Selective placement disposal of drilling fluids in west Texas

Abstract

Burial of spent drilling fluids used in petroleum and natural gas exploration was evaluated as an alternative disposal technique for reducing surface soil contamination in western Texas. Simulated reserve pits were constructed to provide burial depths of 30, 90 (with and without a 30-cm capillary barrier), and 150 cm below the surface, with orderly replacement of stockpiled subsoil and topsoil. Movement of soluble salts and heavy metals from drilling fluids into the overlying soil was monitored over a 20-month period. The effects of depth of drilling fluid burial on establishment, yields, and chemical composition of transplanted fourwing saltbush (Atrinlex canescens (Pursh) Nutt.) and buffalograss (Buchloe dactyloides (Nutt.) Engelm.) were determined for two growing seasons. The drilling fluids contained extremely high concentrations of soluble salts (EC = 155 to 185 dS m^-1) compared to native soil (EC = 0.7 to 1.4 dS m^-1), with Na+ and Cl- being the dominant soluble ions. Migration of soluble salts up to 30 cm into the overlying soil was detected. Salt movement appeared to be governed to a greater extent by diffusive rather than convective flow mechanisms. Capillary barriers of coarse limestone effectively reduced salt movement at only one of the two study locations. Sodium, Ca+2, and Cl- were the dominant mobile ions, while migration of Mg+2, K+, and SO4-2 was observed to a lesser degree. Exchangeable sodium percentages in the 15-cm zone immediately above drilling fluid ranged from 1.9 to 19.0 after 20 months. Total concentrations of Ba, Cr, Cu, Ni, and Zn were greater in drilling fluids than in native soil, but there was no evidence of migration of these metals into overlying soil. Significant increases of Na and K in plant tissues occurred only in the 30-cm depth of burial treatment, indicating the potential for uptake of drilling fluid constituents. However, there was no evidence of accumulation of Ba, Cr, Cu, Ni, or Zn by either plant species after 17 months. Survival of fourwing saltbush and buffalograss transplants was > 90% and was not affected by depth of drilling fluid burial. Increased canopy cover and biomass production on treated pits compared to control plots were attributed to a tillage effect resulting from pit construction.