Mud Weight Definition / Meaning
Mud weight is the density of the drilling mud, quoted in pounds per gallon in North America and as specific gravity or kilograms per cubic metre elsewhere. Fresh water is 8.33 pounds per gallon; drilling muds run from a little above that to 18 or more in high-pressure wells. It is the single most important number on a rig, because the weight of the mud column is what holds back the pressure in the rock while the hole is open.
The hydrostatic pressure of the column is simple arithmetic: mud weight in pounds per gallon times 0.052 times true vertical depth in feet gives pressure in pounds per square inch. Ten-pound mud at ten thousand feet exerts 5,200 psi at the bottom of the hole.
The Window It Has to Fit
Every open-hole section has a pressure window. Below it lies the pore pressure, the pressure of the fluid in the rock; if the mud column exerts less, formation fluid enters the well and the rig has a kick. Above it lies the fracture pressure, the pressure at which the rock splits; if the mud column exerts more, the mud leaks away into the formation as lost circulation. The mud weight has to sit between the two, with a margin, for every formation exposed at once. When the window narrows to nothing, the section is cased off and drilling continues with a new mud weight below it, which is the reason wells are built in casing stages.
Static Weight and Circulating Density
The number on the mud report is the static density. While the pumps are running the friction of moving mud up the annulus adds to the pressure on the formation, and the total is expressed as an equivalent circulating density, the mud weight that would give the same bottom-hole pressure without flow. In a narrow window the difference between static weight and circulating density is what decides whether the well kicks when the pumps are shut off or fractures when they are turned on.
Measuring and Adjusting It
| Action | How |
|---|---|
| Measure | A mud balance: a calibrated beam with a cup of mud at one end and a sliding rider, read every few minutes at the shakers and the suction pit. A pressurised balance corrects for entrained gas. |
| Increase | Add a weighting material, almost always barite (barium sulfate), through the mixing hopper; hematite for very high densities. |
| Decrease | Dilute with base fluid, or remove fine solids with the solids-control equipment, since drilled solids raise density unintentionally. |
| Kill a kick | Calculate the kill mud weight from the shut-in drill-pipe pressure, weight up the active system, and circulate the heavier mud round while holding back-pressure on the choke. |
Usage Example
“Pore pressure came in at 12.4 and the leak-off test gave us 14.1, so we drilled the section at 13.0 and never went above 13.4 circulating.”
What Goes Wrong
Too light and the well kicks; too heavy and it loses returns, and in weak rock the mud can fracture the formation, lose the column, and let the well kick on top of the loss. Drilled solids creep the weight up over a section if the shakers and cones are not keeping pace, and gas-cut mud reads light at the shakers while the column below is still full weight. The driller’s defence is a mud weight checked constantly, a pit-volume system that flags any gain or loss, and a blowout preventer for the day the column is not enough.