Patent Application
20060062081
The invention relates generally to instruments for subsurface logging and exploration. More particularly, the invention relates to techniques for compressing log data for transmission via a selected telemetry format.
The oil and gas industry uses various tools to probe the formation penetrated by a borehole in order to locate hydrocarbon reservoirs and to determine the types and quantities of hydrocarbons. Among these tools, sonic tools have been found to provide valuable information regarding formation properties. In sonic logging, a tool is typically lowered into a borehole, either after the well has been drilled or while the well is being drilled, and sonic energy is transmitted from a source into the borehole and surrounding formation. The sonic waves that travel in the formation are then detected with one or more receivers.
A typical sonic log can be recorded on a linear scale of slowness versus depth in the borehole, and is typically accompanied by an integrated-travel-time log in which each division indicates an increase of one microsecond of the total travel time period. Sonic logs are typically used as direct indications of subsurface properties or—in combination with other logs or other data of the subsurface properties—to determine the formation porosity and other parameters which cannot be measured directly.
Various analysis methods are available for deriving formation properties from the sonic log data. Among these, the slowness-time-coherence (STC) method is commonly used to process the monopole sonic signals for coherent arrivals, including the formation compressional, shear, and borehole Stoneley waves. See U.S. Pat. No. 4,594,691 issued to Kimball et al. and Kimball et al., Geophysics, Vol. 49 (1 984), pp. 264-28.
For logging-while-drilling (LWD) sonic logging, it is desirable to send selected data uphole or wherever desired in real-time via mud pulse telemetry. Mud telemetry is a common method used in LWD operations to transmit log data to the surface. Mud telemetry makes use of the modulations of the pressure of a drilling fluid pumped through the drilling assembly to drill the wellbore. The fluid pressure modulation, however, can only transmit data at a rate of a few bits per second. A typical LWD sonic job requires too much bandwidth to transmit all the desired measured sonic data in real-time.
The limitations imposed on data transmission by a lack of adequate bandwidth are commonly encountered in various logging operations, not just sonic logging. Therefore, various methods for data compression have been developed to reduce the bandwidth requirement of conventional telemetry schemes. For example, U.S. Pat. No. 5,381,092 issued to Freedman describes methods for compressing data produced from NMR well tools. The methods first subdivide a plurality of input signals into multiple groups, where the number of groups is much less than the number of input signals. The method then generates one value for each group. Thus a plurality of values corresponding to the plurality of groups represent the compressed input signals transmitted uphole.
U.S. Pat. No. 5,031,1 55 issued to Hsu describes methods for compressing sonic data acquired in well logging. Samples of each digitized formation wave component are characterized as a vector. Eigenvectors based on the formation wave component vectors are obtained, and selected wave components are correlated to the eigenvectors to obtain scalar correlation factors. The eigenvectors and correlation factors together provide a compressed representation of the selected formation wave component.
U.S. Pat. No. 6,691,036 issued to Blanch et al. describes methods for processing sonic waveforms. A method proposed in this application transforms an acoustic signal into the frequency domain to produce a frequency domain semblance and display the result in a graph with slowness and frequency axes. Published U.S. patent application No. 2004/0145503 by Blanch et al. describe additional methods for processing sonic waveforms.
U.S. Pat. No. 6,405,136 B1 issued to Li et al. describes compression methods for use in wellbore and formation characterization. The method includes performing a 2D transform on the data in the orientation domain and in a domain related to the recording time.
While these methods are useful in compressing log data and in reducing the bandwidth requirements of mud telemetry, a need remains for efficient techniques for downhole data compression.
One aspect of the invention relates to methods for compression of sonic log data. A method in accordance with one embodiment of the invention includes sorting peak components in the sonic log data; filtering the sorted peak components to remove high-frequency portions in the peak components; and decimating the filtered peak components according to a selected ratio to produce compressed data.
One aspect of the invention relates to methods for telemetry transmission of downhole sonic log data. A method in accordance with one embodiment of the invention includes sorting peak components in the sonic log data; compressing the sorted peak components to produce compressed data; packing the compressed data to produce data packets for telemetry transmission; and sending the data packets using telemetry.
One aspect of the invention relates to systems for compressing sonic log data. A system in accordance with one embodiment of the invention includes a processor and a memory, wherein the memory stores a program having instructions for: sorting peak components in the sonic log data; filtering the sorted peak components to remove high-frequency portions in the peak components; and decimating the filtered peak components according to a selected ratio to produce compressed data
Embodiments of the invention relate to techniques for compressing downhole data (e.g., attributes of sonic coherence peaks). These compression schemes may be used to reduce telemetry bandwidth requirements for sending data uphole (e.g., in LWD operations) or to reduce the memory required for storing data for later retrieval (e.g., in logging-while-tripping operations). Embodiments of the invention may be implemented in existing downhole tools (e.g., sonic instruments or other logging tools) or incorporated with future instruments to transmit real-time information where desired. Sonic tools are available for wireline, while-tripping, long-term monitoring, and LWD operations as known in the art. Sonic tools for LWD logging, for example, are described in U.S. Pat. No. 5,852,587 issued to Kostek et al. When used for sonic implementations, the disclosed techniques are applicable to acoustic wave data produced in all modes of excitation (e.g., monopole, dipole, quadrupole, octupole).
Drill string 4 is suspended from hook 9 by means of swivel 13, which is linked by hose 14 to mud pump 15. Pump 15 permits the injection of drilling mud into well 6, via the hollow pipes of drill string 4. The drilling mud may be drawn from mud pit 16, which may be fed with surplus mud from well 6. The drill string 4 may be elevated by turning lifting gear 3 with winch 12. Drill pipe raising and lowering operations require drill string 4 to be temporarily unhooked from lifting gear 3; the former is then supported by blocking it with wedges 17 in conical recess 18 in rotating table 19 that is mounted on platform 20, through which the drill string passes. The lower portion of the drill string 4 may include one or more tools, as shown at 30, for investigating downhole drilling conditions or for investigating the properties of the geological formations. Tool 30 shown is an acoustic logging tool having at least one transmitter and a plurality of receivers spaced therefrom.
Variations in height h of traveling block 8 during drill string raising operations are measured by means of sensor 23 which may be an angle of rotation sensor coupled to the faster pulley of crown block 7. Weight applied to hook 9 of traveling block 8 may also be measured by means of strain gauge 24 inserted into dead line 10a of cable 10 to measure its tension. Sensors 23 and 24 are connected by lines 25 and 26 to processing unit 27 which processes the measurement signals and which incorporates a clock. Recorder 28 is connected to processing unit 27, which is preferably a computer. In addition, the downhole sonic tool 30 may include a processing unit 30a. The downhole processing unit 30a and/or the surface processing unit 27 may be used to perform the data compression and decompression in accordance with embodiments of the invention.
Sonic data acquired in this manner is typically displayed on a chart, or log, of waveform amplitude over time versus depth. As noted above, the slowness-time-coherence (STC) method is among the most commonly used in sonic data analysis. This method systematically computes the coherence (C) of the signals in time windows which start at a given time (T) and have a given window moveout slowness (S) across the array. The 2D plane C(S,T) is called the slowness-time plane (STP). All the coherent arrivals in the waveform will show up in the STP as prominent coherent peaks. The three attributes of a coherent peak are the peak coherent value (COPK) and the peak location in the slowness-time plane (DTPK and TTPK). The attributes of these prominent coherent peaks represent the condensed information extracted from the recorded waveforms. The attributes show the coherence, arrival time, and propagation slowness of all prominent wave components detected from the waveforms.
The peak attributes can be used uphole as input to a selection process called “labeling” to determine the compressional (P), shear (S), and Stoneley (St) slowness logs. The peak attributes can also be used to generate a synthetic slowness-time-plane projection (STTP) for real-time quality control purpose. In any given zone, if the compressional DT log matches to a group of peaks with high coherence, steady DT value, and consistent arrival time, the likelihood of accurate measurement is high. In order to accommodate the mud telemetry bandwidth, the downhole software onboard a sonic tool can select only a few peaks (e.g., 4 peaks) to transmit uphole. First, the software would search for coherent peaks above a given threshold value (usually 0.4) in the STP. There may be a large number of peaks that have coherence above this threshold. The software would then sort the peaks according to descending order of coherence and retains only the top peaks (e.g. top 4).
The bandwidth required to send the 4 highest coherent peaks uphole is significant. The following table shows the number of bits required to represent typical coherence attributes of a single peak.
It requires 14 bits to represent one peak and 56 bits for 4 peaks at any given data frame. Assuming the data frame rate is 10 second per frame, the bit rate requirement for sending the attributes of the 4 peaks uphole is 5.6 bits/sec, which is a restrictive value for most field jobs.
The disclosed methods can compress data with little loss. Under normal circumstance, a compression factor of 4 can be achieved without significant loss of information. A reduction (data compression) by a factor of 4 will make the bit rate requirement for sending data via mud telemetry possible for many applications, including sonic logs. Using sonic logs as an example, with a factor of 4 compression, the peak attributes of 4 peaks can be transmitted at 1.4 bits/sec for 10-second frame rates.
It is apparent from
First, sonic tools are designed to measure slowness of major wave components regardless of coherence, and the high frequency components in the coherence sorted peak attribute data typically are not related to the major wave components. Thus, the real information of interest do not require high frequency representations. Furthermore, the receiver arrays of conventional sonic tools typically span a few feet (e.g., a 2-ft [0.61 m] aperture) along the longitudinal axis of the tool, and the measured slowness of the wave components is typically averaged over the receiver aperture. Essentially, the 2-ft [0.61 m] aperture acts like a low-pass filter, removing high-frequency components. Therefore, the measured P, S, and St slownesses should be slowly varying functions in both the time domain and the depth domain.
In addition, drilling speeds can also affect maximum spatial frequencies (Nyquist frequency) measurable in sonic logs.
While averaging over the 2-ft [0.61 m] aperture may lose a portion of the information content, it is often impractical to record and transmit the full bandwidth of raw data. The important information content of the log is typically included in the lower portion (e.g. lower 25%) of the spatial frequency spectrum. Therefore, a compression scheme (e.g., a band limited data compression scheme), which keeps only the lower portion (e.g. 25%) of the spatial frequency, should have minimal loss of information. Curve 2 represents the lower 25% of the spatial frequency. Thus, by keeping only the lower portion of the spatial frequency, the data is effectively compressed by a factor of four, without a significant loss of information.
The above observations together suggest that sonic log data can be efficiently compressed without loss of much information by keeping mostly the low frequency components. In addition, it may be advantageous to sort the peak attributes according to the peak components, rather than the magnitudes of the coherences. Based on these considerations, embodiments of the invention present techniques for effective data compression that can be implemented in a downhole tool to reduce the telemetry bandwidth requirements or to reduce the memory requirement for storing log data for later retrieval.
Methods of the invention for data compression are based on resorting of the peak attributes according to wave components, rather than according to coherences.
Once the peak attributes have been filtered and decimated, the remaining portion is ready for transmission uphole. The data that are to be transmitted may be encoded in a suitable bit-encoding format for mud telemetry (or other telemetry) (step 45). For example, one may assign 3 bits to encode the magnitudes of peak coherences, 7 bits to DT, and 4 bits to TT. Next, the encoded bits are packed in frames (data packets) for telemetry transmission (step 46) and the data packets are sent where desired (step 47).
Once the compressed data are sent to the surface, they can be decompressed to “reconstruct” the peak attributes in a process that in most part is a reverse of the compression process used to compress the data.
Note that the specific methods described in
Peak sorting according to wave components: P, S, St, O (other)
One of ordinary skill in the art will appreciate that there are many ways to sort the peaks according to the wave components. The following describes a simple procedure that has been found to work quite robustly on field data.
The wave component peak selection process may be based on factors that reflect peak characteristics. For example, the following factors may be used for peak selection: (a) Coherence, (b) Slowness consistent with arrival time for the given transmitter-to-receiver spacing (TR), (c) Early arrival (for P component only), and (d) Late arrival (for St component only). Each of these factors may be associated with a weighting coefficient to yield a cost function for that factor. The total cost function may then be described as the sum of the cost functions for the individual factors.
In sonic logging, the first signal to arrive at a receiver is generally the compressional wave (P-wave), which travels from the transmitter to the receiver through the formation adjacent the borehole. The second signal arrival is generally the shear wave (S-wave). Then, the Stoneley wave (St) comes next. Because the P-wave comes earlier, it would be easier to sort out the P components first. Thus, in accordance with one embodiment of the invention, the lowest cost peak for P component is determined first. Then, the lowest cost peak for the S component is selected from the remaining peaks. The lowest cost peak for the St component is determined next from the remaining peaks after P and S peak selection. Finally, the remaining peaks after the P, S, and St peak selection are labeled “O” for “others.”
In addition, other rules may be used in conjunction with the selection rules outlined above. For example, the P peaks may be preferentially selected from those having slowness within a practical limit, such as the compressional label limits that are part of the downhole tool configuration parameters. Similarly, the S peaks may be preferentially selected from those having a slowness typically expected of a shear wave. The St peaks may also be preferentially selected from those having slowness higher than the mud slowness.
Sometimes, the P component peak may be missing from the STC processing for a few frames. This situation may arise from a faulty peak search algorithm or noise problems. When the P component is missing, the sorting algorithm may incorrectly assign the S peak as the P peak over these few frames. If this happens, the resulting P peak slowness may have a spike (anomaly) over these few frames. To improve the situation, a de-spiking process may be included in P peak sorting to detect any spike. A spike may be defined as an anomaly having a width of a few frames. Such a spike can be detected, for example, by using a suitable filter. If a P spike is detected, the P peak attributes may be reassigned to a median value. After de-spiking, the attributes of the S, St, and O peaks may be reassigned from the original peak attributes using the minimum cost and slowness range rules.
In accordance with embodiments of the invention, the peak sorting algorithm (and peak de-spiking algorithm) may be implemented in any suitable software, including commercially available packages such as Matlab™ from MathWorks (Natick, Mass.). The peak sorting algorithm may include a quality indicator to indicate the quality of the wave-component peak sorting. A quality indicator may be based on the cost function described above or any other suitable function. One of ordinary skill in the art will understand how to implement appropriate algorithm codes in accord with the techniques disclosed herein.
Band-limited compression/decompression for the wave component sorted peak attributes
The wave-component-sorted peak attributes are slowly varying functions with information content primarily in the lower 25% of the frequency band. Therefore, in accordance with embodiments of the invention, a standard band limited compression algorithm may be selected to compress the sorted peak attributes. For example, a time domain version of the band-limited compression may be used. However, one of ordinary skill in the art will appreciate that other approaches may be used without departing from the scope of the invention.
In accordance with one embodiment of the invention, a time domain based band-limited compression algorithm is used. The algorithm consists of low-pass filtering, followed by a four-to-one (or any other suitable ratio) decimation (see e.g.,
In accordance with some embodiments of the invention, a quality indicator may be derived to provide indication of the quality of the compression. For example, a quality indicator may be based on the ratio of the spectral energy in the lower 25% of the frequency band to that in the upper 75% of the frequency band to indicate the quality of the compression.
The following examples illustrate the utility of methods in accordance with embodiments of the invention as applied to actual sonic log data.
Results from Sonic Data
Similarly, as shown in
Similarly,
Some embodiments of the invention relate to systems for performing methods of the invention. A system of the invention may be implemented on the processor in the downhole tool or on a surface processor, which may be a general purpose computer.
Advantages of embodiments of the invention include methods for effective data compression without significant loss of information. The disclosed compression techniques are based on signal characteristics to preserve the information content of the original signals. The compression methods in accord with embodiments of the invention may enable real-time transmission of downhole data that would otherwise be impossible to transmit using mud telemetry. Embodiments of the invention may also be used to compress data to minimize telemetry bandwidth requirements. Embodiments may also be used to compress data downhole for storage, in order to save memory. The saved data may then be retrieved for later processing (e.g. when the instrument is tripped out of the well).
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. For example, while mud telemetry is described as a transmission means herein, those skilled in the art will appreciate that other telemetry means may be used to implement the disclosed techniques. For the purposes of this specification it will be clearly understood that the word “comprising” means “including but not limited to”, and that the word “comprises” has a corresponding meaning.
