A method for monitoring injection of oil and gas reservoirs by means of casing-activated well-ground potential and induced polarization

Abstract

The application discloses a casing-excited well-ground potential and induced polarization combined oil and gas reservoir injection monitoring method, and relates to the technical field of oil and gas field development dynamic monitoring. The method comprises the following steps: collecting surface potential difference data in a power supply stage and secondary field decay response data after the power supply is turned off; calculating apparent resistivity based on the potential difference data and calculating apparent polarization rate based on the secondary field decay response data; calculating apparent resistivity difference and apparent polarization rate difference before and after injection; and outputting interpretation results of an injection front, an injection influence range, an injection medium migration direction and an advantage channel based on the apparent resistivity difference and the apparent polarization rate difference and in combination with the combined features of high resistance / low resistance and strong polarization / weak polarization. The method of the application enhances deep target layer response by casing power supply, and can effectively supplement the shortage of interpretation by only using resistivity through comprehensive analysis of resistivity and polarization rate, thereby effectively improving the observability and interpretability of reservoir dynamic changes in the injection process.

Description

Technical Field

[0001] This invention relates to the field of dynamic monitoring technology for oil and gas field development, and in particular to a method for monitoring oil and gas reservoir injection that combines casing-excited well-to-ground potential and induced polarization. Background Technology

[0002] Existing oil and gas reservoir injection monitoring methods mainly include well-to-surface potential method and time-domain induced polarization method. (1) Well-to-surface potential method usually uses metal casing or downhole electrodes to supply power to the vicinity of the target layer, and sets up multiple receiving electrodes on the surface to observe the potential. The resistivity is calculated through the potential data, thereby obtaining the spatial distribution of the electrical properties around the well. This technology supplies power through the casing and perforation positions, and the current is directly supplied to the deep reservoir. Compared with surface power supply, it is easier to enhance the primary field response of the target layer. During water / gas injection in casing wells, the direction of fluid migration, the range of influence, and the dominant channel of high mineralization will cause resistivity changes. (2) Time-domain induced polarization method observes the secondary field decay response after the power supply is turned off, and can obtain information such as gate voltage, apparent polarizability, time constant, and decay rate. Different fluids, mineralization, and pore structure will affect the polarization strength and decay pattern. Therefore, induced polarization parameters can provide supplementary constraints for resistivity interpretation. In water injection scenarios, the resistivity of the target layer may decrease, but its polarization response may not be consistent with that of low resistivity. In gas injection scenarios, the target layer may exhibit high resistivity, but its polarization rate and decay time may differ from those of low resistivity. Therefore, combining potential observation with the secondary field decay response after power failure is beneficial to improving the reliability of dynamic monitoring interpretation in casing wells.

[0003] However, existing technologies have the following shortcomings: Well-to-surface potential methods mainly rely on potential difference or resistivity for interpretation, which can easily lead to multiple interpretations for high-resistivity oil / gas layers, low-resistivity water layers, and high-salinity water channeling. For dynamic monitoring, simply using "increase or decrease in resistivity" is often insufficient to determine fluid type, affected area, and dominant channel. Existing time-domain induced polarization techniques are mostly applied under surface power supply and reception conditions, and do not utilize the secondary field response of downhole power supply in perforated sections of casing wells. Therefore, drawing on the advantages of surface induced polarization electromagnetic detection, and fully utilizing the observation system of well-to-surface potential methods, the apparent polarizability is obtained by calculating the relaxation decay curve during power outages.

[0004] Therefore, for the injection monitoring scenario of casing wells, a joint well-to-ground potential-time-domain induced polarization (DC-TDIP) method is proposed. This method establishes a joint observation technology using the same well-to-ground observation system and the same power supply method. The comprehensive analysis of the obtained resistivity and polarizability can provide more accurate monitoring and judgment of the injection effect. Summary of the Invention

[0005] To address the aforementioned issues, this invention discloses a method for monitoring oil and gas reservoir injection that combines well-ground potential and induced polarization induced by casing. This method utilizes casing power supply to enhance the response of the deep target layer. Through comprehensive analysis of resistivity and polarizability, it can effectively supplement the shortcomings of interpreting solely using resistivity, thereby effectively improving the observability and interpretability of reservoir dynamic changes during the injection process.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for monitoring reservoir injection combining casing-excited well-to-surface potential and induced polarization includes the following steps:

[0008] s1. Before and after injection in the casing well, power is supplied to the target layer through the casing, and multiple receiving electrodes are deployed on the surface centered on the wellhead to collect surface potential difference data during the power supply phase and secondary field attenuation response data after power supply is turned off.

[0009] s2. Calculate apparent resistivity based on potential difference data, and calculate apparent polarizability based on secondary field attenuation response data;

[0010] s3. Calculate the difference in apparent resistivity and apparent polarizability before and after injection;

[0011] s4. Based on the apparent resistivity difference and apparent polarizability difference, combined with the combined characteristics of high resistance / low resistance and strong polarization / weak polarization, the interpretation results of the injection leading edge, injection influence range, injection medium migration direction and dominant channel are output.

[0012] Optionally, in step s1, supplying power to the target layer through the casing involves using a distributed line source with a limited length of the casing perforation section, allowing current to flow into the formation near the target layer through the perforation section.

[0013] Optionally, in step s1, the receiving electrodes are arranged in a concentric measuring ring with the wellhead as the center.

[0014] Optionally, in step s2, the apparent resistivity is calculated based on the governing equations of the forward modeling of DC resistivity:

[0015] ;

[0016] In the formula, For electrical conductivity, Potential, For source terms, These are mathematical operators;

[0017] A node potential discretization scheme based on an octree finite volume grid is adopted to locally refine the power supply, receiving area, and vicinity of the anomaly, in order to balance the simulation accuracy of deep targets with overall computational efficiency; in the discretization, a node potential discretization scheme is set... For discrete gradient operators, For discrete divergence operators, Given the surface conductivity mass matrix, the system of linear equations is as follows:

[0018] ;

[0019] In the formula, The conductivity-related system matrix is ​​the result of discretization of a finite volume.

[0020] The potentials at all nodes in the field are obtained by solving the linear equations, and the voltage difference is extracted at the receiving point. , , All are surface electrodes, combined with device geometric factors Calculate apparent resistivity :

[0021] ;

[0022] In the formula, This is the supply current.

[0023] Optionally, in step s2, the calculation of apparent polarizability includes:

[0024] A Cole–Cole polarization parameter model is introduced based on the same spatial grid and discrete operator as DC resistivity, using frequency domain complex conductivity. The expression is:

[0025] ;

[0026] In the formula, DC conductivity Polarizability It is a time constant. For frequency index, For frequency, It is an imaginary number;

[0027] The time response of the secondary field after shutdown is denoted as , for the Time window Integral voltage Defined as:

[0028] ;

[0029] The apparent polarizability is obtained by normalizing the absolute value of the primary field potential difference. :

[0030] ;

[0031] In the formula, Let be the apparent polarization rate of the g-th time window.

[0032] Optionally, in step s3, the apparent resistivity difference And the difference in apparent polarization The calculation formulas are as follows:

[0033] ;

[0034] ;

[0035] In the formula, and , respectively, represent the apparent resistivity and apparent polarizability responses for the g-th time window.

[0036] Optionally, in step s4, the combined features include four typical anomalies related to dynamic monitoring of injection in casing wells: high resistivity / medium-high polarization anomaly, low resistivity / high polarization anomaly, high resistivity / low polarization anomaly, and low resistivity / low polarization anomaly, used to identify different injection responses and background anomalies.

[0037] The beneficial effects of this invention are that it introduces the induced polarization method based on the well-to-situ potential method. After acquiring the primary field, it continues to acquire the secondary electric field after shutdown. Through data processing, it obtains the apparent resistivity and apparent polarizability, as well as the difference in apparent resistivity and apparent polarizability before and after injection. By comprehensively analyzing the two types of differential data, more reliable interpretation conclusions can be obtained. The first protection point is the cased well-to-situ DC-TDIP joint detection method and observation scheme, especially the addition of induced polarization measurement after shutdown. The second protection point is to obtain the apparent resistivity and apparent polarizability through processing the well-to-situ potential and induced polarization data. The third protection point is to identify four types of anomalies—high resistivity / strong polarization, low resistivity / strong polarization, high resistivity / weak polarization, and low resistivity / weak polarization—by combining the two parameters of apparent resistivity and apparent polarizability, effectively solving the problem of multiple solutions when relying solely on high and low resistivity for discrimination. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the overall scheme for the joint DC-TDIP monitoring of casing well excitation wells according to the present invention;

[0039] Figure 2 This is a schematic diagram illustrating the working principle of the DC-TDIP dynamic monitoring system for casing wells according to the present invention.

[0040] Figure 3 For monitoring the primary and secondary potential curves of the well-to-surface DC-TDIP;

[0041] Figure 4 This is a flowchart illustrating the implementation steps of the DC-TDIP dynamic monitoring of casing wells according to the present invention.

[0042] Figure 5This is a schematic diagram of the four typical anomaly models of the present invention;

[0043] Figure 6 The image shows the response diagram of the well-to-surface DC-TDIP joint monitoring of the present invention, where (a) to (f) are the apparent resistivity and apparent polarizability responses under different combinations of anomalies, respectively. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] The overall scheme of the present invention is as follows Figure 1 As shown, for dynamic monitoring scenarios involving injection in casing wells, this study leverages the power supply from the casing well and the surface receiver to create well-to-surface electrical potential (BTP) observation conditions. It also incorporates the concept of induced polarization observation to capture secondary field attenuation after power outages, resulting in a combined well-to-surface potential and induced polarization observation method and system. During data processing, parameters such as apparent resistivity and apparent polarizability are extracted from the surface observation network. During construction, two observations are typically conducted: one before injection and one after. By calculating the difference in apparent resistivity and apparent polarizability before and after injection, the injection front, the injection influence range, migration direction, and dominant channel are determined from the differential data characteristics.

[0046] The technical principle of this invention is to integrate well-to-surface potential and time-domain excitation polarization into the same well-to-surface observation system. The energy flows into the formation at the perforation point of the target subsurface layer; surface measuring points are arranged using concentric measuring rings centered on the wellhead, with M and N electrodes positioned as follows: Figure 2 As shown. The ground potential observation section establishes a primary field potential equation based on charge conservation and Ohm's law, and calculates the apparent resistivity and its change through the potential difference between the M and N electrodes on the ground surface. Under the above power supply and receiving system, a time-domain excited polarization method is introduced. After the power supply is turned off, the potential decay over time is measured, the secondary field time response after the power supply is turned off is calculated, and the Cole-Cole polarization parameter is introduced to calculate the apparent polarizability. Figure 3The DC-TDIP monitoring system was used to monitor the primary and secondary potential curves of the well site. If two observations are conducted before and after water / gas injection, the differences in apparent resistivity and apparent polarizability obtained from the two observations can be used to comprehensively analyze the differences in these two physical parameters, thereby analyzing the effectiveness of water / gas injection in underground oil and gas reservoirs.

[0047] In a casing well, the current flows into the formation through the perforation location, rather than spreading uniformly outward from an ideal point source. Instead, it primarily enters the formation near the target layer through the perforated section. Therefore, this invention treats the perforated section as a finite-length distributed line source, aligning the source term with the downhole construction structure. This approach maintains the continuity of near-wellbore current injection and reduces the impact of non-physical source concentration on the secondary field attenuation curve.

[0048] In repeated observations, if the grid, source term, or receiving system is changed with each interpretation, spurious anomalies caused by changes in computational conditions may be mixed into the differential response. This invention emphasizes comparing the pre-injection and post-injection responses under the same well-to-surface observation system, the same power supply, and the same computational grid, so that the differential anomalies mainly reflect changes in reservoir electrical and polarization characteristics.

[0049] The physical significance of time-domain excited polarization response lies in the secondary field formed by the release of polarization charge after power failure. Different fluids and pore structures will change the polarizability, time constant, and decay rate. Therefore, the secondary field curve not only reflects whether there is an anomaly, but also the polarization difference between the anomaly and the background. This invention uses time window integration and primary field normalization to make the responses between different observation points and different construction stages comparable.

[0050] The surface measuring points are arranged in concentric measuring rings with the wellhead as the center, which is beneficial for forming angular and radial response coverage around the casing well. For the water injection front sweep range, response changes at measuring points with different radii and azimuths can be compared; for the dominant channels between wells, areas with enhanced or abnormally attenuated azimuth response curves on the same measuring ring can be observed.

[0051] A method for monitoring oil and gas reservoir injection that combines casing-excited well-to-surface potential and induced polarization, such as... Figure 4 As shown, it includes the following steps:

[0052] s1. Obtaining ground observation response: Before and after injection into the casing well, power is supplied to the target layer through the casing, and multiple receiving electrodes are deployed on the surface centered on the wellhead to collect surface MN potential difference data during the power supply phase and secondary field attenuation response data after power supply is turned off. The completeness and consistency of the observation records are then checked.

[0053] s2. Extract apparent resistivity and apparent polarizability parameters: Calculate apparent resistivity based on potential difference and supply current, and obtain apparent polarizability based on secondary field to form electrical data that can be used for comprehensive analysis.

[0054] The calculation of apparent resistivity is based on the governing equations of DC resistivity forward modeling:

[0055] ;

[0056] In the formula, For electrical conductivity, Potential, For source terms, These are mathematical operators;

[0057] A node potential discretization scheme based on an octree finite volume grid is adopted to locally refine the power supply, receiving area, and vicinity of the anomaly, in order to balance the simulation accuracy of deep targets with overall computational efficiency; in the discretization, a node potential discretization scheme is set... For discrete gradient operators, For discrete divergence operators, Given the surface conductivity mass matrix, the system of linear equations is as follows:

[0058] ;

[0059] In the formula, The conductivity-related system matrix is ​​the result of discretization of a finite volume.

[0060] The potentials at all nodes in the field are obtained by solving the linear equations, and the voltage difference is extracted at the receiving point. Combined with device geometric factors Calculate apparent resistivity :

[0061] ;

[0062] In the formula, This is the supply current.

[0063] The calculation of apparent polarizability includes:

[0064] A Cole–Cole polarization parameter model is introduced based on the same spatial grid and discrete operator as DC resistivity, using frequency domain complex conductivity. The expression is:

[0065] ;

[0066] In the formula, DC conductivity Polarizability It is a time constant. For frequency index, For frequency, It is an imaginary number;

[0067] The time response of the secondary field after shutdown is denoted as , for the Time window Integral voltage Defined as:

[0068] ;

[0069] The apparent polarizability is obtained by normalizing the absolute value of the primary field potential difference. :

[0070] ;

[0071] In the formula, Let be the apparent polarization rate of the g-th time window.

[0072] s3. Perform differential discrimination: Calculate the difference in apparent resistivity and apparent polarizability before and after injection; apparent resistivity difference And the difference in apparent polarization The calculation formulas are as follows:

[0073] ;

[0074] ;

[0075] In the formula, and , respectively, represent the apparent resistivity and apparent polarizability responses for the g-th time window.

[0076] s4. Output interpretation results: Based on the apparent resistivity difference and apparent polarizability difference, combined with the combined characteristics of high resistivity / low resistivity and strong polarization / weak polarization, the interpretation results of the injection leading edge, injection influence range, injection medium migration direction and dominant channel are output, providing a basis for subsequent retesting and observation parameter adjustment.

[0077] Specifically, the combined features include four typical anomalies related to dynamic monitoring of injection in casing wells: high resistivity / medium-high polarization anomalies, low resistivity / high polarization anomalies, high resistivity / low polarization anomalies, and low resistivity / low polarization anomalies, which are used to identify different injection responses and background anomalies.

[0078] During implementation, pre-injection observations should be completed as early as possible before construction or during the stable construction phase, and conditions such as surface electrode grounding resistance, power supply current, received noise, and weather conditions should be recorded. Post-injection observations should maintain consistency in electrode position, power supply segment, time window, and number of stacks; if adjustments are necessary, corrections should be made in the interpretation to avoid misinterpreting changes in construction conditions as changes in reservoir dynamics.

[0079] Using the above methods, a repeatable well-to-ground observation system is established under casing well conditions. For injection monitoring, well-to-ground potential-induced polarization measurements are performed before and after injection. The changes in apparent resistivity and apparent polarizability obtained provide a basis for interpreting the injection front, injection sweep range, and dominant channel direction.

[0080] To verify the identification effect of the method of the present invention, four typical anomalies related to dynamic monitoring of injection in casing wells were constructed, such as... Figure 5 As shown, four anomaly models—high resistivity / medium-high polarization anomalies, low resistivity / high polarization anomalies, high resistivity / low polarization anomalies, and low resistivity / low polarization anomalies—cover various combinations of resistivity and polarizability. The data processing results are used to interpret injection monitoring data. In cases where apparent resistivity is insufficient to distinguish the nature of anomalies, introducing apparent polarizability improves response discrimination. The four anomaly models decompose complex field responses into understandable end-member characteristics. Actual reservoirs may simultaneously contain injection media, residual oil, and high-salinity water, resulting in non-uniform electrical properties. However, by combining high / low resistivity with strong / weak polarization responses, a preliminary discrimination framework can be established, which can then be corrected based on construction data and geological understanding. In the four anomaly combination model, apparent resistivity response reflects the large-scale location of high and low resistivity anomalies. Apparent polarizability and anomaly differential decay curves further reflect the differences in strong polarization, weak polarization, fast decay, and slow decay, providing clearer discrimination criteria between different injection responses and background anomalies.

[0081] Figure 6 The results show the joint monitoring response of well-to-surface DC-TDIP for a combined model of four types of anomalies. It can be seen that when observing the apparent resistivity map alone, only two types of anomalies can be distinguished: high resistivity (A, C) and low resistivity (B, D). However, by adding apparent polarizability, high polarization (A, B) and low polarization (C, D) anomalies can be further distinguished. This result demonstrates that the present invention can output more informative joint discrimination results.

[0082] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for monitoring oil and gas reservoir injection combining casing-excited well-ground potential and induced polarization, characterized in that, Includes the following steps: s1. Before and after injection in the casing well, power is supplied to the target layer through the casing, and multiple receiving electrodes are deployed on the surface centered on the wellhead to collect surface potential difference data during the power supply phase and secondary field attenuation response data after power supply is turned off. s2. Calculate apparent resistivity based on potential difference data, and calculate apparent polarizability based on secondary field attenuation response data; s3. Calculate the difference in apparent resistivity and apparent polarizability before and after injection; s4. Based on the apparent resistivity difference and apparent polarizability difference, combined with the combined characteristics of high resistance / low resistance and strong polarization / weak polarization, the interpretation results of the injection leading edge, injection influence range, injection medium migration direction and dominant channel are output.

2. The method for monitoring oil and gas reservoir injection combining casing-excited well-ground potential and induced polarization as described in claim 1, characterized in that, In step s1, powering the target layer through the casing involves using a distributed line source with a limited length of the casing perforation section, allowing current to flow into the formation near the target layer through the perforation section.

3. The method for monitoring oil and gas reservoir injection combining casing-excited well-ground potential and induced polarization as described in claim 1, characterized in that, In step s1, the receiving electrodes are arranged in a concentric measuring ring with the wellhead as the center.

4. The method for monitoring oil and gas reservoir injection combining casing-excited well-ground potential and induced polarization as described in claim 1, characterized in that, In step s2, the apparent resistivity is calculated based on the governing equations for the forward modeling of DC resistivity: ； In the formula, For electrical conductivity, Potential, For source terms, These are mathematical operators; A node potential discretization scheme based on an octree finite volume grid is adopted to locally refine the power supply, receiving area, and vicinity of the anomaly, in order to balance the simulation accuracy of deep targets with overall computational efficiency; in the discretization, a node potential discretization scheme is set... For discrete gradient operators, For discrete divergence operators, Given the surface conductivity mass matrix, the system of linear equations is as follows: ； In the formula, The conductivity-related system matrix is ​​the result of discretization of a finite volume. The potentials at all nodes in the field are obtained by solving the linear equations, and the voltage difference is extracted at the receiving point. , , All are surface electrodes, combined with device geometric factors Calculate apparent resistivity : ； In the formula, This is the supply current.

5. The method for monitoring oil and gas reservoir injection combining casing-excited well-ground potential and induced polarization as described in claim 1, characterized in that, In step s2, the calculation of apparent polarizability includes: A Cole–Cole polarization parameter model is introduced based on the same spatial grid and discrete operator as DC resistivity, using frequency domain complex conductivity. The expression is: ； In the formula, DC conductivity Polarizability It is a time constant. For frequency index, For frequency, It is an imaginary number; The time response of the secondary field after shutdown is denoted as , for the Time window Integral voltage Defined as: ； The apparent polarizability is obtained by normalizing the absolute value of the primary field potential difference. : ； In the formula, Let be the apparent polarization rate of the g-th time window.

6. The method for monitoring oil and gas reservoir injection combining casing-excited well-ground potential and induced polarization as described in claim 1, characterized in that, In step s3, the apparent resistivity difference and apparent polarization difference The calculation formulas are as follows: ； ； In the formula, and , respectively, represent the apparent resistivity and apparent polarizability responses for the g-th time window.

7. The method for monitoring oil and gas reservoir injection combining casing-excited well-ground potential and induced polarization as described in claim 1, characterized in that, In step s4, the combined features include four typical anomalies related to dynamic monitoring of injection in casing wells: high resistivity / medium-high polarization anomaly, low resistivity / high polarization anomaly, high resistivity / low polarization anomaly, and low resistivity / low polarization anomaly, which are used to identify different injection responses and background anomalies.

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