Handling the low-gravity solids overload

Conventional solids control methods are inadequate to manage low-gravity solids smaller than 8-10 microns. M-I Swaco’s Rich Cook discusses a new solution, recently successfully employed in West Africa, that helps manage these solids while keeping excess rig space to a minimum.

Low-gravity solids buildup is becoming an increasing problem for operators, and mud plant and fluid storage capacity is becoming a larger – or perhaps smaller – problem. This is particularly true for offshore rigs where space is at a premium. The ability to remove these ultra-fine solids is paramount to reducing the amount of disposable drilling fluid and the expensive task of rebuilding new fluid. Conventional solids-control equipment and methods cannot remove low-gravity solids smaller than 8 to 10 microns (ì) that accumulate and degrade the performance of costly oil- and synthetic-base drilling fluids.

When drilling into the reservoir itself, the use of specially designed oil- or synthetic-base reservoir drill-in fluids (RDF) becomes increasingly important. These fluids are designed to drill the reservoir zone with minimal damage while maximizing production of the exposed zones. To achieve these goals, the RDFs should not contain clays or acid-insoluble weight materials that might migrate into the formation, plugging pores. They should be formulated with breakable or acid-soluble viscosifiers and other materials that limit the fluid loss to the formation and ensure good cleanup. Finally, they should prevent clays from swelling and plugging the formation.

Despite these cautions, solids contained in the RDFs can plug formation pore throats and cause reduced permeability and formation damage if those solids cannot be adequately removed. Even with an aggressive, but conventional, solids-control process, some solids and bridging agent particles can degrade and become so small as to slip through the solids-control devices. Often the sized calcium carbonate solids used for weight and bridging are ground down to less than 8-10 microns, too small for most solids-control equipment to capture and remove.

The result is contaminated fluids that must be disposed of or diluted with new mud to create usable product. In areas like West Africa, where strict zero discharge rules and regulations govern the proper disposal of used mud, this presents a significant and expensive problem.

Simply diluting the old mud is problematic as well. Limited mud plant space onshore means supply boats often have nowhere to offload their fluids because the dilution and rebuilding of the old mud uses up the mud plant capacity.

Conventional treatment of old mud, in an attempt to make it reusable, is to dilute the mud system to reduce the level of the fine solids. The dilution process requires three to four times the fluid volume, which can take up most if not all of the mud plant capacity. It is an impossible solution onboard an offshore rig, even with excess fluid storage capacity. Even if the space exists at the mud plant, there are increased costs associated with handling, transportation and storage.

The only other option is to build new mud. Beyond the simple expense associated with new mud, the old mud must be properly disposed of, once again adding to the handling and transportation costs.

A new solution
Recognizing the need for a system that would provide operators with the ability to remove these fine solids, M-I Swaco began developing the Reclaim system, a chemically-enhanced technology capable of eliminating the majority of fine solids from oil- or synthetic-base drilling fluids.

For years, this type of dewatering system had been available for water-base drilling fluids but nothing existed that provided this type of separation for oil and synthetic-base fluids. The Reclaim process is sometimes referred to as ‘dewatering for oil-base muds’.

The system is designed to remove the bulk of fine colloidal particles from non-aqueous drilling fluids and can increase the oil-to-water ratio. The technology is chemically enhanced with Refloc flocculants and Resurf surfactants that effectively flocculate the fine solids in the non-aqueous drilling fluids. A variety of Resurf surfactant products allow engineers to tailor the solution to fit the individual needs of the mud system to be treated while ensuring maximum integrity of the recovered base oil.

Surfactants weaken the emulsion and flocculating polymers agglomerate the fine solids so the centrifuges can easily remove them. The polymers also promote demulsification of the brine droplets, creating the secondary effect of removing water along with the solids, raising the oil-to-water ratio.

At the mud plant or shore base, the process begins when the fluids from the rig are transferred to the Reclaim unit feed pump that moves the fluid to the mixing system. At the mixing stage, variable-frequency drive controlled pumps add the surfactant and flocculating polymers to the mud in predetermined concentrations. Here, the surfactants reduce the emulsion stability of the fluid and water-wet the solids to allow the polymer to adhere to the ultrafines. The flocculated mud stream is then fed into high-speed centrifuges, returning virtually solids-free, reusable, non-degraded recovered oil that can be returned to the mud system for reuse. The solids stream, which also contains a portion of the water phase, is discarded.

Although the fine solids must be disposed of, the volume transported to the disposal site is significantly less than transporting the used up mud. OE

About the Author

Rich Cook is the senior technical communication supervisor at M-I Swaco. Based in Houston, he works closely with the engineers to develop technical documentation, including trade articles and papers.

Case study
An operator working offshore West Africa was using a calcium carbonate (CaCO3) weighted oil-base mud (OBM) drilling fluid system that had been specifically designed to protect the production zone with a D-50 between 10ì and 16ì. The OBM was brought into the mud plant multiple times between wells and treated by running it through a centrifuge, which was unable to remove the ultrafine solids in the system. The D-50 was less than 5ì with some analysis showing 65% of the solids were less than 5ì.

The OBM had high concentrations of ultrafine solids that could not be removed by the centrifuges and existing mud plant volumes were high, preventing further treatment by dilution due to the lack of available space. The operator approached M-I Swaco and asked for a solution that would allow them to treat 700m3 of OBM and recover as much of the base oil as possible while minimizing waste disposal costs. The operator wanted to relieve pressure on the mud plant capacity and avoid expensive dilution to rebuild the mud.

The Reclaim system was brought online at the onshore mud plant that serviced the offshore rig. The goal was to reduce mud weight from 10lb/gal to 7.2lb/gal, reduce solids to 1% v/v or less and increase the oil:water ratio from 70:30 to 97:3 or higher.

The Reclaim unit processed 100m3 of CaCO3 weighted OBM, recovering 78% of the base oil that was in the original mud. The mud weight was reduced from 9.85lb/gal to 7.1lb/gal, and the oil/water ratio was increased to 98:2 with solids less than 1% v/v. Formation testing showed using the recovered oil that had been processed through the Reclaim system increased the return permeability in the production zone from 64% to 86%, increasing the potential productivity of the pay zone.

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