Equivalent Circulating Density
While pumping, annular friction adds to the static mud weight, and the formation feels the sum: the equivalent circulating density.
The equation
ECD = MW + Pann / (0.052 × TVD)
- MW is static mud weight in ppg
- Pann is annular friction pressure loss in psi
- TVD is true vertical depth in feet
Worked example
A 12 ppg mud with 300 psi of annular friction at 10,000 feet circulates at an ECD of 12 + 300/520 = 12.58 ppg. If the fracture gradient sits at 12.5 ppg equivalent, the well fractures only while the pumps are on, a classic and confusing symptom.
When to use it
The tighter the mud window, the more ECD management dominates: narrow margins in deepwater turned ECD from an afterthought into the design driver for entire casing programs.
Related Calculators and Reference
- Mud Weight and Pressure Gradient Calculator: convert between ppg, SG, and psi per foot.
- Probabilistic STOIIP Calculator: volumes with uncertainty ranges.
- PetroWiki by SPE: the industry reference for petroleum engineering.