An evaluation method for reservoir capacity of lamina interface by combination of iodine element tracer and XRF

Abstract

The present application belongs to the technical field of unconventional oil and gas reservoir seepage experiment and reservoir evaluation, and particularly relates to a kind of lamina interface reservoir capacity evaluation method using iodine element tracer and XRF. By selecting suitable shale samples and drilling piston samples, 20% iodine-containing tracer fluid is prepared for displacement experiment. After the experiment, the core sample is cut along the axial direction, the cut sample profile is scanned by XRF element, and the single lamina and lamina interface tracer element distribution characteristics are compared. Finally, the lamina interface opening and reservoir capacity evaluation are carried out. The present application uses sodium iodide tracer fluid combined with XRF scanning, which can directly identify the spatial distribution of fluid in single lamina and lamina interface. By combining the lamina interface opening evaluation with the porosity and permeability contribution analysis, the reservoir significance of the transformation of shale reservoir space from "matrix pore" to "matrix pore + lamina interface crack" dual pore system can be further understood.

Description

Technical Field

[0001] This invention belongs to the field of unconventional oil and gas reservoir seepage experiments and reservoir evaluation technology, and particularly relates to a method for evaluating the reservoir capacity of the laminar interface using iodine tracer and XRF combined. Background Technology

[0002] Shale reservoirs exhibit distinct lamellar structures, with significant differences in mineral composition, grain size, and crystal structure among different lamellar layers, resulting in diverse lamellar assemblages. As the most basic sedimentary structural unit of shale, lamellar layers form distinct interfaces. These interfaces are a result of varying sedimentary environments and diagenetic processes, and are a key manifestation of the heterogeneity of shale reservoirs. In low-permeability shale, lamellar interfaces not only serve as contact or transition zones between adjacent sedimentary units but may also form lamellar fractures with a certain degree of opening under compaction, diagenesis, or tectonic stress. When lamellar interfaces are open or semi-open, they act as preferential fluid migration channels, enhancing the reservoir's horizontal permeability and providing favorable space for the migration and enrichment of shale oil and gas between different lamellar layers. Therefore, the degree of opening and conductivity of lamellar interfaces directly affects the effective permeability and storage capacity of shale reservoirs.

[0003] Currently, conventional permeability testing mostly obtains overall seepage parameters from the core, making it difficult to distinguish the contribution of individual laminae and laminae interfaces to fluid migration. Meanwhile, while traditional methods such as thin section analysis, scanning electron microscopy, and mineral analysis can identify laminae type, mineral composition, and micropore structure, they are mostly static observations and cannot reflect whether fluids exhibit dominant migration along laminae interfaces under simulated formation temperature and pressure conditions. Therefore, there is an urgent need to develop a method capable of identifying laminae interface openness under high temperature and pressure conditions and evaluating its impact on permeability.

[0004] The existing technologies have the following problems: (1) Conventional permeability testing usually takes the entire core plunger as the test object. The results mainly reflect the overall permeability of the sample. It cannot distinguish the contribution of a single laminae and the laminae interface to fluid migration, and it is also difficult to determine whether the fluid migrates preferentially along the laminae interface; (2) Traditional rock thin sections, scanning electron microscopy or mineral analysis are mostly static characterizations. They cannot directly characterize the actual migration process of fluid in the laminae interface under high temperature and high pressure conditions, and it is difficult to reflect the real conductivity under the temperature and pressure conditions of the formation. Summary of the Invention

[0005] To overcome the problems existing in the prior art, the present invention provides a method for evaluating the storage capacity of laminar interfaces using iodine elemental tracing combined with XRF. The present invention includes the following steps:

[0006] S1. Shale sample selection and lamellar structure identification: Observe the shale samples to identify the lamellar type, lamellar combination mode and lamellar interface type in the samples. Drill plunger samples parallel to the shale bedding and grind the sidewalls and end faces of the plunger samples to avoid stress concentration caused by uneven end faces during sample clamping.

[0007] Laminar texture types include organic clay-rich laminar texture, felsic laminar texture, felsic-clay mixed laminar texture, and fibrous columnar spar calcite laminar texture. Laminar interface types include abrupt interfaces, gradual interfaces, cemented interfaces, and transitional interfaces. Cylindrical samples are drilled parallel to the shale bedding using a benchtop mechanical drill, minimizing the use of water to prevent microcracks caused by the interaction of water and rock. The cylindrical core plunger sample has a diameter of 2.5 cm and a preferred length of 4.5 cm, with both ends ground smooth and perfectly parallel.

[0008] S2. Prepare an iodine-containing tracer fluid with a mass fraction of 20%. Place the plunger sample with completed lamellar structure identification in the core holder and establish a temperature and pressure environment under simulated formation conditions. Conduct a displacement experiment by synergistic control of temperature, confining pressure and fluid pressure to allow the iodine-containing tracer fluid to enter the core.

[0009] The iodine mass fraction in the iodine-containing tracer fluid is 20%. It is fully dissolved in deionized water and stirred until the solution is homogeneous and transparent. If the concentration of the tracer fluid is too low, the iodine in the solution will not be clearly detected, thus making it impossible to clearly identify some of the real fluid transport paths. If the concentration is too high, the fluid viscosity will increase, thus increasing the fluid resistance. It is fully dissolved in deionized water and stirred until the solution is homogeneous and transparent.

[0010] S3. Sample cutting and XRF scanning: After the displacement experiment, the inlet pressure was gradually reduced to 0 MPa while maintaining the confining pressure tracking condition. Then the confining pressure was released, and the system was allowed to cool naturally to room temperature. The core sample was taken out and cut along the axial direction. XRF elemental scanning was performed on the cut sample profile to obtain the spatial distribution characteristics and specific content data of iodine.

[0011] S4. Comparison of tracer element distribution characteristics between single laminae and laminae interfaces: Based on the spatial distribution characteristics and specific content data of iodine obtained from XRF elemental scanning, the enrichment degree of tracer iodine at single laminae and laminae interfaces is compared and analyzed to determine the actual migration differences of iodine-containing tracer fluids within shale samples.

[0012] S5. Evaluation of the openness and reservoir capacity of the laminar interface: Based on the comparative analysis of the enrichment degree of iodine at a single laminar surface and the laminar interface, the openness and reservoir capacity of the laminar interface are evaluated. The difference in iodine content obtained by XRF scanning is converted into the evaluation basis of the conductivity and reservoir capacity of the laminar interface, thereby determining whether the laminar interface can serve as an effective reservoir space and dominant seepage channel in shale reservoirs.

[0013] Specifically, the spatial distribution characteristics of iodine obtained from XRF scanning are combined with the opening width, interface density, and connectivity of the laminar interface to comprehensively determine whether the laminar interface can serve as an effective reservoir space and dominant flow channel in shale reservoirs. When the iodine content at the laminar interface is higher than that of the adjacent single laminar layer, and the iodine is continuously distributed along the laminar interface direction, it indicates that iodine-containing tracer fluid preferentially enters and migrates along the laminar interface. In this case, the shale reservoir space can be expanded from a single matrix pore system to a dual pore system of "matrix pores + laminar interface fractures".

[0014] In terms of porosity evaluation, the fracture space formed after the opening of the laminar interface is included in the calculation of the total shale reservoir space; assuming the shale matrix porosity Φ, shale thickness H, shale distribution area S, laminar interface opening width A, and laminar interface density D, the final porosity Φ' can be expressed as:

[0015]

[0016] In permeability evaluation, the fissures formed after the opening of the fracture interface exhibit significant channel properties, and their impact on seepage capacity is closely related to the interface opening width. According to the cubic law of fracture seepage: K=e 2 / 12, where K represents permeability, and e represents the equivalent fracture width, i.e., the opening width A of the laminar interface; the influence of fracture geometry on permeability is corrected by introducing a connectivity factor C. Combined with the density function D, the final equivalent permeability formula of the shale reservoir affected by the laminar interface network can be expressed as:

[0017]

[0018] The present invention has the following beneficial effects: (1) It can simulate the real temperature and pressure environment of shale reservoirs under high temperature and high pressure conditions, and the evaluation results are closer to the actual underground seepage state; (2) By combining sodium iodide tracer fluid with XRF scanning, the spatial distribution of fluid in single laminae and laminae interfaces can be intuitively identified; (3) A method for judging the openness of laminae interfaces based on the enrichment degree of iodine is proposed, which can determine the openness of laminae interfaces under actual temperature and pressure conditions; (4) By combining the evaluation of the openness of laminae interfaces with the analysis of the contribution of porosity and permeability, the reservoir significance of the transformation of shale reservoir space from "matrix pores" to a dual pore system of "matrix pores + laminae interface fractures" can be further understood. Attached Figure Description

[0019] Figure 1 A schematic diagram of the process for combining iodine elemental tracer and XRF under high temperature and high pressure provided in this invention example;

[0020] Figure 2 The diagram showing the difference in iodine element distribution in different striation combinations is provided as an example of the present invention.

[0021] Figure 3 A schematic diagram illustrating the porosity calculation parameters for layered shale reservoirs provided as an example of the present invention. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] This embodiment targets a Paleogene shale formation in the Bohai Bay Basin. This shale formation contains a large number of different laminar types and their combinations, providing a good basis for the verification of the invention. Figure 1 As shown, the specific implementation steps of the present invention are as follows:

[0024] S1. Shale Sample Selection and Laminar Structure Identification: Shale samples without obvious artificial cracks, exhibiting typical laminar structures, and well-preserved were selected as the research objects. Cylindrical plunger samples with a diameter of 2.5 cm and a preferred length of 4.5 cm were drilled, ensuring both ends were parallel. The sample sidewalls and end faces were polished to ensure uniform force distribution within the clamping device, preventing stress concentration due to uneven end faces during sample clamping. Before the experiment, laminar combinations in the samples were identified, including fibrous columnar sparry calcite laminar combinations + organic-rich clay laminar combinations, felsic-clay mixed laminar combinations + organic-rich clay laminar combinations, felsic-clay mixed laminar combinations + felsic laminar combinations, and clay laminar combinations + felsic laminar combinations.

[0025] S2. High-temperature and high-pressure tracer displacement experiments were conducted, with sodium iodide solution being the preferred method. Sodium iodide is readily soluble in water and can form a stable iodine-containing solution. During the displacement process, it can enter the pores, single laminae, and laminae interfaces of the shale sample along with the fluid. Iodine is generally not an inherent element in the original mineral composition of shale, and there is virtually no background signal of iodine in the sample before the experiment. Therefore, the iodine detected after displacement can be mainly attributed to the entry and retention of the tracer fluid, effectively avoiding interference from the original rock element background on the experimental results. At the same time, iodine can be effectively identified by XRF elemental scanning. Therefore, the iodine-containing tracer fluid can be used to characterize the actual migration path of the fluid inside the shale and provide a basis for subsequent evaluation of laminae interface openness and reservoir capacity. The higher the enrichment of iodine, the higher the degree of entry and retention of the tracer fluid at that location, reflecting a strong fluid migration and reservoir capacity in that area.

[0026] During preparation, weigh the appropriate mass of sodium iodide according to the required volume for the experiment, add deionized water to dissolve it completely, and stir until the solution is homogeneous and transparent to obtain a 20% sodium iodide tracer solution. To ensure the stability of the displacement experiment, the prepared tracer solution should be sealed and stored to avoid concentration changes caused by evaporation. Before injecting into the storage system, the solution should be thoroughly mixed and checked for undissolved particles or precipitates to ensure stable tracer fluid concentration and good flowability.

[0027] Inject the prepared 20% sodium iodide solution into the storage system. Before the experiment, close all valves and check the sealing of the storage system, injection pipeline, core holder, and pressure acquisition system. Set the injection pump to constant pressure mode and the pressure to 5 MPa. Perform a stability test on the pipeline and stop the test after confirming stable fluid output.

[0028] The processed core plunger sample was placed in the center of the holder, and the fixing device was tightened gradually in a symmetrical manner to ensure uniform axial force. Then, the confining pressure system was turned on, and the tracking pressure was set to 5 MPa to stabilize the sample under the initial confining pressure conditions.

[0029] After the confining pressure stabilized, a staged loading method was adopted to gradually increase the fluid pressure at the inlet. This was achieved by injecting sodium iodide solution into the system to gradually increase the pressure on the sample, with increments of 5 MPa, until the inlet pressure reached the preset experimental value. This staged loading method avoids the generation of non-native fractures due to sudden stress changes, more closely resembling the gradual loading process of the formation. Simultaneously, a tracking pressure of 5 MPa (confining pressure differential control) was set to suppress non-target seepage of the high-pressure displacing fluid along the sample sidewall, ensuring that the fluid primarily flows axially through the pores, laminae, and laminae interfaces within the core.

[0030] After pressure loading is completed, the temperature control system is activated to raise the holder temperature to the preset formation temperature and maintain it at a constant temperature. During the experiment, the inlet and outlet pressures are continuously recorded, with a preferred data acquisition interval of 2 minutes. When the outlet pressure reaches a response of 0.1–0.2 MPa, it is determined that the tracer fluid has penetrated the sample.

[0031] After the experiment, the inlet pressure was slowly reduced to 0 MPa while maintaining the tracking pressure. Then the confining pressure was released and the system was allowed to cool naturally to room temperature before the core sample was taken out for subsequent analysis.

[0032] S3. Sample Sectioning and XRF Scanning: The extracted core sample is sectioned axially to fully display the internal laminar structure and laminar interfaces. The sectioning direction is consistent with the fluid displacement direction to observe the migration path of the iodine-containing tracer fluid from the inlet to the outlet. During sectioning, the profile should be kept as flat as possible to minimize the impact of mechanical disturbance, sample breakage, or external contamination on the tracer element distribution results. Subsequently, XRF elemental scanning is performed on the sample profile to obtain the spatial distribution characteristics and content data of iodine in different individual laminae and laminar interfaces. Since iodine mainly originates from the iodine-containing tracer fluid that enters the core during displacement, the iodine distribution obtained by XRF scanning can reflect the entry, migration, and retention of the tracer fluid within the sample.

[0033] In XRF scans, regions with higher iodine content indicate a higher enrichment of tracer fluid, suggesting stronger fluid penetration and better transport capabilities in these areas. Conversely, regions with lower iodine content indicate weaker tracer fluid penetration or restricted transport, indicating relatively weaker permeability in these areas. By correlating the iodine distribution results with the locations of individual laminae and laminar interfaces in the sample, a basis can be provided for subsequent comparisons of the storage and conductivity of individual laminae and laminar interfaces.

[0034] S4. Comparison of Iodine Distribution Characteristics in Single Fractures and Fracture Interfaces: Based on the spatial distribution characteristics and content data of iodine obtained from XRF elemental scanning, single fractures and fracture interfaces in the sample were used as evaluation units to compare and analyze the enrichment degree of tracer iodine in both. Specifically, combining the fracture type, fracture combination method, and fracture interface location identified before the experiment, the iodine content inside a single fracture and at adjacent fracture interfaces was extracted from the XRF scanning results. The differences in iodine content, enrichment intensity, and distribution continuity in different locations were compared.

[0035] Figure 2As shown, the iodine content of individual laminae and laminae interfaces differs significantly in different laminae combinations. In the combination of fibrous columnar sparry calcite laminae and organic clay-rich laminae, the iodine content in individual laminae is relatively low, while the iodine content at the laminae interfaces is significantly higher. This indicates that the mechanical difference between rigid calcite laminae and plastic clay laminae is conducive to the formation of open interfaces, and iodine-containing tracer fluids preferentially migrate along the laminae interfaces, demonstrating that the laminae interfaces have a significant controlling effect on fluid migration.

[0036] In the assemblage of felsic-clay mixed laminae and organic-rich clay laminae, the felsic-clay mixed laminae exhibit a significant skeletal support, resulting in a more stable overall pore structure. Iodine storage capacity at the laminae interfaces is improved compared to single laminae. The mineral compositions of the two types of laminae in the assemblage tend to be similar, leading to reduced stress concentration at the laminae interfaces and a decrease in interfacial reinforcement. In this case, fluid transport is no longer entirely dependent on the interfaces but is more controlled by the internal pore structure of the laminae.

[0037] In the combination of felsic-clay mixed laminae and felsic laminae, both individual laminae and laminae interfaces exhibit high iodine content, indicating that the rigid mineral framework improves the overall pore connectivity. The seepage process is controlled not only by the interface but also by the pore structure inside the laminae.

[0038] In the combination of clay-laminated and felsic-laminated surfaces, the iodine content at the gradient interface is higher than that at the abrupt interface, indicating that the interface openness and conductivity are not entirely consistent. Although the abrupt interface has strong mechanical discontinuity, its effective openness and connectivity may be limited due to cementation or filling, resulting in a weak conductivity state. While the gradient interface has a lower degree of mechanical weakening, the superposition of multiple types of pores in its transition zone is more conducive to the formation of continuous seepage channels.

[0039] S5. Evaluation of the openness and storage capacity of the laminar interface: Based on the comparison results of iodine content at the single laminar surface and the laminar interface obtained in step S5, the openness of the laminar interface and its contribution to the storage capacity and permeability are evaluated. Figure 3 As shown, in this embodiment, the overall permeability of the core is no longer used as the evaluation criterion alone. Instead, the iodine enrichment characteristics obtained from XRF scanning are combined with the opening width, interface density, and connectivity of the laminar interface to comprehensively determine whether the laminar interface can serve as an effective reservoir space and dominant flow channel in the shale reservoir. Specifically, when the iodine content at the laminar interface is significantly higher than that of the adjacent single laminar layer, and the iodine is continuously or relatively continuously distributed along the laminar interface direction, it indicates that iodine-containing tracer fluid preferentially enters and migrates along the laminar interface, indicating that the laminar interface has good opening and connectivity. At this time, the shale reservoir space can be expanded from a single matrix pore system to a dual pore system of "matrix pores + laminar interface fractures".

[0040] In porosity evaluation, the fracture space formed after the opening of the laminar interface is included in the calculation of the total shale reservoir space. Let the shale matrix porosity be Φ, the shale thickness be H, the shale distribution area be S, the laminar interface opening width be A, and the laminar interface density be D. The final porosity Φ' can be expressed as:

[0041]

[0042] Therefore, the interfacial density D and the opening width A are the key factors controlling the increase in total porosity.

[0043] In permeability evaluation, the fissures formed after the opening of the fracture interface exhibit significant channel properties, and their impact on seepage capacity is closely related to the interface opening width. According to the cubic law of fracture seepage: K=e 2 / 12, where K represents permeability and e represents the equivalent fracture width, i.e., the opening width A of the laminar interface. The influence of fracture geometry on permeability is corrected by introducing a connectivity factor C. Combined with the laminar interface density D, the final formula for the equivalent permeability of the shale reservoir affected by the laminar interface network can be expressed as:

[0044]

[0045] This relationship indicates that the opening width A of the laminar interface is the dominant controlling factor for permeability. Furthermore, due to its cubic relationship, changes in the micron-level interface opening can also significantly impact the macroscopic permeability of shale reservoirs. The connectivity factor C and the laminar interface density D further amplify the contribution of interface opening to permeability.

[0046] In this embodiment, the enrichment characteristics of iodine obtained by XRF scanning can be combined with the calculation of the contribution of porosity and permeability to determine whether the laminar interface forms an effective reservoir space and a dominant seepage channel, and further evaluate its contribution to the reservoir capacity and seepage capacity of shale reservoir.

[0047] Taking the Paleogene Shale of the Lower Sha-S3 to Upper Sha-S4 sub-member in the Dongying Depression as an example, its matrix porosity Φ is about 5%, and the density D at the laminar interface ranges from 1×10⁻⁶. 4 ~2.5×10 4 The laminar interface opening width A is mainly distributed between 200 nm and 1 μm. Substituting this into the porosity conversion formula, the range of Φ' is 1.04Φ to 1.5Φ. For every 1 μm increase in laminar interface opening width, the porosity increases by 0.2% to 2.5%. Under the overpressure background of the Dongying Depression, the laminar interface opening is concentrated between 3 and 5 μm, resulting in a porosity increase of 0.6% to 12.5%.

[0048] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. 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 or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the protection scope of the claims and specification of the present invention.

Claims

1. A method for evaluating the storage capacity of laminar interfaces using iodine elemental tracing combined with XRF, characterized in that, Includes the following steps: S1. Shale sample selection and lamellar structure identification: Observe the shale samples, identify the lamellar type, lamellar combination mode and lamellar interface type in the samples, drill plunger samples parallel to the shale bedding, and grind the sidewalls and end faces of the plunger samples. S2. Prepare iodine-containing tracer fluid, place the plunger sample with completed lamellar structure identification in the core holder, establish a temperature and pressure environment under simulated formation conditions, and conduct a displacement experiment through coordinated control of temperature, confining pressure and fluid pressure to allow the iodine-containing tracer fluid to enter the core. S3. Sample cutting and XRF scanning: After the displacement experiment, the inlet pressure was gradually reduced to 0 MPa while maintaining the confining pressure tracking condition. Then the confining pressure was released and the sample was allowed to cool naturally to room temperature. The core sample was then cut along the axial direction and XRF elemental scanning was performed on the cut sample profile to obtain the spatial distribution characteristics and specific content data of iodine. S4. Comparison of tracer element distribution characteristics between single laminae and laminae interfaces: Based on the spatial distribution characteristics and specific content data of iodine obtained by XRF element scanning, the enrichment degree of tracer iodine at single laminae and laminae interfaces is compared and analyzed to determine the actual migration differences of iodine-containing tracer fluids inside shale samples. S5. Evaluation of the openness and reservoir capacity of the laminar interface: Based on the comparative analysis of the enrichment degree of iodine element at a single laminar surface and the laminar interface, the openness and reservoir capacity of the laminar interface are evaluated. The difference in iodine element content obtained by XRF scanning is transformed into the evaluation basis of the conductivity and reservoir capacity of the laminar interface, and it is determined whether the laminar interface can serve as an effective reservoir space and dominant seepage channel in shale reservoir. The specific approach of S5 is as follows: The spatial distribution characteristics of iodine obtained by XRF scanning are combined with the opening width, density and connectivity of the laminar interface to comprehensively judge whether the laminar interface can serve as an effective storage space and dominant seepage channel in shale reservoir. The judgment is based on the fact that when the iodine content at the laminar interface is higher than that of the adjacent single laminar layer, and the iodine is continuously distributed along the laminar interface direction, it indicates that the iodine-containing tracer fluid preferentially enters and migrates along the laminar interface. At this time, the shale reservoir space is expanded from a single matrix pore system to a dual pore system of matrix pores + laminar interface fractures. In terms of porosity evaluation, the fracture space formed after the opening of the laminar interface is included in the calculation of the total shale reservoir space, and the final porosity Φ' is expressed as: Where Φ is the porosity of the shale matrix; H is the shale thickness; S is the shale distribution area; A is the opening width of the laminar interface; and D is the density of the laminar interface. In terms of permeability evaluation, the impact of the fractures formed after the opening of the laminar interface on the seepage capacity is closely related to the opening width of the interface, according to the cubic law of fracture seepage: K=e 2 / 12, where K represents permeability, e represents the equivalent fracture width, i.e., the opening width A of the laminar interface. The influence of fracture geometry on permeability is corrected by introducing a connectivity factor C. Combined with the laminar interface density D, the equivalent permeability formula of shale reservoir affected by the laminar interface network is expressed as: 。 2. The method for evaluating the storage capacity of laminar interfaces using iodine elemental tracing combined with XRF as described in claim 1, characterized in that, The plunger sample described in S1 has a diameter of 2.5 cm, a length of 4.5 cm, and parallel ends.

3. The method for evaluating the storage capacity of laminar interfaces using iodine elemental tracing combined with XRF as described in claim 1, characterized in that, The iodine-containing tracer fluid in S2 has a mass fraction of 20% iodine, which is fully dissolved in deionized water and stirred until the solution is homogeneous and transparent.

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