Well Log Data
Geophysical well logs in paper format are scanned, saved, and distributed in tagged image file format (TIFF). Converting well logs from TIFF to Log ASCII Standard (LAS) formatted files is resource intensive, requiring knowledge about software and geological characteristics. Some geological surveys send log scans to private vendors convert the TIFF to LAS files, because the task is too labor intensive to be cost effective. LAS file size is smaller than TIFF, and provides ability to perform calculations, such as determining porosity and lithology, and use in other applications. Some state geological surveys convert TIFF to LAS files for selected wells of interest to project investigations. Kentucky Geological Survey uses PETRA® and LAS Certify software, and Python scripts for the conversion. The following content describes Kentucky’s QA-QC process, kindly shared by Brandon Nuttall and Liz Adams.
Kentucky Geological Survey (KGS) receives some geophysical well data as digital data files submitted by operators. Operators are encouraged, but not required, to submit their logs in digital formats. The digital data files are commonly formatted in 2 ways: company proprietary formats and LAS. Proprietary formats include Schlumberger’s *.DLIS (digital log information system) file and Weatherford’s *.DPK (“data pack”) files. Both file formats include software developed by (for) the company that enable exporting LAS-formatted files. Schlumberger provides several separate digital data formats. The DLIS format is the basic data and the Schlumberger software can be used to export either a LAS file or a PDS formatted file. The PDS files can be distributed separately and are used by Schlumberger utilities BlueView or PDSView to produce printable well log files. Weatherford’s PreView software is an integrated package that can read a DPK file and export either a TIFF image or a LAS file. Some service companies (including Norris Well Logging) use the Warrior software system for acquiring digital well log data, and outputting a LAS file. Kentucky Geological Survey endeavors to distribute files in LAS format. However, raw data files in original format are saved in a shared drive to address future needs, if they arise.
Kentucky Geological Survey also obtains LAS files by digitizing scanned images. PETRA® software offers a basic module for digitizing curve traces from images. Neuralog Inc. offers same capabilities, with advanced functionality for autotracing, detecting and managing wrapped curves, and utilities for editing digitized curves (spike and gap detection, smoothing, etc.) In the 1990s, Kentucky Geological Survey used TerraStation program that included a module for digitizing logs. The program included a built-in lithology analysis module that computed percent of limestone, sandstone, dolomite, and shale, reporting the LS, SS, DOL, and SH fraction traces.
The PETRA® software can be used to import and export digital log data formats and recognizes DLIS and LAS file types. PETRA® is convenient for accumulating a collection of digital log data and using the data for stratigraphic analysis, cross sections, and mapping. For Kentucky Geological Survey, problems were introduced into an online collection of LAS files when the files were exported from PETRA®. The PETRA® export resulted in two primary problems. Problem 1) a duplicate depth track was exported for many wells, which resulted in the number of columns of digital well log data in the ~ASCII data section did not match the number of curves listed in the ~CURVE section. This will cause LAS Certify to fail and can result in misidentified data columns when the data are imported into another software package. Problem 2) the duplication of other log curves occurred. Sometimes curve data, either digitized or imported into PETRA®, get identified by different mnemonics, but the data are identical. In a few cases, the curves are the same data digitized at two different scales. An odd problem is also known related to digital data from Weatherford. Sometimes the field engineer may not have “zeroed” the depth curve. This can make the depths in the LAS file not evenly divisible by the increment between the depths. For example, with depths = 4050.229, 4050.729, 4051.229 … the interval is 0.5, but 4050.229 isn’t an even multiple of 0.5. LAS Certify fails these files.
Kentucky Geological Survey uses Python scripts to harvest and analyze digital well log data from existing LAS files. In some cases, a new LAS file is output that includes newly computed log data (interpretations). Jupyter Notebook explains some LAS QA-QC Utilities. This interactive Python script uses a module called LASIO to read and write LAS files. The file also includes information about how to use the Welly utility to make a quick side-by-side plot of well log curves. One of the most important things this notebook script does is to add some uniform well identifiers to the ~WELL section of the LAS (if they are not there). This is a workbook in progress, for example, a future addition would be to add the tops data to the LAS file (using the LAS 3.0 specification). It would also be useful to obtain the well header items directly from the database rather than from a separate well header file. Microsoft SQL implementations that use ODBC are difficult to set up and access using Python code (most people using Python use distributed SQL implementations like MySQL or PostGres). LASIO may be used to harvest information for a database of digital well curves available (curve, top and bottom of logged interval, etc.)
Access LAS QA QC Utilities Jupyter Notebook
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