Page 155 - Basic Well Log Analysis for Geologist

CHAPTER VIII

ee ee eee eee eee een nne e

LOG INTERPRETATION CASE STUDIES

Introduction
A major hurdle for geologists ts learning how the theory use petrophysical log interpretation as a way to tip the risk
and the many formulas of log interpretation are translated scale toward economically successful wells.
into practice. The learning process is slow, and only takes The list of formulas which follows is an important part of
place after a good deal of dreary effort is supported by petroleum exploration. A derivation of variables is included
actual experience with logs. There are absolutely no in this listing, although in most instances, the reader will
short-cuts. Memorizing log patterns and curve values just not be asked to solve them. Instead, the values will be given
doesn ‘t work, and can, in fact, prove disastrous. However, and will need to be “plugged” into their position in the
to assist the process of changing what's done into how it’s formulas.
done, case studies can be an invaluable asset. The ones
presented here cover a variety of geographical areas, Formulas Used for Calculating
geologic ages, lithologies and different log packages. These Volumetric Producible Hydrocarbon Reserves
are not necessarily a classic representation of any of the
variations. The reader will need to actively engage his or OIL
her intelleet in finding appropriate answers for each
x DA
X
hx
N= 2758. 6X S,< RE
problem. and will also need to apply matertal already
a BOI
learned from a thorough study of the preceding text.
Although all the case studies are based on actual field Where:
examples, in some studies the amount of log data analyzed N, = volumetric recoverable oil reserves in stock tank
would be superfluous in real-life. A pipe-setting decision barrels (STB)
doesn “t always require a full-blown log analysis. The DA = drainage area in acres
unusual anount of detall ty offered here as a learning h = reservoir thickness in feet
experience, Nevertheless, careful study of log can always cd = porosity
a
yield information which will further the development of an Sh = hydrocarbon saturation (1.0 — S,,)
CXPCTLise. RF = recovery factor
These case studies are offered as a progressive series of BOI= oil volume factor or reservoir barrels per stock
problems. In the first example the reader is asked to do very tank barrel
little. but in the final example the reader should be prepared
GOR \
to proceed most of the way alone. An effort is made to BOI = 1.05 + 0.5 x ( 100 /
define variables and terminology appearing in the case
studies, even though they may already have been defined Where:
earlicr in the text.
gas in cubic feet
In cach of the case studies the moveable hydrocarbon GOR (gas oil ratio) =
oil in barrels
index (S,,/S,,) is calculated. Sometimes, the water
saturations of the flushed zone (S,,) may exceed 100%.

Whenever this happens, a value of 100% is then used to GAS
calculate the Moveable hydrocarbon index. Most geologists use formula I; but an alternative.
No matter how significant log interpretation is to a Formula IJ, is offered.
geologist as an exploration tool, it must also be viewed in
the larger context of exploration decision making. That's I. G, = 43.560 X DA X hx b x S, (FF) “RE
why each case study includes a volumetric calculation of
reserves. And, in several studies, the reader is asked to Where:
determine a rate of return on investment. As everyone G, = volumetric recoverable gas reserves in standard
inumately connected with petroleum exploration knows, cubie feet (SCF)
wells can simultaneously be geologic successes and DA = drainage area in acres
economic failures. A successful exploration program will h = reservoir thickness in feet