Lyons W.C. (ed.). Standard handbook of petroleum and natural gas engineering.2001- Volume 1
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Geological Engineering
265
Subsurface
Rock
Fracture Pressure (Fracture Pressure Gradient).
The subsurface
rock fracture pressure can be approximated by utilizing the known pore pressure at
the same depth. The relationship between rock fracture pressure pt (psi) and pore
pressure p, (psi) is
[34]
(2-173)
V
PI
=
(O<h
-
PPI(
G)
+
Pp
where
or,,
is overburden stress (psi) and
v
is Poisson’s ratio.
The subsurface rock fracture pressure gradient is
(2-
174)
where
d
is the depth to the subsurface zone
(ft).
locations, the West Texas region and the Gulf Coast region.
Figure
2-58
shows the variation of Poisson’s ratio versus depth for two general
Figure
2-58.
Poisson’s ratio
vs.
depth (from Eaton, “Fracture Gradient Prediction and
Its
Application in Oilfield Operations,” Journal of Petroleum Technology, October
1969).
266
General Engineering and Science
The constant value of 0.25 for Poisson’s ratio versus depth reflects the geology
and the rock mechanics of the mature sedimentary basin in the West Texas region.
Since mature basins are well cemented, the rock columns of West Texas will act as
compressible, brittle, elastic materials.
The Cenozoic portions of the Gulf Coast sedimentary basins are immature;
therefore, little cementing
of
the sediments has taken place. Poisson’s ratio varies
with depth for such sedimentary columns, reflecting the variation of properties
through the column. At great depth (Le., approaching 20,000 ft), Poisson’s ratio
approaches that of incompressible, plastic materials (i.e., 0.5) [35].
Figure 2-59 gives typical total overburden stress gradients versus depths for several
regions in North America
[36].
The rock fracture pressure gradient at depth can be approximated by using Equation
2-174 and the variable Poisson’s ratios versus depth data (Figure 2-58) and the variable
total overburden stress gradients versus depth data (Figure 2-59).
Example
2-26
In Figure 2-57 the pore pressure gradient has been given as a function of depth for
a typical Gulf Coast well. Determine the approximate fracture pressure gradient for
a depth of
10,000
ft. From Figure 2-57, the pore pressure gradient at
10,000
ft is
pp
=
0.66
psi/ft
d
From Figure 2-58, Poisson’s ratio at 10,000 ft is (i.e., Gulf Coast curve)
v
=
0.45
From Figure 2-59, the total overburden stress gradient is (Le., Gulf Coast curve)
--
Oob
-
0.95 psi/ft
d
Substituting the foregoing values into Equation 2-174 yields
=
(0.95-0.66)(0.45)+0.66 1-0.45
=
0.90 psi/ft
d
This value of
0.90
psi/ft falls on the dashed line
of
Figure 2-57. The entire
dashed line (fracture pressure gradient) in Figure 2-57 has been determined by
using Equation 2-174.
In general, Equation 2-174 can be used to approximate fracture pressure gradients.
To obtain an adequate approximation for fracture pressure gradients, the pore pressure
gradient must be determined from well log data. Also, the overburden stress gradient
and Poisson’s ratio versus depth must be known for the region.
There is a field operation method by which the fracture pressure gradient can be
experimentally verified. Such tests are known as leak-off tests. The leak-off test will
be discussed in Chapter
4.
Basic
Engineering
Soil
Properties
The surface rock formations of the earth are continually exposed to the weathering
process of the atmosphere that surrounds the earth. The weathering process through
Geological Engineering
267
0.7
0.75
0.8
0.85
0.9 0.95
1.0
1.05
OVERBURDEN GRADIENT, G,
psi/ft.
Figure
2-59.
Total overburden stress gradient
vs.
depth (from
Engineering
of
Modem
Dri//ing,
Energy Publication Division
of
Harcourt
Brace
Jovanovich, New
Yo&,
1982,
p.
82).
268
General Engineering and Science
time can change the rock exposed at the surface. Such changes are both mechanical
and chemical. The altered surface rock is often used by lifeforms, particularly by
plant life that directly draws nourishment from minerals at the surface.
As
the
weathering process proceeds at the surface of the earth, rock at the surface
disintegrates into small separate particles and is carried off and deposited at various
locations around the original rock. Some particles are carried great distances
to
the
sea to become source sediments for new rock formations. Some particles move only
a few feet or a mile or two to be deposited with fragments from other nearby rock
locations. These weathered particles, once deposited on the surface of the earth in
land locations, are often referred to as soil
[37].
To a farmer, soil is the substance that supports plant life.
To
a geologist soil is an
ambiguous term that refers
to
the material that supports life plus the loose rock from
which it was derived. To the engineer, soil has a broader meaning.
Soil, from the engineering point
of
view, is defined as any unconsolidated material
composed of discrete solid particles that has either liquids or gases in the voids between
the individual particles.
In general, soil overlays rock formations, and the soil is related to the rock since
the rock was its source. Where the soil ends (in depth) and rock begins is not a well-
defined interface. Basically, the depth
to
which soil is found is that depth where
excavation by land methods can be employed. The area where the removal of material
requires drilling, wedging, and blasting is believed
to
be the beginning of rock (in the
engineering sense). The engineering properties of soil are of importance
to
petroleum
engineering because it is soil that the drilling engineer first encounters as drilling is
initiated. But, more important,
it
is soil that must support the loads
of
the drilling rig
through an appropriately designed foundation. Further, the production engineer
must support the well head surface equipment on soil through an appropriately
designed foundation.
Soil Characteristics and Classification
The engineer visualizes a soil mass as an ideal, real, physical body incapable of
resisting tensile stresses.
The ideal soil is defined as a loose, granular medium that is devoid of cohesion
but possesses internal friction. In contrast, an ideal cohesive medium is one that
is devoid of internal friction. Real soils generally fall between the foregoing two
limiting definitions.
Soils can consist of rock, rock particles, mineral materials derived from rock
formations, and/or organic matter.
Bedrock is composed of competent, hard, rock formations that underlie soils.
Bedrock is the foundation engineer’s description of transition from soils to rock
at depth. Such rock can be igneous, sedimentary, or metamorphic. Bedrock is
very desirable for foundation placement.
Weathered rock is bedrock that is deteriorating due to the weathering process.
Usually this is confined
to
the upper layers of the bedrock.
Boulders are rock fragments over
10
in. in diameter found in soils.
Cobbles are rock fragments from
2-4
in. in diameter found in soils.
Pebbles are rock fragments from about
4
mm
to
2
in. in diameter found in soils.
Gravel denotes unconsolidated rock fragments from about
2
mm
to
6
in. in size.
Sand consists of rock particles from
0.05-2
mm in size.
Silt and clay are fine-grained soils in which individual particle size cannot be
readily distinguished with the unaided eye. Some classification systems distin-
guish these particles by size, other systems use plasticity to classify these particles.
Geological Engineering
269
Plasticity is defined as the ability of such particle groups to deform rapidly without
cracking or crumbling. It also refers to the ability of such groups to change
volume with relatively small rebound when the deforming force is removed.
Silt, in one particle classification system, consists of rock particles from 0.005
to
0.05
mm in size.
Clay, in one particle classification system, consists of inorganic particles less
than 0.005 mm in size. In another system, clay is a fine-grained inorganic soil
that can be made plastic by adjusting the water content. When dried, clay
exhibits considerable strength (i.e., clay loses its plasticity when dried and
its
strength when wetted). Also, it
will
shrink when dried and expand when
moisture is added.
Figure 2-60 shows a classification system developed by the Lower Mississippi Valley
Division,
U.S.
Corps of Engineers. Percentages are based on dry weight.
A
mixture
with 50%
or
more clay
is
classified as clay; with
80%
or
more silt, as silt; and with
80%
or more sand, as sand.
A
mixture with
40%
clay and
40%
sand
is
a sandy clay.
A
mixture with 25% clay and 65% silt is
a
clay-silt (see intersection of dashed lines in
Figure 2-60).
100
0
10
20
30
40
50 60
70
80
90
100"
SILT,
PERCENT
Figure
2-60.
Classification chart for mixed
soils
(from
Lower Mississippi Valley
Division,
U.S.
Corps
of
Engineers).
270
General Engineering and Science
Index
Properties
of
Soils
Easily observed physical properties of soils often are useful indexes of behavior.
These index properties include texture and appearance, specific weight, moisture
content, consistency, permeability, compressibility, and shearing strength
[37,38].
Soil texture, or appearance, depends on particle size, shape, and gradation.
Therefore, using the classification in Figure 2-60 the soil texture can be specified
as sandy clay or clay-sand.
Soil specific weight is the measure of the concentration of packing
of
particles in a
soil mass. It is also an index of compressibility. Less dense, or loosely packed, soils
are much more compressible under loads. Soil specific weight may be expressed
numerically as soil ratio and porosity (porosity for soils being basically the same
definition as that for rocks discussed earlier in this section). Soil porosity e is
(2- 175)
e=-
'h
-'s
Vb
where
V,
is bulk volume of undisturbed soil
(ft'),
and
V,
is volume
of
solids in
soil
(ft).
The specific weight (unit weight) (lb/ft') of undisturbed soil is
W
y=--1
wh
(2-176)
where Ws is the weight
of
the
soil
solids relative to the undistributed soil
bulk
volume (lb).
Relative density
D,
(a percent) is a measure of the compactness of a soil with void
ratio e when the maximum void ratio
is
emax and the minimum emin. Relative density is
(2-1
77)
Percentage compaction usually is used to measure soil density in a fill situation.
Generally, the maximum Proctor specific weight (dry, lb/ft')), determined by a standard
laboratory test, is set up as the standard for the soil. Normally, 90-100% compaction
is
specified.
Moisture content, or water content, is an important influence on soil behavior.
Water content w, dry-weight basis percent, is
(2-1
78)
where Ww
is
weight of water in soil (Ib) and W, is weight
of
solids in soil (Ib).
Total net weight of soils, W (lb), is
we=ww+w,
(2-179)
Geological Engineering
271
Degree of saturation,
S
(percent) is
where
V
is volume of water in soil (fti).
Saturation, porosity, and moisture content are related by
S,.
=
WG
(2-1
80)
(2-181)
where
G
is the specific gravity
of
solids in the soil mass.
Consistency describes the condition of fine-grained soils: soft, firm, or hard.
Shearing strength and bearing capacity vary significantly with consistency. In
consistency there are four states: liquid, plastic, semisolid, and solid.
Permeability is the ability
of
a soil to conduct or discharge water under a pressure,
or hydraulic gradient (permeability for soils being basically the same definition
as that for rocks discussed earlier in this section). For soils the definition of
coefficient of permeability is slightly different than that discussed earlier for
petroleum reservoir rocks. Since civil engineers and hydrologists are always
dealing with water, the coefficient of permeability, or more precisely, the
hydraulic conductivity,
k’
(cm/s)* is
(2-1
82)
where
k
is permeability
as
defined in Equation
2-162,
y,
is the specific weight of
water, and
pw
is
the viscosity of water.
Therefore, substituting Equation
2-162
into Equation
2-182
yields
where i (cm/crn) is the hydraulic gradient and is expressed by
(2-183)
(2-
184)
and
A
(crn’) is the total cross-sectional area of soil through which flow occurs.
Table
2-29
gives some typical values for hydraulic conductivity and drainage
characteristics for various soil types.
Soil compressibility is important for foundation engineering because it indicates
settlement. Settlement or deformation of the soil under the foundation occurs
because
of
change of position
of
particles in a soil mass.
Shearing strength is the shear stress in a soil mass at failure
of
the soil mass
(usually cracking), or when continuous displacement can occur at constant stress.
*Note:
k‘.
hydraulic conductivity, is
used
for
water
flow
in
rocks
as
well
as
in soils.
272
General Engineering and Science
Table
2-29
Hydraulic Conductivity and Drainage Characteristics
of
Soils
[37]
Approximate
Coefficient
of
Permeability
k,
Drainage
Soil
Type cm per sec Characteristic
~~~
Clean gravel
Clean coarse sand
Clean medium sand
Clean fine sand
Silty sand and gravel
Silty sand
Sandy clay
Silty clay
Clay
Colloidal Clay
~
5-1
0
0.4-3
0.05-0.15
0.004-0.02
1
0-5-0.0
1
1
0-5-1
04
i
04-1
0-5
10-7
i
0-9
10"
Good
Good
Good
Good
Poor
to
good
Poor
Poor
Poor
Poor
Poor
Shearing strength of a soil is usually important in determining bearing capacity
of the soil. Shearing stress,
T
(lb/in.2), of a soil is expressed as
T
=
c
+
on
tan
8
(2-185)
where
C
is cohesion of the soil (lb/in.*),
on
is the normal, effective stress
perpendicular to shear surface (lb/in.2), and
8
is the angle of internal friction
of
soil.
The shear strength of coarse particle soils like gravel and sand depend on the
interlocking of their particles and, thereby, on intergranular friction. The shear
strength of pure clay soils depends basically on cohesion. Figure
2-61
gives a
graphical representation of the two limits of soil types, i.e., coarse particle soils
are denoted as
e
soils, pure clay soils are denoted as c soils. Most real soils are a
combination, i.e., (e-c) soils. Some approximate friction angles for typical
cohesionless soil types are as follows
[37]:
n
rA
c
L
0,
6
r-c
=const
Y
c
-soil
C
1
wn
Effective
mmol
stress
0
Figure
2-61.
Graphic shear strength
of
soils.
Geological Engineering
273
Silt, or uniform fine to medium sand 26-30 0.5-0.6
Well-graded sand
30-34
0.6-0.7
Sand and gravel 32-36 0.6-0.7
Site Investigations and Laboratory Tests
Prior to the construction
of
a foundation, field investigation should be carried out
to determine surface and subsurface conditions at the site.
Site Investigations.
Numerous techniques are used for site investigations. The
techniques vary in cost from relatively low-cost visual investigations to costly subsurface
explorations and laboratory tests [39,40].
Visual inspection is an essential primary step. Such inspections should provide
data on surface soils, surface waters, and slopes.
Probing, driving a rod or pipe into the soil and measuring the penetration
resistance, obtains initial subsurface information. This
is
a low-cost method, but
in general it is likely to supply inadequate information about subsurface
conditions, especially on the depth and nature of bedrock.
Augers provide subsurface data by bringing up material for detailed examination.
Augers disturb the soil; therefore, little or no information can be obtained on
the character of the soil in its natural undisturbed state.
Test pits permit visual examination of the soil in place. Such pits also allow
manual sampling of “undisturbed” soil samples. These samples can be taken
from the side walls of the pit.
“Dry” spoon sampling is a technique that is often used in conjunction with auger
drilling. At certain depths in the augered borehole a spoon
is
driven into the
undisturbed bottom of the borehole. The spoon sampler
is
a specified size
(usually a 2-in. OD). The number of blows per foot to the spoon samples
frequently are recorded, indicating the resistance of the soil. The spoon sampler
is
driven into the bottom of the hole with a free-falling weight.
A
140-lb weight
falling
30
in. onto the 2 in. O.D. spoon sampler is the standard method of driving
the sampler into the borehole bottom. Table 2-30 shows a system of correlation
of this technique of sampling. Figure 2-62 shows a typical subsurface soils log
that describes the soils encountered at depth and number of blows per
6
in.
(instead of per foot)
on
the spoon sampler.
Laboratory Tests.
In addition to the site investigations just described and the
on-site tests which can be carried out through test pits and augering, often specific
laboratory tests are required to identify soils and determine their properties. Such labora-
tory tests are conducted on the soil samples that are recovered from various subsurface
depths. These laboratory tests are used when there are questions as to the structural
supporting capabilities of the soils at a particular site. Numerous types of laboratory
tests are available that can aid the engineer
in
designing adequate foundation support
for heavy or dynamic loads.
In
general, however, few tests are necessary for most
foundation designs
[37-401.
Mechanical analyses determine the particle-size distribution in a soil sample.
The distribution of coarse particles is determined by sieving, and particles finer
than a 200 or 270-mesh sieve and found by sedimentation.
274
General Engineering and Science
Table
2-30
Correlation
of
Standard-boring-spoon Penetration with Soil
Consistency and Strength
(2
in
OD
Spoon,
140
Ib)
Unconfined
Compressive
Soil
Number of
Blows
Strength,
Consistency
per Ft on Spoon Tonsm2
Sand:
Loose
Medium
compact
Compact
Very
compact
Very
soft
soft
Stiff
Hard
Clay:
15
16-30
30-50
Over
50
3
or
less
4-1
2
12-35
Over
35
0.3
or
less
0.3-1
.o
1
.o-4
4
or
more
GROUNDWATER
LEVEL AT DATE
OF
BORING
SPOON' BOREHOLE
CLAY, SAND, GRAVEL
FINE RED
SAND
AND SILT
MEDIUM BROWN SAND
EL
252.0
3%
RECOVERY EL
250.0
LIMESTONE
30%
RECOVERY
EL.245.0
EL
240.0
-ROCK
BO%
RECOVERY
'2-IN
SPLIT-BARREL
SPOON
140-LB HAMMER
30-IN
DROP
Figure
2-62.
Typical
soils
boring
log.