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Petrophysical Analysis
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A significant number of wells were completed in the Dakota Formation in the 1950s and 1960s;
many of which are still producing today. To assess the remaining potential, either in these
existing wells or in stepout locations or infill locations, one option is to evaluate the core
and log information gathered during that past time frame. The provided data could be relevant
to an operator with the same vintage of data, or could be used to supplement recently gathered
data in a specific area. The discussion below will cite several possible examples of using the
old data for petrophysical interpretation
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A small subset of the core data available was selected for analysis. The vintage of the
majority of core and log data used is from the 1950s through 1960s, with a few from the
1970s and early 1980s time period. Routine core analysis consisting of porosity,
permeability and saturation measurements was available from core plugs. Also, some
special core analysis was obtained and included vertical and/or maximum horizontal
permeabilities, and grain density measurements. The logging suite varied, with gamma
ray-spontaneous potential-induction-sonic the common assemblage.
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Example 1
The first example well was drilled in 1960 with an oil emulsion mud. Approximately
80 feet of core was obtained from the Dakota Formation, with the majority analyzed for
porosity, permeability and saturations. The logging suite consisted of gamma ray,
spontaneous potential, induction and sonic logs. Figure 1 illustrates the final results
of the log analysis for this well, with the core water saturation and porosity measurements
included for comparison. The reasonable match with water saturations was obtained by
adjusting the formation water resistivity, Rw, for each zone. Accurate values for Rw
are unknown and thus become a variable in the analysis process. In this example, Rw
was determined to be ~ 0.07 ohmm.
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The perforated zones are indicated on the far left-hand side of the figure. Notice an
excellent correspondence between these perforated zones to the clean, porous, gas-bearing
zones on the logs; confirming the log analysis. This well tested at a CAOF of 2.2 mmscfd
with a cumulative production of 1.7 Bscf. According to the logs and core data,
the lower zones, below 6925 ft, are wet and thus have no potential.
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Also on figure 1 are possible fracture intervals indicated by anomalously high porosity spikes.
This is believed to be in response to cycle skipping on the sonic log. Core descriptions
tend to confirm the existence of fractures. Subsequently, low water saturation values
are incorrect in these zones and should be ignored.
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Figure 1. Log analysis for exmaple well 1 in the Dakota Formation. Core water saturation and
porosity values are shown as discrete points. Perforated zones are indicated on the left-side
of the log. (Digital Formation, IncTM)
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Example 2The second example well was drilled through the Dakota in 1980 with
water-based mud. Core measurements consisted of the routine components; porosity, permeability
and saturations. The logging suite includes gamma ray, spontaneous potential, induction and
density-neutron porosity logs. Figure 2 shows the final logging results with the core measurements
shown for comparison.
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Figure 2Log analysis for example well 2 in the Dakota Formation. Core water saturation
and porosity values are shown as discrete points. Perforated zones are indicated on the left-side of the log.
(Digital Formation, IncTM)
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The reasonable match with the core data was obtained by applying a matrix density of 2.65 gm/cc (
average value obtained from core grain measurements) and adjusting Rw (in this case Rw = 0.10
ohmm). The perforated intervals correlate with the log-calculated productive intervals,
confirming the log analysis. This well tested at a CAOF 0.5 mmscfd, with cumulative gas production
of 1.26 Bscf.
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Notice again the fracture indicated intervals in the lower portion of the Dakota. In this case,
it is the response to the density tool that is causing the anomalous high readings.
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