دانلود رایگان مقاله انتشار عناصر شیمیایی از کنده حفاری شیل به محلول آبی با pH مختلف

abstract

The effect of pH changes on leachability of light and heavy metals from shale drill cuttings generated from unconventional shale gas production was investigated. Cuttings, being the primary byproduct generated from drilling operations, belong to the potentially hazardous type of wastes due to presence of heavy and radioactive elements and remains of drilling fluid. In this regard, assessment of potentially dangerous components (PDCs) from rock waste materials was performed by application of batch leaching tests, which has provided information on the sensitivity of leaching under externally imposed changes in pH (natural or caused by treatment) in specific scenarios. The description of shale rocks mineralogical and chemical properties was performed by means of X-ray fluorescence spectroscopy, diffractometry as well as scintillation spectrometry. The concentrations of released constituents due to the leaching tests were measured by atomic absorption spectrophotometry. Results were compared and discussed accordingly with the waste acceptable criteria of elution limits. Analysis of the substrate revealed that the elemental composition was dominated by light elements, whereas heavy metals were present in trace amounts. However, noticeable release of barium (2.0e4.6%) was also recorded, which has originated from not only rock material but also drill mud. Minor mobility was observed for transition elements such as Cr, Co, Fe, Mn, Ni, Zn, Cu and Pb. Results revealed that drill cuttings follow the requirements for other than hazardous and municipal type of deposition, with exception for barium. Moreover, content of radioactive isotopes fulfill the requirements range of acceptable concentrations.

4. Conclusions

Drill cuttings as a secondary material from extraction sites were investigated and discussed in order to assess the potential release of dangerous components under externally imposed changes in pH (natural or caused by treatment) in specific conditions. The rockforming silicates such as plagioclase, micas and illite, chlorite, kaolinite and alkali feldspar were found to constitute to dominant mineral phases in drill cuttings. The overall analysis of the substrate indicated that the elemental composition was dominated by Si > Al > Fe > Ba > Ca > K > Mg > S > Na elements, whereas heavy metals were in trace amounts. Leaching batch test as a quality control and compliance tool was used to describe solubility and release of selected chemical elements to aqueous solution at different pH (3e12). Despite low cation exchange capacity values, shale leachates were found to contain elements of potential concerns. Metals partitioning was strongly affected by pH value of leaching medium and controlled by the mineral forms as well as individual constituent properties (chalcophilic, lithophilic, siderophilic). Furthermore, the differences between mobility ranges depended on both type of submineral connection and atomic size. Cationic elements were more prone to migrate to the acidic eluent than to the alkali one. According to the mobility levels, the constituents can be divided into two main groups. The major release was recorded for Ca, Mg, Na, K under acidic conditions and reached about 3% comparing with the initial concentration. Moreover, noticeable release, in the range of 2.0e4.6% was also recorded for barium, however it does not only originate from rock material but also from drill mud. Minor mobility was observed for transition elements such as Cr, Co, Fe, Mn, Ni, Zn, Cu and Pb. Mobility of metals with the predominant charge 2þ (Cu, Ni, Pb, Zn) was especially enhanced under acidic conditions. The highly mobile group in pH 8e9, being relevant for hydraulic fracturing fluids commonly used in shale gas production, included sodium, calcium, barium and potassium. The subsequent group with lower mobility consisted of copper, chromium and nickel (concentration ranges below 4 mg/l), followed by magnesium, zinc, iron, manganese, cobalt and lead with elution below 0.5 mg/l.