Many of the components in drilling fluids can affect the efficiency of solids-control devices. As discussed in the previous section, fluid rheology, shale inhibition potential, wetting characteristics, lubricity, and corrosivity can all affect both the properties of cuttings and the performance of solids-control equipment. Key components that affect those properties include colloidal materials, macropolymers, conventional polymers, and surface-active materials.
1.Colloidal and Fine Solids
Clay solids (e.g., bentonite, attapulgite) along with clay-laden drilled solids, weighting material, and fine grades of bridging material (e.g., limestone, CaCO3) will all affect the viscosity of a drilling fluid at moderate to high shear rates, thereby increasing the retention of drilling fluid on solids. Although elevated low-shear-rate viscosity is beneficial in many ways (improved hole cleaning, etc.), nothing is gained by having elevated high-shear-rate viscosity. Thinners are often used in both waterbased and oil-based muds to reduce high-shear-rate viscosity, especially in high-density muds. Unfortunately, thinners tend to reduce the lowshear-rate viscosity more. One of the best solutions is to minimize the reliance on large quantities of bentonite and other clays for viscosity control and utilize mud systems that provide high yield stress and Bingham-like rheology with low thixotropy, that is, muds with high lowshear-rate viscosity and nonprogressive gels. Examples include xanthanbased
polymer muds and mixed metal systems, such as mixed metal hydroxide, mixed metal oxyhydroxide, mixed metal oxide, and mixed metal silicate.
Weighting materials increase the entire rheology profile, and some weighting materials tend to be abrasive not only to the drill string and casing, but also to solids-control devices. Shaker screen life tends to be shorter with weighting materials like hematite, magnetite, and ilmenite, all of which are twice as hard as barite. Alternatives include going to a finer grind of weighting material, treating the system with lubricants, and using high-density brines instead of the standard weighting material. The latter option is risky, though, because the brines tend to be corrosive and may present incompatibility problems with polymers in the drilling fluids, especially acrylamide-based polymers like PHPA.
Large solids, such as those typically used as LCM, have irregular shapes that are generally classified as flake, fiber, or granule. These are usually removed at the shakers and discarded along with the cuttings. Treatment of whole mud with LCM (rather than 100-bbl pill or slug) requires reintroduction of fresh LCM into the circulating system before pumping it back downhole. At low concentrations (2 to 15 ppb), LCM tends to have little effect on standard mud properties. However, at higher concentrations, LCM will tend to absorb significant amounts of water (especially noticeable in invert-emulsion muds) and increase high-shearrate viscosities of WBMs and NAFs; a detergent or wetting agent will usually remedy this problem, though addition of base fluid may also be necessary. For applications requiring high concentrations of LCM in the whole mud, a separate set of shakers to remove and recycle the LCM back to the active mud system downstream of the centrifuge may prove economical. High concentrations of LCM will tend to blind shaker screens when the shaker is used to remove both cuttings and LCM, and an additional scalping shaker may be necessary, whether or not the LCM is recycled back to the active mud system.
Like conventional LCM, gilsonite and other asphaltenes will tend to plug shale shaker screens, particularly triple-layer designs; single- or double-layer screens work much better [Amoco].
Highly shear-thinning polymers that are used to provide high viscosity at very low shear rates (e.g., xanthan gum) can be expected to affect the performance of low-shear-rate solids-control devices, such as settling tanks, hydrocyclones, and centrifuges. If these polymers exhibit viscoelasticity, the elastic nature of the polymers at low shear rates may reduce efficiency of these low-shear-rate devices even more. Extensional effects (see the Section 2.3), which can produce a different kind of
viscoelasticity, may also be generated by these polymers. In this case, it is usually extensional viscosity, rather than extensional elasticity, that is of most concern. Extensional viscosity manifests itself at high fluid velocities and may be important in any high-throughput solids-control device that contains orifices or openings.
Shale encapsulators are high-molecular-weight (HMW) polymers that serve to inhibit dispersion of large cuttings, but they also tend to flocculate clay fines. Some of the more popular shale encapsulators are PHPA, AMPS-acrylamide copolymers, and HMW polypropylene glycol. Generating a coarser distribution of solids in the mud can improve the efficiency of the solids-control equipment. On the other hand, in the absence of fines, these polymers exhibit extensional viscosity, a property that can cause blinding of shaker screens. They also tend to adsorb strongly on solid surfaces, such as shaker screens, thus exacerbating the problem. Addition of a small quantity (less than 3% by volume) of fine solids (bentonite, barite) to the mud will deplete the highest-molecularweight fraction of the polymer first, which is the fraction that is mainly
responsible for the extensional viscosity and strong adsorption. At the same time, at that low level of solids, shear viscosity (the type of viscosity normally measured) will not be affected very much. Thus, addition of a small quantity of fine solids as soon as possible to a polymer mud will increase shaker screen conductivity and throughput of hydrocyclones and centrifuges.
Another problem that can occur with dry HMW long-chain polymers like PHPA is formation of ‘‘fish eyes’’ and ‘‘strings’’ in the mud that are attributable to improper mixing. When incompletely hydrated and dispersed, the polymer cannot perform as expected and can ‘‘gunk up the works,’’ blinding screens and plugging hydrocyclones and centrifuges.
The following guidelines can help to prevent such problems:
. Add all mud products after initiation of the solids-control equipment.
. Add HMW dry polymers like PHPA slowly over a full circulation, using a venturi hopper.
. If bentonite is part of the formulation, mix some in with the polymer before adding the latter through the hopper.
. If possible, coat HMW polymers with an oil, or better yet, predisperse
the polymer in oil or mutual solvent before adding it to the system.
In all cases, consult the product manufacturer for proper addition.
The surface-active additives in mud products are designed to adsorb on a specific type of substrate: cuttings (shale inhibitors, antiaccretion agents, and oil wetting agents), weighting material and LCM (oil-wetting agents), liquid internal phase (emulsifiers), and drill string/casing/well bore (lubricants, ROP enhancers). However, none of these additives is perfectly selective for its designed target substrate and every one adsorbs on various types of surfaces. Lubricants and ROP enhancers, in
particular, will generally adsorb on anything, including shaker screens and the internal surfaces of hydrocyclones and centrifuges. They may accumulate to such an extent that they reduce throughput in these devices. Little can be done to alleviate this other than to avoid overtreatment of the mud with surface-active additives; alternatively,
a permanent coating of a low-energy substance (e.g., TeflonTM) on the exposed surfaces of the equipment will discourage adsorption, but that solution can prove expensive. On the other hand, a very thin coating of mud lubricant, ROP enhancer, or wetting agent can actually be beneficial, inhibiting corrosion and solids accumulation in and on solidscontrol equipment surfaces. Using modest treatment levels will promote formation of relatively thin coatings without forming thick deposits that can reduce solids-equipment performance.
Low-molecular-weight (LMW) shale inhibitors (e.g., various amines) serve as clay intercalators and hydration suppressants and keep cuttings from swelling and dispersing; like shale encapsulators, they can inhibit degradation of drilled solids and make them easier to remove. Thinners and deflocculants, which include lignosulfonates, lignites, polyphosphates,and LMW acrylamides, improve solids-separation efficiency of all the solids-control devices by reducing the overall viscosity of the drilling fluid.