Flocculation

Key Takeaways

• Flocculation of drilling mud causes a dramatic and problematic increase in viscosity and gel strength that can halt drilling operations: when bentonite-based water-based mud is contaminated with divalent cations (calcium from cement, anhydrite, or hard formation water, or magnesium from dolomite dissolution), the cations replace the monovalent sodium ions that keep the bentonite platelets dispersed; the divalent cations act as bridges between adjacent clay platelet surfaces, causing them to aggregate into a three-dimensional network that dramatically increases the mud's apparent viscosity and gel strength; a well-viscosified bentonite mud that circulates normally at 50-100 centipoise viscosity can become a non-flowing gel with yield point exceeding 100 lb/100 sq ft after severe calcium contamination, making it impossible to circulate through the drill string without dangerously high pump pressures; the treatment for calcium-induced flocculation is the addition of soda ash (sodium carbonate) to precipitate the calcium as insoluble CaCO3 and remove it from solution, restoring the sodium-dominated environment that keeps the bentonite dispersed.
• Polymer-induced flocculation in produced water and tailings treatment uses synthetic polymers or biopolymers as flocculants that adsorb onto multiple particle surfaces simultaneously, physically bridging between particles and drawing them together into the loose, open-structured flocs that settle rapidly under gravity; the mechanism requires careful optimization of polymer molecular weight (higher molecular weight polymers can bridge greater distances between particles but may be too large to access small pore spaces between tightly packed particles), polymer dosage (too little does not provide enough bridging; too much causes re-stabilization as every particle surface is coated with polymer and there is no unoccupied surface for polymer-to-particle bridging), and mixing intensity (gentle, low-shear mixing allows flocs to grow to large, fast-settling sizes, while excessive shear breaks flocs apart and reduces their settling rate); the optimal flocculant and dosage for a specific produced water or tailings application must be determined by bench-scale jar testing with the actual water to be treated, because the colloidal stability of fine particles depends strongly on their surface chemistry, which varies between formations and between fields.
• The capillary suction time (CST) test, the standard laboratory method for quantifying the dewaterability of fine-particle suspensions, measures the rate at which filtrate drains from a suspension sample placed on a piece of filter paper, with faster drainage (lower CST value) indicating a more easily dewatered suspension; adding flocculant to the suspension typically reduces the CST dramatically (by a factor of 2-10) compared to the untreated suspension, because the flocculated aggregates have a much higher settling rate and filtration rate than the individual colloidal particles; this improvement in CST translates directly to higher throughput in centrifuge or belt-press dewatering equipment in produced water treatment plants and in the cuttings dryers used to reduce retained oil on OBM cuttings before offshore discharge; the flocculant type and dosage that minimizes CST in a jar test is the starting point for optimizing the full-scale dewatering process.
• Flocculation in oil sands tailings management is one of the most challenging and commercially important applications of flocculation chemistry in the petroleum industry: the Athabasca oil sands hot water extraction process generates large volumes of fluid fine tailings (FFT), a stable colloidal suspension of kaolinite and illite clay particles (1-10 micrometers diameter), residual bitumen, and process water that does not consolidate under gravity at practical time scales without chemical treatment; the Alberta Energy Regulator's increasingly stringent tailings management requirements have driven intensive research into flocculant systems capable of converting FFT into a trafficable material (solid enough to support equipment) within months rather than decades; polymer flocculants (primarily polyacrylamide and its derivatives) are the primary treatment, but the optimal polymer requires careful matching to the specific clay surface chemistry of the tailings, which varies between mines and between tailings sources within the same mine; the enormous volumes of tailings requiring treatment (hundreds of millions of cubic meters accumulated across the Athabasca oil sands mining operations) make flocculant cost per cubic meter treated one of the most important economic parameters in tailings management program design.
• Starch and biopolymer flocculants are used in drilling fluid applications where synthetic polymers are undesirable: in deepwater drilling where environmental regulations restrict the discharge of synthetic polymer-containing cuttings, naturally occurring biopolymers (guar gum, xanthan gum, starch derivatives) can provide some flocculant activity without the environmental concerns associated with polyacrylamide and similar synthetic materials; in wellbore bridging and fluid loss control, starches with flocculant properties can help aggregate fine drilled solids and form a tighter, less permeable filter cake on the wellbore face than unflocculated polymer systems; however, biopolymer flocculants are generally less effective per unit mass than synthetic polyacrylamide flocculants, are susceptible to enzymatic degradation by bacteria in the circulating mud system (requiring biocide addition to maintain performance), and have higher cost per unit flocculation activity in most applications, limiting their use to situations where the environmental or regulatory constraints on synthetic polymers outweigh their cost and performance disadvantages.