What mud does
- Cleans the hole — carries rock cuttings up to surface.
- Cools and lubricates the bit and drill string.
- Seals the formation with a filter cake on the wellbore wall.
- Controls pressure — its weight (density) balances formation pore pressure.
The safe range of mud density: heavy enough to balance pore pressure (preventing a kick) but light enough not to fracture the rock (preventing lost circulation).
Types of mud
Mud systems fall into three families: water-based mud (WBM), the most common and lowest-cost; oil-based mud (OBM), which lubricates better and handles tricky shales; and synthetic-based mud (SBM), which performs like OBM with a lower environmental footprint.
Hydrostatic pressure from the mud column equals mud weight (ppg) × 0.052 × true vertical depth (ft). Get the mud weight wrong and you risk a kick or a fractured formation.
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Frequently asked
Drilling mud is a mix of a base fluid (water, oil or synthetic), weighting material like barite to control density, clays for viscosity, and chemical additives. The exact recipe is engineered for each well.
Mud weight is the fluid's density in pounds per gallon. Fresh water is about 8.33 ppg; muds are weighted heavier to control formation pressure.