Page 207 - Basic Well Log Analysis for Geologist

Case Study 5 Answer
The behavior of the Combination Neutron-Density log, Sandstone. even after using the shaly sand correction. are
which was run on a sandstone matrix, alerts you to the the result of the very fine grain size of the sand (Fig. 95).
presence of shale in the reservoir. You know that tfa sand The reservoir’s grain size, along with its shale centent. is
contains oil or water or both, the two curves—neutron and the reason for the low relative permeability to gas (Fig. 96)
density-—should have the same porosity values if the sand is and the fairly low reservoir permeabilities (Fig. 97). The
logged ona sandstone matrix and if itis shale-free. moveable hydrocarbon index is less than 0.7 and the
However, if a sand contains gas (as in the case of the moveable oil saturation is high. These parameters cause vou
Pictured Cliffs) and is shale-free. the density log should to be reasonably certain gas will move out of the reservoir.
have higher perosity values than the neutron log. (See High bulk volume water values (0.11 to 0.14) also result
Chapter [Vabout gas effect.) Your examination of the from the very fine-grained shaly sand. But, the lack of
neutron-density log (Fig. 93) reveals neutron porosity (by) data-point scatter from the hyperbolic line (Fig. 08) shows a
is higher than density porosity (bp). The higher neutron reservoir at irreducible water saturation (Sy, ;,.). And, at
porosity is recorded because shale has a high hydrogen ion irreducible water saturation, the reservoir will not produce
concentration. ‘The neutron log responds primarily to this water. despite high water saturation values.
concentration of hydrogen ions. As you write your report on the Pictured Cliffs Sandstone
High values on the gainma ray log (Fig. 93) also alert you in-fill wel], you summarize all of the favorable indicators.
to the presence of shale in the Pictured Clitfs Sandstone. First of all, the zone ts approximately 30 ft thick and has
High gamma ray readings are caused by the greater levels of permeability as shown by the presence of mudcake,
radioactivity found in shale. indicated by the caliper log. Reservoir permeability is also
Beeause shale can create permeability problems. as you indicated by the moveable hydrocarbon parameters (Sy/S,.
analyze the well. vou decide to pay careful attention to the and MOS). Next. good porosities, ranging from [4 to 22%
various ways permeability can be detected or estimated. are determined from the Combination Neutron-Density log.
You note a decrease in hole diameter occurring from The bulk volume water plot is of special significance
1,926 to 1.958 fton the caliper log (Figs. 93 and 94). The because it shows a reservoir at irreducible water saturation,
decrease in hole diameter indicates the presence of Consequently, water-tree production can be expected even
mudcake, which means invasion has taken place. though there are high water saturations. Volumetric
A calculation of water saturation by the standard Archie calculations give a production estimate of approximately
The high water saturations in the Pictured Cliffs
equation gives values ranging from 62 to 83%. These high 0.322 BCE All of the information you have developed.
water saturation values result from the shale content of the along with the shallow depth, low-cost, and very low-risk
reservoir, Which is 6 to 29%. By using the Schlumberger of the well, make a completion recommendation inevitable.
(1975), shaly sand equations to correct for the effect of The Pictured Cliffs was perforated from 1,926 to 1.954
shale, the water saturations values can be lowered to 48 to ft. After an acid treatment and a sand frac, the well initially
67%. Such a substantial adjustment to the water saturation produced 350 mcfgpd with no water on a 3/4” choke.
values illustrates the importance of shaly sand analysis in During the first year of production, the well produced
the evaluation of certain reservoirs. approximately 60 million cubic feet (60,000 incf) of gas.

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