A DAS system based on the evaluation of CO 2 Ways to contain the risk of leakage

Abstract

The present invention discloses a method for evaluating CO in formation based on DAS system. 2 The method of sealing leakage risk is to use the DAS system to complete the real-time dynamic monitoring of the wellbore structure health and the creep, activation, and fracture of the cover layer through the underground optical cable. At the same time, by obtaining the disturbance information of all positions along the optical fiber, the low-frequency deformation caused by the broadband vibration of the wellbore and the movement of the formation is spatially densely sampled. The present invention uses the DAS system as a remote monitoring method. The demodulation host can be placed indoors. Only the relatively cheap optical cable needs to be placed in the wellbore to achieve three-dimensional spatial positioning. It has a high reuse rate, low comprehensive cost, and long service life. In addition, the sensor optical cable can be permanently retained, and can monitor the entire life cycle of oil and gas extraction and carbon storage, effectively reducing CO 2 Risk of large-scale leakage.

Description

Technical Field

[0001] The present invention relates to a method for assessing the leakage risk of CO2 stored in a formation, and in particular to a method for assessing the leakage risk of CO2 stored in a formation based on a DAS system. Background Art

[0002] Carbon capture, utilization and storage (CCUS) is a large-scale carbon reduction technology and one of the most effective means to mitigate global climate change and achieve carbon peak and carbon neutrality goals. Carbon dioxide (CO2) geological storage is a CO2 emission reduction method that injects CO2 and permanently stores it in geological bodies such as underground aquifers or abandoned oil and gas reservoirs. Among them, abandoned oil and gas reservoirs are relatively ideal geological storage sites. The most popular enhanced oil recovery (EOR) technology currently mainly drives away the residual oil and gas in the oil field by pressurizing and sealing CO2 in oil and gas reservoirs, and then forms a CO2 seal similar to the original oil and gas reservoir, which is convenient for CO2 storage while improving oil and gas recovery. In the long-term CO2 storage process, the wellbore system and reservoir-caprock system in the oil and gas reservoir are affected by fluid flow, temperature and pressure changes, external force damage, geological faults and chemical corrosion, and its storage safety is gradually reduced, resulting in the risk of leakage in CO2 storage. Therefore, during the injection and storage process, continuous monitoring is required to identify whether leakage may occur and prevent it.

[0003] At present, the existing monitoring methods mainly focus on monitoring the impact of CO2 on the ecological environment in the event of leakage, such as monitoring the CO2 content in the atmosphere, soil and groundwater. This type of monitoring method is mainly point-based, with a limited monitoring range and cannot be monitored continuously in real time. Underground migration monitoring technology (four-dimensional seismic monitoring, time-shift VSP, etc.) is subject to the influence of monitoring costs and formation environment and is not suitable for large-scale monitoring scenarios. Because optical fiber has the characteristics of small diameter, light weight, non-conductivity, simple structure, good stability, and suitability for long-distance signal transmission. Compared with traditional monitoring methods, the distributed optical fiber sensing (DAS) system that relies on optical fiber to realize sensing functions is basically not restricted by space, and can withstand harsh environments such as high temperature, high humidity and strong electromagnetic interference. The phase-sensitive optical time-domain reflectometer (Phi-OTDR) system is the most sensitive sensing technology for strain and vibration monitoring in the DAS system, which can meet the needs of long-distance and large-scale strain monitoring under a large time span. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a method based on a DAS system that can effectively evaluate the risk of CO2 storage leakage in the formation by real-time dynamic monitoring of the health of the wellbore structure and the creep, activation, and fracture of the formation through underground optical cables.

[0005] Technical solution: The method of the present invention for reducing the risk of CO2 leakage uses underground optical cables to conduct real-time dynamic monitoring of the health of the wellbore structure and the creep, activation and fracture of the formation, and mainly includes the following steps:

[0006] S1, after the exploration finds the suitable CO2 storage block, determine the monitoring plan and design the monitoring layout network;

[0007] S2, at a pre-selected surface location, according to the geological structure and the expected monitoring plan, complete the drilling process from the surface down to the cap layer; at the cap layer, drill a cylindrical hole of a certain diameter to one side and drill to the underground reservoir;

[0008] S3, lower the casing and reinforce the optical cable on the outside of the outermost casing to ensure that the optical cable is connected to the well along with the casing;

[0009] S4, sealing the annular space between the casing and the well wall, making the casing the only passage for oil and gas to enter the well, and at the same time directly coupling the optical cable outside the casing with the formation;

[0010] S5, installing a casing head shell at the top of the surface casing to lead out the optical cable; at the same time, the optical cables between different wellheads are connected in series;

[0011] S6, connect the head end of the optical cable to the DAS system, obtain the backscattered Rayleigh scattering signal along the optical cable, perform three-dimensional spatial modeling of oil and gas wells and gas injection wells, sense the surrounding environment signals, and carry out preliminary formation and wellbore background noise monitoring;

[0012] S7, after the CO2 reservoir is put into use, the DAS system is used to carry out well CO2 leakage monitoring and formation structure health monitoring.

[0013] Furthermore, in step S7, the DAS system separates the low-frequency deformation component of the formation movement, the transient strong vibration component of the formation seismic and the continuous broadband vibration component generated by the wellbore leakage, and combines the spatial deployment form of the optical fiber to determine the corresponding space and frequency information of the formation movement and wellbore leakage through multi-point positioning, thereby judging whether there is a risk of CO2 leakage and the distribution of dangerous locations.

[0014] Furthermore, analysis is performed based on the low-frequency deformation component of the formation movement, the transient strong vibration component of the formation seismic, and the continuous broadband vibration component generated by the well leakage. When obvious abnormal signals are found, it indicates that CO2 leakage is likely to occur here, and it will be used as a key observation area.

[0015] Furthermore, the DAS system estimates the pressure intensity and vibration frequency sensed by the optical cable, and estimates the blowout rate and its corresponding level;

[0016] At the same time, the accident danger level is determined based on the monitoring signal obtained in step S6 and an early warning signal is set.

[0017] Furthermore, the leakage of the cap layer can be monitored by analyzing the low-frequency deformation and the intensity of seismic waves generated when the cap layer faults;

[0018] By monitoring the energy intensity, density, timing of event occurrence and response time of small vibration events caused by changes in formation pressure or formation fractures, the monitoring level of cap rock creep, activation and fault is divided.

[0019] Compared with the prior art, the present invention has the following significant effects:

[0020] 1. By adopting the DAS system, it is possible to naturally image the low-frequency deformation of the formation and the transient strong vibration generated by the formation earthquake, and monitor the broadband vibration generated by the wellbore leakage; by monitoring the low-frequency components in the underground reservoir and cap layer activities, combined with the underground optical cable layout to perform multi-point spatial positioning, the real-time dynamic monitoring of the CO2 seal can be achieved.

[0021] 2. The DAS system can effectively obtain disturbances at all positions along the optical fiber, thereby achieving continuous distributed spatial intensive sampling of low-frequency deformations caused by broadband vibration of the wellbore and formation movement, and high formation imaging resolution;

[0022] 3. As a remote monitoring method, the demodulation host can be placed indoors. Just place the relatively cheap optical cable in the wellbore to achieve three-dimensional spatial positioning, with high reuse rate, low comprehensive cost and long service life;

[0023] 4. As a sensor head, the optical sensor cable has the advantages of low cost, passive underground part, no networking requirements, strong survivability, and can be permanently cast underground to monitor the entire life cycle of oil and gas extraction and CO2 storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a flow chart of laying optical cables of the present invention;

[0025] Figure 2 It is a monitoring schematic diagram of the present invention;

[0026] Figure 3 It is a schematic diagram of the spatial relative position of the optical cable and the steel casing of the present invention;

[0027] Figure 4 is a schematic cross-sectional view of a tube well monitored by the present invention;

[0028] Figure 5 It is the monitoring flow chart of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0030] The optical cable cloth flow chart of the present invention is as follows Figure 1 As shown, the specific process is as follows:

[0031] Step 1: After the exploration finds a suitable CO2 storage block (reservoir), conduct investigation and research according to the monitoring project, collect necessary basic data, determine the monitoring plan, and design the monitoring layout network. The monitoring diagram is as follows: Figure 2 As shown. CO2 is injected into the reservoir through gas injection wells 1-3 after being pressurized. Monitoring optical cables are laid out during the construction of underground wells such as oil and gas wells and gas injection wells. It is required that the optical cable network be reasonably laid out to quickly locate the formation movement and wellbore leakage in three dimensions.

[0032] Step 2, drilling, using special equipment and technology, at a pre-selected surface location, according to the geological structure and the expected monitoring plan design, from the surface down to the cap layer (impermeable layer), from the cap layer to one side, drill a cylindrical hole of a certain diameter, and drill to the underground reservoir (underground aquifer or abandoned oil and gas reservoir and other geological bodies);

[0033] Step 3: Lower the casing. The steel casing is lowered at once. The outermost steel casing is reinforced with optical cables, which are connected to the well along with the casing. The optical cable is coiled in a ring around the outermost layer of the steel casing and fixed in a fixed position. Figure 3 As shown, the optical cable must be protected during the laying process, and it is strictly prohibited to bend, twist, or stretch the optical cable; dual-path loop optical cable is required to suppress the influence of fading noise and improve the signal-to-noise ratio.

[0034] Step 4, inject cement to seal the annular space between the casing and the well wall to isolate the oil, gas and water layers, making the casing the only channel for oil and gas to enter the well, and at the same time directly coupling the optical cable outside the casing with the formation and being fixed and protected;

[0035] Step 5, wellhead installation, the casing head shell is installed on the top of the surface casing, the sensor optical cable is led out, the optical cables between different wellheads are connected in series, and finally the wellbore optical cable is laid. The cross-sectional diagram of the wellbore structure is shown in Figure 4 As shown;

[0036] Step 6: Connect the head end of the optical cable to the DAS system. The sampling rate of the DAS system is set to 1000Hz, the pulse width is 100ns, and the spatial resolution is 10m. Obtain the backscattered Rayleigh signal along the sensing optical cable, perform three-dimensional spatial modeling of oil and gas wells and gas injection wells, sense the surrounding environmental signals, and conduct preliminary formation and wellbore background noise monitoring; the DAS system used has the ability to monitor low-frequency deformation and broadband vibration information simultaneously. The monitoring objects are not limited to monitoring oil and gas wells and CO2 injection wells, but can also be used to monitor all abandoned wells, monitoring wells and other underground wells around carbon storage.

[0037] Step 7: After the CO2 reservoir is put into use, CO2 leakage monitoring of the well and formation structure health monitoring are carried out. The monitoring process diagram is as follows: Figure 5 By separating the low-frequency deformation component of stratum movement, the transient strong vibration component of stratum seismicity, and the continuous broadband vibration component caused by wellbore leakage, combined with the spatial deployment form of optical fiber, multi-point positioning is used to determine the corresponding spatial and frequency information of stratum movement and wellbore leakage, thereby judging whether there is a risk of CO2 leakage and the distribution of dangerous locations.

[0038] Carbon storage leak warning method:

[0039] CO2 injected into the reservoir generally leaks through wellbore damage and caprock faults or cracks. Combined with the low-frequency deformation component of formation movement obtained in step 7, the transient strong vibration component of formation seismicity, and the continuous broadband vibration component generated by wellbore leakage, analysis is performed. When obvious abnormal signals are found, it indicates that CO2 leakage is likely to occur here, and this area can be observed in particular.

[0040] Leaking wellbore monitoring:

[0041] Among them, leakage along underground wellbore is the main leakage path. Generally, due to chemical or external force, CO2 leaks along the hollow cement ring in the wellbore, the wellbore bridge plug or the surrounding rock crushing. Wellbore is the most direct and fastest channel for CO2 to leak into the atmosphere. A large amount of CO2 leakage in a short period of time will cause a sudden increase in pressure in the wellbore and generate broadband vibration. The blowout rate can be estimated by the pressure intensity and vibration frequency sensed by the sensor cable, and the blowout rate and its corresponding level can be estimated based on the estimated blowout rate.

[0042] Drilling leakage usually uses the blowout rate as an indicator to grade the CO2 diffusion risk level. The accident risk level is usually divided into three levels. The maximum blowout rates are 2.5m / s, 10m / s, and 20m / s, respectively, and the corresponding diffusion distances are 0-130, 130-180, and 180-420m, respectively. The DAS system should also use this standard to grade after obtaining the vibration signal. Level 1 is the lowest risk and level 3 is the highest. Based on the local formation conditions, wellbore width, permeability and other factors, the fluctuation degree of the formation movement signal is matched with the blowout rate. The accident risk level is determined based on the monitoring signal obtained in step 6 and an early warning signal is set. When the risk level reaches level 1, 2, or 3, all staff and residents within 130, 180, or 420m must evacuate immediately. Repair or seal the relevant wellbore.

[0043] Leakage from caprock faults or cracks:

[0044] The effectiveness and sustainability of carbon storage in reservoirs depends on the sealing integrity of the caprock. Large-scale injection of CO2 into reservoirs will cause the caprock and reservoir pressure to increase and the stress state to change, mainly in the following three aspects: ① The slow and smooth creep of the caprock may produce cracks; ② Faults formed in early tectonic movements will become active again after the caprock pressure increases; ③ Excessive pressure will cause the caprock to produce fracture surfaces and cause serious relative displacement to both sides. CO2 in the reservoir will leak through the caprock into the aquifer, which will cause groundwater pollution and seriously affect the surface ecosystem. Therefore, pressure and stress must be limited to ensure that no leakage occurs.

[0045] Caprock leakage can be monitored by the intensity of seismic fluctuations generated just after low-frequency deformation and caprock faulting. Monitoring levels 1, 2, and 3 are divided according to caprock creep, activation, and faulting. By measuring the energy intensity, density, timing of event occurrence, and response time of small vibration (microseismic) events caused by changes in formation pressure or formation cracks, the pressure change pattern and the movement and movement of gas in the reservoir and caprock are reflected. Through long-term continuous observation, the DAS system can realize statistical analysis of historical data and form an intuitive description of the intensity and frequency of formation movement at any location in the monitoring area, which helps to determine whether there are potential dangers such as weak points or cracking and leakage in the caprock at that point. Once determined to be a weak point or leakage point, it can be directly remedied by underground grouting.

[0046] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the scope of protection of the present invention.

Claims

1. A method for evaluating the leakage risk of CO2 storage in a formation based on a DAS system, characterized in that: Real-time dynamic monitoring of wellbore structural health and formation creep, activation, and fractures through underground optical cables includes the following steps: S1, after the exploration finds the suitable CO2 storage block, determine the monitoring plan and design the monitoring layout network; S2, at a pre-selected surface location, according to the geological structure and the expected monitoring plan, complete the drilling process from the surface down to the cap layer; at the cap layer, drill a cylindrical hole of a certain diameter to one side and drill to the underground reservoir; S3, lower the casing and reinforce the optical cable on the outside of the outermost casing to ensure that the optical cable is connected to the well along with the casing; S4, sealing the annular space between the casing and the well wall, making the casing the only passage for oil and gas to enter the well, and at the same time directly coupling the optical cable outside the casing with the formation; S5, installing a casing head shell at the top of the surface casing to lead out the optical cable; at the same time, the optical cables between different wellheads are connected in series; S6, connect the head end of the optical cable to the DAS system, obtain the backscattered Rayleigh scattering signal along the optical cable, perform three-dimensional spatial modeling of oil and gas wells and gas injection wells, sense the surrounding environment signals, and carry out preliminary formation and wellbore background noise monitoring; S7, after the CO2 reservoir is put into use, the DAS system is used to carry out well CO2 leakage monitoring and formation structure health monitoring; In step S7, the DAS system separates the low-frequency deformation component of the formation movement, the transient strong vibration component of the formation seismic, and the continuous broadband vibration component generated by the wellbore leakage, and determines the space and frequency information corresponding to the formation movement and the wellbore leakage through multi-point positioning in combination with the spatial deployment form of the optical fiber, thereby judging whether there is a risk of CO2 leakage and the distribution of dangerous locations; The low-frequency deformation component of the formation movement, the transient strong vibration component of the formation earthquake and the continuous broadband vibration component generated by the well leakage are analyzed. When the signal is obviously abnormal, it indicates that CO2 leakage is likely to occur here, and it will be regarded as a key observation area. Monitor the leakage of the cap rock by analyzing the low-frequency deformation and the intensity of seismic waves generated when the cap rock faults; By monitoring the energy intensity, density, timing of event occurrence and response time of small vibration events caused by changes in formation pressure or formation fractures, the monitoring level of cap rock creep, activation and fault is divided.

2. The method for assessing the leakage risk of CO2 storage in a formation based on a DAS system according to claim 1, characterized in that: The DAS system estimates the pressure intensity and vibration frequency sensed by the optical cable, and estimates the blowout rate and its corresponding level; At the same time, the accident danger level is determined based on the monitoring signal obtained in step S6 and an early warning signal is set.

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