Multi-well data sets are normalized for consistency. Specific to the application, various combinations of well log data are combined using contour crossplots. These crossplots are interpreted using various techniques (usually polygon analysis techniques). Then, on a well-by-well basis, the results of the polygon analysis are generated. These data can then be translated to maps or spreadsheets for further analysis.
The example below shows the results of a large multi-well data scan related to coal bed methane in shallow formations of central Alberta. Both modern openhole and cased hole well log data were used. All data were normalized using the openhole data for reference.
The objective of the scan was to target the thickest intervals where gas potential exists based on the interpretation of normalized neutron vs. normalized acoustic data, which were the common the data among the openhole and cased hole data sets.
The polygons outline expectations based on knowledge of the physics of the well logging tools as labeled on the crossplot. The listing below shows the results on a per well basis of the various constituents for a given stratigraphic interval.
The next display shows the scanned interval for one well. The depth column to the right is shaded based on the color of the polygons. For example, the depths shaded black refer to coal. The depths shaded orange represent silts.
For a given well with a selected scan (depth) interval, computer programs can summarize the thickness of the interval that falls in each of the polygons. That is, the thickness of data that falls in the 'gas' polygon or the thickness of data that falls within the 'coal' polygon. In addition to thickness, there are programs available that will yield average values of other data for each polygon. For instance, the average porosity in the 'sand' polygon or the average gamma ray value for the 'shale' polygon.
For multiple wells, the single well polygon net pay summaries are collected into one major summary, examples of which are shown below. These types of applications are very useful for determining accurate sand-shale ratios for mapping purposes.
Multiple well summary of polygon thickness for neutron-acoustic crossplot scanning
Multiple well summary of polygon thicknesses and well log curve averages based on neutron-acoustic crossplot scanning
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