The overview paper examines environmental control techniques associated with borehole technology. The overview includes two aspects of environmental control: surface discharge of pollutants from the drilling process, and subsurface interzonal isolation resulting from well completion.

The definition and objectives of the environmental control technology for oilfield drilling operations are presented first. Its preventive function is derived from modem concepts of source separation and recycling in addition to the conventional remedial function of waste disposal.

The use of systems analysis to define the environmental constraints in the drilling process places environmental control technology as an integral part of modem mud engineering. The technology serves to control volume and toxicity of active mud systems. In well completions, environmental control comprises well cementing technologies to control the hydraulic integrity of annular seals.

Described is the technological mechanism of the mud system volume expansion resulting from inefficient cuttings removal. Special consideration is given to drilling fluid properties such as dispersivity and dewaterability; their physical nature and control measures are discussed. Also presented are examples of recent research efforts to develop new mud systems that have properties of low dispersivity and high dewaterability.

The discussion on drilling fluids toxicity control includes recent developments in toxicity measurements as well as toxicity reduction through the use of low-toxicity substitutes or remediation mechanisms. Examined are recent research efforts to develop a simple toxicity test which can be deployed at well sites. These efforts are discussed in view of the search for a correlation between the field tests and the conventional Mysid test Also presented are examples of recently developed substitutes for diesel oil, metal and salt-related toxicity. Further discussion covers remediation methods for oil-base mud cuttings, and the role of surface mechanisms in oil removal from solids.

Part of the overview focuses on the mechanisms related to the integrity of borehole annular seals and subsurface migration of pollutants. Specifically discussed are recent developments in the methodology of Mechanical Integrity Test using the Cement Bond Log and the neutron activation techniques. Also included is a short discussion of potential applications of recently developed techniques to design the annular cement seal integrity.

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