Page 202 - Basic Well Log Analysis for Geologist

LOG INTERPRETATION CASE STUDIES

Case Study 5

Cretaceous Pictured Cliffs Sandstone
San Juan Basin
A large block of acreage in the San Juan basin, much of it
Oncor = on ~ [(bnelay/O.45) x 0.30 X Von!
with gas production, has been purchased by the company
peor = bp ~~ [(bnetay/0.45) x 0.13 x Vonl
you work for. The company is pursuing an active
development drilling program of in-filling with new
= LbNeore)® + Lbpeore
obn-p = s
Cretaceous Pictured Cliffs Sandstone wells on 80 acre units,
rather than on the 160 acre units. previously used.
— Von J Gey ee Pn.
You are presented with a log from a new in-fill well and
Ron i Ron 9. 2x Ry x (1.0 — Vin) X R,
are asked to evaluate it. Depth of the Pictured Cliffs
Sandstone in the well is from 1,920 to 1,964 ft. The log oe OR.
package includes: an induction log with a Spherically 0.4x Ry, « (1.0 — Vy)
Focused Log (SFL*) and an SP log (Fig. 92), and a
Where (Formula Variables):
Combination Neutron-Density log (recorded in sandstone
porosity units?) with a gamma ray log (Fig. 93). GRigg = Gamma ray reading trom various depths in
You already know from previous experience that shale in : Pictured Cliffs Sandstone (for depths picked.
a zone can adversely affect logging measurements. Water see Log Evaluation Table: work Table FE).
saturations calculated by the Archie formula will have GRyax = 134 API gamma ray units; units are read on
values which are too high (i.e. pessimistic values) if shale is gamma ray log at a depth of 1,838 ft (shale).
present (see Chapter V1). Furthermore, shale in the GRyin = 64 API gamma ray units; units are read on
reservoir will cause permeability problems. gamma ray log ata depth of 1,921 ft (clean
A careful examination of the neutron porosity (dy). sand).
density porosity (dp). and gamma ray log convinces you Ry. = 4ohm-meters; resistivity of adjacent shale ata
that you are dealing with a shaly Cretaceous Pictured Cliffs depth of 1.915 ft.
sand (Fig. 93). A shaly sand analysis will be necessary. But, Onelay = 0.53: neutron porosity of adjacent shale ata
before following this investigative path, you decide to "depth of 1.866 ft. :
cheek the R, value given for the area against a log Ven = Volume of shale
calculated R,,. (See Chapter II, Table 3. for R,, formulas. ON com = Neutron porosity corrected for shale
Also see charts with Figures 12, 13, and 14). bp com = density porosity corrected for shale
R,, Calculation—-Determine R,, using the following ob = porosity corrected for shale
information: depth = |.936; BHT = 89°F at 2.145 ft:
Other information you use to coniplete your log work is:
surtace temperature = 65°F; Rap = 2.26 at 65°F: SSP =
—S7Tmv; Tp = 87°F: Ray at 75° = 1.984, K = 71.526; Rye = 1.734 at T;
RintefRue = 6-255: Ring = 1.687; Ry. = 0.269. Therefore: Tr = 87°F
R,, at 75°F = 0.3026 Because of the in-fill drilling program, your company Is
Ry, at Ty = 0.264 particularly interested in having you calculate recoverable
reserves based on 80 acre units. Reservoir depletion will not
or
be a problem. since Pictured Cliffs Sandstone wells
Vsh =p FP: = 0.51; temperature (estimated) = 99°F; initial bottom hole
R, at Ty == 0.26 normally don't drain 160 acres. The volumetric recoverable
To do a shaly sand analysis you use: a formula for gas reserves are estimated from the following parameters:
drainage area (DA) = 80 acres; reservoir thickness (h) = 30
calculating volume of shale. formulas for correcting both ft: effective porosity (b,.) = 17%: water saturation (S,,) =
the neutron and density porosity for volume of shale, a 53%; recovery factor (RF) = 0.75; gas gravity (estimated)
formula for calculating neutron-density porosity, and
finally, a formula which corrects water saturation for the pressure (TBHP estimated) = 770 PSI: Z factor = 0.928:
effect of shale. The procedure is: geothermal gradient = 0.051 * depth; and pressure
Shaly Sand Analysis—Shaly Sand Formulas; gradient = 0.395 x depth.
Schlumberger (1975): Table (work Table E) assists you with your evaluation.
A Cretaceous Pictured Cliffs Sandstone Log Evaluation
GRiog — GRnin
GRmax ~ GRrin

+See Chapter IV for a discussion of different matrix units used on the
Combination Neutron-Density Log.
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