Mud weight is the primary well control barrier
The drilling fluid column is the primary well control barrier. Its density, called mud weight, sets the hydrostatic pressure at the bottom of the hole. That pressure must stay above the formation pore pressure, or formation fluids flow into the wellbore. It must also stay below the fracture gradient, or the rock breaks down and mud is lost to the formation. Field crews measure mud weight with a mud balance and record it in pounds per gallon. The check takes minutes, and it runs every few hours while drilling. A bad number caught in the morning beats a kick at midnight.
Viscosity, gel strength, and filtration
The Marsh funnel gives a fast funnel viscosity in seconds per quart. A direct-indicating viscometer adds plastic viscosity, yield point, and gel strength after the pumps stop. A filter press measures filtration at low temperature and pressure, and at high temperature and high pressure when the well requires it. These results show how well the mud carries cuttings, suspends solids when circulation stops, and builds a thin, low-permeability filter cake. pH, alkalinity, chloride, and sand content complete the routine. API RP 13B-1 sets the standard procedure for every one of these tests.
A test only counts when it is written down
API RP 13B-1 exists so a mud test means the same thing on every rig. Its annexes cover rig-site sampling and the calibration of the mud balance, thermometers, and viscometers, and the standard includes a report form for water-based drilling fluids. The habit matters more than any single number. Test at the same intervals, record results the same way, and watch the trend instead of the latest reading.
The mud log and the daily report should tell the same story. When they drift apart, the crew is flying on memory. The same discipline covers the rest of the operation, from rig inspection and API standards to annular pressure monitoring. Cuttings handling ties the mud system to waste, which we cover in our guide to drill cuttings and waste management. If the field record and the mud log live in different places, start with an operations audit.