- Cooling and lubricating the bit
- Suspending and transporting cuttings
- Stabilizing the hole
- Control fluid loss and loss circulation
- Reducing torque associated with sticky clays
- Controlling subsurface pressure
Any driller can tell you that no two holes are alike. It is of key importance to match your drilling fluid to the ground conditions. In order to do this, a driller must be able to identify the characteristics of the ground they are drilling through and maximize their drilling performance by managing their drilling fluids correctly.
The driller can work with a distributor of bentonite drilling muds and polymers, as well as the manufacturer, to ensure the use of proper products based on the ground condition.
Once the driller is on site, managing the drilling fluids requires a working knowledge of the properties of the fluids (viscosity, gel strength, fluid loss, sand content, density) to ensure that the best possible mix is achieved to drill through the local stratigraphy. For example, if drilling through a loose unconsolidated formation, the driller will likely want to have a drilling fluid with a viscosity greater than 45 seconds and, depending on the formation, may want to enhance the filter cake with a PAC polymer. If there are larger particles such as gravel, it may be wise to enhance the gel strength of the drilling mud with an additive.
Once the proper drilling fluid is obtained, it is extremely important that the driller test the mud as they are recirculating it. This article will touch on the basic tests as well as some advanced tests that a driller can perform in the field.
Marsh Funnel and Cup
This is a very easy test that measures the viscosity of the drilling fluid. Note that this instrument only measures viscosity under one
flow condition. As fluid is recirculated, regular measurement using the Marsh funnel and cup provides the driller with a trend to monitor gradual or sudden changes in the viscosity of the drilling fluid. The driller will then be able to determine when to add more bentonite and additives to the mixture to keep the drilling fluid characteristics constant.
The mud scale is a useful tool to measure the weight of the mud. By comparing this value to the original mud weight, the driller can quickly identify the amount of additional solids that are being held in suspension in the drilling fluid. An ideal mud weight is 8.6-9 pounds per gallon. Allowing the weight of the mud to increase significantly may result in formation damage and higher filtration rates. High-density muds can wear on pumps and tooling as well. Additionally, a heavy mud cannot be adequately displaced by grout when sealing the annular space between the borehole and casing. The driller may have to evaluate the pit design to better allow for cuttings to fall out of suspension.
Sand Content Kit
The sand content kit allows the driller to measure the percentage of abrasive solids that are rejected by a 200 mesh screen that is in the drilling fluid. It is imperative that the driller control the sand content of the recirculated drilling fluid. This abrasive grit will cause damage to pump parts as well as increase the mud weight.
Every driller should seriously consider owning the three tools above. They are all simple to use in the field and provide the driller with insight into the trend of the characteristics of the drilling fluid. These instruments can give much more detailed information than relying on the use of site and touch in the field. Slurry test kits that contain all three instruments in a hard-sided case are readily available. Videos showing the proper use of these instruments can be seen on YouTube.
More advanced tools are also available to the driller. These include a field rheometer and a filter press. These tools allow the driller to better understand the rheology and filtrate control of the drilling fluids.
A field rotational rheometer (viscometer) is a hand-operated instrument that allows rheological measurements to be taken at two speeds, 300 rpm and 600 rpm. Utilizing the Bingham plastic model, an assumption is made that the shear rate has a linear relationship to the shear stress. Then, these two measurements are plotted on a graph where the slope is the plastic viscosity (PV) and the yield point (YP) is the point where the shear rate is zero. A fluid with a yield point of zero would not be able to suspend cuttings without flow.
A high plastic viscosity is correlated with increased solid content in the drilling fluid. This typically can also be seen by an increasing mud weight. However, if an increasing plastic viscosity is seen without noticeable change in mud weight, then there is an increase in ultra-fine solids in the fluids. This early indication of ultra-fine solids can allow the driller to take early corrective action. The yield point relates to the stress required to start the fluid movement and correlates with inter-particle interactions including bentonite interactions and polymer-bentonite bridging. The higher the yield point, the higher the ability for the fluid to suspend cuttings. In the field these measurements are used to keep the characteristics of the drilling fluid on trend by maintaining low measurements for plastic viscosity (control solids content) and maintaining a yield point high enough to suspend cuttings without increasing pressure excessively.
Another tool that can be used in the field is a filter press. The filter press allows the driller to evaluate the ability of the drilling fluid to create an effective filter cake and control fluid loss. A sample of the drilling mud is pressurized to 100 psi against a filter paper. The driller can then see how effective the filter cake is and get an indication of the ability for the filter cake to control fluid loss (filtrate). The filter cake should be very thin and dense. It should measure between 1⁄32 inches and 2⁄32 inches. This allows the driller to make sure that the bentonite is properly yielded and determine if a PAC polymer may be required to tighten up the filter cake. A thick filter cake can have negative implications. The contact between the filter cake and the drill stem can be increased leading to differential sticking or an increase in rotational torque.
The use of drilling fluids is a science that enables a driller to have much more control over the success of a borehole. The slurry test kit, rheometer and filter press are all tools that enable a driller to ensure the understanding of the drilling fluid and to monitor the effectiveness of the mud during the drilling process. Keeping the characteristics of the drilling fluid consistent and correcting problems early lead to the successful and timely completion of a drilling project.
Michael Kleespies is Territory Sales Manager for CETCO Drilling Products. For more Drilling Fluids columns, visit www.thedriller.com/drillingfluids.
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