Depth Reference Systems
There are a number of datum systems used in the depth reference systems. The datum systems which are most widely used are :
• Mean Sea Level, MSL
• Rotary Table Elevation, RTE
• 20” Wellhead Housing
The Mean Sea Level, MSL is a permanent, national and well documented datum whereas datum such as the Rotary Table Elevation, RTE only exists when the drilling rig is on site. The top of the 20” Wellhead Housing is only available when the wellhead housing has been installed and will be removed when the well is abandoned. Hence, since the only permanent datum is the MSL (the rig will be removed and the wellhead may be removed on abandonment) the distance between the MSL and the rotary table on the drillfloor and the MSL and the wellhead housing must be measured and recorded carefully on the well survey documents. The elevation of the rotary table above the MSL will be measured when the drilling rig is placed over the drilling location.
The depths of the formations to be penetrated are generally referenced, by the geologists and reservoir engineers, to MSL since the Rotary Table Elevation will not be known until the drilling rig is in place. In most drilling operations the Rotary Table elevation (RTE) is used as the working depth reference since it is relatively simple, for the driller for instance, to measure depths relative to this point. The elevation of the RTE is also referred to as Derrick Floor Elevation (DFE). Depths measured from these references are often called depths below rotary table (BRT) or below derrick floor (BDF). The top of the kelly bushing is also used as a datum for depth measurement. In this case the depths are referred to as depths below rotary kelly bushing (RKB).
The depth of any point in the wellpath can be expressed in terms of the Along Hole Depth (AHD) and the True Vertical Depth (TVD) of the point below the reference datum. The AHD is the depth of a point from the surface reference point, measured along the trajectory of the borehole. Whereas the TVD is the vertical depth of the point below the reference point. The AHD will therefore always be greater than the TVD in a deviated well. Since there is no direct way of measuring the TVD, it must be calculated from the information gathered when surveying the well. The techniques used to survey the well will be discussed in the chapter on wellbore surveying.