Drilling string displacement, abbreviated as DSD, is the volume of drilling fluid displaced by the portion of the drill string that is submerged in the wellbore. It is the amount of mud pushed out of the way when drill pipe, drill collars, heavy-weight drill pipe, bottom-hole assembly components, casing, or other tubulars are run into the well. This displacement occurs because the steel components occupy space that was previously filled by drilling fluid. The displaced fluid causes the mud level in surface pits or tanks to rise unless the fluid is intentionally removed or redirected. Conversely, when the drill string is pulled out of the hole, that occupied volume is removed from the wellbore, and drilling fluid flows back into the well, causing the surface pit level to decrease if no replacement fluid is added.
Drilling String Displacement Formula |
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\( DS_d \;=\; \dfrac{ ( D_{pod}^2 - D_{pid}^2 ) \cdot l }{ 1029.4 }\) (English) \( DS_d \;=\; \dfrac{ ( D_{pod}^2 - D_{pid}^2 ) \cdot l }{ 1273.24 }\) (Metric) |
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| Symbol | English | Metric |
| \( DS_d \) = Drillstring Displacement | \( bbl \) | \( l \) |
| \( D_{pod} \) = Casing / Drill Pipe OD | \(in\) | \( mm \) |
| \( D_{pid} \) = Casing / Drill Pipe ID | \(in\) | \( mm \) |
| \( l \) = Length of Drill Hole | \(ft\) | \( m \) |
Knowledge of drilling string displacement is essential for well control and drilling operations. It is used to calculate expected pit volume changes while tripping pipe, verify that observed pit gains or losses are consistent with normal pipe movement, estimate the volume of fluid that must be transferred to or from the pits during tripping operations, and distinguish normal displacement effects from abnormal events such as kicks, lost circulation, or equipment leaks. Accurate displacement calculations are also important when planning trips, maintaining proper wellbore fill, and monitoring mud inventory throughout drilling operations.
Drilling string displacement should not be confused with drill string capacity. Capacity is the internal volume of the drill string that can hold drilling fluid, whereas displacement is the external volume occupied by the drill string that displaces fluid in the annulus and wellbore. These are separate properties and are calculated using different dimensions of the tubulars
