Zero discharge drilling has been one of the "holy grails" of the industry since offshore drilling commenced. Millions of dollars are spent to drill the upper portions of wells through an operation that is well known as "pump and dump" in the drilling parlance. This practice has continued unabated for lack of a suitable and cost effective method for recovering the mud while drilling this interval. An alternative to "pump and dump" provides savings not only in the cost of the mud, but also alleviates any environmental impact concerns. Also, current practice provides limited direct pressure management during the drilling of upper sections of the hole, allowing everything from borehole washouts, shallow water flows to loss of the wellbore itself to occur. Now is a good time for the industry to look at the new options available to achieve zero discharge drilling.
This paper presents a methodology for achieving both elimination of "pump and dump" and borehole pressure management using a single tool. The paper discusses a unique subsea pumping apparatus that could be deployed at the sea floor to divert and pump back return drilling fluid through a separate conduit, as well as having the capability to apply a back pressure to the well and thus manage the pressure in the annulus during the drilling of the upper hole sections. Advantages include full returns of the mud during the drilling of the upper hole sections, opportunities to use specialty muds for the health of the borehole, and pressure management for mitigating shallow water incidents. The potential effect is a smoother drilling operation creating options to drill to deeper depths and set the conductor casing deeper. The paper will highlight the processes of pressure management, the tools and methods to deploy the technology as well as touch upon the costs involved. The paper will also explore options to deploy this technology from a purpose built rig specially for drilling the top holes with zero discharge and annular pressure management.
The term "zero discharge" is perhaps known to have come from the Governmental White Papers1 that appeared in Norway in 1996-97. Although the impetus then was to move towards the idea of sustainable development for efficient management of natural resources with stringent controls on discharge of effluents in all sectors of the industry and not necessarily restricted to the petroleum industry, it nevertheless may not be wrong to assume that the idea of "zero discharge" drilling probably originated there.
The practice of "pump and dump", which involves discharging the drilled cuttings as well as the drilling fluid at the seafloor, has been continuing in the offshore drilling industry primarily because of the absence of suitable options to bring the mud back up to the rig safely and at a reasonable cost. Initially, "pump and dump" comprised of drilling the upper sections of the hole utilizing primarily seawater.
