But what do we mean when we say that?
By Douglass Keller
Water well contractors who use fluids to drill must keep in mind they are not merely drilling holes in the ground. The final objective is to supply water and that requires a hole in the ground.
Water and air by themselves do not qualify as engineered muds. Additions must be incorporated to fresh water, changing the initial properties to create an engineered fluid. Yes, the mud costs will increase, but not as much as operating costs will decrease. Fluid manufacturers sincerely believe this will be a benefit for all involved.
Properties concerned when mixing include: viscosity, density, filtrate, cake thickness, rheology, hardness, and pH.
Viscosity is defined as the resistance to flow (pumped). Viscosity is recorded as seconds divided by a quart using a Marsh funnel viscometer and a graduated cup. The viscosity of fresh water is 26 seconds/quart.
Density measures the weight of the mud in a given volume of fluid using a mud balance, recorded as pounds divided by a gallon. The weight of fresh water is 8.345 pounds/gallon.
Filtration control is designed to prevent free water loss to the formation and minimize wall cake thickness. These two properties are tested with a filter press.
Rheology measures the amount of solids, carrying capacity, and gel strengths. A rheometer is used to test these properties. Total hardness and pH are determined with test strips.
Fluids are water or air-based and adding anything to change their properties is a form of engineering. When air drilling, a foaming agent is commonly introduced to give the air stream properties to remove cuttings and provide hole stability.
The drilling foam can be enhanced with bentonite or polymer additives to achieve better results in adverse drilling conditions. Knowing now that air/foam drilling fluids can be engineered, we will continue our discussion with the focus on water-based drilling fluids.
Fresh water, commonly used as make-up water for drilling fluids, has a viscosity of 26 seconds/quart as measured with a Marsh funnel and quart cup. The density of water, measured with a mud balance, is 8.345 pounds per gallons.
Calcium hardness and pH, measured with strips, are important properties of the make-up water that can be easily adjusted using soda ash. Fresh water is treated with soda ash to remove calcium hardness and increase pH. Calcium is the primary contaminant in fresh water that inhibits the mixing of bentonite and polymer additives. Given all its characteristics, water alone does not provide rheological or filtration control properties.
Bentonite is the base additive to improve the properties of water. Filtration control, tested with a filter press, measures free water loss from the drilling fluid. The addition of bentonite improves water loss, but not to the recommended desired target of 15 cubic centimeters or less regardless of how much is added. However, as the quantity of bentonite is increased, other properties are altered.
Viscosity can become too high, reducing pumping efficiency. In fact, pump pressures can increase to the point that the formation feels the increase, which can create driller-induced hole instability.
Too much bentonite gel increases development time and can reduce well production. Gel strengths also increase with the addition of bentonite, resulting in the propensity of the fluid to circulate cuttings rather than dropping them from the active system, slowing penetration and requiring more time and equipment to remove cuttings.
After the addition of bentonite to fresh water, PAC (polyanionic cellulose) polymers can be added to the drilling fluid. The primary function of PAC polymers is to improve filtration control properties. The filtrate is the measured free water from the drilling fluid that flows from the fluid.
A decrease in the amount of free water available to the formation aids in controlling hole stability. As contractors, we want our drilling fluid to stay in the wellbore to maintain desired fluid levels and hole stability.
The filtration property also correlates with wall cake thickness. Generally speaking, the lower the filtrate, the thinner the cake thickness. Both properties are measured with a filter press. The desirable limit of filtrate is 15 cubic centimeters or less with a wall cake 2/32 of an inch.
Some contractors try to reach these properties with bentonite and water alone. This is like building a brick house without any mortar. PAC polymers act like the mortar, strengthening and sealing the bentonite platelets, or bricks, by creating a wall cake that greatly reduces filtration with a moderate viscosity increase. Filtration control, to many fluids experts, is the most important fluids property to achieve and maintain.
Following the incorporation of PAC polymers to the bentonite fluid, shale-stabilizing or clay-inhibiting polymers may be mixed in the existing fluid system to combat reactive shale/clay in the formation.
Geological information through experience, drilling logs, or exploration is the best way to prepare for drilling through clays. In other words, there is not a property that specifically addresses what additive to introduce to the fluid system until the clay is discovered.
PHPA (partially hydrolyzed polyacrylamides) polymers encapsulate (wrap up) clay and shale, preventing them from reacting with water. To determine if a clay/shale is contaminating the drilling fluid, a full mud test may be needed to get the full aspect of the effects the drilled solids are having on the fluid properties.
An important thing to remember when using PHPA polymers is that bentonite is also clay. Bentonite must be pre-hydrated before introducing it to an active fluid system that contains a PHPA polymer. A pre-mix tank is highly recommended.
PHPA properties are usually present in a fluid as indicated by an increase in viscosity. With small chain PHPA polymers, filtration control can also improve. If using mud-cleaning equipment with shaker screens, a small chain PHPA polymer may be compatible with select shaker screens.
A key point to make about PHPA polymers is the amount of PHPA may not be determined until the reactiveness of the shale/clay is known to the contractor. Remember, more is not always better. In many cases, on-site visits result in a reduction of certain additives, usually bentonite.
Maintaining the Fluid
As the drilling process continues, fluid properties can be monitored regularly by testing for the properties initially designed into the drilling fluid. Often changes in the test results are used to determine if the correct amount and types of additives are being introduced into the active fluid system.
The primary objective, of course, is to maximize the borehole value. Testing is required to monitor the properties; undesirable test results can lead to appropriate changes to the active fluid system. Monitoring penetration rates and inspection of cuttings is also an important visual test that fits in nicely with fluids property testing.
This is a basic approach to designing an engineered fluid that can be maintained throughout the drilling process. The properties and testing procedures are derived from the oil field industry and still in use. If the fluid meets the desired properties, the correct quantity and combination of additives is adequate.
We are working together to drill water wells for our communities. Yes, mud costs may increase, but operation costs will certainly decrease—less development time, fewer hole problems, and less time on the job. Time is money as they say.
Douglass Keller is a field service representative for Baroid IDP. He has been with Baroid for nine years and has done training for the National Ground Water Association, the Florida Ground Water Association, Baroid’s week-long program in Texas, as well as distributors. He has worked in the water well industry, mineral exploration, and construction.