Are rheological properties important in the drilling fluid on HDD projects?

By Trenchlesspedia Staff | Last updated: July 7, 2023

Yes, very important! The following is a basic explanation based on using what is known as The Bingham Plastic Model.

Plastic Viscosity (PV), Yield Point (YP) and Gel Strengths

Plastic Viscosity

PV is known as the plastic viscosity of the drilling fluid. A low PV indicates that the mud is capable of drilling rapidly because of the low viscosity of the mud exiting at the bit. A high PV is caused by a viscous base fluid and by excess colloidal solids. The only ways to lower PV is to dilute or remove (dump) mud from the system and to have adequate solids control equipment

Yield Point

YP is known as the yield point of the drilling fluid. It is an indication of the drilling fluids ability to carry the cuttings from the wellbore to the surface. If the YP is not properly adjusted, the result is poor hole cleaning. Poor hole cleaning can ultimately result in severe operational problems such as high torque/drag, reduced rates of penetration and stuck pipe.

PV and YP readings are calculated from viscometer data taken at 600 and 300 rpm. The PV equals the 600 rpm reading minus the 300 rpm reading. The YP equals the 300 rpm reading minus the PV.


600 reading – 300 reading = PV

300 reading – PV = YP

If 600 rpm reading is “46” and 300 rpm reading is “35:”

46 – 35 = 11 This is your PV.

35 – 11 = 24 This is your YP.

Gel Strengths

Gel strengths are indications of the drilling fluids ability to suspend cuttings when circulation is ceased. By definition, the gel strength is the shear stress of a drilling fluid that is measured at a low shear rate after the drilling fluid has been static for a certain period of time.

There are operational impacts worth noting when excessive gel strengths and low gel strengths are present. Excessive gel strengths can lead to high pump initial pressures when breaking circulation after the drilling fluid is static for a period of time. The high pump pressure may lead to formation fractures and lost circulation.

Low gel strengths can lead to problematic conditions such as stuck pipe and hole pack off due to insufficient cuttings suspension.

10-Second Gel

Gel strength readings are obtained by mixing drilling fluid at high rate in a viscometer for at least 10 seconds, then stopping to let the fluid stand undisturbed for about 10 seconds. At the 10-second mark, the hand wheel is turned slowly and steadily (3 rpm) in the direction to produce a positive dial reading. The maximum reading is the initial gel strength (10-second gel) reading.

10-Minute Gel

The 10-minute gel is obtained by taking the same sample and stirring at a high rate for at least 10 seconds, stopping and letting the fluid stand undisturbed for 10 minutes. Right at the 10-minute mark, the hand wheel is turned slowly and steadily (3 rpm) in the direction to produce a positive dial reading. The maximum reading is recorded as the 10-minute gel.

The daily readings of rheological properties of the drilling fluid on horizontal directional drilling projects are essential in the success of the drilling process. There are many drilling fluid products available to control the rheological properties of the drilling fluid on the market today.

It is highly advised that only certified mud engineers carry out the duties of executing the testing and monitoring these properties with daily data reports.

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Trenchlesspedia Staff
At Trenchlesspedia, we deliver educational content about trenchless construction and trenchless rehabilitation technologies and methods. Our aim is to connect trenchless industry professionals to the solutions and issues that matter most with our complete source of industry-specific information and answers.

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