ABSTRACT
Bentonite is an essential component of drilling fluids
and geosynthetic clay liners, which are mostly used in the geotechnical
engineering industry for oil well drilling and lining of tailings storage
facilities as well as landfill sites. The Ghana Environmental Protection Agency
(EPA) is advocating for a shift from the compacted clay liners to geosynthetic
clay liners since the latter has superior hydraulic properties and longer
service life in terms of preventing groundwater and soil pollution than the
former. In Ghana, several metric tonnes of bentonite are imported for drilling
purposes costing millions of dollars. In order to save much needed foreign
exchange for other sectors of the economy, there is the need to source for
local material to substitute commercial bentonite. Bentonite is used due to its
unique properties, which are dictated by its mineralogical composition. It is
mainly composed of the clay mineral montmorillonite. Montmorillonite is
reported to be the major clay mineral in black cotton soils occurring in Ghana.
Black cotton soils are reported to occur widely in Ghana covering an area of
over 168,000 hectors, which are largely unexploited. This study therefore
looked at black cotton soil with the aim of using it as substitute for
bentonite in liner and drilling applications. Sample of black cotton soils were
collected from Dawhenya, Prampram and Tsopoli which are located within the
Accra Plains of Ghana. The samples were air dried, crushed and sieved to
-0.075mm. The physical, chemical, mineralogical properties as well as the
plastic viscosity, apparent viscosity, gel strength and yield point were
determined and the results compared to those of commercial bentonite and some
local and international standards. The physical properties included textural characteristics,
colour, lithological characteristics by visual inspection and some index
properties such as particle size distribution, Atterberg’s limits, specific
gravity and moisture content by BS 1377. The chemical and mineralogical
properties were determined using x-ray fluorescence and x-ray
diffractometry respectively. The organic matter content,
cation ion exchange capacity (CEC) and exchangeable ions were also determined
by the Walkley and Black method and ammonium displacement method respectively.
The pH of the soils was determined using the glass electrode. Swell index and
permeability were determined in accordance with IS 1498 and ASTM D5887
respectively. The plastic viscosity, gel strength and yield point were
determined as stipulated in the API 13B-1 standard. The rheological properties
were determined for concentrations of 22.5g/350ml, 32.5g/350ml, 42.5g/350ml,
52.5g/350ml and 62.5g/350ml. In order to improve the rheological properties,
the tests were repeated for the same concentrations dosed with 10% Na2CO3. The
results of physical, chemical and mineralogical test showed that the black
cotton soils contain calcium montmorillonite as the dominant clay mineral. The
permeability results obtained were of the order of 10-9cm/s, which
compare well with those of commercial bentonite and are within the Ghana
Minerals Commission LI 2182 requirements as well as the USA EPA (most widely
used in other parts of the world) requirements. The plastic viscosity, gel
strength and yield point before the addition of Na2CO3 were 5 mPa.s, 4.8 mPa
for 10sec and 10mins, 5-10 mPa respectively for a concentration of 22.5g/350ml.
Upon the addition of 10% of Na2CO3, the plastic viscosity, gel strength and
yield point increased to between 24-28.8 mPa.s, 5-12 mPa for 10sec, 11-26 mPa
for 10mins, and 4.8-33.6 mPa respectively at the concentration of 22.5g/350ml
and these values fulfill the requirements of the API 13B-1. An economic
evaluation indicated a possible cost savings of about 58% when black cotton
soil is used as drilling fluid and a cost saving of about 94% when used to
substitute bentonite in geosynthetic clay liners.
CHAPTER ONE
INTRODUCTION
1.1 General
All over the world, hydrocarbons
and minerals industries are very lucrative sectors and international companies
are always looking to invest in these sectors. The discovery of oil in
commercial quantities in Ghana brought in many international companies who
obtained licenses for offshore exploration. Since the hydrocarbons can only be
extracted through the drilling of wells, clay will continue to be used in
drilling activities (Omole et al., 2013). The clay component used in drilling
fluid is bentonite and the cost of importing commercial bentonite for drilling
purposes runs into millions of dollars. The total expenditure in drilling an
oil well is estimated to be between 1 million to 100 million dollars out of
which 5 to 15% is spent on drilling fluids (Bloys et al., 1994). There is
therefore the need for a local substitute that may reduce cost.
In Ghana, huge quantities of rock
are mined in open pit mines and processed to extract precious metals, which
results in the generation of a lot of waste known as tailings. The waste
material generated by mining activities contains chemicals that are harmful to
the environment and therefore need to be stored in a manner that the
environment will not be adversely affected.
It has therefore become important
to safely dispose tailings; hence, mining companies are compelled to contain
them behind specifically designed dams with basin liner systems.
The purpose of the liner system is
to prevent hazardous chemicals from reaching the soil and ground water (Akayuli
et al, 2013).
Natural soil barriers containing
clay minerals have been the most widely used material in containment facilities
in Ghana. These barriers are referred to as compacted clay liners (CCLs) or
recompacted clay liners. Rowe et al, (1995) stated however that, a clay’s
ability to act as a barrier to chemical, municipal and mining waste could not
be determined only from laboratory permeability measurements. They further
explained that the permeability of compacted clay depends on factors such as
placement and compaction of the clay, the mineral composition of the clay, or
the effect of leachate on hydraulic conductivity resulting from the mineral
composition.
The use of geosynthetics in Ghana
is increasing rapidly. While the use of geosynthetics in civil engineering
industry is low, the mineral industry long appreciated the advantages
associated with the use of these geosynthetics (Bouazza et al. 2013).
Geosynthetic Clay Liners (GCLs) are
made of bentonite glued or stitched between two geotextiles or glued to a
geomembrane. The permeability of bentonite dictates the permeability of
geosynthetic clay liners except those with geomembranes (Bouazza et al. 2013).
Bentonite is any clay, which has
smectite as the most abundant clay mineral and the two main classes of
bentonite that are mostly used in industry, are Sodium and Calcium bentonite.
Bentonite is also used as a component of drilling fluids for the purposes of
lubrication, cooling cutting tools and removal of cuttings from wells. The flow
and deformation properties of bentonite are very important in geotechnical
engineering (Hosterman et al, 1992). These properties stem from the swelling
ability of the bentonite that also depends on the type of smectite clay within
the bentonite.
Black cotton soils are defined as
being black or greyish black in colour and containing clay of over 50%. The
predominant clay mineral in the soil is the smectite group (Morin, 1971).
Black cotton soils are generally
expansive; therefore, pose serious engineering challenges when engineering
structures are to be founded on them. The presence of the smectite group in
Black cotton soil makes it capable of possessing some of the properties of
bentonite that is widely used as drilling mud and in Geosynthetic clay liners.
There is therefore the need to evaluate these properties of the black cotton
soils with the aim of using them as substitute for bentonite in Geosynthetic
clay liners and drilling mud. This study considers the physical, chemical,
mineralogical, geotechnical and rheological properties of some typical black
cotton soils found in Ghana.
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Item Type: Ghanaian Topic | Size: 89 pages | Chapters: 1-5
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