Page 183 - Basic Well Log Analysis for Geologist

LOG INTER PRETATION CASE STUDIES

Case Study 3
Eocene Wilcox Sandstone
Gulf Coast
. . At — 55
You are assigned the log evaluation of a lower Wilcox II. sonic porosity (b,) = At 22
1.4
sandstone in South Texas. A fairly typical Gulf Coast log
package was used in the well. It consists of an induction and
electric log with an SP and a R,,, quick look curve and a
conic poros} — 3At= Atma)
sonic log. The induction electric log has a deep induction TW. sonic porosity (6) = gx At
log used to Measure resistivities in the uninvaded zone (R,)
and a short normal to Measure resistivities in the invaded Where:
zone (R;). The log package is run ona single combination Atma = 55.5 ysec/ft for all sandstones.
tool, and because it requires only one run in the well, it has
Because the sonic log is strongly affected by gas. you need
saved your company valuable rig time.
to use the equation for sonic porosity gas correction:
Drilling operations, on your company’s lower Wilcox
wildeat well, halted at 10.936 ft after penctration of a dy 0.7
=
bX
sandstone. There was a sudden, large. gas increase in the
The recoverable reserves of gas are calculated from the
drilling mud; gas increased by 3,200 units over background
gas volumetric equation and the following parameters are
on the chromatograph. As a response to this, mud weight
needed to complete the equation: drainage area = 240 acres:
had to be increased from [4.8 Ibs/gal to 15.4 tbs/gal to
reservoir thickness = [5 ft: porosity = 18%: water
contain the gas within the formation. When drilling
saturation = 57%; gas gravity (estimated) = 0.62: recovery
Operations were resuined, gas continued to cut the mud with
factor = 0.6: temperature (estimated) = 279°F; initial
a weight of 15.4 Ibs/gal going into the hole and 15.2 Ibs/gal
bottom hole pressure (IBHP, estimated) = 8,103 PST: Z
coming out of the hole. Also. the mud logger’s
factor = 1.229; geothermal gradient = 0.0255 * formation
chromatograph maintained about 100 units of gas, even
depth: and pressure gradient = 0.74 x formation depth.
when the well was deepened beyond the zone of initial gas
Your company has purchased a 25% working interest
show.
(WI) in the well, which has a net revenue interest (NR of
You select five depths (or points) from 10,930 to 10,970
82.5%. Net revenue interest its the total interest (total
ft within the Wilcox sand interval. Depths are picked on the
interest equals 100%) minus any royalties such as an
basis of an even distribution through the interval being
interest granted to a mineral rights owner.
evaluated. Here. you use a distribution of points every 12 ft
The estimated cost of the well ts 1.8 million dollars. You
from 10,930 to 10,970 ft.
use a product price of $1.90 per mef to find out the
Because the only porosity tool at your disposal is the
projected return your company can expect on its
sonic log, you use one of the following equations to help
investment,
you find porosity. The perferred equation is II because of
You have acquired certain information needed to help
your experience with tts use in the Gulf Coast.
with an evaluation of the logs: R, = 0.022 at Ty F =
Equation Variables --At = ysee/ft and is interval transit
0.62/2-15 (formation factor for Gulf Coast sands): Ryy =
time read froni the sonic log: Aty, = pesec/ft and is interval 0.222 at Ts; Tp = 260°F: At,, = 116; and surface
= 56.7 determined for you. However, this information for the last
transit time of shale read on a sonic log from a clean shale temperature = 80°F.
zone up the borehole from the Wilcox sand; At,,, = wsec/ft The following Eocene Wilcox Sandstone Log Evaluation
and is interval transit time of the matrix for sandstone Table (work Table C) ts designed to assist you with your
(known for Wilcox sands in Gulf Coast); and Aty = yasee/ft work. The first three depths and their deep induction, short
and is interval transit time of freshwater-based muds (see
Chapter I'V). normal, and interval transit time (At) values have been
Sonic Porosity Equations: two depths is left for you to complete.
At Mma x = 100 |
I. sonic porosity (@) =< Ath
Aty — Atna
°
Where:
At; = 189
At, ma

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