Page 177 - Drilling Technology in Nontechnical Language

168 Drilling Technology in Nontechnical Language Second Edition

Sand content. Sand is normally the most abrasive solid present in
the mud, and a high sand content will increase wear on pumps, valves,
and other equipment. However, all solids in the mud will contribute to
mud abrasiveness. Sand content should be kept as low as possible by using
the solids control equipment properly. The sand content is measured by

passing a fixed volume of mud through a 200-mesh sieve into a marked
glass container. The sand sits in the bottom, and the sand content is
measured directly from the marks.

Mud rheology. Rheology is the science of the deformation and flow

of fluids. When discussing the rheology of fluids in the well (mud, cement,
or brines), what is of interest is the relationship between how fast the fluid

flows and the pressure required to maintain that flow rate (either in the

pipe or in the annulus). The relationships between these properties will
affect circulating pressures, surge and swab pressures, and hole cleaning
ability. Surge and swab pressures occur when the drillstring is lowered
in the hole and fluid is displaced upwards. This imposes temporary extra

pressure on the hole and is called surge pressure. When the drillstring is
lifted upwards, fluid has to flow downwards as a pressure drop or suction

is created by the withdrawal of the steel volume. This causes a temporary
pressure reduction on the hole and is called swab pressure. Hole cleaning

ability refers to the ability of a drilling fluid to lift cuttings out of the hole at
a certain flow rate. This ability is related to the fluid density and rheology.

To fully specify a drilling fluid that must perform specific functions,
the required rheology must also be defined. The rheology not only affects

the relationship between flow rate and pressure, it also affects the flow

rates at which the flow regimes change.

At very low flow rates inside the pipe and annulus, the fluid will move
with all parts of the fluid moving in the same direction and at roughly the

same speed. This is called plug flow, shown as “Stage 2” in figure 7–6.

If the flow rate is increased, the drag at the edge will cause the central

part of the fluid to flow faster than the edges. The resultant flow is called

laminar flow. It is less efficient for transporting cuttings up the hole.

If the flow rate is further increased, the flow will become chaotic.

The average flow direction will be in one direction, but within the fluid,

there are numerous eddies and swirling flow. This is turbulent flow and is

the most efficient for hole cleaning.
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