the deep oil and gas production formation. As oil and gas are extracted from the formation, the formation pore pressure decreases, which leads to an increase in the effective stress of the reservoir skeleton. The skeleton undergoes compressive deformation, and the formation undergoes compression and subsidence. At this stage, due to the small amount of deformation, it has not yet affected the overlying formation. Seabed deformation and subsidence stage: As oil and gas are continuously extracted from the formation, the deformation of the producing formation continues to accumulate, the effective stress on the reservoir skeleton continues to increase, and the overlying strata will subside significantly. The subsidence phenomenon is transmitted upward to the seabed surface, and significant seabed surface subsidence occurs. Wellhead instability and subsidence stage: As formation oil and gas continue to be produced, the seabed undergoes severe deformation and subsidence, and the suspended height of the wellhead tree structure continues to increase, posing a risk of tilting instability and subsidence of the wellhead tree structure.

[0009] In some possible implementations, the three different stages are divided based on the dominant anomaly response of the corresponding monitoring parameters and their preset criteria. The preset criteria include the monitoring parameter amplitude threshold, the rate of change threshold, and the anomaly duration threshold. Specifically, when the formation strain anomaly reaches the set threshold of the first stage but the seabed deformation and wellhead subsidence have not yet reached the corresponding threshold, it is determined to be the formation elastic deformation stage; when the seabed deformation or subsidence reaches the second stage threshold but the wellhead subsidence has not yet reached the corresponding threshold, it is determined to be the seabed deformation and subsidence stage; when the wellhead subsidence or tilt reaches the third stage threshold, it is determined to be the wellhead instability and subsidence stage.

[0010] In some possible implementations, based on the three different stages identified, and combining multi-source data and corresponding monitoring methods, a phased and graded early warning system is implemented for formation changes and instability evolution during the development of deep-water shallow gas reservoirs. The process is as follows: risk identification and early warning are achieved in the first and second stages; the instability evolution of the wellhead tree structure is suppressed by adjusting the production system, thereby preventing the risk from developing into the third stage. Specifically: The first-stage warning is triggered when the formation microstrain value or its growth rate reaches a preset threshold. The response measures include reducing the production pressure differential, slowing down the production ramp-up, shortening the monitoring cycle, and strengthening trend analysis. The second-stage warning is triggered when seabed deformation, subsidence, or their rate of change reaches a preset threshold. The measures include further controlling production, optimizing production systems, increasing the frequency of seabed and wellhead monitoring, and conducting wellhead structural stability reviews, all aimed at preventing the risk from developing into the third stage.

[0011] In some possible implementations, for the formation elastic deformation stage, the fiber optic micro-strain method is used to monitor formation changes and instability evolution during the development of shallow gas reservoirs in deep water. Specifically, the fiber optic cable is installed on the outside of the oil and gas well casing and effectively coupled with the formation by cement sealing, in order to monitor formation micro-strain changes and identify formation elastic deformation.

[0012] In some possible implementations, for the seabed deformation and subsidence stages, optical imaging methods are used to monitor the formation changes and instability evolution process during the development of shallow gas reservoirs in deep water. Specifically, optical imaging ranging equipment is installed on the wellhead tree, and the seabed deformation and subsidence around the wellhead are monitored and the seabed deformation is identified through multi-laser beam ranging imaging.

[0013] In some possible implementations, for the wellhead instability and subsidence stages, a depth monitoring scheme is adopted to monitor formation changes and instability evolution during the development of shallow gas reservoirs in deep water. Specifically, a depth gauge is installed on the wellhead tree, and through the high-precision pressure and acceleration monitoring built into the depth gauge, the depth changes are interpreted by eliminating tidal patterns, and the acceleration change characteristics are integrated to monitor the state and subsidence changes of the wellhead and identify the subsidence and instability information of the wellhead tree structure.

[0014] Secondly, the present invention also provides a subsea wellhead and near-wellbore area stability monitoring system, which includes an optical fiber, an optical imaging ranging device, and a depth gauge. The optical imaging ranging device and the depth gauge are mounted on the Christmas tree and integrated with the subsea Christmas tree (SCM) system; wherein: The optical fiber is laid on the outside of the sleeve and sealed with cement. The elastic deformation of the formation is monitored through the strain transfer between the sleeve, cement ring and formation. The optical imaging ranging device monitors the deformation and subsidence of the seabed around the wellhead using a multi-laser beam ranging imaging method; The depth gauge is used to obtain information on the settlement and instability of the wellhead tree structure.

[0015] In some possible implementations, the monitoring system relies on the subsea production tree (SCM) system for power supply, communication, and control, and transmits the monitoring data to the upper display terminal via an umbilical cable, forming a communication link based on "subsea production tree (SCM) - umbilical cable - display terminal" to realize the transmission of underwater monitoring data.

[0016] Because the present invention adopts the above technical solution, it has the following characteristics: 1. This invention achieves multi-source fusion and phased monitoring: By integrating information such as formation deformation, seabed subsidence and wellhead subsidence through fiber optic micro-strain monitoring, optical imaging ranging monitoring and depth gauge monitoring, combined with parameters such as pressure and acceleration, it realizes continuous perception, phased identification and graded early warning of instability evolution during the development of shallow gas layers in deep water, and adapts to the application needs of deep water engineering.

[0017] 2. This invention solves the problems of installation, deployment, long-term service and stable operation of monitoring systems in deep water environments by relying on the combined installation method of wellbore, wellhead and Christmas tree; at the same time, it fully considers the characteristics of deep water low temperature and high pressure and shallow weak cementation formations, and is more suitable for wellhead and near-wellbore safety monitoring and early warning in the development of deep water shallow gas layers.

[0018] In summary, this invention can be widely applied to wellhead and near-wellbore safety monitoring and early warning during the development of shallow gas reservoirs in deep water. Attached Figure Description

[0019] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings: Figure 1 This is a schematic diagram illustrating the three stages of formation changes and instability evolution during the development of deep-water shallow gas reservoirs according to an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of the underwater wellhead and near-wellbore area stability monitoring system according to an embodiment of the present invention.

[0021] Figure 3 This is a schematic diagram of a communication scheme according to an embodiment of the present invention. Detailed Implementation

[0022] It should be understood that the terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “described” as used herein may also include the plural forms. The terms “comprising,” “including,” “containing,” and “having” are inclusive and therefore indicate the presence of the stated features, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, elements, components, and / or combinations thereof. The method steps, processes, and operations described herein are not construed as requiring them to be performed in a particular order described or illustrated unless the order of performance is explicitly indicated. It should also be understood that additional or alternative steps may be used.

[0023] Although terms such as first, second, third, etc., may be used in this document to describe multiple elements, components, regions, layers, and / or segments, these elements, components, regions, layers, and / or segments should not be limited by these terms. These terms may be used only to distinguish one element, component, region, layer, or segment from another. Unless the context clearly indicates otherwise, terms such as "first," "second," and other numerical terms used herein do not imply order or sequence. Therefore, the first element, component, region, layer, or segment discussed below may be referred to as the second element, component, region, layer, or segment without departing from the teachings of the exemplary embodiments.

[0024] For ease of description, spatial relative terms may be used in the text to describe the relationship of one element or feature relative to another element or feature as shown in the figure. These relative terms include, for example, "inside," "outside," "middle," "outer," "below," "above," etc. Such spatial relative terms are intended to include different orientations of the device in use or operation, other than those depicted in the figure.

[0025] To address the problems of scattered monitoring targets, inconsistent evaluation, delayed early warning, and insufficient ability to identify wellhead instability in deep-water shallow gas reservoir development, this invention provides a method and system for subsea wellhead and near-wellbore stability monitoring and graded early warning. The aim is to establish a stability monitoring system suitable for this scenario through multi-source data monitoring, accurately identify different evolution stages such as formation elastic deformation, seabed deformation and subsidence, and wellhead tilting and sinking, and to achieve graded and staged early warning based on multi-source information. Simultaneously, it constructs an integrated monitoring system suitable for deep-water environments, encompassing the wellhead, wellbore, and near-wellbore, providing subsea installation schemes based on the wellbore, wellhead, and wellbore, and establishing reliable subsea data acquisition and communication transmission schemes. This provides a basis for on-site wellhead safety management, production system adjustments, and safe and efficient development. Therefore, this invention enables continuous monitoring of the formation, seabed, and wellhead instability evolution process, improving the targeting and effectiveness of monitoring and early warning in deep-water shallow gas reservoir development.

[0026] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0027] Example 1: The method for monitoring and classifying early warning of stability in the subsea wellhead and near-wellbore area for deep-water shallow gas reservoir development provided in this example is as follows: S1: Graded early warning for deep-water shallow gas reservoir exploitation process.

[0028] In this embodiment, the graded early warning system for deep-water shallow gas reservoir extraction includes: S11: Divide the formation changes and instability evolution process during the development of deep-water shallow gas layers into three stages, and implement phased and graded early warning in combination with corresponding monitoring methods.

[0029] Specifically, such as Figure 1 As shown, the formation changes and instability evolution during the development of shallow gas reservoirs in deep water are divided into three stages, and a graded early warning system is implemented in conjunction with corresponding monitoring methods, specifically: The first stage is the elastic deformation stage of the formation: the deformation occurs in the deep oil and gas production formation. As the oil and gas in the formation are extracted, the formation pore pressure decreases, which causes the effective stress of the reservoir skeleton to increase. The skeleton undergoes compression deformation, and the formation undergoes compression and subsidence. In this stage, because the deformation is relatively small, it has not yet affected the overlying strata.

[0030] The second stage is the seabed deformation and subsidence stage: as oil and gas are continuously extracted from the formation, the deformation of the producing formation continues to accumulate, the effective stress on the reservoir skeleton continues to increase, and the overlying strata will subside significantly. The subsidence phenomenon is transmitted upward to the seabed surface, and significant seabed surface subsidence occurs.

[0031] The third stage is the wellhead instability and subsidence stage: Wellhead tree structure instability is the biggest risk to oil and gas well production. As formation oil and gas are continuously produced, the seabed undergoes severe deformation and subsidence, and the suspended height of the wellhead tree structure continues to increase, posing a risk of wellhead tree structure tilting instability and subsidence.

[0032] Furthermore, it should be noted that the above three stages are divided based on the dominant anomaly response of the corresponding monitoring parameters and their preset criteria. The preset criteria include monitoring parameter amplitude thresholds, change rate thresholds, and anomaly duration thresholds, etc. The thresholds can be determined based on baseline monitoring data, field calibration results, numerical simulation results, or engineering experience, and will not be elaborated here. When the formation micro-strain anomaly reaches the first stage threshold while the seabed deformation and wellhead subsidence have not yet reached the corresponding thresholds, it is determined to be the first stage; when the seabed deformation or subsidence reaches the second stage threshold while the wellhead subsidence has not yet reached the corresponding threshold, it is determined to be the second stage; when the wellhead subsidence or tilt reaches the third stage threshold, it is determined to be the third stage.

[0033] S12: Set early warning principles based on the above three different stages.

[0034] In this embodiment, risk identification and early warning are implemented in the first and second stages, and the unstable evolution of the wellhead tree structure is suppressed by adjusting the production system, strictly controlling the development to the third stage.

[0035] Specifically, the first-stage warning can be triggered when the formation micro-strain value or its growth rate reaches a preset threshold. The measures include reducing the production pressure differential, slowing down the production ramp-up, shortening the monitoring cycle, and strengthening trend analysis. The second-stage warning can be triggered when the seabed deformation or subsidence value or its rate of change reaches a preset threshold. The measures include further controlling production, optimizing the production system, increasing the frequency of seabed and wellhead monitoring, and conducting wellhead structural stability verification. The above measures aim to prevent the risk from developing into the third stage.

[0036] S2: Phased monitoring of deep-water shallow gas reservoir development.

[0037] In this embodiment, as Figure 2 As shown, to address the evolutionary characteristics of formation-seabed-wellhead tree stability during the development of the aforementioned deep-water shallow gas reservoirs, a phased monitoring approach was adopted, specifically: Formation elastic deformation stage: Fiber optic microstrain monitoring is employed. The fiber optic cable is installed outside the casing of the oil and gas well and effectively coupled with the formation through cement sealing. This method is used to monitor changes in formation microstrain and is suitable for identifying formation elastic deformation in the early stages of development. Specifically, an optical cable containing at least two grating optical fibers can be laid outside the casing, equipped with two sets of modulation and demodulation equipment to monitor the temperature and overlying formation deformation throughout the well section. After the fiber optic cable is laid outside the casing and sealed with cement, microstrain monitoring can be achieved through strain transfer between the casing, cement sheath, and formation.

[0038] Seabed Deformation and Subsidence Stage: Seabed deformation monitoring based on optical imaging is employed. An optical imaging ranging device is installed on the wellhead Christmas tree, and multi-laser beam ranging imaging is used to monitor seabed deformation and subsidence around the wellhead. This method is suitable for short-distance, high-precision testing, meeting the needs of monitoring seabed deformation near the wellhead. Specifically, relying on the Christmas tree, an optical imaging ranging device is installed to image the surface around the production well, completing the monitoring of seabed subsidence in the near-wellbore area.

[0039] Wellhead instability and settlement stage: A depth gauge is used for monitoring. The depth gauge is installed on the wellhead tree and uses built-in high-precision pressure and acceleration monitoring. The depth change is explained by tidal pattern inversion and the acceleration change characteristics are integrated to monitor the state and settlement changes of the wellhead, thereby obtaining information such as settlement instability of the wellhead tree structure.

[0040] Example 2: Figure 2 As shown, the subsea wellhead and near-wellbore stability monitoring system provided in this embodiment includes an optical fiber 1, an optical imaging ranging device 2, and a depth gauge 3. The monitoring system adopts a combined deployment method of "earthworks-mounted installation and wellbore-supported installation". For production wells, the optical imaging ranging device 2 and the depth gauge 3 are installed on the earthworks-mounted system and integrated with the SCM, wherein: An optical fiber 1 is laid outside the well casing 4 and sealed with cement grout 5 to achieve continuous monitoring of formation micro-strain. Specifically, the optical fiber 1 is inserted into the outside of the well casing 4 and sealed with cement grout 5 to form an effective coupling with the formation. The optical imaging ranging device 2 monitors the deformation and subsidence of the seabed around the wellhead using a multi-laser beam ranging imaging method; Depth gauge 3 is used to obtain information such as settlement and instability of the wellhead tree structure.

[0041] Furthermore, such as Figure 3 As shown, all monitoring equipment, including the underwater industrial control computer 6, relies on the subsea wellhead (SCM) 7 for power supply, communication, and control. The SCM is then connected to the umbilical cable 8 via the SCM to the display terminal 9 on the production platform. The production well's SCM has a reserved monitoring interface for power supply and data communication. Subsequently, the production well's SCM transmits the monitoring data to the upper display terminal 9 via the umbilical cable 8, thus forming a communication link based on the "subsea wellhead SCM - umbilical cable - display terminal" to achieve underwater monitoring data transmission. Therefore, this invention, through an integrated monitoring system constructed using production wells, achieves comprehensive monitoring of the formation, subsea wellhead, and near-wellbore area stability.

[0042] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In the description of this specification, the terms "a preferred embodiment," "furthermore," "specifically," "in this embodiment," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the embodiments in this specification. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0043] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for monitoring and grading early warning of the stability of the underwater wellhead and near wellbore area suitable for deep water shallow gas reservoir exploitation, characterized in that, include: The formation changes and instability evolution process during the development of deep-water shallow gas reservoirs are divided into three stages: the first stage is the formation elastic deformation stage, the second stage is the seabed deformation and subsidence stage, and the third stage is the wellhead instability and subsidence stage. Based on the three different stages obtained, combined with multi-source data and corresponding monitoring methods, a phased and graded early warning system is implemented for the formation changes and instability evolution process during the development of deep-water shallow gas reservoirs. Based on the three different stages identified, different monitoring methods were used to conduct phased monitoring of stratigraphic changes and instability evolution during the development of deep-water shallow gas reservoirs.

2. The method for monitoring and grading early warning of the subsea wellhead and near wellbore area stability suitable for deep water shallow gas reservoir production according to claim 1, characterized in that, The three different stages are as follows: Elastic deformation stage of formation: Deformation occurs in the deep oil and gas production formation. As oil and gas are extracted from the formation, the formation pore pressure decreases, which leads to an increase in the effective stress of the reservoir skeleton. The skeleton undergoes compressive deformation, and the formation undergoes compression and subsidence. At this stage, due to the small amount of deformation, it has not yet affected the overlying formation. Seabed deformation and subsidence stage: As oil and gas are continuously extracted from the formation, the deformation of the producing formation continues to accumulate, the effective stress on the reservoir skeleton continues to increase, and the overlying strata will subside significantly. The subsidence phenomenon is transmitted upward to the seabed surface, and significant seabed surface subsidence occurs. Wellhead instability and subsidence stage: As formation oil and gas continue to be produced, the seabed undergoes severe deformation and subsidence, and the suspended height of the wellhead tree structure continues to increase, posing a risk of tilting instability and subsidence of the wellhead tree structure.

3. The method for monitoring and grading early warning of the subsea wellhead and near wellbore area stability suitable for deep water shallow gas reservoir production according to claim 2, characterized in that, The three different stages are divided based on the dominant anomaly response of the corresponding monitoring parameters and their preset criteria. The preset criteria include the monitoring parameter amplitude threshold, the rate of change threshold, and the anomaly duration threshold. Specifically, when the formation strain anomaly reaches the set threshold of the first stage but the seabed deformation and wellhead subsidence have not yet reached the corresponding threshold, it is determined to be the formation elastic deformation stage; when the seabed deformation or subsidence reaches the threshold of the second stage but the wellhead subsidence has not yet reached the corresponding threshold, it is determined to be the seabed deformation and subsidence stage; when the wellhead subsidence or tilt reaches the threshold of the third stage, it is determined to be the wellhead instability and subsidence stage.

4. The method for monitoring and grading early warning of the subsea wellhead and near wellbore area stability suitable for deep water shallow gas reservoir production according to claim 1, characterized in that, Based on the three distinct stages identified, and combining multi-source data and corresponding monitoring methods, a phased and graded early warning system is implemented for formation changes and instability evolution during the development of shallow gas reservoirs in deep water. The process is as follows: In the first and second stages, risk identification and early warning measures are implemented. Instability evolution of the wellhead tree structure is suppressed by adjusting production systems, thereby preventing the risk from progressing to the third stage. Specifically: The first-stage warning is triggered when the formation microstrain value or its growth rate reaches a preset threshold. The response measures include reducing the production pressure differential, slowing down the production ramp-up, shortening the monitoring cycle, and strengthening trend analysis. The second-stage warning is triggered when seabed deformation, subsidence, or their rate of change reaches a preset threshold. The measures include further controlling production, optimizing production systems, increasing the frequency of seabed and wellhead monitoring, and conducting wellhead structural stability reviews, all aimed at preventing the risk from developing into the third stage.

5. The method for monitoring and grading early warning of the subsea wellhead and near wellbore area stability applied to deep water shallow gas reservoir production according to claim 1, characterized in that, For the formation elastic deformation stage, the fiber optic micro-strain method is used to monitor formation changes and instability evolution during the development of shallow gas reservoirs in deep water. Specifically, the fiber optic cable is installed on the outside of the oil and gas well casing and effectively coupled with the formation by cement sealing, in order to monitor formation micro-strain changes and identify formation elastic deformation.

6. The method for monitoring and grading warning of the subsea wellhead and near wellbore region stability applied to deep water shallow gas reservoir production according to claim 1, characterized in that, To monitor the formation changes and instability evolution during the development of shallow gas reservoirs in deep water, optical imaging methods are used to target the seabed deformation and subsidence stages. Specifically, optical imaging ranging equipment is installed on the wellhead tree, and multi-laser beam ranging imaging is used to monitor the seabed deformation and subsidence around the wellhead and identify seabed deformation.

7. The method for monitoring and grading early warning of the subsea wellhead and near wellbore area stability applied to deep water shallow gas reservoir production according to claim 1, characterized in that, To address the wellhead instability and subsidence stages, a depth monitoring scheme is employed to monitor formation changes and instability evolution during the development of shallow gas reservoirs in deep water. Specifically, a depth gauge is installed on the wellhead tree. Through the high-precision pressure and acceleration monitoring built into the depth gauge, the depth changes are interpreted by eliminating tidal patterns and integrating acceleration variation characteristics to monitor the wellhead status and subsidence changes, and to identify subsidence and instability information of the wellhead tree structure.

8. A subsea wellhead and near wellbore region stability monitoring system, characterized by, The system includes optical fibers, an optical imaging rangefinder, and a depth gauge. The optical imaging rangefinder and the depth gauge are mounted on the wellhead and integrated with the subsea wellhead (SCM) system. The optical fiber is laid on the outside of the sleeve and sealed with cement. The elastic deformation of the formation is monitored through the strain transfer between the sleeve, cement ring and formation. The optical imaging ranging device monitors the deformation and subsidence of the seabed around the wellhead using a multi-laser beam ranging imaging method; The depth gauge is used to obtain information on the settlement and instability of the wellhead tree structure.

9. The subsea wellhead and near-bore region stability monitoring system of claim 8, wherein, The monitoring system relies on the subsea production tree (SCM) system for power supply, communication, and control, and transmits the monitoring data to the display terminal above via an umbilical cable, forming a communication link based on "subsea production tree (SCM) - umbilical cable - display terminal" to realize the transmission of underwater monitoring data.

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