Drilling Fluid Parameters

The mud system and the various components of the mud chemistry affect the interaction between the drill bit and formation. The Wellsite Geologist should have an understanding of these interactions and be aware of the properties that influence them.
The mud system serves many purposes and though functions are more relevant to the drilling operation, those that are of interest to the geologist include:
• Controlling subsurface pressures (mud density)
• Cuttings removal (viscosity)
• Suspending cuttings when not circulating (gel strength)
• Providing an impermeable wall cake (filtrate)
• Releasing cuttings at surface (viscosity & gel strength)
1.Mud Density
One of the main functions of mud density is to provide a balance between the fluid column and formation pressures. The pressure at the bottom of the borehole is a function of the vertical height of the fluid column and the density of the fluid. This pressure is defined as the “mud hydrostatic pressure” and is referenced to the flowline.
P=0.0519 x W x TVD
where:
P = hydrostatic pressure (psi)
W = mud density (ppg)
TVD = vertical depth (ft)
or
P 0.0098 x W x TVD
where:
P = hydrostatic (kPa)
W = mud density (kg/m3)
TVD = vertical depth (m)
When circulating, the pressure exerted by the mud column increases due to frictional pressures losses within the borehole annulus. Those pressure loses can be calculated using:
Annular Pressure Loss in laminar flow (Bingham):

where:

L = length of annular section (ft) 

V = annular flowrate (ft/min)

Dh = Hole diameter (inches)

Dp = Drill pipe diameter (inches)

YP = Yield point (lbs/100ft2)

PV = Plastic velocity (cps)

This annular pressure loss is calculated for each individual annular section. The equivalent circulating density is then calculated by adding those pressure losses to the mud density.

Should the mud hydrostatic pressure fall below the formation pressure, (i.e. become underbalanced), the possibility of fluid incursions into the wellbore is greatly increased. The amount of incursion will be dependent upon the formations porosity and permeability. When formation pressure is greater than the mud hydrostatic pressure the following may be seen:

• Large quantities of cavings (due to wellbore instability)

• Connection gases

• Trip gases

• Increased background gas

• Fluid incursions to the mud system

If the hydrostatic pressure is greater than the formation pressure, (i.e. overbalanced), then flushing of the formations will take place.The degree of overbalance by the mud column will affect both the ROP (impeding cuttings removal) and the gases recorded at surface (due to flushing).As a general rule, the drill rate will decrease as the mud weight is increased.

2.Viscosity:

As has been mentioned earlier, viscosity has an effect on cuttings recovery. Intuition suggests that a more viscous fluid would have an increased carrying capacity and hence retrieve cuttings more effectively. Viscosity also affects the flow regime of the fluid, which has an even greater effect upon the carrying capacity of the mud system. Generally, the higher the viscosity of the fluid, the more likely the fluid is to be in laminar flow. Though annular flow is not as effective as turbulent flow for removing cuttings, using turbulent flow throughout the annulus would result in a washed out hole.

Plastic viscosity (PV) is a measure of resistance to flow caused by the mechanical friction between the drilled solids in the mud, the make-up solids and liquids, and the shearing properties of the mud itself. This is one of the primary reasons for controlling mud solids.

In the mud system there are solids that are an integral part of the mud (bentonite, starch, CMC, etc.) and solids that are undesirable (sand, limestone, dolomite, etc.). As the mud density is increased, by the addition of barite or hematite (more solids), the plastic viscosity will automatically increase. The PV is also a function of the viscosity of the fluid phase of the mud. So as temperature rises, the viscosity of water decreases, hence PV will decrease also.

3.Yield Point:

The yield point (YP) is a measure of the electro-chemical attractive forces within the mud under flowing conditions. These forces are a result of positive and negative charges located near or on the particle surfaces within the mud. The yield point then is a function of the surface properties of the mud solids, the volume concentration of the solids and the concentration and type of ions within the fluid phase.

It is also measure of flocculation, which can be caused by the introduction of soluble contaminants such as salt, cement, anhydrite, or gypsum. The breaking of clay particles by the grinding action of the bit and pipe also causes flocculation. Introduction of inert solids causes the particles to be closer together and hence have an increased attraction and the drilling of hydratable clays introduces active solids which may flocculate.

4.Gel Strength:

Gel strength is a measurement that denotes the thixotropic properties of the mud. It is a measurement of the attractive forces of the mud while at rest or under static conditions. As this and yield point are both measures of flocculation they will tend to increase and decrease together.

5.Filtrate/Water Loss:

Loss of fluid (usually water and soluble chemicals) from the mud system into the formation occurs when formation permeability is such that it allows fluid to pass through the pore spaces. As fluid is lost a build up of mud solids occurs on the face of the wellbore. This is filter cake. Two types of filtration occur, dynamic, while circulating and static, while the mud is at rest. Dynamic filtration reaches a constant rate when the rate of erosion of the filter cake due to circulating matches the rate of deposition of the filter cake. Static filtration causes the cake to grow thicker with time, which results in a decrease in fluid loss with time.

Mud measurements are confined to the static filtration. Excessive filtration and thick filter cake build up are likely to cause the following problems:

• Tight hole, causing excessive drag.

• Increased pressure surges due to reduced hole diameter.

• Differential sticking due to an increased area of pipe contact in filter cake.

• Excessive formation damage and evaluation problems.

Most of these problems are caused by the filter cake and not the amount of filtration. The aim is to deposit as thin and impermeable a filter cake as possible. A low water loss may not do this, since the filter cake is also dependent upon the solids size and their distribution.

6.Other Mud System Properties:

There are other mud properties that should be observed by the Wellsite Geologist. Changes in chlorides content may indicate the drilling of a permeable formation with a different fluid content. Small incursions may not be spotted by the drill crew if excessive mud treatment is being carried out, but changes in the chlorides content may indicate their presence.