Assessment of black cotton soil as substitute for bentonite in liner and drilling applications.

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.

Source: KNUST