Invert-Fluid Flocculation: A Method for Recycling Drilling Fluids

A new technology is now in use to increase the efficiency of traditional solids-control equipment in removing colloidal fines and water from invert drilling fluids. This method makes it possible to extend fluid lifespan and recover and reuse valuable components. Colloidal fines—particles measuring less than 2 µm equivalent spherical diameter—are typically difficult to remove through the exclusive use of mechanical devices relying on screening or gravity. Through the application of chemistry and equipment packages, these colloidal solids now can be removed from systems with a hydrocarbon-continuous phase through a process of flocculation, which aggregates these fines to facilitate their removal. The use of this process results in a high-performance fluid with lower density, increased oil content, and no colloidal solids. The benefits go beyond those associated with traditional treatment methods and include the alternative provided to the disposal of unusable stock.

The invert-fluid flocculation technology, called Reclaim, was introduced recently by M-I Swaco. Invert drilling fluids are water-in-oil emulsions containing a hydrocarbon-continuous phase with an emulsified internal aqueous “brine” phase in addition to clays, weighting agents, and other additives (Darley and Gray 1988). These fluids serve many functions during drilling, one of which is to remove excavated rock, or “drill cuttings,” from the borehole. These solids are incorporated continuously into the mud, with solids-control equipment employed at the rigsite to remove them and return the drilling fluid to the active system in a usable condition. Typical equipment, such as shakers and centrifuges, remove solids greater than 5 to 7 µm (ASME 2005). However, as drilling fluids are used to drill multiple holes, fine particles can build up and ultimately create problems for efficient drilling. Typically, the drilling fluid is diluted and reformulated to reduce the concentration of these fine solids and return the fluid to acceptable physical properties. This is often costly and also increases the volume of drilling fluid, leading to problems of storage, transport, and disposal. In severe cases, in which dilution is uneconomic, the drilling fluid will be disposed of as waste and fresh fluid mixed. This involves significant cost and environmental impact.

The new technology uses polymeric inverse emulsions and surfactants coupled with defined-shear and mixing systems to flocculate fine solids in invert drilling fluids, enabling them to be removed by use of standard oilfield centrifuges. The result is the reclamation of the hydrocarbon component with no fine-contaminant solids, and this reclaimed hydrocarbon stream can be used as base fluid for new drilling formulations. This significantly benefits the processes of fluids recycling and inventory control, reduces disposal volume, and improves drilling performance.

The invert-fluid flocculation process (Fig. 1) can be used on active drilling projects to enhance solids-control-equipment efficiency, as well as at storage facilities and waste-treatment plants for reconditioning drilling fluid returned from field operations. The process involves injection of a proprietary chemical treatment into the drilling fluid, optimized mixing systems, and centrifugation. This is formalized as a single-unit standard-equipment package containing all pumps, pipe work, mixers, chemical storage, and process-control systems to ensure optimal process reaction.

Fig. 1—A process depiction of the invert-fluid flocculation technology.

Treatments Tailored to Fluids

The drilling fluid is treated with a polymeric inverse emulsion tailored for each type of drilling-fluid system. The polymer particles adhere to multiple particles of solids suspended in the invert fluid, creating larger aggregates and enhancing the solid/liquid separation performed with a decanting centrifuge. The polymeric inverse emulsion is often diluted further to varying degrees in compatible oil, depending on the treatment stream. This dilution serves to reduce the viscosity of both the flocculation chemical and the drilling fluid to enhance the dispersal and mixing process. The process can be qualitatively understood in terms of wettability differences, and as such, a demulsifying surfactant and water may be required to weaken the water-in-oil emulsion present in the drilling fluid and water-wet the solids. In these cases, the water droplets will be demulsified and also removed from the drilling fluid during centrifugation.

The flocculation polymer, base oil, surfactant, and water are injected into the drilling fluid, in-line, using a series of flowmeters to control the effective dose. To derive the maximum benefit from the products, intimate contact of the treatment chemicals with all the solids present in the drilling fluid is ensured by use of a chemical-injection and mixing package designed and optimized for the viscosity of the chemical products and the drilling fluid. The mixing package employs a series of static mixers that consist of fixed geometric element(s) within a pipe and that use the energy of the flowstream to drive the mixing process. A dynamic mixer also is used.

Once the mud has been chemically treated and the solids have been flocculated, it is passed through a decanting centrifuge. The choice of centrifuge is important for an efficient process. Greater centrifugal force and longer residence time within the centrifuge generate increased separation efficiency of the flocculated solids from the drilling fluid and reduce the chemical consumption required. Fig. 2 shows an example of typical solids expelled from a centrifuge and a picture of the solids, once the flocculation chemical package has reacted with the solids and water present. The solids have become bulky and dry, with a significant yield stress showing that flocculation is working effectively. As these solids are removed, the treated fluid recovered will have a low density and a very high (98 to 99%) oil content, which can be used as a base for new drilling fluids. Tests have shown that this stream also contains valuable components of the original drilling fluid, resulting in less product required to convert the treated fluid to fully formulated drilling fluid when compared with the use of clean base fluid.

Fig. 2—Solids expulsion from typical centrifuging (left) and flocculation after reaction of chemicals with the solids and water present (right).

Alberta Case Study

The invert-fluid flocculation process can be used for fluid management at the rigsite to increase the efficiency of the total-solids-removal equipment package available and prevent the formation of unusable stock. At storage sites, the process can be used to improve the properties of the drilling fluid, with benefits seen in higher drilling rates, lower well costs, and increased fluid lifespan. For example, the invert-fluid flocculation treatment of mineral-oil-based drilling fluid in Alberta, Canada, reduced the fluid density from 1.20 to 0.95 g/cm3, saving rig time through increased drilling rate. It also eliminated the need to dispose of 7,063 ft3 (200 m3) of unusable drilling-fluid stock mud per well, and with an average of 25 wells per year, this technology has eliminated the need to dispose of ≈176,500 ft3 (5000 m3) of fluid annually. It has also reduced the considerable chemical consumption and cost associated with building fresh systems to achieve the lower required drilling-fluid density.

Abu Dhabi Case Study

The invert-fluid flocculation process also can form part of an inventory-management system to reduce the stock of spent fluid, reduce storage requirements and costs, and recover valuable components of the fluid for reuse in building new stock. It will reduce the volume-increase, chemical-consumption, and transportation costs compared with traditional dilution methods used to reduce solids. The system has associated environmental benefits, as the volume of spent or excess fluid requiring disposal will be reduced as fluids are reused. For example, the invert-fluid flocculation technology was successfully used in Abu Dhabi to treat diesel-based drilling fluid, which was heavily contaminated with drilled solids, by decreasing the density of the fluid from 1.2 to <0.9 g/cm3, increasing the oil content to as high as 98%, and successfully removing the fine solids. This recovered fluid was used instead of neat diesel as a dilution fluid in subsequent operations. It effectively eliminated the high dilution volumes previously required, decreased the storage pressure on the facility, and significantly reduced the volume and associated cost for disposal either on land or into dedicated storage wells.

Information provided by Karen McCosh, Alan Gilmour, and G.A. Addicks, all of M-I Swaco, and Ray Farinato, Kemira Group.

References

ASME Shale Shaker Committee. 2005. Drilling Fluids Processing Handbook, Burlington, Massachusetts, USA: Elsevier.
Darley, H.C.H. and Gray, G.R. 1988. Composition and Properties of Drilling and Completion Fluids. 5th edition. Houston: Gulf Publishing Company