The Borehole and Drilling Mud
Learning objectives
- Explain why the borehole is drilled overbalanced and what the mud does
- Describe how mudcake forms on permeable beds
- Explain borehole washout in shale
- Read the caliper as a permeability and lithology indicator
An Imperfect Cylinder
Before a single property can be read, remember how the hole was made. It is drilled with mud, a fluid held at a pressure a little above the formation pressure (overbalance) so the well does not blow out. That overbalance is the engine behind almost everything in this chapter: it pushes mud against the rock, and the rock pushes back differently depending on what it is. The borehole is never a clean cylinder, and its imperfections are a record worth reading.
Mudcake on the Permeable Beds
Against a permeable bed the overbalanced mud forces its liquid (the filtrate) into the rock and strands its solids on the wall as a mudcake. The cake is thin but it matters twice: it pulls the hole slightly under bit size, and, because it only forms where fluid can move, it is a direct flag that the bed is permeable. The cake also chokes off further invasion, which is why invasion is shallow in good rock and the deep logs can still see past it.
Washout, and Reading the Caliper
Against shale the story reverses: the mud reacts with the clays, the wall caves, and the hole washes out well over bit size. A tight, competent bed stays in gauge. The tool that records all this is the caliper, which simply measures the hole diameter with depth. Read it first: under gauge means mudcake and permeability, over gauge means washed-out shale, on gauge means a tight or competent bed. It is the cheapest lithology and permeability flag in the whole suite, and it tells you which other readings to trust.
References
- Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
- Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.