High-performance non-aqueous drilling fluids (NADFs) are required to meet the challenging technical requirements of many offshore wells. Significant advances have led to the development of advanced NADFs, such as paraffins, olefins and esters, that are less toxic and more biodegradable than early generation diesel and mineral oil base fluids. Advanced NADFs provide the necessary drilling performance while ensuring environmentally-sound operations.
Three options exist to manage waste from NADF-coated drill cuttings: marine discharge, downhole injection, and hauling to shore for land disposal. All options have advantages and disadvantages with regard to total life cycle environmental impact, safety, cost, and operational performance. Marine discharge of cuttings associated with advanced NADFs, however, is the option with the highest safety and operational flexibility. Further, the improved environmental performance of advanced NADFs broadens the acceptability of marine discharge.
Field monitoring studies at multiple offshore drilling sites reveal a relatively consistent picture of the fate and effects of discharging drill cuttings associated with NADFs. The degree of impact is a function of local environmental conditions (water depth, currents, temperature), the amount of material discharged, and the type of drilling fluid used. More significant temporal and spatial effects were observed at sites that used early generation drilling fluids. Wells drilled with advanced NADFs resulted in smaller zones of seafloor impact and more rapid recovery of the benthic communities.
This paper, based on the work performed by a task force of the International association of Oil and Gas Producers (OGP)1, summarizes our knowledge base of the environmental-effects related to the discharge of NADF-coated drill cuttings. In addition, it describes tools and techniques for assessing environmental effects including laboratory methods to determine drilling fluid toxicity, biodegradability, and bioaccumulation potential and numerical models to predict the seafloor distribution of cuttings.
The paper is intended to aid regulatory development and project environmental assessments by providing information to help balance environmental, operational, and cost considerations when choosing waste-management options for NADF drilling.
Introduction
Exploration and development drilling activities have expanded globally into such regions as the Caspian Sea, the UK Atlantic margin, offshore Brazil and West Africa, and the deep waters of the Gulf of Mexico as technology has improved the economics of finding and extracting oil and gas. New drilling concepts, including extended reach, horizontal and multi-lateral wells, enable development to proceed with fewer platforms allowing these resources to be developed more economically. These techniques also have an environmental benefit of reducing the zone of seafloor disturbance.
New drilling concepts are technically challenging and require high-performance drilling fluids with capabilities exceeding those available from water based fluids (WBFs). As a result, non-aqueous drilling fluids (NADFs), for which the continuous phase is primarily a non-water soluble base fluid i.e. non-aqueous base fluid (NABF), have been used extensively by the petroleum industry. Access to a full range of drilling fluid technology is necessary to achieve drilling performance objectives and to support cost-effective development, especially in deep water or where horizontal or extended reach drilling is employed.
