American Oil and Gas Reporter - January 2012 - 154

Natural fractures, mostly mineralized,
are apparent in cores and well bore images
ranging from microscopic to as wide as
0.25 inches. These fractures often can
facilitate hydraulic fracture complexity
by dilating during the fracturing process.
This dilation will be a function of their
orientation to the stress field and the variations in horizontal stresses. In addition
to natural fractures, subtle textural features
can induce complexity to the hydraulic
fracture system. Near to the bore hole,
these fractures can be problematic as perforating on a fracture swarm may result
in narrow fractures because of competition
for fluid. This can lead to high injection
pressures and inability to place proppant.
Most importantly, stratigraphic sequence
boundaries, mineralized fractures and other
weak interfaces reorient locally the direction
of fracture propagation, resulting in fracture
branching, fracture stepovers and general
fracture complexity. These features may
promote height growth containment and
higher surface area for reservoir volume,
but also promote the development of obstacles and constrictions for proppant transport. The creation of surface area outside
good-quality rock (i.e. lack of containment)
and the creation of unsupported surface
area (poor proppant transport) are two
fundamental conditions of poor fracture
efficiency and loss of surface area.
Rock composition and texture complicate horizontal completions by introducing
variability. This leads to changes in fracture
propagation and containment that are not
anticipated but can be clearly observed
during microseismic monitoring.

FIGURE 3
Tangential Stress Concentration for Barnett Shale Horizontal Well
2,000

Hoop_Map

6,000

T
S
B
S
T

acoustic velocity and permeability that
arise from their laminated depositional
texture and their final diagenetic texture.
This results in a contrast in elastic properties between directions parallel and
perpendicular to bedding that can be as
high as 400 percent (Figure 2).
Changes in properties related to direction to bedding result primarily from
changes in texture even when composition
may be relatively the same. Anisotropy
in mechanical properties substantially
changes the rock stress-strain relationship,
and affects the development of far-field
stress, near-well bore stress concentrations,
breakdown pressure, hydraulic fracture
width, potential for solids production and
stability of the completion.
As the magnitude of the elastic
anisotropy increases, the difference in
Young's modulus and Poisson's ratio
measured normal and parallel to bedding
also increases. This impacts far-field completion parameters of fracture containment,
fracture complexity, proppant transport
and fracture closure stress, as well as
near-well bore completion parameters of
breakdown pressures, near-well-bore fracture width and proppant entry. Because
of its strong influence in near-well-bore
and far-field completion parameters, adequate characterization of elastic
anisotropy is of fundamental importance

Stresses
Organic-rich mudstones can exhibit
pervasive and strong anisotropy in elasticity, tensile/compressive strength,

FIGURE 2
Ratios of Horizontal and Vertical Young's Modulus (EH/EV)
And Poisson's Ratios (µH/µV)
#

Matrix Composition

EH/EV

µH/µV

Carbonate

1.03

0.97

Calcareous
Mudstone

1.05

0.88

Silty
Mudstone

1.52

1.25

Siliceous
Mudstone

2.06

1.47

Organic/Argillaceous
Mudstone

2.49

1.34

Argillaceous/Siliceous
Mudstone

2.99

1.60

Argillaceous/Calcareous
Mudstone

3.79

1.26

Argillaceous
Shale

4.01

2.07

154 THE AMERICAN OIL & GAS REPORTER

for completion design in tight shales.
In the absence of tectonic deformation,
the two horizontal stresses that develop
in a layered formation are uniform. Structural deformation such as folding leads
to differences in minimum and maximum
horizontal stresses. When considering
stress variations along horizontal well
bore trajectories, these will change with
changes in rock texture and composition,
and when the lateral bore hole intersects
multiple beds (Figure 3). In addition, in
regions subjected to tectonic deformation,
the two horizontal stresses will be different
in proportion to their elastic anisotropic
properties.
Heterogeneity in texture and composition, elastic anisotropy of the rock types,
textural complexity and structural deformation by tectonic strains are the fundamental sources of in situ stress. As rock
types change along the well bore, so do
local stresses. In addition, the presence
of the well bore and its mode of completion
(including open hole, cased or cemented,
uniform or nonuniform cementing) creates
different local stress conditions. This results in varying stress and breakdown
pressure regimes along the horizontal
well and complicates the completion design. Understanding the rock and the geologic conditions of basin development
are fundamental for evaluating near-wellbore, and far-well-bore stress conditions,
and for designing effective completions.
As a well bore is drilled, the stresses
previously carried by the rock are redistributed around the bore hole wall. In a
typical extensional stress state (overburden
stress greater than horizontal stresses),
compressive stresses increase at the sides
of the well bore to compensate for the
missing material, and are maximum at
the well bore sides and minimum at the
top and bottom. For example, in the
Barnett Shale, anisotropy and relatively
low horizontal stresses reduce the compressive stresses to the point that the top
and bottom of the well bore are placed in
tension, often resulting in "drilling induced
fractures" parallel to the well bore-often
evidenced on imaging tools.
The near-well-bore conditions have
important consequences for hydraulic

American Oil and Gas Reporter - January 2012

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