Geochemical Partitioning of Metals in Spent Drilling Fluid Solids

Metals in oil and gas fluids have been of concern to the environmental and industrial communities since 1976. As a result, metals in 31 spent drilling fluids representative of the major oil and gas production provinces in the Continental United States, were fractionated into exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter, and residual forms using a sequential extraction (summation of fractions) technique. Bioavailability and mobility of metals in solid matrices follow in sequence of the operational defined fractions with chemical reactivity decreasing in the order of exchangeable > bound to carbonate > bound to Fe-Mn oxide > bound to organic matter > residual fractions. Metals evaluated in this study include arsenic, barium, cadmium, chromium, lead, and zinc. The summation of fractions was compared to independent total metals analysis using the United States Environmental Protection Agency (USEPA) SW-846 Method 3050 digest procedure to evaluate metal recoveries. No difference was observed in the summation of fractions and EPA Method 3050 total metal values for arsenic, barium, and cadmium. EPA Method 3050 digest was about 28 percent lower in chromium, and about 19 percent lower in lead and 16 percent lower in zinc than the total by summation of fractions. Almost all of the barium (95.6 percent) was recovered in the residual fraction. Arsenic was recovered primarily in the residual fraction (74.3 percent) and the Fe-Mn oxide fraction (16.1 percent). The highest quantity of cadmium was recovered in the residual fraction (43.3 percent), followed by the bound to organic (27.9 percent), and bound to Fe-Mn oxide (21.1 percent) fractions. Chromium was observed primarily in the residual (40.4 percent) and bound to Fe-Mn oxide (34 percent) fractions. Lead was distributed primarily in the bound to Fe-Mn oxide (49.3 percent), and residual (27 percent) fractions. Zinc was almost equally distributed in the bound to organic (36.2 percent), and bound to Fe-Mn oxide (33.1 percent) fractions. Cadmium (3.9 percent) and arsenic (2.7 percent) were the only metals with an exchangeable fraction >1 percent of the total. Low total and/or low exchangeable metal concentrations ultimately control the bioavailability and mobility of metals in spent drilling solids and limit the potential for an adverse impact on the environment.