Three-dimensional resistivity reservoir mapping

Abstract

A method for three-dimensional (3D) reservoir mapping includes obtaining a 3D resistivity volume in a point set format and transforming the 3D resistivity volume in the point set format into a 3D resistivity map in a geological prospecting learning (SEG-Y) format. A method for identifying an interface between a high resistivity volume and a low resistivity volume may also include applying a first resistivity threshold to the 3D resistivity map in the SEG-Y format to generate a binary resistivity volume, wherein a first high resistivity value is assigned to a cell having a resistivity value greater than the first resistivity threshold value in the 3D resistivity map, and a second low resistivity value is assigned to a cell having a resistivity value less than the first resistivity threshold value; and evaluating the binary resistivity volume to identify the interface between the high resistivity volume and the low resistivity volume.

Description

Cross Reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 499,092, filed April 28, 2023, entitled “THREE-DIMENSIONAL RESISTIVITY RESERVOIR MAPPING,” the disclosure of which is hereby incorporated by reference herein. BACKGROUND

[0002] Subsurface reservoirs are often mapped to facilitate development of oil fields, for example, to determine optimal well locations and to calculate oil field reserves. Reservoir mapping has historically relied on expert interpretation of seismic data, as well as various well log data obtained from test and production wells. During production, well locations can rely at least in part on reservoir maps and the placement of boundary layers within those maps in order to enable drilling of wells through oil and gas bearing formations.

[0003] Ultra-deep azimuthal resistivity (UDAR) measurements can be used in a reservoir mapping while drilling (RMWD) operation. In such an operation, the resistivity environment around the wellbore can be mapped by making deep or ultra-deep electromagnetic logging measurements in the wellbore. The depth of investigation of such measurements is generally a function of the transmitter-receiver spacing and the electromagnetic frequency used in the measurement (maximum depth of investigation is obtained with large transmitter-receiver spacing and low frequency). The electromagnetic measurements can be inverted using one-dimensional (ID) longitudinal resistivity inversion (along the tool axis) and two-dimensional (2D) transverse resistivity inversion (perpendicular to the tool axis). The inversion can then be used to create a three-dimensional (3D) resistivity volume or map around the wellbore.

[0004] While the above 3D resistivity volume provides a wealth of information about the subsurface formations penetrated by the wellbore, interpretation of this information is not straightforward. One of the main challenges in interpreting the 3D resistivity volume is correlating it with existing seismic maps, including seismic data and corresponding seismic interpretations, inversions, attributes, etc. Updating or modifying the seismic maps with the 3D resistivity volume is further difficult. Such correlation with and updating of the seismic maps can be of significant interest for making informed, proactive geosteering decisions during drilling operations, as well as improving the understanding of the oil field for further development and field planning. There is a need in the industry for improved RMWD methods that provide improved integration with existing seismic maps. BRIEF DESCRIPTION OF DRAWINGS

[0005] For a more complete understanding of the disclosed subject matter and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: Figure 1 An example drilling rig including a depth-reading electromagnetic while-drilling logging tool is depicted.

[0006] Figure 2 An example depth-reading electromagnetic while-drilling logging tool is depicted.

[0007] Figure 3 A flowchart showing one example method for generating a SEG-Y formatted 3D resistivity map and identifying at least one horizon in a formation.

[0008] Figure 4 A flowchart depicting one example method for generating a Society for Exploration Geophysics (SEG-Y) formatted 3D resistivity map.

[0009] Figure 5 A flowchart depicting another example method for generating a SEG-Y formatted 3D resistivity map and identifying at least one horizon in a formation.

[0010] Figure 6 A flowchart depicting one example method for extracting resistivity amplitudes from point cloud data.

[0011] Figure 7A 、 Figure 7B 、 Figure 7C and Figure 7D (collectively referred to as FIG. 7) depict Figure 5 an example implementation of the method shown in FIG. 6, wherein Figure 7A a point cloud formatted 3D resistivity volume is depicted, Figure 7B a SEG-Y formatted 3D resistivity map is depicted, Figure 7C a binary volume is depicted, and Figure 7D a horizon derived from the binary volume is depicted. DETAILED DESCRIPTION

[0012] A method for three-dimensional (3D) reservoir mapping is disclosed. The method can include obtaining a point cloud formatted 3D resistivity volume and transforming the point cloud formatted 3D resistivity volume into a Society for Exploration Geophysics (SEG-Y) formatted 3D resistivity map. A method for identifying an interface between a high resistivity volume and a low resistivity volume can also include applying a first resistivity threshold to the SEG-Y formatted 3D resistivity map to generate a binary resistivity volume, wherein cells in the 3D resistivity map having a resistivity value greater than the first resistivity threshold are assigned a first high resistivity value and cells having a resistivity value less than the first resistivity threshold are assigned a second low resistivity value; and evaluating the binary resistivity volume to identify the interface between the high resistivity volume and the low resistivity volume.

[0013] The disclosed embodiments can advantageously provide a SEG-Y formatted 3D inverted resistivity volume from deep reading resistivity measurements, and can further enable resistivity contrasts indicative of reservoir boundaries in the 3D volume to be readily identified and mapped. Moreover, the disclosed techniques can be advantageously used during drilling operations, and can update reservoir boundary locations as drilling progresses. Moreover, it will be appreciated that the disclosed embodiments can advantageously enable a rock physicist to evaluate and interpret the inverted resistivity data with mature seismic tools.

[0014] Figure 1 An example drilling rig 20 positioned over an oil or gas formation 70 is depicted. The drilling rig can include a derrick and hoisting equipment (not shown) for raising and lowering a drill string 30, which extends into a wellbore 40 and includes a drill bit 32 deployed at a lower end of a bottom hole assembly (BHA) 35, as shown. In the depicted embodiment, the drill string 30 also includes a deep reading electromagnetic logging tool that includes different transmitter 50 and receiver 60 subassemblies configured to make deep, directional, electromagnetic logging measurements in the wellbore 40, e.g., of the formation 70 penetrated by the wellbore.

[0015] It will be appreciated that, Figure 1 The illustrated deployment is merely an example. The drill string 30 can include substantially any suitable downhole tool components, e.g., including a steering tool such as a rotary steerable tool, a downhole telemetry system, and one or more additional MWD and / or LWD tools, including various sensors for sensing downhole characteristics of the borehole and surrounding formation. The disclosed embodiments are in no way limited to any particular drill string configuration.

[0016] It will be further appreciated that the disclosed embodiments are not limited to use with a land rig 20 as Figure 1 illustrated, but can be equally well adapted for use with land or offshore operations. Moreover, the disclosed embodiments are not limited to use with the logging while drilling embodiment as Figure 1 illustrated. The disclosed embodiments are equally well adapted for use with any deep reading electromagnetic logging tool, including wireline logging tools and logging while drilling tools.

[0017] Further reference is made to Figure 1, the drilling rig 20 can also include a field operations or oilfield evaluation facility 80 (e.g., a control room or field office). In the depicted embodiment, the operations facility can include a system for processing the deep-reading electromagnetic logging data, such as including a computer or computer system. The computer system can include one or more processors (e.g., microprocessors) that can be connected to one or more data storage devices (e.g., hard drives or solid state memory) and a user interface, as well as to cloud-based storage or additional processors. It will also be appreciated that the disclosed embodiments can include processor-executable instructions stored in the data storage devices. The executable instructions can be configured to, for example, perform the methods 100, 120, and 150, which will be described in greater detail below with respect to FIGS. 1-3. Thus, the computer system can be configured to invert the electromagnetic logging data to generate a 3D resistivity volume, extract a 3D resistivity map in SEG-Y format from the 3D resistivity volume, and evaluate the 3D resistivity map to identify and / or characterize formations, faults, fluid contacts, interfaces, etc. therein. Of course, it will be appreciated that the disclosed embodiments are not limited to the use of any particular computer hardware and / or software. Figures 3-5

[0018] Figure 2 An example embodiment of a deep-reading electromagnetic logging tool is depicted that includes different transmitter and receiver subassemblies 50 and 60. In the depicted example embodiment, the transmitter subassembly (or tool) 50 includes at least one electromagnetic transmitter 52 (including one or more transmit antennas) deployed on a transmitter collar 51. The receiver subassembly (or tool) 60 includes at least one electromagnetic receiver 62 (including one or more transmit antennas) deployed on a receiver collar 61. When deployed in a drill string (e.g., the drill string 30), the transmitter and receiver subassemblies 50 and 60 can be axially spaced apart by substantially any suitable distance to achieve a desired depth of measurement (e.g., in a range of about 10 to about 50 meters or more, depending on the measurement target). Although not shown, one or more other BHA tools can be deployed between the subassemblies 50 and 60.

[0019] Continuing with reference to Figure 2 The transmitters 52 and receivers 62 can each include substantially any suitable antenna configuration, such as including tri-axial antennas (e.g., an axial antenna and first and second transverse antennas that are orthogonal to each other) or one or more tilted antennas. As known to those of ordinary skill in the art, an axial antenna is one whose dipole moment is substantially parallel to the longitudinal axis of the tool. A transverse antenna is one whose dipole moment is substantially perpendicular to the longitudinal axis of the tool. A tilted antenna refers to one whose dipole moment is at an angle (e.g., an angle of about 45 degrees) with respect to the tool axis. Such antenna configurations are well known and commonly used in the industry.

[0020] ​Depth-reading electromagnetic measurements can be made in a wellbore (e.g., wellbore 40) by exciting a transmitting antenna and measuring the voltage response in one or more receiving antennas. As also known to those of ordinary skill, the time-varying current (alternating current) in the transmitting antenna (obtained by exciting the antenna) generates a corresponding time-varying magnetic field in the local environment (e.g., tool collar and formation). The magnetic field, in turn, induces currents (eddy currents) in the conductive formation. These eddy currents further generate secondary magnetic fields, which can produce a voltage response in the receiving antennas. The measured voltage in the receiving antennas can be processed, as known to those of ordinary skill in the art, to obtain one or more properties of the formation, such as formation resistivity.

[0021] The electromagnetic voltage measurements (or ratios of various voltage measurements) can be inverted using a multi-dimensional resistivity inversion process (e.g., including ID, 2D, 2.5D, and / or 3D inversion). The inversion results (e.g., spatial resistivity values) can then be evaluated to assemble a 3D resistivity volume around the wellbore in point-set format. It will be appreciated that the point-set format can be considered a large text file including the measured depth, resistivity, and x, y, and z-axis coordinates at each point in the 3D resistivity volume (e.g., distributed in a cylinder or cuboid around the wellbore).

[0022] Turning now to Figure 3 , a flowchart depicting an example method for extracting a 3D resistivity map in Society of Exploration Geophysicists (SEG-Y) format and identifying resistivity contrasts as boundaries representing at least one of a bed, an interface, a fluid contact, and / or a fault in a formation is depicted. For example, depth-reading electromagnetic logging measurements are inverted at 102 using a 2D lateral inversion. The 2D inversion results are assembled into a 3D resistivity volume in point-set format at 104. A 3D resistivity volume in SEG-Y format is extracted from the 3D resistivity volume at 106 (e.g., the 3D resistivity volume in point-set format can be transformed into a 3D resistivity map in SEG-Y format). The 3D resistivity map can be output as shown at 108, and the 3D resistivity map can be further evaluated at 110 to generate a resistivity contrast map or volume, for example, by generating a binary 3D resistivity volume, as described in more detail below. The resistivity contrast map can then be further evaluated at 112 to generate a 3D structural interpretation of the 3D resistivity map, which provides an indication of formation beds, faults, interfaces, fluid contacts, and the like.

[0023] Turning now to Figure 4 , a flowchart depicting an example method 120 for generating a 3D resistivity map in SEG-Y format is depicted. For example, by making and inverting ultra-deep or depth-reading electromagnetic measurements (e.g., as described above with respect to Figure 2The 3D resistivity volume in point set format is obtained at 122. A blank SEG-Y grid can be defined at 124 that contains the obtained 3D resistivity volume. The blank SEG-Y grid can be overlaid on the point set data (3D resistivity volume) at 126. Resistivity amplitudes along individual vertical seismic traces can be extracted at 128 for each cell in the blank SEG-Y grid (e.g., at the center location of each cell) to obtain a preliminary resistivity map. As described below with respect to Figure 6 In more detail, the resistivity amplitudes can be extracted (or captured or computed) as vertical seismic traces (e.g., z-direction seismic traces) by compressing the point set data along the vertical axis of each cell and organizing the compressed point set data into vertical seismic traces that extend downward at each cell location (e.g., each center location) in the blank SEG-Y grid. Each cell location refers to each x-y location in the blank SEG-Y grid.

[0024] It will be appreciated that because the resistivity measurements in the 3D resistivity volume are distributed along the wellbore trajectory, the point set data can not completely overlap the blank SEG-Y grid. Therefore, projecting (or overlaying) the regularized blank SEG-Y grid onto the irregularized point set data can result in smearing or distortion of the resistivity amplitudes along the edges of the grid (away from the wellbore trajectory). Accordingly, the method 120 can further include discarding resistivity amplitudes that are located outside of a predetermined spatial volume of the wellbore trajectory at 130 to obtain a 3D resistivity volume in SEG-Y format.

[0025] Figure 5 A flowchart depicting another example method 150 for generating a resistivity map in SEG-Y format and identifying at least one horizon in a formation is depicted. For example, by conducting and inverting ultra-deep or deep reading electromagnetic measurements (e.g., as described above with respect to Figure 2 The 3D resistivity volume in point set format can be obtained at 152. The 3D resistivity volume can be evaluated at 154 to determine the spatial extent of the data (e.g., by extracting spatial statistics from the point set data). The spatial statistics can include, for example, the minimum and maximum x, y, and z-axis spatial coordinates of the 3D resistivity volume. A blank SEG-Y grid can then be defined at 156 that contains the obtained 3D resistivity volume. For example, the blank SEG-Y grid can be defined by dividing the difference between the maximum and minimum x, y, and z-axis spatial coordinates by a predetermined or user-selected grid spacing (such as 0.1, 0.5, or 1 meter). The blank SEG-Y grid can be overlaid on the point set data at 158. Resistivity amplitudes along individual vertical seismic traces can be extracted at 160 for each cell in the blank SEG-Y grid (e.g., at the center location of each cell) to obtain a preliminary resistivity map, for example, as described below with respect to Figure 6In more detail.

[0026] As noted above, superimposing the regularized SEG-Y grid onto the irregular point set data can result in smearing or distortion of the resistivity amplitudes along the grid edges (away from the wellbore). To address this difficulty, certain of the resistivity amplitudes can be discarded, e.g., those that are too far away from the wellbore (e.g., greater than a threshold distance from the wellbore). In one embodiment, the wellbore trajectory (also referred to as the well path) can be obtained at 162, e.g., from survey measurements (e.g., static and / or dynamic inclination and azimuth measurements made during drilling operations). At 164, a probe volume (or surface) around the well trajectory can be computed, e.g., from the depth reading resistivity measurement probe limit. The probe volume (or surface) can be, for example, a cylindrical volume or surface around the well trajectory with a radius equal to the probe limit (or some predetermined fraction or multiple of the probe limit). As noted above with respect to Figure 2 the probe limit can be related to one (e.g., a fraction or multiple) of the transmitter-receiver spacing in the depth reading resistivity EM logging tool. The resistivity amplitudes that are outside the probe volume can be discarded at 166 to obtain a 3D resistivity map in SEG-Y format. The probe volume can be superimposed, for example, on the preliminary resistivity map. The amplitudes outside the probe volume can be discarded, while the amplitudes inside the probe volume can be retained.

[0027] Continuing with reference to Figure 5 , the method 150 can further include converting the 3D resistivity map in SEG-Y format to a 3D contrast map at 168, e.g., to a 3D binary volume. For example, a predetermined or user-defined resistivity threshold can be applied to the 3D resistivity map. Cells with resistivity values greater than the threshold can be assigned a first high resistivity value (e.g., 100 ohm-m), while cells with resistivity values less than the threshold can be assigned a second low resistivity value (e.g., 1 ohm-m). Thus, the resulting 3D binary volume can indicate a first high resistivity volume and a second low resistivity volume in the 3D resistivity map and an interface or surface between them. It will be appreciated that the disclosed embodiments are not limited to converting the 3D SEG-Y resistivity map to only a single binary volume. In other embodiments, the map can be converted to, e.g., a tri- or quad-valued volume by applying first and second or first, second, and third resistivity cutoff values to the map to define three or more different resistivity volumes and corresponding interfaces between them. Further, in other embodiments, the 3D resistivity map can be converted to a plurality of different binary volumes by repeating the conversion at 168 at a plurality of different threshold resistivity values (e.g., using corresponding first, second, and third thresholds to generate first, second, and third 3D binary volumes).

[0028] Further reference is made to Figure 5The binary (or ternary or quaternary) volume can then be further evaluated at 170 to identify and characterize interfaces or boundaries between different resistivity volumes. For example, the binary volume can be further evaluated using a 3D automatic tracking tool to interpret one or more horizons indicated by the interfaces. The 3D binary volume can still be further evaluated at 170 to calculate the geological volume of each region in the binary volume (e.g., to calculate the geological volume of the high resistivity and possibly oil-bearing volume).

[0029] Figure 6 A flowchart depicting an example method 180 for transforming resistivity data from a discrete space representation in x, y, and z axes to a vertical amplitude seismic trace representation with varying (e.g., continuously varying) amplitude values for a given x-axis and y-axis position (referred to herein as SEG-Y format) from point set data at 128 or 160 of Figure 4 Figure 5 The method 180 includes defining a plurality of headers at 182 that include auxiliary set information such as sampling rate, trace size, trace length, X&Y coordinates, etc. This auxiliary set information can be obtained from the empty SEG-Y grid defined at 124 and 156 of Figure 4 Figure 5 The point set data is then compressed / transformed at 184 into a more efficient format (binary in one example embodiment). The point set data can be fed into a compression algorithm by defining a sliding window across different bins (e.g., at each cell location) of the point set data. In one example embodiment, a window length of at least 10% of the total trace length is used. The window can be slid by approximately 50% of the window size. The compression algorithm can be configured to perform dimensionality reduction using data transformations from one domain to another such that the maximum amount of information (variance) is preserved while the rest of the information is ignored.

[0030] Continuing with the example method 180 of Figure 6 The compressed data generated in 184 is then organized at 186 into vertical seismic traces for each cell location in the empty SEG-Y grid. The record of the compression process is preserved and can be used in the future to process the data back to the original point set format. Sinusoidal interpolation can also be applied to the data to provide visual seismic traces (e.g., to provide smooth or continuous seismic traces). Each seismic trace can then be stored and / or output at 188 in SEG-Y format, including related information used to generate the seismic trace, e.g., including extended binary coded decimal interchange code (EBCDIC) headers, binary headers, and trace headers. It will be appreciated that the size of the stored (or output) product can increase compared to the original point set data due to the additional compression information stored with the data.

[0031] Turning now to the example method 180 of Figure 7A Figure 7B ​​​、 Figure 7C and Figure 7D (collectively, FIG. 7) depict an example implementation of the method 150. Figure 7A A 3D resistivity volume in point-set format is depicted at 202. In the depicted example, the 3D resistivity volume includes a 3D array of formation resistivity values (shown in grayscale) distributed around a wellbore 204. Note that the 3D resistivity volume is displayed in three-dimensional space, with the z-axis representing total vertical depth (down from the surface), and the x- and y-axes representing north and east directions. Figure 7B A 3D resistivity map in SEG-Y format 210 is extracted from the 3D resistivity volume shown above at 208. In the depicted example, the 3D resistivity map is displayed as a cuboid in the three-dimensional space described above. Figure 7A A 3D resistivity map in SEG-Y format 210 is extracted from the 3D resistivity volume shown above at 208. In the depicted example, the 3D resistivity map is displayed as a cuboid in the three-dimensional space described above. Figure 7C A binary volume is obtained by applying a resistivity threshold to the 3D resistivity map shown above at 210. Cells with resistivity values greater than the threshold are displayed in dark gray at 212. Note that the region 212 can represent a potential reservoir. Cells with resistivity values less than the threshold and cells with discarded resistivity values are displayed in light gray at 214. Figure 7B A binary volume is obtained by applying a resistivity threshold to the 3D resistivity map shown above at 210. Cells with resistivity values greater than the threshold are displayed in dark gray at 212. Note that the region 212 can represent a potential reservoir. Cells with resistivity values less than the threshold and cells with discarded resistivity values are displayed in light gray at 214. Figure 7D A binary volume is obtained by applying a resistivity threshold to the 3D resistivity map shown above at 210. Cells with resistivity values greater than the threshold are displayed in dark gray at 212. Note that the region 212 can represent a potential reservoir. Cells with resistivity values less than the threshold and cells with discarded resistivity values are displayed in light gray at 214. Figure 7C A binary volume is obtained by applying a resistivity threshold to the 3D resistivity map shown above at 210. Cells with resistivity values greater than the threshold are displayed in dark gray at 212. Note that the region 212 can represent a potential reservoir. Cells with resistivity values less than the threshold and cells with discarded resistivity values are displayed in light gray at 214.

[0032] It should be understood that the present disclosure includes many embodiments. These embodiments include, but are not limited to, the following embodiments.

[0033] In a first embodiment, a method for generating a three-dimensional (3D) resistivity map of a subsurface formation in a Society of Exploration Geophysicists (SEG-Y) format includes: obtaining a 3D resistivity volume in a point-set format, the 3D resistivity volume including a three-dimensional array of formation resistivity values around a subsurface wellbore; defining an empty SEG-Y grid, the grid containing the obtained 3D resistivity volume; superimposing the empty SEG-Y grid on the obtained 3D resistivity volume; extracting resistivity amplitudes from the 3D resistivity volume along a single vertical seismic trace for each cell in the empty SEG-Y grid to generate a preliminary resistivity map; and discarding unwanted resistivity amplitudes among the extracted resistivity amplitudes in the preliminary resistivity map to generate the 3D resistivity map in the SEG-Y format.

[0034] The second embodiment can include the first embodiment, wherein the obtaining the 3D resistivity volume in the point cloud format comprises: taking depth-reading electromagnetic measurements in the subterranean wellbore; inverting the depth-reading electromagnetic measurements to inverted resistivity values using a multi-dimensional resistivity inversion algorithm; and assembling the 3D resistivity volume in the point cloud format from the inverted resistivity values.

[0035] The third embodiment can include the second embodiment, wherein taking the depth-reading electromagnetic measurements in the wellbore comprises: rotating a depth-reading electromagnetic logging tool in the subterranean wellbore; emitting electromagnetic energy into the subterranean wellbore using emitters on the depth-reading electromagnetic logging tool while rotating; and receiving the emitted electromagnetic energy at receivers on the depth-reading electromagnetic logging tool, wherein the receivers are spaced at least 10 meters apart from the emitters.

[0036] The fourth embodiment can include any of the first through third embodiments, wherein defining the empty SEG-Y grid comprises: extracting maximum and minimum x, y, and z-axis spatial coordinates from the 3D resistivity volume; and dividing the difference between the maximum and the minimum x, y, and z-axis spatial coordinates by a predetermined or user-selected grid spacing.

[0037] The fifth embodiment can include any of the first through fourth embodiments, wherein extracting the resistivity amplitudes along the single vertical seismic trace for each cell in the empty SEG-Y grid comprises: compressing the point cloud data along a vertical axis of the cell; and organizing the compressed point cloud data into the single vertical seismic trace.

[0038] The sixth embodiment can include any of the first through fifth embodiments, wherein discarding the resistivity amplitudes comprises discarding resistivity amplitudes in the extracted resistivity amplitudes that are greater than a predetermined distance from the subterranean wellbore.

[0039] The seventh embodiment can include any of the first through sixth embodiments, wherein discarding the resistivity amplitudes comprises: obtaining a trajectory of the subterranean wellbore; determining a detection limit of a depth-reading electromagnetic resistivity tool; calculating a detection volume around the subterranean wellbore from the trajectory and the detection limit; and discarding resistivity amplitudes outside the detection volume.

[0040] The eighth embodiment can include any of the first through seventh embodiments, further comprising: applying a first resistivity threshold to the 3D resistivity volume in SEG-Y format to generate a binary resistivity volume, wherein locations with resistivity values greater than the first resistivity threshold are assigned a first high resistivity value and locations with resistivity values less than the first resistivity threshold are assigned a second low resistivity value.

[0041] The ninth embodiment can include the eighth embodiment, further comprising evaluating high resistivity volumes and low resistivity volumes in the binary resistivity volume to characterize at least one interface between the high resistivity volumes and the low resistivity volumes.

[0042] The tenth embodiment can include the ninth embodiment, further comprising repeating the applying and the evaluating at a second resistivity threshold, wherein the second resistivity threshold is different than the first resistivity threshold.

[0043] In an eleventh embodiment, a system for generating a three-dimensional (3D) resistivity map of a subsurface formation in Society for Exploration Geophysics (SEG-Y) format includes a depth-reading electromagnetic logging tool configured for taking measurements in a wellbore that penetrates the subsurface formation, and at least one computer processor configured to: apply a multi-dimensional inversion algorithm to the measurements to generate a 3D resistivity volume in point-set format; define an empty SEG-Y grid that encompasses the 3D resistivity volume; overlay the empty SEG-Y grid on the 3D resistivity volume; extract resistivity amplitudes from the 3D resistivity volume along a single vertical seismic trace for each cell in the empty SEG-Y grid to generate a preliminary resistivity map; and discard unwanted resistivity amplitudes among the extracted resistivity amplitudes in the preliminary resistivity map to generate the 3D resistivity map in the SEG-Y format.

[0044] The twelfth embodiment can include the eleventh embodiment, wherein defining the empty SEG-Y grid includes: extracting maximum and minimum x, y, and z-axis spatial coordinates from the 3D resistivity volume; and dividing differences between the maximum and the minimum x, y, and z-axis spatial coordinates by a predetermined or user-selected grid spacing.

[0045] The thirteenth embodiment can include any of the eleventh through twelfth embodiments, wherein extracting the resistivity amplitudes along the single vertical seismic trace for each cell in the empty SEG-Y grid includes: compressing the point-set data along a vertical axis of the cell; and organizing the compressed point-set data into the single vertical seismic trace.

[0046] The fourteenth embodiment can include any of the eleventh through thirteenth embodiments, wherein discarding the resistivity amplitudes includes: obtaining a trajectory of the wellbore; determining a detection limit of the depth-reading electromagnetic resistivity tool; calculating a detection volume around the wellbore according to the trajectory and the detection limit; and discarding the resistivity amplitudes outside the detection volume.

[0047] The fifteenth embodiment can include any of the eleventh through fourteenth embodiments, wherein the at least one processor is further configured to: apply a resistivity threshold to the 3D resistivity volume in SEG-Y format to generate a binary resistivity volume, wherein locations where resistivity values are greater than the resistivity threshold are assigned a first high resistivity value and locations where resistivity values are less than the resistivity threshold are assigned a second low resistivity value; and evaluate the binary resistivity volume for high resistivity volumes and low resistivity volumes to characterize at least one interface between the high resistivity volumes and the low resistivity volumes.

[0048] In a sixteenth embodiment, a method for identifying an interface between a high resistivity volume and a low resistivity volume includes: obtaining a three-dimensional (3D) resistivity volume in point-set format, the 3D resistivity volume comprising a three-dimensional array of formation resistivity values around a subterranean wellbore; transforming the 3D resistivity volume in point-set format to a 3D resistivity volume in Society of Exploration Geophysicists (SEG-Y) format; applying a first resistivity threshold to the 3D resistivity volume in SEG-Y format to generate a binary resistivity volume, wherein cells of the 3D resistivity volume where resistivity values are greater than the first resistivity threshold are assigned a first high resistivity value and cells where resistivity values are less than the first resistivity threshold are assigned a second low resistivity value; and evaluating the binary resistivity volume to identify the interface between the high resistivity volume and the low resistivity volume.

[0049] A seventeenth embodiment can include the sixteenth embodiment, further comprising: repeating the applying and the evaluating for a second resistivity threshold, wherein the second resistivity threshold is different than the first resistivity threshold.

[0050] An eighteenth embodiment can include any of the sixteenth through seventeenth embodiments, wherein obtaining the 3D resistivity volume in point-set format includes: conducting depth-reading electromagnetic measurements in the subterranean wellbore; inverting the depth-reading electromagnetic measurements using a multi-dimensional resistivity inversion algorithm to obtain a resistivity inversion; and assembling the 3D resistivity volume in point-set format from the resistivity inversion.

[0051] A nineteenth embodiment can include any of the sixteenth through eighteenth embodiments, wherein transforming the 3D resistivity volume includes: defining an empty SEG-Y grid, the grid containing the obtained 3D resistivity volume; superimposing the empty SEG-Y grid on the obtained 3D resistivity volume; extracting resistivity amplitudes from the 3D resistivity volume along a single vertical seismic trace for each cell in the empty SEG-Y grid to generate a preliminary resistivity volume; and discarding unwanted resistivity amplitudes of the extracted resistivity amplitudes in the preliminary resistivity volume to generate the 3D resistivity volume in SEG-Y format.

[0052] The twentieth embodiment can include the nineteenth embodiment, wherein: extracting the resistivity amplitudes includes compressing the point set data along a vertical axis of the cell; and organizing the compressed point set data into the single vertical seismic trace for each of the cells; and discarding includes obtaining a trajectory of the subterranean wellbore; determining a detection limit of the depth-reading electromagnetic resistivity tool; calculating a detection volume around the subterranean wellbore from the trajectory and the detection limit; and discarding the resistivity amplitudes outside the detection volume.

[0053] As described above, the disclosed embodiments include methods and systems for transforming resistivity data from a discrete spatial point set representation to a SEG-Y format. It will be understood that the disclosed embodiments can also include 3D resistivity maps. For example, the disclosed embodiments can include a 3D resistivity map composed of a 2D array or grid of resistivity amplitude seismic traces, where the amplitude seismic traces define resistivity values along a vertical dimension orthogonal to the 2D array.

[0054] While three-dimensional resistivity reservoir mapping has been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Claims

1. A method for generating a three-dimensional (3D) resistivity map of a subterranean formation in Society for Exploration Geophysics (SEG-Y) format, the method comprising: obtaining a 3D resistivity volume in point cloud format, the 3D resistivity volume comprising a three-dimensional array of formation resistivity values around a subterranean wellbore; defining an empty SEG-Y grid, the grid encompassing the obtained 3D resistivity volume; superimposing the empty SEG-Y grid on the obtained 3D resistivity volume; extracting resistivity amplitudes from the 3D resistivity volume along a single vertical seismic trace for each cell in the empty SEG-Y grid to generate a preliminary resistivity map; and discarding unwanted resistivity amplitudes in the extracted resistivity amplitudes of the preliminary resistivity map to generate the 3D resistivity map in the SEG-Y format.

2. The method of claim 1, wherein obtaining the 3D resistivity volume in the point cloud format comprises: conducting depth-reading electromagnetic measurements in the subterranean wellbore; inverting the depth-reading electromagnetic measurements into inverted resistivity values using a multi-dimensional resistivity inversion algorithm; and assembling the 3D resistivity volume in the point cloud format from the inverted resistivity values.

3. The method of claim 2, wherein conducting the depth-reading electromagnetic measurements in the wellbore comprises: rotating a depth-reading electromagnetic logging tool in the subterranean wellbore; emitting electromagnetic energy into the subterranean wellbore using an emitter on the depth-reading electromagnetic logging tool while rotating; receiving the emitted electromagnetic energy at a receiver on the depth-reading electromagnetic logging tool, wherein the receiver is spaced at least 10 meters from the emitter.

4. The method of claim 1, wherein defining the empty SEG-Y grid comprises: extracting maximum and minimum x, y, and z-axis spatial coordinates from the 3D resistivity volume; and dividing the difference between the maximum and the minimum x, y, and z-axis spatial coordinates by a predetermined or user-selected grid spacing.

5. The method of claim 1, wherein extracting the resistivity amplitudes along the single vertical seismic trace for each cell in the empty SEG-Y grid comprises: compressing the point cloud data along a vertical axis of the cell; and organizing the compressed point cloud data into the single vertical seismic trace.

6. The method of claim 1, wherein discarding the resistivity amplitudes comprises discarding resistivity amplitudes in the extracted resistivity amplitudes that are more than a predetermined distance from the subterranean wellbore.

7. The method of claim 1, wherein discarding the resistivity amplitudes comprises: obtaining a trajectory of the subterranean wellbore; determining a detection limit of a depth-reading electromagnetic resistivity tool; calculating a detection volume around the subterranean wellbore from the trajectory and the detection limit; and discarding the resistivity amplitudes outside the detection volume.

8. The method of claim 1, further comprising: ​ ​ ​ ​ ​ ​ applying a first resistivity threshold to the 3D resistivity volume in SEG-Y format to generate a binary resistivity volume, wherein locations with resistivity values greater than the first resistivity threshold are assigned a first high resistivity value and locations with resistivity values less than the first resistivity threshold are assigned a second low resistivity value.

9. The method of claim 8, further comprising: evaluating the high resistivity volumes and the low resistivity volumes in the binary resistivity volume to characterize at least one interface between the high resistivity volumes and the low resistivity volumes.

10. The method of claim 9, further comprising repeating the applying and the evaluating at a second resistivity threshold, wherein the second resistivity threshold is different than the first resistivity threshold.

11. A system for generating a three-dimensional (3D) resistivity volume of a subterranean formation in Society for Exploration Geophysics (SEG-Y) format, the system comprising: a depth-reading electromagnetic logging tool configured to make measurements in a wellbore that penetrates the subterranean formation; and at least one computer processor configured to: apply a multidimensional inversion algorithm to the measurements to generate a 3D resistivity volume in point-set format; define an empty SEG-Y grid that encompasses the 3D resistivity volume; superimpose the empty SEG-Y grid on the 3D resistivity volume; extract resistivity amplitudes from the 3D resistivity volume along a single vertical seismic trace for each cell in the empty SEG-Y grid to generate a preliminary resistivity volume; and discard unwanted resistivity amplitudes in the extracted resistivity amplitudes of the preliminary resistivity volume to generate the 3D resistivity volume in SEG-Y format.

12. The system of claim 11, wherein defining the empty SEG-Y grid comprises: extracting maximum and minimum x, y, and z-axis spatial coordinates from the 3D resistivity volume; and dividing differences between the maximum and the minimum x, y, and z-axis spatial coordinates by a predetermined or user-selected grid spacing.

13. The system of claim 11, wherein extracting the resistivity amplitudes along the single vertical seismic trace for each cell in the empty SEG-Y grid comprises: compressing the point-set data along a vertical axis of the cell; and organizing the compressed point-set data into the single vertical seismic trace.

14. The system of claim 11, wherein discarding the resistivity amplitudes comprises: obtaining a trajectory of the wellbore; determining a detection limit of the depth-reading electromagnetic resistivity tool; calculating a detection volume around the wellbore from the trajectory and the detection limit; and discarding the resistivity amplitudes outside the detection volume.

15. The system of claim 11, wherein the at least one processor is further configured to: apply a resistivity threshold to the 3D resistivity volume in SEG-Y format to generate a binary resistivity volume, wherein locations with resistivity values greater than the resistivity threshold are assigned a first high resistivity value and locations with resistivity values less than the resistivity threshold are assigned a second low resistivity value; and evaluating the binary resistivity volume to characterize at least one interface between the high resistivity volume and the low resistivity volume.

16. A method for identifying an interface between a high resistivity volume and a low resistivity volume in a subterranean formation, the method comprising: obtaining a three-dimensional (3D) resistivity volume in point cloud format, the 3D resistivity volume comprising a three-dimensional array of formation resistivity values around a subterranean wellbore; transforming the 3D resistivity volume in point cloud format into a 3D resistivity map in Society of Exploration Geophysicists (SEG-Y) format; applying a first resistivity threshold to the 3D resistivity map in SEG-Y format to generate a binary resistivity volume, wherein cells in the 3D resistivity map having resistivity values greater than the first resistivity threshold are assigned a first high resistivity value and cells having resistivity values less than the first resistivity threshold are assigned a second low resistivity value; and evaluating the binary resistivity volume to identify the interface between the high resistivity volume and the low resistivity volume.

17. The method of claim 16, further comprising: repeating the applying and the evaluating for a second resistivity threshold, wherein the second resistivity threshold is different than the first resistivity threshold.

18. The method of claim 16, wherein obtaining the 3D resistivity volume in point cloud format comprises: conducting depth-reading electromagnetic measurements in the subterranean wellbore; inverting the depth-reading electromagnetic measurements using a multi-dimensional resistivity inversion algorithm to obtain a resistivity inversion; and assembling the 3D resistivity volume in point cloud format from the resistivity inversion.

19. The method of claim 16, wherein transforming the 3D resistivity volume comprises: defining an empty SEG-Y grid containing the obtained 3D resistivity volume; superimposing the empty SEG-Y grid on the obtained 3D resistivity volume; extracting resistivity amplitudes from the 3D resistivity volume along a single vertical seismic trace for each cell in the empty SEG-Y grid to generate a preliminary resistivity map; and discarding unwanted resistivity amplitudes among the extracted resistivity amplitudes in the preliminary resistivity map to generate the 3D resistivity map in SEG-Y format.

20. The method of claim 19, wherein: extracting the resistivity amplitudes comprises compressing the point cloud data along a vertical axis of the cell; and organizing the compressed point cloud data into the single vertical seismic trace for each of the cell; and the discarding comprises obtaining a trajectory of the subterranean wellbore; determining a detection limit of a depth-reading electromagnetic resistivity tool; calculating a detection volume around the subterranean wellbore from the trajectory and the detection limit; and discarding the resistivity amplitudes outside the detection volume.

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