The Composition Functions and General Nature of Rotary Drilling Fluids

6.1 Introduction

Early spring-pole drillers, including the ancient Chinese, added water to the borehole as an aid to rock softening and cutting removal. Since the drilling fluid is a comparatively unimportant feature of the cable tool method, little thought was given this subject until the advent of rotary drilling. As pointed out previously, the continuous circulation of the drilling fluid (mud) was a principal reason for the early success of rotary tools in areas considered undrillable by the percussion method. Consequently, the advancement of drilling fluid technology has contributed greatly to the success of rotary drilling.

The first rotary drilling fluid was water. When quicksand threatened the progress of the famous Spindletop well, a muddy fluid was mixed by driving cattle through a shallow water pit. When used as a drilling fluid, this clay-water mixture lined the borehole, prevented caving of the quicksand, and allowed drilling to continue.1 From this crude beginning has evolved the drilling fluid technology of today.

The extent to which drilling mud properties must be controlled varies with geologic conditions. In soft rock areas successful completion of a well may require very precise control of mud properties, and the mud used is often an expensive and complicated chemical mixture.

In hard rock areas plain water may be a satisfactory and even superior drilling fluid. In addition to liquid muds, both air and gas are used as drilling fluids with spectacular results in many areas. Therefore, we may surmise that the selection of mud type is governed by the specific requirements of the geologic area in question, and depends on the drilling fluid's ability to perform the functions necessary in that area.

6.2 Testing of Drilling Fluids

The purpose of testing drilling muds is to determine their ability to perform certain necessary functions. The industry standard for mud testing is the API Recommended Practices No. 29, from which some of this section is taken. Also, several excellent manuals and books are available which cover testing procedures in detail.2-6

6.21 Mud Density

The density of drilling muds is normally measured with a mud balance or scale such as shown in Figure 6.1. These instruments are rugged, easily calibrated, and lend themselves to field use. Hydrometers are also used, although not so commonly. Calibration of either device is performed by using fresh water having a known density of 8.33 lb/gal (or 62.4 lb/ft3). The general practice is to report density in pounds per gallon, hence a 10 pound mud is one having a density of 10 lb/gal. It has been suggested that density be reported as a pressure gradient; this, however, is not yet common.



Mud Balance
Fig. 6.1. Diagram of typical mud balance. 6.22 Mud Viscosity

Mud viscosity is difficult to measure; several measuring devices are in common use. Further complication generally ensues because each of the methods yields a different value. Before discussing the modes of measurement, let us define what is meant by drilling mud viscosity.

Most liquid drilling muds are either colloids and/or emulsions which behave as plastic or non-Newtonian fluids. The flow characteristics of these differ from those of Newtonian fluids (such as water, light oils, etc.) in that their viscosity is not constant, but varies with the rate of shear. (Recall Figure 2.8, Chapter 2, for the definition of viscosity.) This general behavior is shown in Figure 6.2. Note that for plastic fluids a certain value of stress (true yield point) must be exceeded in order to initiate movement. This is followed by a transition zone of decreasing slope in which the flow pattern changes from plug to viscous flow. The viscosity of a true (Newtonian) liquid is constant and equal to the slope of the line depicting its stress-strain behavior. Therefore, if the viscosity of a plastic fluid is measured in a conventional manner, i.e., the ratio of shearing stress to rate of shearing strain, the value obtained will depend on the rate of shear at which the measurements were taken.

Marsh Funnel

Various field instruments used for viscosity measurements are shown in Figure 6.3. The oldest of these is the Marsh funnel. In this method the funnel is filled to the upper mark (1500cc) with freshly collected, well agitated mud. The operator then notes the time, removes his finger from the discharge and measures the time for one quart (946 cc) to flow out. This time in seconds is recorded as the Marsh funnel viscosity of the mud. Marsh funnels are manufactured to precise dimensional standards and may be calibrated with water which has a funnel viscosity of 26 ± 0.5 sec. Funnel viscosities are of little quantitative use but have general comparative value. From experience^ it is known that for certain mud types, certain funnel viscosities are desirable. Oil base mud viscosities are sometimes reported as the 500 cc discharge time, although the tendency is to standardize on the quart basis.

Apparent Viscosity Plastic Viscosity

Rate of shear or velocity

Fig. 6.2. Flow behavior of plastic and Newtonian fluids.

Rate of shear or velocity

Fig. 6.2. Flow behavior of plastic and Newtonian fluids.

Stormer Viscosimeter

A more quantitative measurement of viscosity is obtained with the Stormer viscosimeter. This device consists of a spindle which is rotated in a test cup by a set of gears driven by a falling weight. The revolutions of the spindle are recorded by a revolution counter. By trial and error, weights are added to the line until a stabilized rotating speed of 600 rpm is obtained. The weight or driving force in grams is then used with a calibration chart to obtain the mud viscosity. This value is the apparent viscosity of the mud measured at a rate of shear corresponding to 600 rpm of the Stormer instrument.

Multispeed Rotational Viscosimeters

Realization that neither funnel nor Stormer viscosity measurements adequately defined the flow properties of plastic fluids led to the development of multispeed measuring devices.6-10 One of these is the Fann V-G (viscosity-gel) meter. In principle, the Fann meter is like the Stormer, in that the basic measurement is the torque necessary to revolve an inner rotor in a stationary, mud-filled test cup. The spindle is driven by a two speed synchronous motor. Gear changes plus the two motor speeds allow six operating spindle rpm's. Torque readings are obtained directly from a dial on the instrument. This multiplicity of measurements allows a more complete definition of the plastic flow curve (shearing stress versus rate of shearing strain). The instrument constants have been adjusted so that the slope of the linear portion of the flow curve may be obtained as the difference between the 600 and 300 rpm torque readings.8 This slope is defined as plastic viscosity (or rigidity) and is given in centipoise.

Apparent viscosity at 600 rpm is obtained as one half the 600 rpm torque reading. These and other relationships are illustrated by the following equations and Figure 6.4.

Fann Meter
Diagram Fann

Fig. 6.3. Viscosity measuring devices. (A) Marsh funnel and mud measuring cup. (B) Stormer viscosimeter and calibration chart. (C) Fann V-G meter.

Fig. 6.3. Viscosity measuring devices. (A) Marsh funnel and mud measuring cup. (B) Stormer viscosimeter and calibration chart. (C) Fann V-G meter.

PaF — 5$600

  • 6.1) (6.2)
  • 6.3) Yb = $300 - Hp where nP = plastic viscosity, cp

Paf = apparent viscosity, cp

Yb = Bingham yield point, lb/100 ft2

$ = torque readings from instrument dial at 600 and 300 rpm.

From these relationships:

  • 6.4) Yb = 2(naF - up) or
  • 6.4a) naF = Hp + JFj

True yield point (Figure 6.2) is normally defined by equation (6.5) for plastic or Bingham fluids.4'11

In summary, the Marsh funnel viscosity is useful as a comparative value and is recorded in seconds. Stormer viscosity is an apparent viscosity expressed in centipoise, but has limited value since it is valid at only one rate of shear. The Fann viscosity is the plastic viscosity and represents the rate of change of shearing stress with respect to shearing strain over the linear portion of the consistency curve. The latter, along with yield point, is useful in hydraulic calculations, as is shown in the next chapter.

Image Fann Meter

300 Setting, rpm

Fig. 6.4. Measurement of plastic flow properties with the Fann V-G meter. By adjustment of instrument constants, m* and Yb are readily obtained from 600 and 300 rpm dial readings.

300 Setting, rpm

Fig. 6.4. Measurement of plastic flow properties with the Fann V-G meter. By adjustment of instrument constants, m* and Yb are readily obtained from 600 and 300 rpm dial readings.

Plastic viscosity is due to friction between solid particles in the mud and the viscosity of the dispersed phase (base liquid). Normally, it is not appreciably changed by chemical treatment and is almost entirely dependent on solid content. Exceptions to this rule are surfactant muds in which the clay solids exist as dispersed floes (see Figure 6.17). This state of dispersion lowers plastic viscosity by reducing inter-solid friction.12 Ordinary thinners (viscosity reducers) lower apparent viscosity by reducing yield point, and have little or no effect on plastic viscosity.3 (Note Eq. 6.4a). No means of computing either apparent or plastic viscosity from Marsh funnel time is available. The mud temperature at which these measurements are made should be recorded.

6.23 Gel Strength

The gel strength of a mud is a measure of the shearing stress necessary to initiate a finite rate of shear. These measurements are normally taken and reported as initial gel strength (zero quiescent time) and final gel strength (ten minutes quiescent time). Fundamentally, for Bingham fluids initial gel strength and true yield value should be the same; however, such is not the observed case.7 This discrepancy is probably due to

  • 1) the failure of drilling muds to behave as Bingham fluids at low rates of shear
  • 2) the impossibility of measuring initial shear at exactly zero quiescent time.

Initial gel strength has qualitative usefulness but should not be confused with true yield point. A correlation between initial gel strength and Bingham yield is shown in Figure 6.5. Gel strengths are measured with either the Stormer or Fann instruments as the shear stress necessary to cause spindle movement at a very low shearing rate.

6.24 Filtration Test

The filtration, water loss, or wall building test is conducted with a filter press as indicated in Figure 6.6.

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  • eoin
    Why do we use gel strength bingham plastic standard api 10 minutes?
    7 years ago

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