An inversion method for characterizing the connectivity of the internal reservoirs of a fractured-collapsed body

Abstract

The application provides an inversion method for describing the connectivity of the internal reservoir of a faulted karst body. The method uses seismic attributes reflecting the outline of the faulted karst body, the holes and cracks in the faulted karst body, establishes a comprehensive low-frequency model reflecting the carbonate bedrock, the faulted karst body, and the crack and hole system in the faulted karst body, uses the model to perform post-stack constrained sparse pulse inversion, obtains a longitudinal wave impedance data body of the underground, and uses the longitudinal wave impedance data to give a longitudinal wave impedance threshold value of the reservoir, and describe the connectivity of the internal reservoir of the faulted karst body in space.

Description

Technical Field

[0001] This invention relates to the field of oil and gas exploration technology, and in particular to an inversion method for characterizing the connectivity of reservoirs within fractured solutions. Background Technology

[0002] In 2015, Lu Xinbian et al. proposed the concept of fault-knot traps, which mainly refer to the dissolution and modification of fracture zones by karst water seeping downwards or surging upwards along the fault direction, forming karst fracture-cavity systems in different three-dimensional spaces. These traps are formed under the cover of overlying marl or laterally dense limestone. In the Tarim Basin's outer slope area of ​​the Tahe River, influenced by the Upper Ordovician strata, the development of karst fracture-cavities shows a good match with fault zones and tectonic deformation, exhibiting obvious fault-controlled karst characteristics. Dissolution and expansion occur around deep, large fault zones, making these areas favorable for the development of karst fracture-cavities in the Middle to Lower Ordovician strata. Guided by the fault-knot reservoir model, with the deepening of exploration and development, the Shunbei large-scale fault-knot type oil and gas field was discovered. Currently, the Shunbei oil and gas field has identified and confirmed 18 strike-slip fault zones related to fault-knot development, with a resource scale of 17×10⁸ t oil equivalent, demonstrating the good exploration and development prospects of the Shunbei area.

[0003] Previous studies have suggested that the main reservoir spaces within fault-dissolved bodies are caves associated with strike-slip faults, high-angle tectonic fractures, and cavities formed by dissolution along fractures. Tectonic fracturing is the primary controlling factor for the development of fault-dissolved reservoirs, while dissolution further promotes reservoir development (Jiao Fangzheng 2018). Fault-dissolved bodies exhibit strong heterogeneity, distributed laterally along fault zones in segments, and longitudinally in a network pattern with irregularities. "Beaded" high-energy seismic facies are strongly correlated with cave reservoirs, exhibiting low P-wave impedance; dissolution cavities exhibit strong energy or blank reflection seismic facies, corresponding to lower P-wave impedance; fractured reservoir development segments show good correlation with seismic properties such as AFE, ant bodies, and likelihood bodies.

[0004] A fault-knot is a geological body composed of fractures and cavities. Its characterization includes the external morphology of the fault-knot's boundary space, as well as the internal structure of cavities, dissolution pits, and fractures connecting these pits. Reservoir development within a fault-knot is controlled by multiple factors, and the seismic wave field is complex, making accurate quantitative characterization of the reservoir within a fault-knot challenging.

[0005] Patent CN110794476B provides an inversion method based on fault-dissolved body phasing control. It primarily utilizes tensor properties to characterize the fault-dissolved body boundary, establishing a low-frequency model of the boundary. This low-frequency model is then incorporated into the conventional inversion process to obtain the wave impedance volume under the seismic phasing control of the fault-dissolved body, thereby achieving a quantitative or semi-quantitative characterization of the "fault-dissolved body" reservoir and ultimately completing the effective reservoir quantification. However, this method only considers the external morphology of the fault-dissolved body and does not account for the complex fracture-vuggy system within it.

[0006] Patent CN110007344B provides a seismic identification method and apparatus for fault-dissolved reservoir connectivity. It primarily relies on a three-dimensional seismic data volume to construct a maximum likelihood model. The maximum likelihood model is then spatially sculpted using threshold values ​​of its attributes to obtain the spatial outline of the fault-dissolved reservoir. Within this sculpted space, two points at the same elevation are selected, and the root mean square velocity of the seismic data at these two points is calculated and converted into formation pressure. The formation pressure is then used to determine whether the reservoirs at these two points are connected. However, this invention only indicates whether two points are connected; it cannot spatially characterize whether the entire reservoir is connected.

[0007] Patent CN107390264B provides a method for characterizing the internal structure of carbonate fractured-dissolved bodies. It primarily utilizes post-stack conventional inversion to obtain subsurface P-wave impedance data, determines the P-wave impedance data threshold representing fractured-vuggy reservoirs, and performs a hollowing-out process on the subsurface P-wave impedance data. It then calculates the tensor properties of seismic data and performs spatial smoothing on the tensor properties to determine the tensor property threshold representing the fractured region of the fractured-dissolved body, thus obtaining the contour of the fractured-dissolved body. Using the contour of the fractured-dissolved body as the boundary, the tensor properties and the subsurface P-wave impedance data are displayed within the contour, achieving the characterization of the external contour and internal structure of the fractured-dissolved body. However, this method uses P-wave impedance properties to characterize the porous reservoirs inside the fractured-dissolved body and tensor properties to characterize the external morphology of the fractured-dissolved body boundary. It cannot characterize the spatial distribution and connectivity of the reservoirs inside the fractured-dissolved body.

[0008] The aforementioned background research indicates that previous methods for characterizing reservoir connectivity within fault-deposited bodies were limited to planar or point-to-point connectivity, failing to reflect spatial connectivity within the reservoir. Alternatively, methods superimposed three seismic attributes—tensor, P-wave impedance, and ant-like mass—reflecting the external morphology and internal structure of fault-deposited bodies for different targets, to characterize their external morphology and internal structure. Due to the significant dimensional differences between these attributes, they could only qualitatively reflect the external morphology and internal structure of fault-deposited bodies, unable to quantitatively describe the spatial distribution of reservoirs and the scale of connected reservoir spaces within the fault-deposited bodies. Summary of the Invention

[0009] To address the shortcomings of existing technologies in characterizing reservoir connectivity within fracture-dissolve bodies, this invention proposes a seismic facies-constrained inversion technique to reflect the distribution of connected reservoirs within fracture-dissolve bodies. This technique can quantitatively characterize the scale and distribution of fracture-cavity connectivity within fracture-dissolve bodies, facilitating oil and gas reserve calculations and guiding well location deployment within fracture-dissolve bodies, thereby improving drilling success rates and exploration and development efficiency.

[0010] In a first aspect, the present invention proposes an inversion method for characterizing the connectivity of the internal reservoir of a dissolved body, comprising the following steps:

[0011] S1: Establish a straight-plate low-frequency model of underground strata using logging data from actual drilling.

[0012] S2: Determine the boundary of the broken solution, assign longitudinal wave impedance value to the broken solution, and expand the straight plate low-frequency model obtained in step S1 to obtain the low-frequency model of the spatial distribution of the broken solution.

[0013] S3: Combine actual well logging data to obtain the background value of the longitudinal wave impedance of the fracture, assign the background value of the longitudinal wave impedance of the fracture to the maximum likelihood property, and obtain the low-frequency model of the fracture.

[0014] S4: Using the straight plate low-frequency model from step S1, constrained sparse pulse inversion is performed on the original seismic data to obtain subsurface P-wave impedance data and a low-frequency model reflecting pore and cavern reservoirs is obtained.

[0015] S5: Based on the low-frequency models obtained in steps S1-S4, establish a comprehensive low-frequency model of bedrock, fracture-collapse, and fracture-cavity system;

[0016] S6: Using the integrated low-frequency model of bedrock, fracture-knot, and fracture-cavity system obtained in step S5, post-stack constrained sparse pulse inversion is carried out to obtain underground P-wave impedance data. The P-wave impedance data is then sculpted to characterize the connectivity of the reservoir within the fracture-knot.

[0017] As a specific embodiment of the present invention, step S1 includes using logging data from actual drilling to establish a straight-plate low-frequency model of the underground formation, given the background value of the longitudinal wave impedance of the underground formation.

[0018] Specifically, a P-wave impedance histogram is established using the P-wave impedance of drilled underground strata wells. The median value of the histogram is read as the uniform impedance background value of the underground strata, and a low-frequency model of the underground strata P-wave impedance as a straight plate (constant) is established.

[0019] As a specific embodiment of the present invention, in step S2, determining the boundary of the fractured solution includes: using the volume element density value corresponding to the intersection of the smoothed seismic reflection layer and the original seismic reflection layer as the threshold value of the fractured solution boundary to determine the boundary of the fractured solution.

[0020] Specifically, the volume element density value corresponding to the intersection of the smoothed seismic reflection layer and the original seismic reflection layer is used as the threshold value of the fault-dissolved body boundary (less than the background value of the P-wave impedance of the underground strata and greater than the background value of the P-wave impedance of the porous reservoir) to determine the boundary of the fault-dissolved body.

[0021] As a specific embodiment of the present invention, in step 2, the smoothing process includes structural filtering, edge detection and volume element density enhancement processing of the seismic data, and then large smoothing processing of the enhanced seismic reflection layer.

[0022] As a specific embodiment of the present invention, in step S3, the maximum likelihood attribute of the longitudinal wave impedance background value of the fracture is determined by using actual drilling results to determine the threshold value of the maximum likelihood body and obtain the spatial distribution of the maximum likelihood attribute of the fracture; the maximum likelihood body calculation is performed by constructing filtered data from seismic data to calculate the maximum likelihood body and obtain the maximum likelihood attribute of the fracture.

[0023] As a specific embodiment of the present invention, in step S4, when performing constrained sparse pulse inversion on the original seismic data, the P-wave impedance threshold value of the pore and cavern reservoir is also given.

[0024] As a specific embodiment of the present invention, step S5 includes: embedding the low-frequency model of holes, cavities and cracks in step S4 into the low-frequency model in step S2, and then embedding it into the straight plate low-frequency model in step S1 to obtain the integrated low-frequency model of the bedrock, fracture-dissolved body and fracture-cavity system.

[0025] In a specific embodiment of the present invention, in step S6, the longitudinal wave impedance data is engraved, and the threshold value of the effective carbonate reservoir is determined using the drilling results, and the longitudinal wave impedance data is engraved.

[0026] Secondly, the inversion method for characterizing the connectivity of reservoirs inside fractured solutions provided by this invention is applicable in fields with significant formation heterogeneity.

[0027] As a specific embodiment of the present invention, the areas in which the stratigraphy exhibits significant homogeneity include carbonate rock strata, igneous rock strata, and / or salt dome strata.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] 1. After implementation, this invention can characterize the spatial connectivity of reservoirs within carbonate rock fracture-dissolved bodies. Macroscopically, a fracture-dissolved body is a favorable reservoir relative to the surrounding rock, its interior consisting of a fracture-cavity system composed of fractures, dissolution pores, and caves. The interior of a fracture-dissolved body is composed of fracture-cavity units of varying sizes that are separated from each other. This invention can clearly and intuitively characterize these interconnected fracture-cavity units of different sizes within a fracture-dissolved body.

[0030] 2. After the implementation of the present invention through the embodiments, in actual drilling, it is preferable to drill a single large-scale fractured cavity unit that is interconnected. The development of a single interconnected fractured cavity unit can reduce the number of drilling wells, reduce drilling costs, and achieve direct benefits.

[0031] 3. This invention has a wide range of applications. It is not only suitable for carbonate rock fractures, but also for igneous rocks, salt bodies, and paleochannels, which are strata with significant spatial anisotropy. Attached Figure Description

[0032] Figure 1 This is a diagram illustrating the implementation steps for characterizing the connectivity of the internal reservoir of the broken solution in an embodiment of the present invention;

[0033] Figure 2 This is a seismic profile through well B in an embodiment of the present invention;

[0034] Figure 3 This is a cross-section of the straight plate model through well B in an embodiment of the present invention;

[0035] Figure 4 This is a cross-section of the straight plate model embedded in the B-well fault solution in an embodiment of the present invention;

[0036] Figure 5 This is a low-frequency model profile of the fracture in well B in an embodiment of the present invention;

[0037] Figure 6 This is a low-frequency model profile of the well hole in embodiment B of the present invention;

[0038] Figure 7 This is a low-frequency model profile of the surrounding rock, fracture-collapse, and fracture-cavity system of well B in this embodiment of the invention;

[0039] Figure 8 This is the longitudinal wave impedance profile obtained by inverting sparse pulses through well B in an embodiment of the present invention;

[0040] Figure 9 The root mean square attribute of the volume element density T74_2_300ms in the north-south three-dimensional region of this invention;

[0041] Figure 10 This is the longitudinal wave impedance profile of wells A, B, C, and D connected in Embodiment 1 of the present invention;

[0042] Figure 11 The thickness of the T74_20ms_T76 longitudinal wave impedance engraving connected reservoir in Embodiment 1 of the present invention;

[0043] Figure 12 The thickness of the T74_20ms_T76 longitudinal wave impedance engraving connected reservoir in Embodiment 1 of the present invention is shown. Detailed Implementation

[0044] The present invention will be further described below with reference to specific embodiments, but this does not constitute any limitation on the present invention.

[0045] Example

[0046] Reference Figure 1This embodiment provides an inversion method for characterizing the connectivity of reservoirs within fault-dissolved bodies, applied to oil and gas exploration in Ordovician carbonate fault-dissolved bodies in the Shunbei 3D area of ​​the Tarim Basin. The specific details of applying this invention to the currently relevant blocks in the Tarim Basin where fault-dissolved bodies are the exploration targets are as follows:

[0047] Within the Shunbei three-dimensional region, the fault-dissolved bodies extend in a northeast-southwest direction on the plane. Figure 2 To illustrate the implementation process of this invention using the seismic profile through well B as an example, the following describes the longitudinal profile through well B:

[0048] Step 1: Using logging data from actual drilling, the following example of the longitudinal profile of well B illustrates the implementation process of this invention. Given the P-wave impedance background value of the carbonate formation, a low-frequency model of the carbonate rock straight plate is established, such as... Figure 3 As shown, the background value of the P-wave impedance of the carbonate rock formation in this area is 10ms below layer T74, which is a uniform constant value.

[0049] Step 2: As Figure 2 As shown in the cross-section, there is a "beaded" pattern of seismic reflection anomalies below well B, with the well bottom located at the apex of the "beaded" reflections; Figure 2 The seismic data shown undergoes structural filtering, edge detection, and volumetric density enhancement. The interpreted seismic reflection layers are smoothed extensively. The volumetric density values ​​at the intersections of the smoothed and original seismic reflection layers are used as threshold values ​​for the fault-knot body boundaries to determine these boundaries. A P-wave impedance value (less than the background value of carbonate rocks and greater than the background value of porous reservoirs) is assigned to the fault-knot body to obtain a low-frequency model of its spatial distribution, such as... Figure 4 As shown, the lower part of the trajectory of well B is located within a fractured solution, indicating that well B encountered a fractured solution during drilling.

[0050] Step 3: Using seismic data to construct filtered data, perform maximum likelihood volume calculations to obtain information reflecting fracture properties. Using actual drilling results, determine the threshold value of the maximum likelihood volume to obtain the spatial distribution of the maximum likelihood properties reflecting the fracture. Combine this with actual well logging data to obtain the P-wave impedance background value of the fracture. Assign this background value to the maximum likelihood properties to obtain the low-frequency model of the fracture, such as... Figure 5 As shown in the figure, drilling fluid loss occurred at the bottom of well B during drilling, confirming that a fracture was encountered. The bottom of well B is located at the top of the fracture.

[0051] Step 4: Using the straight-plate low-frequency model, constrained sparse pulse inversion is performed on the original seismic data to obtain subsurface P-wave impedance data. Given the P-wave impedance threshold values ​​for pore and cavern reservoirs, a low-frequency model reflecting the pore and cavern reservoirs is obtained, such as... Figure 6 As shown, the holes and cavities are located below well B.

[0052] Step 5: Establish a comprehensive low-frequency model of the bedrock, fault-knot, and fracture-cavity system: Embed the low-frequency model of the fault-knot into the straight plate model, and then embed the low-frequency models of pores, cavities, and fractures into the fault-knot. This yields a comprehensive low-frequency model of the fracture-cavity system, where fault-knots develop within carbonate strata, and pores, cavities, and fractures develop within the fault-knots. Figure 7 As shown, this reflects the development of fractured solutions in a carbonate rock background, with cracks, pores, and cavities inside the fractured solutions.

[0053] Step 6: Post-stack constrained sparse pulse inversion: Using a comprehensive low-frequency model reflecting the carbonate bedrock, fault-dissolved body, and fracture-cavity system, post-stack constrained sparse pulse inversion is performed to obtain subsurface P-wave impedance data, such as... Figure 8 As shown, the longitudinal impedance profile obtained by constrained sparse pulse inversion through well B is also the result profile after the implementation of this invention. It can be seen that the fractured solution encountered by well B is longitudinally and laterally connected. Using the drilling results, the threshold value of the effective reservoir of carbonate rock is determined, and the longitudinal impedance data is sculpted to depict the connectivity of the reservoir inside the fractured solution.

[0054] Within the Shunbei three-dimensional region, the fault-dissolved body extends in a northeast-southwest direction on the plane. Viewed from the plane (projection), this fault-dissolved body appears connected, such as... Figure 9 As shown, the root mean square density of volume elements in the Shunbei 3D area at T74_2_300ms is an example. Currently, four wells (A, B, C, and D) have been drilled for exploration of the fault-dissolved body. Using this method, the connectivity of the reservoir within the fault-dissolved body is characterized, suggesting that wells A, B, and C form a connected reservoir. Figure 10 As shown, the longitudinal wave impedance profiles of wells A, B, C, and D show that the formation below T74 layer (10ms) is carbonate rock, while the formation above it is clastic rock. The carbonate rock portion is the primary focus. From the profile, wells A, B, and C are connected in the lateral fracture-cavity system within the carbonate rock portion, forming a single fracture-cavity system. Well D is not connected laterally to these three wells and is a separate fracture-cavity system.

[0055] like Figure 11 As shown, T74_20ms_T76 longitudinal wave impedance sculpts the thickness of the connected reservoir. Given the longitudinal wave impedance threshold value of this section, the spatial distribution of the fractured solution is characterized, depicting two independent fracture-cavity systems. The figure shows that wells A, B, and C are spatially a fracture-cavity system, while well D is not within this fracture-cavity system.

[0056] The reservoir of well D is not connected to the reservoirs of these three wells, such as Figure 12As shown, the T74_20ms_T76 longitudinal wave impedance model depicts the thickness of the connected reservoir. Wells A, B, and C are not within this fractured-cavity system, while well D is. Well D is a separate reservoir. The straight-line distance from well A to well C is 2.43 km, and the straight-line distance from well C to well D is 1.9 km. During drilling, these four wells experienced venting or mud loss in the target formation of the fractured-cavity system, ultimately achieving high-yield oil and gas flow. The interference test results indicate that wells A, B, and C belong to the same connected reservoir, while well D is not connected to these three wells. The test results are consistent with the conclusions drawn from using this method to characterize the connectivity of the reservoir within the fractured-cavity system.

[0057] Any numerical value mentioned in this invention, if there is only a two-unit interval between any minimum and any maximum value, includes all values ​​that increase by one unit each time from the minimum to the maximum value. For example, if the amount of a component, or the value of a process variable such as temperature, pressure, or time, is stated as 50-90, in this specification it means specifically listing values ​​such as 51-89, 52-88… and 69-71 and 70-71, etc. For non-integer values, it may be appropriately considered that a unit is 0.1, 0.01, 0.001, or 0.0001. These are merely some specifically specified examples. In this application, in a similar manner, all possible combinations of numerical values ​​between the listed minimum and maximum values ​​are considered to have been disclosed.

[0058] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. An inversion method for characterizing the connectivity of internal reservoirs of a karst body, characterized in that, The method comprises the following steps: S1: establishing a straight plate low-frequency model of the underground formation by using well logging data of actual drilling; S2: determining the boundary of the fault-dissolved body, assigning a longitudinal wave impedance value to the fault-dissolved body, and unfolding the straight plate low-frequency model obtained in step S1 to obtain a low-frequency model of the spatial distribution of the fault-dissolved body; S3: combining the actual well logging data to obtain a longitudinal wave impedance background value of the fracture, assigning the longitudinal wave impedance background value of the fracture to a maximum likelihood attribute, and obtaining a low-frequency model of the fracture; S4: using the straight plate low-frequency model in step S1 to perform constrained sparse pulse inversion on the original seismic data to obtain underground longitudinal wave impedance data and obtain a low-frequency model reflecting the pore and cave reservoir; S5: based on the low-frequency models obtained in steps S1-S4, establishing a comprehensive low-frequency model of the bedrock, fault-dissolved body and fracture-cave system; S6: using the comprehensive low-frequency model of the bedrock, fault-dissolved body and fracture-cave system obtained in step S5 to perform post-stack constrained sparse pulse inversion to obtain underground longitudinal wave impedance data, and carving the longitudinal wave impedance data to depict the connectivity of the internal reservoir of the fault-dissolved body.

2. The inversion method of claim 1, wherein, The step S1 comprises using well logging data of actual drilling to assign a longitudinal wave impedance background value to the underground formation and establish a straight plate low-frequency model of the underground formation.

3. The inversion method according to claim 1 or 2, characterized in that, In the step S2, the determination of the boundary of the fault-dissolved body comprises using the body element density value corresponding to the intersection part of the smoothed seismic reflection layer and the original seismic reflection layer as a threshold value of the fault-dissolved body boundary to determine the boundary of the fault-dissolved body.

4. The inversion method of claim 3, wherein, In the step S2, the smoothing processing comprises performing structural filtering, edge detection and body element density enhancement processing on the seismic data, and then performing large smoothing processing on the enhanced seismic reflection layer.

5. The inversion method according to claim 2 or 4, characterized in that, In the step S3, the longitudinal wave impedance background value of the fracture is assigned to the maximum likelihood attribute by using the actual drilling results to determine the threshold value of the maximum likelihood body and obtain the spatial distribution of the maximum likelihood attribute reflecting the fracture; the maximum likelihood body calculation is performed by using the structural filtering data of the seismic data to calculate the maximum likelihood body and obtain the maximum likelihood attribute reflecting the fracture.

6. The inversion method of claim 2 or 4, wherein, In the step S4, when performing the constrained sparse pulse inversion on the original seismic data, a longitudinal wave impedance threshold value of the pore and cave reservoir is also given.

7. The inversion method of claim 2 or 4, wherein, The step S5 comprises embedding the pore, cave and fracture low-frequency models in step S4 into the low-frequency model in step S2, and then embedding the low-frequency model into the straight plate low-frequency model in step S1 to obtain the comprehensive low-frequency model of the bedrock, fault-dissolved body and fracture-cave system.

8. The application of the inversion method in any one of claims 1-7 in the field of obvious formation heterogeneity.

9. Use according to claim 8, characterized in that, The field of obvious formation heterogeneity comprises carbonate rock formation, igneous rock formation and / or salt dome formation.

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