Page 156 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION CASE STUDIES

(b) = porosity YT; = formation temperature in °F
S, = hydrocarbon saturation (1.0 — S,,) Ty = Ty + 2 & (depth 100)
RF = recovery factor T, = = mean surface temperature
Pf, = surface pressure depth = formation depth in feet
Pf, = reservoir pressure
As you begin trying out the various formulas in the case
_ UA3t depth studies, keep in mind that many values such as water
Pf, 15 saturation (S,) and porosity (¢)) are given in percent. So,
even though this will be immediately obvious to you.
*0.43 is auniversal average pressure gradient
remember to change percentages to decimals before
which may need to be adjusted for local
entering the numbers into your calculator.
conditions.
HL. G, = 43,5600 x DA x hx @ x S, x By X RF
Where:
G, = volumetric recoverable gas reserves in standard Case Study |

cubic feet (SCF)
DA = drainage area in acres Pennsylvanian Atoka Sandstone
h = reservoir thickness in feet Permian Basin
ch = porosity Your company has just finished drilling a 15,900 ft
Sy, = hydrocarbon saturation (1.0 — S,)
RF = recovery factor wildcat well in the Permian basin. The primary target,
Morrowan sandstones, are not sufficiently developed for
B,, = gas volume factor in SCF/cu ft
commercial production and you are facing a decision of
a a Tey PL declaring the well dry and abandoned (D&A) unless you
e P. ZX T, can find another zone which will produce hydrocarbons.
Sample cuttings from the well indicate that at 14,600 ft
=| 9-7 +60) PL | the Atoka Sandstone is predominantly loose, subrounded,.
+
15 “2x (459.7 Te) coarse to very coarse. quartz sandstone with minor, tan,
arkosic sandstone—medium-grained to coarse-grained,
unsorted and friable. This information indicates that the
Where:
Atoka Sandstone may have permeability because of its
T.. = temperature at standard conditions
larger pore space developed from the coarse grain size.
P.. = surface pressure at standard conditions
Better permeability is also indicated by poor cementation
P= reservoir pressure
because the sands are Iriable.
Z = gas compressability factor
Other information from the Atoka which you consider
T; = formation temperature (°F)
favorable ts the emission of a few gas bubbles from sample
cuttings and also gas on the mud fogger’s chromatograph,
FORMATION PRESSURE ESTIMATION
an instrument designed to measure the amount and type of
1. Static mud column pressure = depth X mud weight x gas in drilling muds. Total gas background on the
O52 chromatograph inereased from: 10 units to 40 units of gas
2. Rule of thumb for static bottom hole pressure: with a trace of C-2 and C-3 hydrocarbons during drilling of
the sandstone 's bottom four feet.
Py. = Pun + 0.25 X (Pyp/}00) x (depth/ 100)
Even though you are optimistic about the productive
Where: potential of the Atoka Sandstone because of the nature of
P,. = static bottom hole pressure the sample cuttings and the gas shows, you are concemed
Py» = well head pressure about a reverse drilling break which occurred as the Atoka
penetration rate through the Atoka was [5.5 min/ft and
was penetrated. The drill penetration rate was 8 to 10 min/ft
GEOTHERMAL GRADIENT ESTIMATION before, and again after, the Atoka Sandstone. However, the

Ty
=
(Tp & 100. decreased to 20 min/ft through the bottom 10 ft. The slower
~ depth drilling times may mean that the sandstone doesnt have the
Where: porosity and permeability suggested by sample
g = temperature gradient in °F/100 ft examination.

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