INTRODUCTION Wellbore erosion. Physical impact damage. Mud weights and

INTRODUCTION

Wellbore instability is one of the major challenge faced by
the drilling and mud engineers. A radical change in both the mechanical stress
and the chemical interactions results in the wellbore or hole instability which
can often take the form of formation caving and sloughing resulting in hole
enlargement, fill and bridges. This can cause some serious problems such as
stuck pipe, sidetracks, logging and interpretation challenges, poor cement jobs
and loss of circulation.

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The main factors that contribute to wellbore instability
are: 

• Mechanical stress.

Tension failure which includes fracturing and lost
circulation.
Compression failure results in spalling and collapse or plastic flow.
Abrasion and impact.

• Chemical interactions with the drilling fluid.

Shale hydration, swelling, and dispersion.
Dissolution of soluble formations.

• Physical interactions with the drilling fluid.
Erosion.
Wetting along previously existing fractures (brittle shale).
Fluid invasion due to which pressure transmits.

For a mud engineer, the fundamental working knowledge in
geo-mechanics as well as fluid-rock interaction chemistry is important.
Wellbore instability can be caused by several wellbore conditions. Therefore,
it is necessary to diagnose the condition causing failure and then an
appropriate remedy can be applied. These include mechanical conditions as well
as chemical conditions. Some of them are listed below:

Hole cleaning problems.
Wellbore erosion.
Physical impact damage.
Mud weights and pore pressures.
Surge and swab pressures.
Wellbore stresses.
Reactivity of the failing formation.
Chemical compatibility with the mud system.
Possible wellbore dissolution.

As we know that oil-based or synthetic mud systems provides
more wellbore stability than water-based drilling fluids but on another hand,
there are many limitations that can cause problems in using them. These
limitations include:

Individual formations evaluation must be required.Health,
safety, and environmental concerns.
More cost and unavailability of materials.
Loss of circulation.

SHALE DEPOSITION AND SEDIMENTARY ROCK

Sedimentary rocks are major of two types:

Non-Clastic Sedimentary Rock: The rocks which are the
results of precipitates either chemical or organic are known to be non-clastic
sedimentary rocks. They haven’t traveled distance from the place of weathering
to the depositional basin.

Clastic Sedimentary Rocks:The rocks which traveled
sufficient distance from the place of erosion to the place of deposition are
commonly termed as clastic sedimentary rocks. The transportation can be caused
by water, wind or due to gravity. It must have to be gone through the phases of
sedimentation and diagenesis.

Sedimentation:
Flowing water has the tendency to carry particles along with itself resulting
in the erosion of larger particles and sediments into smaller ones. These
particles settle down according to their sizes ultimately in the form of
layers. This process is said to be as sedimentation. Sedimentation process
determines the grain size of the sediment as well as certain features that may
be present in a sediment, such as bedding planes.

Diagenesis
Sediments, after being deposited pass through many processes including
compaction and consolidation, minerals dissolution and precipitation and
changes in their composition. This phase is known to be diagenesis process. The
sediments are compacted and the remaining water due to carried along through
the river and streams begin to squeeze out. Due to diagenesis, the sand, silt
and clay minerals are bind together to form sandstone, siltstone, and shale
rock respectively. As time passes, more and more layers are deposited over the
lower ones, increasing their temperature and pressure and hence changes in the
sediments.

Shale Deposition
Shale is the clastic sedimentary rock composed primarily of particles that
are in the clay-size class (on average, smaller than 4 microns). The term
“clay” has two definitions. One definition of clay is a size class of
sedimentary particles. The other definition refers to a class of minerals known
as clay minerals. Clay or shale formations were accumulated in marine
depositional environments. As being clastic in nature, clay minerals traveled
distance from the place of erosion to the marine basins. The difference in different
types of shale depends upon the variability in these depositional basins. For
example, mild climate lands contain more smectite minerals than kaolinite clays
while tropical climate lands contain more kaolinite than smectite clay
minerals.

Clay accumulation as sediment depends upon the flow velocity
of the water suspending the minerals. In quiet water, clay-sized particles
settle down and deposit. These environments occur in offshore beneath the waves
and in bays or lagoons. Lakes and river floodplains may also be included in
clay depositional environments.

Features of Marine Sedimentation

FLOCCULATION
Flocculation is when fine clays aggregated and create into a floc (larger clays
particles) that can be deposited more easily than dispersed clays. It occurs
when fine clays are traveled from freshwater environment to saline water
environment. The flocculated clays often deposited along with a quantity of
quartz and feldspar due to which marine deposited clays contain the significant
amount of silt-sized quartz or feldspar grains.

BIOTURBATION
Bioturbation is the disturbance of the sediment by organisms that live in
the sediment. Organisms including mollusks and worms obtain nutrients by
digesting the sediment. Due to this, the actual structure of the sediment
is changed and in some cases, pelletized which increases their permeability.
Shale which contains high amount of organic matter known as carbonaceous shale
has not been bioturbated and hence deposited as laminated shale.

The compaction of clay minerals is one the main factor for
the generation of geo pressures. With the increase in depth, the temperature
and pressure increases due to which the changes in minerals occur. The clay
minerals become less permeable and start to trap pressure. There are two types
of changes in the minerals of the clay or shale sediments diagenetically which
includes:

1)     
The conversion of smectite clay minerals into a
mixed layer illite-smectite

2)     
The precipitation of mineral cement.

The transformation of smectite into illite-smectite clay
results in the clay minerals in the clay sediment or shale becoming less
chemically active. When the temperature increases above 200°F, the smectite
clay becomes illite, therefore, more deeper shale is less chemically reactive
than younger and shallow shale. The rigid and brittle nature of shale comes due
to the mineral cement precipitation. Silica, which is a by-product of the
conversion of smectite into illite- smectite hence, silica cementation is more
dominant than carbonate cementation.

There are many other shale-like formations that come outside
the marine and non-marine shale. One of the types is volcanic tuff.

VOLCANIC TUFF
Tuff is the accumulation of a volcanic ash eruption. The volcanic ash, when
it falls to the surface of the ocean or sedimentary basin, is primarily
composed of silicate glass. This volcanic glass is chemically unstable and it
crystallizes to form clay materials. The bed of altered volcanic ash is
sometimes called a bentonite bed. Wyoming bentonite is mined from a deposit of
altered volcanic ash.

IDENTIFYING SHALE Gamma-ray logs are commonly used to
identify shale formations. The tool run by logging measure the natural gamma
rays emitted by the formations. These gamma rays are the results due to the
disintegration of potassium, thorium, and uranium. Potassium is commonly
present in shale. When a spontaneous potential log is run, the deflection to
the right indicates the presence of shale while to the left denotes sandstone.
In oil and gas basins, 50 to 75% of rock drilled is shale.

 

 

Different rocks have different properties and drilling
problems associated with them. Some of them are listed in the chart below:

 

CLAY CHEMISTRY

As referred to minerals, clays are crystalline materials
with a layered structure of silica and alumina minerals. Smectite, illite,
chlorite, and kaolinite are the most occurring minerals found in shale rock.
Kaolinite, Illite, and chlorite can adsorb water as well as cations on their
crystalline external surfaces. On the contrary, smectite has greater ability by
absorbing on its surface as well as within its layers. The phenomena associated
with clay absorbing water, exchanging cations and the specific surface area are
called colligative properties of clay which determines the how much reactive
the shale is. This reactivity of clays can easily be evaluated by measuring
Cation Exchange Capacity (CEC). The range of CEC for pure clay mineral
materials is:

 

Clay

CEC (meq/100g)

Smectite

80 – 150

Illite

10 – 40

Chlorite

10 – 40

Kaolinite

3 – 10

 

Smectite It consists of minerals having sandwich-type
structure and containing water between the aluminosilicate layers. Smectite is
composed of montmorillonite, hectorite, saponite, nontronite and many other
specific minerals. They are very sensitive to water and can exchange ions
between the layers.

Illite
This is a particular clay mineral with an aluminosilicate backbone structure.
There is a layer of potassium ions but no water between the layers. The
potassium ions between the layers are fixed while on the external surfaces they
are exchangeable.

Chlorite
The chlorite minerals structure consists of an alumina layer sandwiched between
two layers of silica and a layer of magnesium or iron oxide and free from interlayer
water. Typically, chlorite mineral clays are unreactive but some of them can
hydrate and slough.

Kaolinite
It is the most less reactive mineral. The structure consists of alternating
layers of silica and alumina. It has much larger crystallite size than those of
smectite or illite, less specific surface area, absorbing water and exchanging
cations ability. They can be dispersed in water-based drilling fluids.

 

THE EARTH’S STRESS

Wellbore instability is much dependent on the earth’s
stresses. Overburden pressure, pore pressure, and tectonic forces are the main
factors that influence on the instability of the hole drilled.

OVERBURDEN PRESSURE
It is the total overlying weight of all the formations plus fluids on a subject
formation. The pressure created by the overburden is called the geostatic,
lithostatic or total overburden pressure. It can be calculated by the following
formula:
                                                                
PO = ?B x TVD
Where:
              ?B = Bulk
density of the sediments
              TVD = Total Vertical Depth
              PO = total
overburden pressure (formation + fluids)

Overburden pressure can be calculated in English units by
the following equation:

                                                      
PO (psi) = 0.052 x ?B (lb/gal) x TVD (ft)

Pressure and depth are related to each other by a term known
as “gradient” which is the pressure divided by depth. Bulk density changes with
depth and location. The overburden pressure gradient (POG) can be
calculated by:

                                                     
POG (psi/ft) = 0.052 x ?B (lb/gal)

 

PORE PRESSURE & INTERGRANULAR PRESSURE
Pore pressure and intergranular pressure are the two ways in which total
overburden pressure is supported by a rock.
Intergranular Pressure (PI)The pressure
exerted by the matrix. It is transmitted through the grain to grain
contact.
Pore Pressure (PP)It is the pressure exerted
by the formation fluids (water, oil, and gas) present inside the rock
pores. It must be balanced with mud weight. The pore pressure gradients
changes with the salinity of the water. Normally it is 0.465 psi/ft. The
normal pore pressure can be calculated by:
PP (psi) = 0.052
x pore fluid density (lb/gal) x TVD (ft)
 

Abnormal Pressure
When the pore pressure is greater than the calculated hydrostatic pressure, it
is called abnormal pressure. Abnormal pressure conditions are caused when the
formation fluids are trapped by some seal rock to penetrate further as
overburden pressure increases. The seal may consist of pure shale, salt,
dolomite or any other impermeable formations.

 

ORIENTATION OF STRESSES
A vertical stress is created due to the overburden resulting in outward
horizontal stress characterized by rock’s mechanical properties. These
stress are divided into 3 principal stresses perpendicular to each other
(SEE fIGURE 2) which are as follow:
(1) maximum principal stress (?MAX)
(2) intermediate principal stress (?INT)
(3) minimum principal stress (?MIN)

Basically, the minimum principal stress is equal to the
fracture gradient. In a non-tectonic stressed region, the maximum stress acts
vertically while intermediate and minimum principal stresses act horizontally
having the same magnitude.

In deviated wells, the wellbore becomes less stable due
to these stresses and hence more mud weight is required. In these wells,
the principal stresses are divided into different orientations of being
radial (?R), tangential (?T) and axial (?A)
to the good trajectory, as shown in Figure 3.

If the differential stress is less than the rock’s
tensile strength (shown as a negative number), tensile failure or fracture will
occur. If the mud weight is less than the fracture gradient, the fracture or
failure will die out near the wellbore. If the mud weight exceeds the fracture
gradient, lost circulation will occur. If the differential stress is greater
than the rock’s compressive strength, spalling and wellbore collapse or plastic
intrusion (salt) will occur.

TECTONIC FORCES

These stresses damage rock materials. These are caused by
the movements of the earth’s plated and other geological forces. Due to this,
the two horizontal stresses have different values. Folds and faults are the
results of tectonic forces. Compressional tectonic stress causes brittle rocks
to fall into the whole plastic formations to squeeze out the hole while
extensional tectonic stress results in fractures and lost in circulation. Fold
belt mountains consequences of compressional tectonic stress and extensional
tectonic stresses are responsible for faulting in basins. In structures like
salt domes, mud weights are kept large to provide maximum stability to the
wellbore as stresses are reformed more by the upward intrusion and penetration
of salt through the rock.