Drilling Fluids Rheology: Basics and Definitions

Rheology is a part of science of matter flow and deformation. It can help us understand and explain the behavior of the fluids under a verity of conditions including temperature, pressure and external forces. 


It is among the most used rheological terms. It can be defined as the resistance of the substance to flow. In the oil field, many terms related to viscosity are used to describe the viscosity of drilling fluids

- Funnel viscosity 
- Apparent viscosity 
- Plastic viscosity 
- Yield point 
- Low shear rate viscosity 
- Gel Strengths 

Funnel Viscosity 

The funnel viscosity is measured using the March funnel. It is used as a relative indicator of drilling fluid condition, it does not provide accurate information about the flow characteristics. It is used in the oil field to detect any changes in the drilling fluids properties. 

Apparent Viscosity 

It is reported in oil field as the reading 300 RPM or the half of the reading 600 RPM of the mud viscometer. 

Plastic Viscosity 

This drilling fluid parameter describes the resistance to flow caused by the mechanical friction and it is affected by: 

- Solids concentration 
- Size and shape of the solids 
- Viscosity of the fluid phase 
- The oil water ratio in invert emulsion drilling fluids 

Any increase in the plastic viscosity can be related to an increase in solids concentration, reduction of the solids size, a change in shape of solids or the combination of all these parameters. The increase of solids concentration can be confirmed by density changes or retort analysis. 

Some solids are added to the drilling fluids to adjust fluids properties such bentonite for viscosity and barite for density. Drilled solids or cuttings are additional particles which can affect the planned properties of the drilling fluids and have to be removed from the system using: 

- Settling 
- Dilution or displacement 

Plastic viscosity is also related the viscosity of the fluid phase. The viscosity of water decreases with the increase of the temperature causing plastic viscosity decrease. The plastic viscosity should be kept as low as possible because low level of plastic viscosity results in higher energy at the drilling bit, better hole cleaning and less wear on equipment. 

At the rig site the plastic viscosity is calculated as follows: 

PV (cp) = Reading 600 - Reading 300 

Yield Point 

The yield point can represent the level of the electro-chemical forces in the drilling fluids. These forces are a result of charges on the particles surfaces. The yield point can be also defined as the resistance to flow which can be controlled by proper chemical treatment. The yield point decreases as the attractive forces are reduced by chemical treatment and increases with use of some viscosifier. 

The yield point is usually used as an indicator of the ability of the drilling fluids to suspend the solids and remove them from the wellbore. The yield point is calculated as follows: 

YP(lb/100 ft²) = 2 x Reading 300 – Reading 600 

Low shear rate viscosity 

It has been proved through laboratory studies and field experience that the low shear viscosity values (Readings 6 RPM and 3 RPM) give better understanding on hole cleaning than yield point in highly deviated, horizontal and extended reach wells. This parameter fill the gap in understanding the drilling fluids behavior between the dynamic viscosity measurements of PV, YP and the static measurements of gel strength. 

Gel Strengths 

Some fluids have the ability to form gels in static situation and come to flow when the shear is applied, this property is called thixotropy. Gel strength reading in the oil field is taken for 10 seconds and 10 minutes. The gel strength is related to the amount and types of solids in suspension, time, temperature and chemical treatment. The gel strength should be controlled to do not exceed the normal trend of gelation because excessive gel strength can lead to many problems such as: 

- Entrapment of gas and air in the drilling fluids 
- Excessive pressure when breaking circulation 
- Excessive swabbing while tripping out the hole 
- Excessive surging when tripping in the hole 
- Reduction of solid control equipment efficiency 

Gel strength and yield point are both measured to give information about the attractive forces in the drilling fluids, but the gel strength is measured for static forces and yield point is measured for dynamic forces. 

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