Impact of Dewatering Technology on the Cost of Drilling-Waste Disposal
Authors: Malachosky Ed , Sanders Robert , McAuley Les 
Published: December 2007

Summary
The use of mobile, chemically enhanced centrifugation dewatering systems toprocess liquid waste from two deep exploratory wells in Kern County, CA, saved $136,900 in waste-disposal costs. Liquid waste in the wells was reduced by 45%. This paper tracks daily costs and chemical usage for each well to isolate keycost factors.

Introduction
The use of different types of dewatering technology to process effluent fromvarious industries has been well documented. others have reported on the use ofa centralized facility to process drilling waste transported from a number ofdrilling sites to the facility, hut the application of the technology at anactive drilling location has been limited by the complexity of dewatering. Theuse of the technology as an on-line addition to the rig's solids controlequipment has been suggested, but the results of such on-line processing havenot been extensively reported in the literature. This paper reviews the use ofone such dewatering paper reviews the use of one such dewatering technique, chemically enhanced centrifugation, at two active drilling locations. Chemically enhanced centrifugation uses chemical coagulants and flocculants toachieve fine-particle flocculation. The flocculated solids are then separatedfrom the waste stream with a high-speed centrifuge. This solids-separationtechnique differs from conventional "closed-loop" mud systems in oneimportant way: conventional closedloop systems rely on additionalsolidsprocessing equipment, such as "microclones" (small-diameterhydrocyclones), and traditional centrifuges to separate water and solids fromthe waste stream. Processing in this manner generally fails because theadditional solids-processing equipment cannot remove extremely fine particlesfrom the waste stream. When the particles from the waste stream. When therecovered water is returned to the mud system, fine solids are ala returned tothe mud. Continued reuse of the recovered water eventually results in afine-particle accumulation in the mud system that can be corrected only throughmassive dilution. Thus, the objective of waste-volume reduction is defeated or, at least, severely compromised. Chemically enhanced centrifugation, however, removes fine solids by treatment with chemical coagulants and flocculants. In July 1988, Arco Oil and Gas Co.'s Western Dist. began field testing of twodewatering units that used chemically enhanced centrifugation to process liquiddrilling waste. Both units were mobile in the sense that the component partscould be loaded onto two trailer trucks and transported to the drillinglocation for assembly. Both units were comparable in design and operation andwere furnished by the same service company. Before the units were selected, alist of checkpoints that were considered key to the successful operation ofsuch a unit was prepared (see Table 1). Available units were prepared (see Table 1). Available units were compared on the basis of a physical examinationfollowing this list. The unit with the best score was selected for the tests. Operational considerations limited the size of the reserve pit at bothfield-trial locations. The dewatering units were used to create aminimum-discharge (closed-loop) solids-control system. Because the cost ofliquid-waste disposal was $10.85/bbl, the plan was to send all the effluents tothe dewatering unit for processing. The recovered water would be returned asneeded to the mud system, and the solids separated by the centrifuge would bedischarged to the reserve pit. Because this recovered water was to be pit. Because this recovered water was to be reused in the mud system, asuspended-solids content of less than 500 mg/L and a residual-polymer contentof less than 0.1 lbm/bbl in the recovered water were dewatering goals. Thepurpose of these field trials was to assess the ability of the dewatering unitto achieve water-clarity goals for less than $10.85/bbl. This assessment wouldprovide one means of evaluating the economic viability of thisminimum-discharge solids-control system.

Well Background Data
The Mettler No. 1 was drilled in the Tejon (North) field, Kern County, CA. The dewatering test began July 26 and was completed Oct. 24. The dewateringunit was on location 96 days. Fig. 1 is a schematic of the well's hole size andcasing programs. The Arco-Texaco No. 801 was drilled in the Midway Sunsetfield, Kern County. The dewatering test began Aug. 18 and was completed Dec.20. Difficulty in setting cement plugs for directional control caused the timeplugs for directional control caused the time on location to be significantlylonger than planned (125 days). Fig. 2 is a schematic planned (125 days). Fig.2 is a schematic of the well's hole size and casing programs.
JPT
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