Method for identifying a fractured zone by using shear and compressional wave logging

Abstract

A method for identifying fractured and porous zones using P-wave and S-wave logging is proposed. First, a nonlinear relationship between P-wave and S-wave logging data of the target formation is fitted based on density, natural gamma, and full-wave logging data. The S-wave logging curves for all wells are then calculated based on this relationship. Second, the P-wave and S-wave velocity ratios of the wells are calculated. Third, the P-wave and S-wave velocity ratios of mudstone with a certain thickness in the target formation are identified and statistically analyzed. If the P-wave and S-wave velocity ratio of a mudstone section is less than 1.7, similar to the sandstone layers above and below, and significantly lower than the corresponding values ​​of the overlying and underlying mudstone, it indicates the presence of fractures or pores in or near that mudstone section. Alternatively, it suggests that the well is located on a strike-slip fault zone, and the fractured and porous low-Vp / Vs mudstone section is near the fault location. This method offers low cost and high accuracy in identifying downhole fractures and pores. It has significant practical value for identifying drilling mud loss sections, sweet spots in tight oil and gas and shale oil and gas, evaluating production capacity, determining reservoir formation models, assessing fracturing, and determining oil, gas, and water permeability.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas reservoir fracture and porosity prediction technology in formation exploration, specifically involving a method for identifying fracture and porosity zones in wells using low P-wave and S-wave velocity ratio logging curves of mudstone. Background Technology

[0002] Near-vertical fractures are often found near strike-slip fault zones in cratonic basins. These fractures significantly impact reservoir properties, reservoir formation and destruction, well leakage, and well kicks. High-yield wells in certain tight oil and gas formations are closely associated with the presence of fractures, cracks, and porosity, while others are related to reservoir destruction and water content. However, aside from expensive and rarely implemented imaging logging, conventional logging generally cannot identify the presence of fractures or porosity in tight lithologies such as mudstone and carbonate rocks.

[0003] In our research project on seismic reservoir prediction in the No. 1 Gas Production Plant of Yanchang Gas Field Group and the bauxite of Changqing Oilfield, our team discovered a method for identifying "fractured-pore mud" in wells. Because the P-wave / S-wave velocity ratio of sandstone is generally lower than that of mudstone, with the boundary typically between 1.6 and 1.8, to obtain accurate elastic parameter logging thresholds for different lithologies in the Upper Paleozoic target strata of this region, we selected three wells with full-wave logging and several wells with simulated converted S-wave logging from 650 wells in the area. We statistically analyzed the logging Vp / Vs values ​​and P-wave / S-wave impedance of different lithologies, such as gas-water dry sandstone, limestone, mudstone, and coal seams, and created attribute cross-plots for different lithologies. This revealed that the Vp / Vs values ​​of most mudstones were higher than those of sandstones, but in some well sections, the Vp / Vs values ​​of mudstone were similar to those of sandstone. Further investigation revealed that the lower Vp / Vs of the latter mudstone was due to the presence of fractures, cracks, or dissolution pores, which we term "fractured-pore mud." Figure 1 ).

[0004] The discovery of "fractured-pore gouge" provides a new technical approach and method for determining the presence of fractures or pores in formations using conventional logging curves. This unique and effective technique has not been proposed or used before, and is particularly suitable for determining whether a well is drilling on or near a strike-slip fault, or whether the formation at the well location contains fracture zones (fault breccia) or solution-filled sections, by using conventional logging curves and simulated shear wave logging time-difference curves. Based on observation and comprehensive interpretation of logging, core, and thin section data, this method for identifying fractured-pore gouge can generally be determined at the scale of mudstone intervals larger than 1 meter. These fractures can exist in intervals of tens of meters and can also be simultaneously verified in core samples at the decimeter level. The size of the fractured interval can be determined from a mudstone Vp / Vs ratio of approximately 1.7. The principle and method for identifying fracture zones or solution-filled zones in mudstone are the same.

[0005] It should be noted that shear wave transit time logging can be obtained from full-wave logging, but most wells in a basin or region do not undergo full-wave logging, or generally only a few wells have full-wave logging data. The shear wave transit time curves for most wells need to be generated through simulation conversion from other logging methods. There are various methods for fitting and calculating shear wave transit time based on other logging methods; nonlinear methods offer higher accuracy, but this is not the focus of this invention.

[0006] Oil and gas geophysicists often use low P-wave / S-wave velocity ratios from pre-stack seismic inversion to predict whether sandstone contains gas. However, using the P-wave / S-wave velocity ratio to predict formation properties (sandstone thickness and its gas content) is unrelated to the fracture identification method of this invention. The two rely on different principles and have different identification or prediction objectives.

[0007] Transverse wave time-of-flight logging measures the velocity of transverse waves propagating parallel to the wellbore. When a transverse wave encounters a fracture, it generally decomposes into two mutually perpendicular polarization directions, propagating at different velocities: a fast wave propagating parallel to the fracture and a slow wave propagating perpendicular to the fracture. When encountering a near-vertical fracture downhole, such as a near-vertical strike-slip fault, it decomposes into a fast wave polarized parallel to the wellbore and a slow wave polarized perpendicular to the wellbore. Since the sound source emitting the sound wave and the receiver detector are arranged parallel to the wellbore, they can only receive the fast transverse wave and not the slow transverse wave. Therefore, compared to non-fractured sections, the measured transverse wave velocity in fractured sections parallel to the wellbore will be increased, while the longitudinal wave velocity in the same direction generally does not change or changes very little. Thus, the ratio of longitudinal to transverse wave velocities in near-vertical fracture zones will decrease.

[0008] Strike-slip faults are the most common type of fault in cratonic basins. For example, 3D seismic profiles reveal numerous strike-slip vertical faults within basins such as the Ordos and Tarim. The accompanying fractures of these vertical faults are also nearly vertical or have a large dip angle. Therefore, fractures identified by the decrease in the P-wave / S-wave velocity ratio should primarily be near-vertical fractures.

[0009] Why is the low P-wave / S-wave velocity ratio of mudstone used to identify fracture zones instead of sandstone or carbonate rocks? This is because sandstone or carbonate rocks can themselves serve as reservoirs, and when they contain natural gas, they also exhibit a decrease in the P-wave / S-wave velocity ratio. Therefore, using them to identify fracture zones can lead to multiple interpretations.

[0010] Conventional seismic profiles typically reveal the presence of fractures. Properties such as dip angle, azimuth, edge, curvature, and ant-like structures of seismic coherence bodies or seismic horizons can also identify certain fracture or crack zones on a plane. However, these differ from the identification of fracture or pore zones in this invention in two aspects. First, seismic methods can only identify large-scale fractures and cracks in three-dimensional seismic zones; they should not respond to small cracks extending vertically by 1-2 meters or micro-cracks less than 1 meter. Second, three-dimensional seismic coherence and curvature methods are used to identify fracture development zones on a plane, but cannot determine whether cracks exist in a specific well section or layer within a well. This invention is primarily used to identify whether fractures, cracks, or pores exist at a specific layer or depth within a well.

[0011] This invention primarily utilizes the P-wave and S-wave velocity ratio to identify the presence of fractures and pores in mudstone formations. This differs from some published literature that describes methods for identifying formation fluid properties, gas-bearing types, and porosity. The latter typically rely on known reservoirs, while mudstone generally is not considered a reservoir, whereas sandstone usually is. Further identification of fluid properties, gas-bearing types, and porosity within sandstone pores is only possible after confirming the existence of a sandstone reservoir. Therefore, this technology specifically addresses fracture and pore identification methods in mudstone, a field previously unexplored.

[0012] Some drilling data from the Changqing Oilfield contain electrical imaging and core sampling data in low Vp / Vs mudstone sections. Detailed analysis of these data reveals ambiguity in fracture detection within these low Vp / Vs mudstone sections; specifically, fractures are not visible in the electrical imaging data of some wells with low Vp / Vs mudstone sections. This is presumably due to the difference in lateral detection depth between shear wave logging and electrical imaging logging, such as in the upper Benxi Formation of well Shaanxi 472 (low Vp / Vs mudstone). Figure 2 Electrical imaging logging typically has a lateral detection depth of less than 1 meter, primarily detecting the presence of fractures near the wellbore surface. Shear wave logging, on the other hand, generally has a lateral detection depth of around 3 meters, though some believe it can reach up to 50 meters, indicating the presence of fractures in deeper formations. Therefore, when the conclusions drawn from the two methods differ, they may indicate the development of fractures at different depths around the wellbore.

[0013] In the Benxi Formation bauxite reservoir area of ​​Longdong, most well sections with gas-bearing bauxite containing dissolution pores exhibit low Vp / Vs responses. Comparison with electrical imaging profiles of the same well section indicates that, in addition to generally well-developed dissolution pores, these sections also show... Figure 3 Sometimes, irregular clastic structures exist, and core sampling reveals either fractures or internal clastic bauxite, all of which exhibit solution pores. Therefore, gas-bearing pure bauxite with solution pores also exhibits logging response characteristics with low Vp / Vs values. Whether there is a necessary relationship between the formation of solution pores in bauxite and the formation of gas reservoirs and fracture zones remains unclear.

[0014] Fault analysis of the 3D seismic coherence reveals that while the Ordos Basin exhibits multiple phases of strike-slip faults, the Ordovician-Permian strike-slip faults are the most developed, with the largest displacement and the highest density, exhibiting multiple directions including northeast, northwest, and near-east-west. Similarly, the Ordovician-Permian vertical fractures are also the most developed. These fractures not only play a crucial role in controlling the formation of dissolution pores in the Ordovician internal carbonate rocks, dolomitization, and hydrocarbon migration and accumulation, but also significantly influence the source-reservoir matching and hydrocarbon accumulation in the Upper Paleozoic. Therefore, fractures identified using the low Vp / Vs ratio of Upper Paleozoic mudstones may indicate the fracture zones related to this phase of strike-slip faults in the Ordovician-Permian system, providing indications for fractures in both the Upper and Lower Paleozoic hydrocarbon accumulation systems. Summary of the Invention

[0015] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a method for identifying fracture zones using longitudinal and transverse wave logging. Identifying the presence of fractures in the wellbore is of significant practical value for predicting sweet spots in drilling formations, evaluating production capacity, determining the causes of fracturing, well leakage and well kick, reservoir formation models, and assessing oil, gas and water permeability.

[0016] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0017] A method for identifying fractured zones using P-wave and S-wave logging includes the following steps:

[0018] Step 1: Select full-wave logging data to fit the nonlinear relationship between the target section shear wave logging time difference DTs and the longitudinal wave logging time difference DTp, density logging DEN and natural gamma logging GR, DTs=f(DTp,DEN,GR), and calculate the shear wave logging curves of all wells.

[0019] Step 2: Calculate the ratio of the longitudinal wave velocity Vp to the transverse wave velocity Vs, Vp / Vs;

[0020] Step 3: Overlay the natural gamma ray logging GR and Vp / Vs onto the integrated logging profile at a scale of 1:200-1:2000 to determine the relative magnitude of the P-wave and S-wave velocities of sandstone and mudstone in the well, and to determine whether there is a fractured zone in the well.

[0021] The calculated P-wave to S-wave velocity ratio for drilling is less than 1.7-1.8 for sandstone and greater than 1.7-1.8 for mudstone and carbonate rock.

[0022] The ratio of longitudinal wave velocity Vp to transverse wave velocity Vs in the calculation of drilling is Vp / Vs, where Vp / Vs = DTs / DTp. For mudstone and carbonate rocks, the ratio is greater than 1.75, while for sandstone, the ratio is less than 1.75.

[0023] To determine the relative magnitude relationship between the P-wave and S-wave velocity ratios of sandstone and mudstone in a well, if the Vp / Vs of mudstone is greater than that of sandstone and is between 1.7 - 2.3, then there are no fractures or pores in this mudstone section; if the Vp / Vs of mudstone is similar to that of sandstone and is between 1.4 - 1.7, then there are fractures or pores in this mudstone section, or the well is located in a fault zone or dissolution zone. When there are fractures or pores, the Vp / Vs of mudstone will be close to that of sandstone and will also be less than 1.75.

[0024] If the formation is pore-free carbonate rock, then 1.7 < Vp / Vs ≤ 2.3. If the formation contains fractures or pores, then Vp / Vs ≤ 1.7.

[0025] Overlap the natural gamma log GR and Vp / Vs on the comprehensive log plot. Set the left and right scales of the GR log curve to 0 and 200 API respectively, and the left and right scales of Vp / Vs to around 1.5 - 2.0. Observe the relative changes in Vp / Vs of mudstone when the GR curve deflects to the right and Vp / Vs of sandstone when the GR curve deflects to the left; if the GR curve deflects to the right and Vp / Vs deflects to the left, that is, the Vp / Vs of mudstone and sandstone are similar in magnitude, then there are fractures or pores in the mudstone; if the GR curve deflects to the right and Vp / Vs also deflects to the right, that is, the Vp / Vs of mudstone is greater than that of sandstone, then there are no fractures or pores.

[0026] To distinguish sandstone and mudstone according to the magnitude of natural gamma GR. The GR demarcation line between sandstone and mudstone is between 70 - 100 API. If the Vp / Vs of mudstone > 1.7, significantly greater than the Vp / Vs of sandstone, then there are no fractures or pores; if the Vp / Vs of mudstone ≤ 1.7, similar in magnitude to the Vp / Vs of sandstone, then there may be fractures or pores; if the target layer interval belongs entirely to argillaceous rock and does not contain sandstone, and the Vp / Vs of argillaceous rock without fractures or pores ≥ 1.7, then the interval with Vp / Vs < 1.7 is the fracture or pore interval; if the target layer interval belongs to a carbonate rock interval, after determining the Vp / Vs of the lithology interval without fractures or pores using a similar method, the abnormal interval with a significantly lower Vp / Vs than this background Vp / Vs is the fracture or pore carbonate rock interval.

[0027] A method for identifying fracture and pore zones using P-wave and S-wave logging is applied to terrigenous clastic rock formations and / or carbonate rock formations.

[0028] The beneficial effects of the present invention are:

[0029] The present invention has the characteristics of being economical, practical, and low-cost, and the results are in good agreement with the translational faults on the seismic profile. Identifying whether there are fractures underground has important practical value for predicting sweet spots in drilling formations, evaluating production capacity, fracturing, judging the causes of well leakage and well kick, and judging reservoir formation models and oil-gas-water seepage performance. Through exploration application and verification in the Upper Paleozoic gas reservoirs in the Yanchang Gas Field of the Ordos Basin, the Changqing Jingxi - Mizhi Gas Reservoir, and the bauxite gas reservoir in the Taiyuan Formation in eastern Gansu, good results have been achieved.

[0030] This method has a wide range of applications in identifying favorable reservoirs for shale oil and gas and carbonate oil and gas in drilling. Attached Figure Description

[0031] Figure 1 Cross plot of P-wave and S-wave velocity ratios (Vp / Vs) and P-wave impedance values ​​(Ip) for different Upper Paleozoic lithologies in the central and eastern Ordos Basin; the plot shows that the Vp / Vs ratio of mudstone with fractures is significantly lower than that of ordinary mudstone, but similar to that of sandstone.

[0032] Figure 2 Two layers of low Vp / Vs mudstone and bauxite were found in the Benxi Formation of well Shan 472 in the Jingbian gas reservoir, at depths of 3596-3603m and 3637-3641m, respectively. The Vp / Vs ratio of the mudstone is between 1.5 and 1.7, significantly lower than the corresponding values ​​of the overlying and underlying mudstone (a); the former showed no fractures in the corresponding well section on the electrical imaging logging profile (b), while the latter is pure bauxite containing solution pores and vertical fractures. The lateral detection depth of full-wave logging is 3-50m, while the lateral detection depth of electrical imaging logging is about 1m. Therefore, the former may reflect the presence of fractures deeper into the wellbore.

[0033] Figure 3 Logging profiles of the Shan 2 section of the Benxi Formation in the Long 58 well of the Changqing Longdong Gas Field, local electrical imaging logging profiles of the bauxite section, and core photographs;

[0034] Figure 4 Comparison of wells without fractures (a) and wells with fractures (b); In Figure b, the mudstone of the Shan 2 3 sub-member has fractures, with a Vp / Vs ratio between 1.5 and 1.75, while the Vp / Vs ratio of mudstone in other unfractured sections is between 1.75 and 2.0.

[0035] Figure 5 Map showing the drilling locations and 2D seismic network distribution of the No. 1 gas production plant in the Yanchang Gas Field. The seismic profiles show that wells experiencing lost circulation in the Upper Paleozoic and those containing faulted mudstone are all located near strike-slip fault zones. (Refer to...) Figure 6 and Figure 7 At 1200km 2 Of the approximately 650 wells drilled in the area, 16 wells have Paleozoic fracture mud on their surfaces. Most of these are high-yield gas wells and have experienced significant mud loss during drilling.

[0036] Figure 6 To extend the relationship between the Y202 well with fractured mud in the gas field and the vertical fracture, (a) the two-dimensional seismic network and the location of the fracture passing through the Y202 well; (b) the translational fracture characteristics on the YC101 seismic profile near the Y202 well.

[0037] Figure 7To extend the relationship between the Y2003-1 well with fractured mud in the gas field and the vertical fault, (a) the two-dimensional seismic network and the location of the fault passing through the Y2003-1 well; (b) the vertical fault characteristics on the YC101 seismic profile near the Y2003-1 well.

[0038] Figure 8 In the Changqing-Jingxi gas reservoir, the Vp / Vs ratio of the Benxi Formation bauxite section of well Tao 76 in the 3427-3430m range is between 1.5 and 1.65, which is significantly lower than the corresponding values ​​of the overlying and underlying mudstone (a); the corresponding well section has near-vertical fractures and fragmented structures on the electrical imaging logging profile (b).

[0039] Figure 9 Log logging response, electrical imaging logging and core images of the low Vp / Vs fracture mud in the bauxite gas reservoir of Well Long 47 in the Changqing Longdong Gas Field;

[0040] Figure 10 Log logging response and coring images of the low Vp / Vs fracture mud in the bauxite gas reservoir of the Changqing Longdong Gas Field, Long 82 well. Detailed Implementation

[0041] The present invention will be further described below with reference to the embodiments and accompanying drawings, but the present invention is not limited to the following embodiments.

[0042] A method for identifying fractured-pore zones in mudstone using shear wave logging curves:

[0043] First, the nonlinear relationship between the P-wave and S-wave logging data of the target section is fitted based on the density, natural gamma, and full-wave logging data. Then, the S-wave logging curves of all wells are calculated based on this relationship.

[0044] Then, calculate the P-wave to S-wave velocity ratio of the well. Generally, the P-wave to S-wave velocity ratio of sandstone is less than 1.7-1.8, while that of mudstone is greater than 1.7-1.8.

[0045] Finally, the longitudinal and transverse wave velocity ratios of mudstone with a certain thickness in the target layer were identified and statistically analyzed.

[0046] If the P-wave to S-wave velocity ratio of mudstone in a certain well section is less than 1.6-1.8, similar to that of sandstone in the upper and lower adjacent layers, it indicates that there are high-angle fractures in the mudstone section and the sandstone above and below, or that the well is drilled on a strike-slip fault zone, and the low Vp / Vs well section of mudstone is near the fault zone.

[0047] Specifically, proceed as follows:

[0048] a. Select full-wave logging data to fit the nonlinear relationship between shear wave logging time difference DTs and longitudinal wave logging time difference DTp, density logging time difference DEN, and natural gamma logging time difference GR, DTs=f(DTp, DEN, GR);

[0049] b. Calculate the P-wave / S-wave velocity ratio (Vp / Vs) for all wells: Vp / Vs = DTs / DTp. Mudstone and carbonate rocks generally have a larger Vp / Vs ratio, often greater than 1.75, while sandstone generally has a smaller Vp / Vs ratio than 1.75. When fractures and pores are present, the Vp / Vs ratio of mudstone will be close to that of sandstone, also less than 1.75. Figure 1 ).

[0050] c. Overlay the GR and Vp / Vs logging curves onto the composite logging chart, such as... Figure 2 As shown in Figure a, for mudstone with GR > 70-100 API (GR curve deflected to the right), and if Vp / Vs > 1.75 (also deflected to the right), there are no fractures or pores.

[0051] d. For example Figure 2 As shown in b, for mudstone with GR>70-100API (GR curve deviates to the right), if Vp / Vs≤1.75 (i.e. curve deviates to the left), then fractures and pores exist.

[0052] e. Regardless of whether it is a terrigenous clastic rock strata or a carbonate rock strata, if the Vp / Vs of the mudstone contained therein is similar to the corresponding value of the sandstone, then the mudstone section of the strata may have steeply dipping fractures.

[0053] Example 1: Application in the No. 1 Gas Production Plant of Yanchang Gas Field in the southeastern Ordos Basin

[0054] The No. 1 Gas Production Plant of Yanchang Gas Field has 86 wells located on the 2D seismic survey line, of which 16 wells encountered fault zones. Figure 5 Taking wells Y202 and Y2003-1, which contain fractured mud, as examples ( Figure 5 Vertical faults can be observed near the wells on the seismic profiles of both wells, namely O6YC1001 and 06YC111. Figure 6 , Figure 7 Wells with fractured zones tend to be high-yield gas wells due to their good permeability; most wells with fractures are also high-yield gas wells. Figure 5 ).

[0055] Mud loss occurred during the drilling process to identify fractured zones in the well logging.

[0056] A survey was conducted on the locations of wells in the study area where well leakage occurred. Four wells in the area experienced large-scale well leakage during drilling: Yan 251E-3, Yan 176W, Yan 177-1, and Yan 154E. Figure 5 These wells, on average, lost over 1,000 cubic meters of mud per well, with the loss occurring mostly between the Shiqianfeng and Benxi Formations. This aligns with the Vp / Vs indicator showing fractured and porous mudstone sections in the corresponding well segments. Furthermore, the seismic profiles of these four wells also reveal fractures of varying scales. Therefore, the fractured and porous zones predicted by the method of this invention are real and objectively exist.

[0057] FMI electrical imaging logging verification

[0058] FMI (Fractional Imaging) logging is an effective method for identifying fractures in wells, but due to its high cost, it is typically only used in a very small number of appraisal wells. Five wells in the Yanchang Gas Field's Production Plant No. 1 have undergone FMI logging operations: Chang 99, Chang 103, Chang 106, Chang 107, and Chang 108. FMI logging of these five wells did not detect fractures in the target Upper Paleozoic strata, consistent with the results obtained using the Vp / Vs method, which also showed no fractures or gouge. However, in the Taiyuan Formation bauxite gas field of the Longdong Formation in the Changqing Oilfield, the fractured zone obtained from FMI logging coincides with the low Vp / Vs anomaly interval (see Example 3).

[0059] Verification of fracture data observed from drilling core samples

[0060] Wells with fractured mudstone sections were selected in the study area, and core sampling was conducted in these sections to observe fracture development. Five wells in the entire area met this condition: Shi 209, Yan 304, Yan 324, Yan 343, and Yan 176. Figure 5 The core logging descriptions of these five wells show that the target formation, Box 8 – Benxi Formation, contains fractures of varying sizes, numbers, or densities. These fractures extend vertically or at high angles, and are unfilled to partially filled with mudstone or calcite. The fracture widths range from 0.5 to 5 mm, and the fracture lengths range from 10 to 20 cm. These fractures are consistent with the fractured mudstone sections and formations with low Vp / Vs. Microfractures can sometimes also be seen on the corresponding core sections.

[0061] Example 2: Jingxi-Mizhi Gas Field Area in the Central and Eastern Ordos Basin

[0062] This region contains industrial gas reservoirs in both the Upper Paleozoic and Ordovician strata. Fractures likely play a significant role in the dissolution pores, dolomitic intercrystalline pores, hydrocarbon charging, and reservoir formation within the Majiagou Formation of the Ordovician. Small vertical strike-slip faults can be observed on seismic profiles, but fractures in wells generally require electrical imaging logging for identification. For most wells where electrical imaging logging is not implemented, the method of this invention can identify fracture zones, and the identification results are largely consistent with the fractures indicated by the electrical imaging logging profiles.

[0063] like Figure 8 As shown, the Vp / Vs ratio of the Benxi Formation bauxite section in the Tao 76 well of the Changqing Jingxi gas reservoir ranges from 1.5 to 1.65 at 3427-3430 m, which is significantly lower than the corresponding values ​​of the overlying and underlying mudstone (a); the corresponding well section has near-vertical fractures and fragmented structures on the electrical imaging logging profile (b).

[0064] Example 3: Longdong bauxite gas field area in southwestern Ordos Basin

[0065] Industrial oil and gas have been discovered in multiple wells in the Taiyuan Formation of Longdong County within the bauxite. Dissolution during the surface formation stage and subsequent dissolution along fractures by acidic formation water during oil and gas charging are the main reasons for the formation of bauxite reservoirs. Gas-producing bauxite well sections generally exhibit low Vp / Vs anomalies, which are also observed in corresponding electrical imaging profiles and core samples, revealing fractures and intraclastic rocks. Figures 9-10 ).

[0066] Figure 9 The images show the logging response, electrical imaging logging, and core images of the low Vp / Vs pore-fractured mudstone in the Long 47 well of the Changqing Longdong Gas Field. The low Vp / Vs anomaly in the bauxite is reflected in both the imaging logging and the core images, indicating near-vertical fractures.

[0067] Figure 10 The image shows the logging response and core images of the low Vp / Vs fractured mudstone reservoir in the Long 82 well of the Changqing Longdong Gas Field. The low Vp / Vs interval contains vertical unfilled fractures.

Claims

1. A method for identifying fractured zones using P-wave and S-wave logging, characterized in that, It includes the following steps: Step 1: Select the non-linear relationship between the shear wave logging time difference DTs, the compressional wave logging time difference DTp, the density logging DEN, and the natural gamma logging GR in the target interval of the full-wave logging data fitting, DTs = f(DTp, DEN, GR), and calculate the shear wave logging curves of all wells; Step 2: Calculate the ratio Vp / Vs of the compressional wave velocity Vp to the shear wave velocity Vs of the well; Step 3: Overlay the natural gamma logging GR and Vp / Vs on the comprehensive logging profile with a scale of 1:200 - 1:2000, and judge the relative magnitude relationship of the compressional and shear wave velocity ratios of sandstone and mudstone in the well, and judge whether there is a fracture and pore zone in the well.

2. The method for identifying fractured zones using P-wave and S-wave logging according to claim 1, characterized in that, For the calculation of the compressional and shear wave velocity ratio of the well, the compressional and shear wave velocity ratio of sandstone is less than 1.7 - 1.8, and that of mudstone and carbonate rock is greater than 1.7 - 1.

8.

3. The method for identifying fractured zones using P-wave and S-wave logging according to claim 2, characterized in that, For the calculation of the ratio Vp / Vs of the compressional wave velocity Vp to the shear wave velocity Vs of the well, Vp / Vs = DTs / DTp. For mudstone and carbonate rock, it is greater than 1.75, while for sandstone, Vp / Vs is less than 1.

75.

4. The method for identifying fractured zones using P-wave and S-wave logging according to claim 1, characterized in that, For the judgment of the relative magnitude relationship of the compressional and shear wave velocity ratios of sandstone and mudstone in the well, if the Vp / Vs of mudstone is greater than that of sandstone and is between 1.7 - 2.3, then there are no fractures and pores in this mudstone section; if the Vp / Vs of mudstone is similar to that of sandstone and is between 1.4 - 1.7, then there are fractures and pores in this mudstone section, or the well is in a fault zone or dissolution zone. When there are fractures and pores, the Vp / Vs of mudstone will be close to that of sandstone and is also less than 1.

75.

5. A method for identifying fractured zones using P-wave and S-wave logging according to claim 4, characterized in that, If the formation is pore-free carbonate rock, then 1.7 < Vp / Vs ≤ 2.

3. If the formation contains fractures and pores, then Vp / Vs ≤ 1.

7.

6. The method for identifying fractured zones using P-wave and S-wave logging according to claim 1, characterized in that, For the overlay of the natural gamma logging GR and Vp / Vs on the comprehensive logging chart, the left and right scales of the GR logging curve are set to 0 and 200 API respectively, and the left and right scales of Vp / Vs are set to about 1.5 - 2.0 respectively. Observe the relative changes of Vp / Vs of mudstone when the GR curve deflects to the right and Vp / Vs of sandstone when the GR curve deflects to the left; if the GR curve deflects to the right and Vp / Vs deflects to the left, that is, the Vp / Vs of mudstone and sandstone are similar in size, then there are fractures and pores in the mudstone; if the GR curve deflects to the right and Vp / Vs also deflects to the right, that is, the Vp / Vs of mudstone is greater than that of sandstone, then there are no fractures and pores.

7. A method for identifying fractured zones using P-wave and S-wave logging according to claim 6, characterized in that, For the distinction between sandstone and mudstone according to the magnitude of the natural gamma GR, the GR boundary between sandstone and mudstone is between 70 - 100 API. If the Vp / Vs of mudstone > 1.7 and is significantly greater than the Vp / Vs of sandstone, then there are no fractures and pores; if the Vp / Vs of mudstone ≤ 1.7 and is similar in size to the Vp / Vs of sandstone, then there may be fractures and pores; if all well intervals of the target layer belong to argillaceous rocks and do not contain sandstone, and the Vp / Vs of argillaceous rocks without fractures and pores ≥ 1.7, then the well interval with Vp / Vs < 1.7 is the fracture and pore section; if the well interval of the target layer belongs to the carbonate rock section, after determining the Vp / Vs of the lithology section without fractures and pores by a similar method, the abnormal well interval significantly less than this background Vp / Vs is the fracture and pore carbonate rock section.

8. The method for identifying fractured zones using P-wave and S-wave logging according to claim 1, characterized in that, It is applied to terrigenous clastic rock formations and / or carbonate rock formations.

Images