The Good, Bad, and Ugly of Wall Cake

Knowledge of wall cake can make it an asset and not a liability during well installation.

By Marvin F. Glotfelty, RG

Figure 1. An undeveloped well (A) will have a wall cake on the borehole face, which will impede the flow of groundwater into the well. In a fully developed well (B), the wall cake will be removed to allow better groundwater flow into the well.

The productivity and efficiency of a well’s performance are the primary objectives for just about every water well installation or redevelopment project.

That goal is applicable whether the water is to be used as a public drinking water source, for industrial supply, for agricultural use, or for any other purpose. To achieve optimal productivity and efficiency, the well must be adequately developed to remove most (if not all) of the wall cake that accumulated on the borehole wall during well drilling.

When groundwater enters a well, it passes through several media types as it flows from the aquifer, through the filter pack and screen, and then into the well. The well screen will generally have an effective porosity much greater than the formation pore spaces, so the screen itself will not pose an impediment to the flow of water or the productivity of the well.

The filter pack sand will be designed and installed in such a manner as to have a hydraulic conductivity much greater than that of the aquifer, so the filter pack envelope won’t impede the flow of water into the well.

Thus, the primary impediment that obstructs groundwater flow into any well is the wall cake (Figure 1). Thus, the singular focus of well development is to break down and remove the wall cake because it is the wall cake that hampers groundwater flow (reduced productivity) and increases water-level drawdown (reduced efficiency).

Removal of the wall cake (also termed filter cake, mud cake, or skin damage) is a critical task, and yet some folks in our industry have an inadequate understanding of just what the wall cake is, how it was formed, and what the effects of wall cakes are.

Figure 2. Conceptual diagram of drilling fluids in a dispersed state (A) and flocculated state (B). Drilling fluid with low filtrate will result in a thin wall cake (C), whereas drilling fluid with a high filtrate will result in a thicker wall cake (D).

Those effects include benefits (the good), challenges (the bad), and sometimes significant problems (the ugly). This column is intended to clarify some good, bad, and ugly aspects of wall cakes so they can be managed by groundwater professionals to maximize the good and minimize the bad and ugly features of wall cakes.

Many people associate wall cakes only with boreholes drilled with the direct mud rotary method. Mud rotary drilling does create a wall cake, but less pronounced wall cakes will also result from just about every other type of drilling.

No matter how effective the drill bit is at breaking up and pulverizing the formation, the drilled cuttings still must be removed from the borehole. To achieve that, the cuttings must be lifted out of the hole by some force.

Hydraulic forces of the drilling fluid are used to circulate the cuttings out of the hole during mud rotary drilling, while various other types of energy can also be used to clear the cuttings. For example, pneumatic energy from compressed air is used while air rotary drilling, and mechanical energy used from bails or auger flights is used with cable tool or auger drilling, respectively.

Regardless of the drilling method used, there will be some amount of wall cake that results from the pressure against the borehole wall, resulting from the process required to remove drilled cuttings.

The Good

Figure 3. Fine-grained silt or clay from the formation can become incorporated into the wall cake.

For wells drilled using circulated drilling fluid, we can focus on the characteristics of that fluid to understand just how the wall cake is formed. The components of drilling fluid (drilling mud) generally include bentonite clay, water, and additives of polymer and soda ash (Na₂CO₃). The bentonite clay thickens the fluid mixture and increases its viscosity (resistance to flow) so that the carrying capacity of the fluid is improved.

At the microscopic scale, bentonite clay particles are platelet-shaped and carry a negative electric charge. The clay platelets can be suspended in the water-clay slurry when they are in a dispersed state, as illustrated in Figure 2-A. When polymer or soda ash additives are introduced to a drilling fluid mixture, the bentonite becomes flocculated (Figure 2-B), as the clay particles are bonded from the forces of cation exchange, hydrogen bonding, or Van Der Waals attractions.

Figure 4. Differentially stuck drill pipe can result when a thick wall cake and poor borehole alignment causes the drill string to lie against the borehole wall. The difference between the higher hydraulic pressure inside the borehole, and the smaller hydraulic pressure within the formation will prevent the drill pipe from being rotated or moved up or down, although fluid circulation will be uninterrupted.

A flocculated drilling fluid is preferable since it will have a greater viscosity and carrying capacity without adding additional bentonite. While the viscosity of a drilling fluid is a critical attribute, that fluid characteristic is independent of wall cake formation. It is possible to have a drilling fluid with high viscosity that results in a thin, relatively hard, and essentially impermeable wall cake, which are the desired wall cake characteristics. It is also possible to have a drilling fluid with low viscosity that results in a thick, fluffy wall cake that will be problematic.

Wall cake formation is the direct result of the filtrate volume characteristic (also termed water loss) of drilling mud. As water migrates from the drilling fluid into the adjacent formation, it will leave behind a layer of bentonite clay platelets on the borehole wall. Therefore, a higher filtrate value will allow more water to migrate into the formation and create a thicker wall cake, and a lower filtrate will result in a thinner wall cake (Figures 2-C and 2-D).

It is important to understand that filtrate volume, and therefore wall cake thickness, is a controllable characteristic of the drilling fluid, and not a characteristic of the formation.

When drilling fluid is being checked by a driller or mud engineer, the filtrate volume is measured with a filter press, during which a small volume of drilling mud is pressurized to 100 psi (using a CO₂ cartridge or nitrogen tank) for 30 minutes (or it can be pressurized for only 7½ minutes with the results doubled).

The water (filtrate) that is pushed out of the drilling fluid through a 200-mesh filter is collected in a graduated cylinder and measured in cubic centimeters (equal to milliliters). The thickness and characteristics of the remaining clay layer on the filter paper will be analogous to the wall cake in a borehole that’s been drilled with this fluid. The filtrate volume should not significantly exceed 10 cubic centimeters, and the wall cake thickness on the filter paper should be no more than about 2/32 of an inch.

When the circulated drilling fluid creates a thin, hard wall cake, there are a number of benefits that result to constitute the good of wall cakes. A thin, hard wall cake will do a good job of stabilizing the borehole during drilling, and such wall cakes can be readily broken down and removed during well development.

The Bad

On the other hand, a thick, fluffy wall cake will do a poor job of stabilizing the borehole and may allow formation material to slough into the borehole during drilling or during well construction. Sloughed formation material can cause stuck drill pipe and may also disrupt the installation of annular filter pack or sealing materials.

A thick, fluffy wall cake will also potentially cause excessive torque along the drill pipe since the thickness of the wall cake grows inward when filtrate volumes are large. Thus, for boreholes with thick wall cakes, the borehole diameter is effectively reduced.

In addition to the reduced effective diameter of the borehole, high filtrate/thick wall cake conditions will result in excessive quantities of filtrate water percolating into the adjacent formation, which may cause swelling of native clays and further destabilize the borehole.

These conditions of high filtrate and thick fluffy wall cakes constitute the bad aspect of wall cakes. These conditions are avoidable with proper drilling fluid maintenance and additives.

The Ugly

In addition to filtrate, another important drilling fluid characteristic that impacts wall cakes is mud weight.

Drilling fluid is circulated through a closed system, wherein the drilling mud is pumped from the bottom of the borehole up to a mud tank at the land surface, and then back down to the drill bit again. The drilled cuttings drop out at the mud tank, but fine-grained native silt or clay particles may remain in the drilling fluid, and if that occurs, it will be reflected by increased solids and weight of the drilling mud.

The percent solids in drilling fluid should generally be no more than about 2%, and the weight of drilling fluid should be in the range of 8.4 to 9 pounds per gallon (clear water is about 8.3 lb./gal).

Elevated mud weight and high solids content is an indicator that the wall cake may be compromised by incorporating fine solids that have been recirculated in the borehole, and those solids may cause the wall cake to become thicker and less stable, as shown in Figure 3. Such conditions are a clear indicator that the driller needs to improve their solids control system (including adequate mud tank size and baffling, shale shaker, desanders, and desilters, as required).

If the drilling fluid is not properly maintained such that the filtrate, percent solids, and weight of the drilling fluid become elevated, conditions may result in the drill string becoming differentially stuck, which is a condition where the drill pipe lies against the wall cake, such that the difference between the higher hydraulic pressure inside the borehole and the lower hydraulic pressure in the adjacent formation will cause the drill string to become tightly stuck against the wall of the borehole (Figure 4).

This is an example of poor wall cake conditions constituting the ugly aspect of wall cakes.

With proper monitoring and maintenance of the drilling fluid system, and with an understanding of the downhole conditions that may lead to good, bad, or ugly results, groundwater professionals can keep wall cakes as an asset and not a liability during well drilling, construction, and development.

Learn The Art of Water Wells

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Marvin F. Glotfelty, RG, is the principal hydrogeologist for Clear Creek Associates, a Geo-Logic Associates Co. He is a licensed well driller and registered professional geologist in Arizona, where he has practiced water resources consulting for more than 35 years. He is author of The Art of Water Wells (NGWA Press, 2019) and was the Groundwater Foundation’s 2012 McEllhiney Lecturer. Glotfelty can be reached at mglotfelty@geo-logic.com.