The occupational exposure risk of chemicals to personnel during well construction operations is managed and controlled. The control of exposure of personnel to chemicals during well construction operations has not always been given the same focus as environmental issues. To some extent the reduction of environmental risks has also resulted in reduced health and safety risks. The containment of fluids for disposal, rather than immediate discharge may increase occupational exposure. It is important to conduct comprehensive risk assessments of drilling fluid systems that consider health, environmental and safety issues.
A significant quantity of the chemicals supplied to the well site are used during drilling operations. As a result, drilling operations give rise to the greatest occupational exposure to drilling fluids. In this paper the general background of drilling fluids and the various categories of base fluids and additives in use today are discussed. The paper examines trends in use, human exposure, best practices in fluid use, monitoring of health hazards and effects, and recommends some risk reduction measures.
Drilling fluids are a key requirement in the vast majority of drilling operations as a means of removing drill cuttings from the hole, controlling downhole pressures, lubricating the bit, and other vital functions to insure a safe and productive well (Fig. 1). Although the most primitive systems used water only, drilling performance was enhanced by using other materials, e.g. clays as filtrate reducers, viscosifying agents and dispersants for rheology control. As drilling became more complex, especially in long sections, with increased temperatures and when drilling more reactive formations, the number of additives have increased rapidly in number and in complexity. By the 1970s and 1980s, the base fluid for the now complex emulsion had evolved from water to hydrocarbon-based fluids and the number of chemicals used in the drilling fluids industry numbered in the thousands.
The functions required of drilling fluids can be summarized as follows:
Fig. 1: Fluids Circulating System of a Rig and Well.
The drilling fluid is recognised as a primary barrier in a well for controlling downhole pressures and for the consequential avoidance of uncontrolled gas or fluid intrusions from the formations being drilled or exposed.
Drilling fluids must be able to remove cuttings from the well bore as they are produced. Drilling fluid is pumped down the drillstring and out through the bit, circulating cuttings to the surface up the annulus where they are removed by solids removal equipment. The fluid is then re-circulated through the hole. This process is repeated as drilling progresses. To lift the cuttings out of the hole effectively, the fluid must have some viscosity. Clays and polymers can provide this viscosity.