Depth Type in a Oil Well
Depth is one of the most fundamental and important measurement performed by wireline logging crews .It is the primary measurement provided for:
- commonly to resolve reservoir thickness,
- wireline logs offer a geological information source for the entire length of a wellbore u , and a major use of logs data is to make well -to-well geological correlations ,
- correlations between wells ,
- geological markers ,
- casing point, reservoir delineation ,
- monitoring of the different contacts (OWC, GWC, FWL .....) .
Logger's depth is often more precise than driller's depth wich is not corrected stretch.
Depth type definitions :
- TWT: "Two Way Time". It is determined by the location of the check shot energy source , typically at or around ground level .
- TVD: "True Vertical Depth" . It is the vertical distance from the wellhead to the wellbore curvature , measured from the directional survey.
- TVD ss "Sub Sea True Vertical Depth". It is the vertical distance from the mean sea level MSL (Z sol ) to the wellbore curvature , measured from the directional survey.
- MD "Measured Depth". It is calculated from the survey depth , along the well path from the wellhead to that reservoir station .
NB:
- In a vertical well ;
MD = TVD TVD ss = MD - KB
KB=GL+RT(RT is the Rotary table )
- In Horizontal or deviated well ;
MD is >>> than TVD .TVD = MD X COS α (α angle deviation of the well )
- In a vertical well ;
- In Horizontal or deviated well ;
