During offshore drilling operations, the disposal of drill cuttings and associated residual drilling fluid is determined by regulatory constraints, which are usually based on environmental risk. The environmental risk of drill cuttings disposal options is influenced strongly by the location of the well, the level of residual drilling fluid, and the type of drilling fluid. The International Association of Oil & Gas Producers (IOGP) has divided drilling base fluids into water-based drilling fluids (WBDFs) and non-aqueous drilling fluids (NADFs), which are categorized as Group I: High Aromatic Content, Group II: Medium Aromatic Content, and Group III: Low/Negligible Aromatic Content. Group III fluids encompass many types of fluids with low or undetectable levels of aromatic, including olefins, synthetic paraffins, and enhanced mineral oils. Laboratory testing and post-drilling environmental surveys clearly show the difference between WBDFs, Group I and Group III NADFs. However, despite laboratory studies differentiating the various Group III fluids, this differentiation is not clearly observable in single-well environmental monitoring studies. The objectives of this research are (1) to model the environmental risk from offshore drill cuttings discharge with several different Group III drilling base fluids, (2) to determine the impact of formation oil on the calculated environmental risk, and (3) to assess the use of modeling to differentiate drilling base fluids.
In this project, DREAM (Dose-related Risk and Effect Assessment Model) was used to simulate the environmental risk from drill cuttings discharge with different drilling base fluids under identical discharge conditions of bore hole diameter, retention on cuttings (ROC), particle size distribution, current, etc. The drilling fluids modeled are: diesel (Group I), four Group III fluids (internal olefin, two enhanced mineral oils, and a synthetic paraffin), and water-based fluid (WBDF), as well as formation oil on cuttings. Benthic environmental risk is quantified using four factors that potentially impact sediment organisms: chemical stress (toxicity), burial, change in sediment grain size, and oxygen depletion due to biodegradation of chemicals present in the drilling base fluid.
The modeling results presented in this paper support the differentiation between different drilling fluids and provides insight into the primary drivers of risk. For all fluids, grain size and burial posed small risk in this modeling scenario. As expected, the largest risk was predicted for diesel based on chemical toxicity while the smallest was for WBDF. Most WBDF toxicity impacts are in the water column and not the sediment. Group III NADFs, except for one enhanced mineral oil, had similar risk, but the main risk contributors were different. For the enhanced mineral oils and synthetic paraffin, chemical toxicity influenced overall risk; internal olefins did not exert risk from chemical toxicity. For all Group III NADFs, the main contributor to environmental risk from the discharged matter is oxygen depletion by degradation of the organic load in the base fluid, with more biodegradable Group III fluids having higher predicted risk. This higher predicted risk assessment runs counter to what is seen in environmental surveys. One shortcoming of DREAM is the inability to accommodate anaerobic biodegradation, which leads to predicted long timeframes for contamination that do not match environmental monitoring results. While DREAM is useful for comparing fluids, the outputs of the model should be assessed in context of available environmental studies and operator experience.