Comparative study of mineralogy and geochemical compositions of commercially imported bentonite and some locally derived bentonitic clays from Anambra Basin, Southeastern Nigeria
Description
The study focused on the mineralogy and chemical properties of the bentonites recovered from the geological units of the Imo Shale and Ameki Formation, compared with commercially imported bentonite (CIC), for its suitability in the formulation of drilling mud. About 50 suspected bentonite clays were investigated, and properties of starting materials such as pH, conductivity, composition, grain size, degree of alteration, and filtration conditions were utilized during screening. The CIC and four samples that met the API specification for drilling mud were subjected to X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. The kaolinite-type consists of 55 % kaolinite, 20 % quartz, 15 % K-feldspar, and 10 % hematite. The smectite-type range from 28-47 % kaolinite, 26-32 % quartz, 12-20 % nontronite. The CIC consists of 12 % quartz, 10 % K-feldspar, 12 % calcite, 41 % nontronite, and 25 % amorphous materials. The SPL15 is predominantly kaolinite, whereas SPL6, SPL8, and SPL11, including the CIC, are smectite-type with a significant amount of kaolinite, except the CIC. The XRF results, show Al2O3 (15.77-25.49) wt%, Fe2O3 (6.61-10.01) wt%, SiO2 (51.67-59.11) wt%, and loss on ignition (7.57-11.22) wt%. Nontronite, one of the smectite group of minerals, was identified from the XRD data. These were supported by elevated concentrations of Fe in some of the samples. More silica may be present in an amorphous phase in the CIC. Palygorskite and basanite are also present in some of the clays. The smectite-type contained an elevated Fe/Al ratio and is rich in Ca-smectite, which differs significantly from the CIC. Based on these results, the primary criteria for the formulation of drilling fluid, using mineralogy, and chemical compositions are achievable. A comparison with the processed CIC has revealed significant levels of compositional disparity/deficits in the local clay. Consequently, treatment/beneficiations with some additives may be necessary to achieve the desired compositions.
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Materials and methods 1. Field sampling method: Fifty (50) representative fresh clay samples were carefully collected from the field at various locations where they were exposed within the study area. The samples were initially air-dried and later pulverized by developing a highly polished surface in the chamber of the specimen holder of the Pw 1800 diffractometer with a copper tube anode. 2. Analytical methods: 50 g of each sample were placed in beakers and soaked with de-ionized water of a pH value of 7.0 at room temperature (25 oC), stirred properly, and left for 24 hours. The pH meter (PC Testr 35 Multi-Parameter) was used to read off the pH and electrical conductivity values of each sample by deepening the electrodes into the beaker containing de-ionized water together with the sample. 3. Mineralogical analysis: Four samples from the various study locations that successfully scaled through the preliminary API standard screening test and one commercially imported drilling clay were analyzed for their mineralogical, using the X-ray diffraction (XRD) approach. The following instrument/conditions were applied: Philips PW1729 X-ray diffractometer, Cu anode X-ray tube, 40 kV 30 mA tube, 40 kV 30 mA tube power, 1 degree 2-theta/min scan speed 5 to 80 degrees 2 theta scan range graphite monochromatic at SpectraChem Analytical, CRL Energy Ltd, New Zealand. Phase identifications and quantification was carried out using Siroquant search/match evaluation. The mineralogy was carried out on unoriented mounts of the total samples. 4. Chemical analysis: XRF analysis was also conducted on the same samples as above to determine the major oxide compositions of the clay, using a Philips PW 1450/20 automatic X-ray fluorescence spectrometer equipped with 60 position sample changer at the same laboratory. Powder samples (<75microns) were dried in an oven at 110 oC, and lost on ignition (LOI) was carried out at 1000 oC. A total of 10 key major oxides: SiO2, TiO2, Al2O3, ∑Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5, and loss on ignition (LOI) of each sample was equally determined by drying the samples at 110 oC overnight followed by calculation of their water and other volatiles content as wt.% at 1000 oC. The accuracy of the major element is +2%.