• Borehole stability – the weight of the fluid in the borehole overcomes the formation pressure and keeps the directional hole from collapsing. But this is a balancing act, if the mud is too dense (heavy) two things may occur.
  • Frack outs – bad.
  • Drilling fluid loss to the formation – formation damage and long development effort.
• Bit cleaning – the drilling mud exits the bit near the cutting face, keeping the teeth and/or cutting surfaces clean of the formation material. Have you ever heard the driller say his bit was “balled up”? That occurs when the drilling fluid is not cleaning the drill bit and clay builds up on the teeth.
• Remove the cuttings from the bit face and borehole – we only want to cut the formation with the bit one time. The mud removes the cuttings from the bit face and carries them to the surface. As the bit advances, the drilling fluid must have enough viscosity to carry the cuttings to the surface.
• Gel strength – the drilling fluid must be able to keep the cuttings in suspension when the mud is not moving up the borehole.
• Cool the guidance tooling – as we discussed in past “Inflection Points” the location of the bit is determined by the steering tools located in the drill pipe or special sub just behind the bit. During drilling, frictional forces generate heat and the drilling fluid helps keep the guidance tooling cool.
• Minimize fluid loss to the formation – the drilling fluid coats the borehole wall with a thin, impermeable, easily removed wall cake. The wall cake minimizes the amount of drilling mud lost to the formation. Large amounts of drilling fluid lost to the formation can lead to decreased permeability near the well, formation damage and increased development time.

Now that we understand the main functions of the drilling fluid, how do we know if the mud is functioning properly? In many cases, if the well is installed on the first try, and development time is short, we inherently know the drilling fluid did its job. But if the well cannot be constructed after multiple attempts and/or the development process is lengthy, poor drilling fluid properties may be the reason.

First things first – we are going to use fresh water and mix/shear either a dry powder or liquid material with the water in order to create a drilling fluid with the properties to successfully drill the borehole and install a well. What type of powder or liquid? You’ll have to wait for the next newsletter for that detail. So what are the correct drilling fluid properties and how do we measure them?

Fluid Density

As we drill, formation materials (cuttings) will be entrained in the drilling fluid. In order to keep the density of the mud from increasing as drilling progresses, we have to remove the solids from the mud at the surface using a fluid cleaning/recycling system. The other option is to use the mud “one time through”. In other words we do not clean the mud and reuse it, we just send the mud to waste after it reaches the ground surface. So how do we measure the mud weight? We utilize a mud balance – a specially designed scale that weighs liquids in pounds per gallon (ppg). Fresh water weighs about 8.3 ppg. Our intent is to mix a drilling fluid that has a weight of less than 9.0 ppg, which helps maintain the other properties discussed below.

Sand Content

During the advancement of the hole, the drilling fluid carries the cuttings to the surface. When we are utilizing a drilling fluid cleaning/recycling system, shale shakers and de-sanding/de-silting cones remove the cuttings from the mud before the mud is sent to the mud pump and back down the hole. The intent of the drilling fluid cleaning system is to remove the sand from the drilling fluid. Sand is detrimental to the drilling process for several reasons:

• Sand is an abrasive – it can cause serious damage to the mud pumps, fluid swivel, drill pipe and any other surface it touches. Drillers need to keep sand out of their drilling system as much as possible.
• Sand is heavy – if we allow sand to build up in the drilling fluids, the density of the mud increases which can lead to frack outs and formation damage.
• Excessive sand can cause a thick, porous wall cake.

We measure sand content with another simple, direct reading tool commonly referred to as a sand content kit. In normal drilling conditions we want to keep the sand content less than 1% in the fluid we are recirculating back down the hole.

Viscosity

Viscosity is simply defined as “resistance to flow”. In everyday terms, honey has a higher viscosity than water. In the drilling business we measure the viscosity of a fluid by timing how long it takes for a fixed volume of fluid to flow through a standard testing funnel (Marsh Funnel) fitted with a specific sized orifice. Fresh water has a Marsh Funnel viscosity of 26 seconds. What is the proper viscosity for drilling fluid? It depends on the formation. Normal drilling fluid viscosities can range from 36 seconds for fine sand to over 80 seconds for coarse gravel.

(I once had a helper on a rig cut the orifice end of the funnel off so he could use it to fill the rig engine with oil. Using that “modified” Marsh Funnel, water had a viscosity of 18 seconds.)

Density, sand content and viscosity are all very important parameters of a drilling fluid. They are also easy to measure; most drillers, helpers and geologists can use the testing kits to properly determine those three physical properties. However, when we start to consider gel strength and filter cake, things can get somewhat complicated. First let’s tackle filter cake/fluid loss.

Filter Cake/Fluid Loss

As we drill, the drilling fluids start to move through the pore spaces of the formation. As drilling continues, some of the solids start to plug off the pore space and form a wall cake or filter cake on the inside of the borehole. Filter cake is a good thing. We want the drilling fluid to stay inside of the borehole, not travel deep into the formation. The best wall/filter cake is thin, non-porous, flexible and easily removable. Drillers can measure both the thickness of the wall cake and how much water is moving across the filter cake by using a filter press.

Most drillers strive to maintain a filter cake thickness of less than 2/32 on an inch and a fluid loss of less than 15 ml. Many things can cause an increase in the thickness of the filter cake and the amount of fluid loss, including cement contamination and high solids (sand content). Note that some drillers may not have a filter press on the rig site as it is expensive, somewhat fragile and takes training to use properly.

Gel Strength

The technical definition of gel strength from “Groundwater and Wells” is “The minimum shearing stresses that will produce permanent deformation of a colloidal suspension.” The driller definition is “how well will the drilling fluid hold particles in suspension when the mud pump stops?” Think about water – when water is moving it can carry stuff, like sand. However, when water stops moving the sand falls out of suspension very rapidly. We want our drilling mud to hold sand in suspension when the mud pump stops (like when we make a connection) or when we have pulled the drill pipe out of the hole, before we build the well. How do we measure gel strength and what are the units of measurement? A rheometer is used and the units of measurement are lbs./100 ft.2 (or we could measure the
plastic viscosity and yield point). How many of you have ever seen this test run on a drill rig? In my thirty plus years I have probably run this test ten times… all in the lab. Most (99%) water well and environmental drillers do not check the gel strength of their mud. However it is still an important parameter of the fluid. Drillers will know their fluid’s gel strength is “good” if they do not get stuck in the hole.

We discussed some of the properties of drilling fluids (mud) and how to measure them, including density, sand content, viscosity, filter cake/fluid loss and gel strength. So what does all of this mean? In a nutshell, drilling with mud can be complicated and we (drillers) just don’t throw some powder or liquid into water and start drilling. We carefully chose the type of drilling fluid (more on the choices in an upcoming newsletter) based on the end use of the well, the local geology, regulations and experience. We then carefully monitor the parameters of that drilling fluid to insure the borehole is drilled quickly, with the minimum amount of formation damage.