Low-Yield Clay

Low-yield clays are naturally occurring clay minerals found in formation rock that contribute little to the viscosity or gel strength of a water-based drilling mud when they enter the mud system as drill solids. Unlike high-yield bentonite (which can build significant viscosity at low concentrations), low-yield clays such as kaolinite, illite, and chlorite have a platelet structure that does not hydrate or expand appreciably in water. When these clays are drilled out of the formation, they become contaminant solids in the mud system. They increase the mud weight but do not contribute useful rheological properties, which means they degrade drilling performance without providing any of the benefits of designed mud additives.

Key Takeaways

Low-yield clays are characterized by their yield point in barrels of viscous mud produced per ton of dry clay. High-yield bentonite produces 85 to 100 barrels of 15-centipoise mud per ton. Medium-yield bentonite produces 35 to 84 barrels per ton. Low-yield clays (kaolinite, illite, chlorite, mixed-layer clays) produce fewer than 35 barrels per ton and are classed as drill solids rather than as useful mud-building additives.
As low-yield clay drill solids accumulate in the mud system, the mud weight increases without a corresponding increase in viscosity. This produces a mud that is heavier than needed (which can cause overbalance and formation damage) while lacking the carrying capacity needed to lift drill cuttings efficiently. The result is slower penetration rates, increased equivalent circulating density, and potential stuck pipe from cuttings accumulation.
The primary response to low-yield clay buildup is mechanical solids control: the shale shaker, hydrocyclone (desander and desilter), and centrifuge are used to remove fine particles from the mud. Low-yield clays that pass through all the solids control equipment must be diluted out with fresh base fluid and new mud additives.
Formation damage from low-yield clay invasion is a secondary concern. Fine clay particles (especially kaolinite) that invade the near-wellbore matrix during overbalanced drilling can block pore throats and reduce permeability. This damage is often irreversible without acid treatment.
In water-sensitive formations, disturbing low-yield clays with water-based mud can cause clay migration, fines plugging, and swelling damage even when the clays are low-yield (low-expanding). Oil-based or synthetic-based muds are preferred in these situations to avoid water contact with the formation clays entirely.

What Makes a Clay "Low-Yield"?

Picture a clay mineral as a flat, microscopic plate, like a tiny playing card. Bentonite (montmorillonite) is a plate that swells dramatically when it gets wet: water molecules push between the layers of the crystal structure, expanding the plate and creating a thick, gooey gel. A small amount of bentonite makes a lot of viscous mud. This is why bentonite is the preferred mud-building additive.

Kaolinite, by contrast, is a stacked plate structure where the layers are strongly bonded together. Water cannot easily push its way in. The plate does not swell. If you put kaolinite into water and stir, you get a suspension of inert particles, not a viscous gel. When kaolinite enters a drilling mud from the formation, it behaves the same way: it adds weight (because the solid particles are denser than water) but not viscosity. You have more solids but no more gel strength to carry cuttings.

Illite falls between bentonite and kaolinite in behavior. It has some interlayer swelling capacity but far less than montmorillonite. Chlorite has even less. Mixed-layer clays (illite-smectite or chlorite-smectite) vary in yield depending on the proportion of smectite in the interlayer. All of these fall into the low-yield category for practical drilling purposes.

Fast Facts

In the Mannville Group of central Alberta, some formations contain 15 to 30 percent kaolinite by volume in the sand fraction. This kaolinite is not cemented into the rock matrix: it sits as booklets of loosely stacked plates in the pore space. When water-based mud contacts this formation, the kaolinite booklets can detach and migrate with fluid flow, physically plugging pore throats far from the wellbore. The result is a skin factor (damage) that can reduce well productivity by 30 to 60 percent even before any formation water is produced. Recognizing low-yield clay damage type on a post-completion well test and correcting it with HCl-HF acid blends has been a standard practice in Alberta tight gas development since the early 1990s.

Solids Control for Low-Yield Clay Drill Solids

Solids control equipment is the first line of defense against low-yield clay buildup in the mud system. The shale shaker is the primary screen, which removes the coarser cuttings before they can be ground finer by the drill bit and bit cones. Shaker screens with a mesh size of 120 to 200 mesh (75 to 120 microns) are commonly used in formations with significant clay content.

Hydrocyclones (desanders remove particles above 40 microns; desilters remove particles above 15 microns) handle the intermediate particle size range. The underflow from the hydrocyclones carries the removed clay particles to a waste pit. The overflow, which is clean mud, returns to the active system.

For the finest low-yield clay particles (below 5 to 10 microns), only a centrifuge provides effective separation. High-speed centrifuges at 2,000 to 3,500 RPM can separate colloidal-sized clay particles from the mud. The centrifuge underflow (the clay-laden fraction) is discarded. The centrifuge overflow returns to the system.

Even with all three stages of solids control running, some low-yield clay accumulates in the mud over time. When solids content measured by retort (the percent by volume of solids in a mud sample) rises above 10 to 15 percent, dilution is needed. Dilution means dumping a volume of mud and replacing it with base water and fresh chemicals. Dilution is expensive in terms of chemical cost and waste disposal, but it is the only way to reset the solids balance when mechanical separation cannot keep up.

Low-Yield Clay Versus High-Yield Clay in Mud Design

A mud engineer designing a water-based mud for a new well in a clay-rich formation has to solve a balance problem. The mud must have enough bentonite (high-yield clay) to build the viscosity and gel strength needed for cuttings lifting. It must not have so many low-yield clay drill solids that the mud weight, plastic viscosity, and equivalent circulating density rise above the formation fracture gradient.

In the Duvernay shale play in Alberta, the overlying Ireton shale is thick and rich in mixed-layer clays. Drilling through several hundred metres of Ireton before reaching the Duvernay target inevitably loads the mud system with low-yield clays. Operators use freshwater polymer muds (polyacrylamide or PHPA-based) rather than bentonite muds because polymer muds are better at inhibiting the dispersion of low-yield shale clays into fine particles that pass through solids control. PHPA molecules adsorb onto the clay surface and keep the clay particles coarser and easier to remove on the shaker.

Low-yield clay is also called a non-commercial clay, drill solids clay, or contaminant clay in mud engineering. Related terms include high-yield clay (a clay mineral, primarily sodium bentonite, that produces a high volume of viscous mud per unit weight; the standard additive for building viscosity and gel strength in water-based drilling muds), solids control (the system of equipment including shale shaker, hydrocyclones, and centrifuge used to remove drill solids from the active mud system; critical for managing low-yield clay contamination), retort analysis (a mud testing method that heats a mud sample to vaporize the liquids, allowing the volume fractions of oil, water, and solids to be measured; used to track low-yield clay buildup in the mud), plastic viscosity (a component of drilling mud viscosity that is directly related to the size and concentration of solid particles; low-yield clay drill solids increase plastic viscosity without increasing yield point, degrading cuttings transport efficiency), and inhibitive mud (a drilling fluid formulated to suppress hydration and dispersion of formation clays; PHPA polymer muds and potassium chloride muds are common inhibitive systems used in low-yield clay-rich formations).

When Low-Yield Clay Buildup Stalled a Cardium Well in West-Central Alberta

A drilling crew was drilling a horizontal Cardium well in the Pembina field area of west-central Alberta. The vertical section drilled through approximately 900 metres of Cretaceous shales before reaching the Cardium sandstone target. The shales in the upper section were illite-rich with some mixed-layer illite-smectite. The low-yield clay from these shales began accumulating in the 350-cubic-metre active mud system from about 600 metres depth.

By the time the drill bit reached the Cardium top at 2,100 metres, retort solids were at 18 percent by volume, up from 5 percent at spud. Plastic viscosity had risen from 12 to 34 mPa·s. The equivalent circulating density in the horizontal leg was approaching 14.2 kPa/m, within 0.5 kPa/m of the Cardium fracture gradient.

The mud engineer called for a dilution cut of 80 cubic metres (dumping 80 m³ of mud and replacing with 60 m³ of fresh water plus 20 m³ of premixed chemicals). The dilution reduced solids to 11 percent and plastic viscosity to 19 mPa·s, restoring a safe circulating density margin. The dilution cost CAD 28,000 in fresh chemicals and 14 hours of non-drilling time. A second dilution was needed halfway through the lateral section.

Post-well review recommended running the centrifuge at higher throughput starting from 400 metres depth rather than waiting until the solids reached the problem threshold. Earlier mechanical removal would have reduced the amount of dilution needed and saved an estimated CAD 19,000 in chemical cost and 8 hours of rig time on the horizontal section.