Page 162 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION CASE STUDIES

Case Study 1 Answer
A careful examination of the logs recorded through the moveable oil saturation (MOS) values are also low. Again,
Atoka Sandstone provides the following information. On this is initially puzzling because, in a good reservoir with
the caliper log (Fig. 55), mudeake development is shown by high porosity and permeability, these values should be high.
the decreasing hole size. This mudcake development is Additional anomalous information comes from the
important because it indicates a permeable zone. Mudcake calculation of the moveable hydrocarbon index. The values
forms by the accumulation of solid particles from drilling are greater than 0.7, but (usually) favorable moveability
muds on the borehole walls when a porous and permeable values are less than 0.7. Analyses of ROS, MOS, and the
zone 1s invaded with drilling fluids. The SP log (Fig. 54) moveable hydrocarbon index provide negative evidence that
shows hydrocarbon suppression because the SP reading is hydrocarbons will move. Indeed, all of these factors suggest
less than SSP Such suppression suggests the presence of that most of the hydrocarbons will remain in place in the
hydrocarbons; sample shows and the neutron-density log reservoir.
response (Fig. 55) through the Atoka tell you that in this While information relating to inadequate hydrocarbon
case the hydrocarbons will probably be gas. The density log moveability should not be ignored in initial log evaluation,
(Fig. 55) reads a much higher porosity than the neutron log. itcan sometimes be explained in the following way. In this
When this occurs, it is because of gas effect. case, what is stgnificant is the high residual gas saturation in
If water saturation Archie (S,,,) and porosity (@) values the flushed zone (1.0 — S,, = residual hydrocarbon
are crossplotted (Fig. 57), a grain size variation of saturation). This high residual gas saturation is the result of
medium-grained to coarse-grained is shown on the plot. the bypassing of gas by drilling fluids invading a reservoir.
This supports sample cutting examination which indicates a The high residual gas saturation left behind after invasion
medium-grained to coarse-grained sandstone. can be erroneously read by the logs as unmoved
Crossplotting irreducible water saturation versus porosity hydrovarbons (S,,, < S,"/5: Chapter V1).
(see Fig. 58) reveals that the Atoka zone has good Even though you are concerned about negative
permeability with values ranging from 10 to over 100 information from the drill penetration rate through the
millidarcies (md). Another crossplot in Figure 59, Atoka Sandstone, and about the pessimistic moveability
comparing a calculated irreducible water saturation (Sj, = data, vou decide that the other evidence from sainple

\F2000) with water saturation Archie. shows a high examination. gas shows, and log interpretation supports a
relative permeability to gas (K,,). The high relative decision to set pipe. Log interpretation information
permeability to gas (K,,) means there is a correspondingly especially significant to your decision is: high porosities on
low permeability to water (K,,). The reservoir, therefore, the neutron-density logs, strong gas effect on the
should not produce water. neutron-density logs. low water saturation calculated by the
The values derived for both water saturation and bulk Archie equation, high log-derived permeabilities from
volume witer are low. These low values also indicate that various permeability crossplots, and the low bulk volume
the Atoka Sandstone is a reservoir with a high gas saturation water values. While you believe the well contains a
at irreducible water saturation. Because water saturation gas-filled Atoka Sandstone reservoir, you want to determine
values are low, it follows that the reservoir must have high whether or not the well will be acommercial success. To do
gas saturation (remember: |.0 — S, = hydrocarbon so. you calculate volumetric gas reserves before making
saturation). your final pipe setting decision.
Other evidence that the reservoir is at irreducible water Anestimation of Atoka Sandstone gas recovery of 11.0
saturation und has high gas saturations can be interpreted BCF is calculated by using the following paraineters:
from a crossplot (Fig. 60) of bulk volume water. This geothermal gradient = 0.014 < formation depth: pressure
expected that hydrocarbons would be moved out instead of factor = 0.85; formation temperature = 205°F; initial
gradient = 0.35 X formation depth; drainage area = 560
crossplot is created by plotting water saturation Archie
versus porosity (cb). The data points cluster along a
acres; reservoir thickness = [5 ft: effective porosity =
hyperbolic line and have, then, approximately equal values
15%, water saturation = 13%; gas gravity = 0.65; recovery
for bulk volume water (BVW). Bulk volume water values
bottom hole pressure = 5,117 PSI; Z factor =
0.988.
plotted along the hyperbolic line range from 0.015 to 0.027.
The Atoka Sandstone was perforated from 14,610 to
Their clustering also supports the conclusion that the
14,615 ft. The calculated absolute open flow (CAOF) was
reservoir has high gas saturation at irreducible water
21,900,000 cu ft of gas per day (21,900 mcfgpd) with a
saturation.
high shut-in tubing pressure (SITP) of 3,758 pounds per
Values calculated for residual] oil saturation (ROS) are
square inch (PSI) and a high initial bottom hole pressure
high. This ts anomalous because other evidence supports
the conclusion that the reservoir has high porosity and
(IBHP) of 5.556 PSL. The gas gravity was 0.599 at a bottom
permeability. Under these conditions, it could ordinarily be
hole temperature of 219°F The well’s first year cumulative
production was 3,268,129 mcf plus 95,175 barrels of
left behind in the rock. Furthermore, calculations of condensate.
147