WELLOG                                 TRUE VERTICAL DEPTH

REVISED 01-05-2008

TRUE VERTICAL DEPTH:

When drilling a well, the possibility exists that the well may not be perfectly vertical. Depending on the drilling technique used and the geological formations that encountered during the drilling process, a well may deviate from the intended vertical direction. The amount of deviation from vertical measured in degrees will affect the True Vertical Depth (TVD) of the well.

HOW TRUE VERTICAL DEPTH IS CALCULATED:

If the inclination of the well is determined, then the true vertical depth can be calculated.

Example 1:

A well is drilled to 1000 feet. It has been determined that the well was inclined 10 degrees from vertical.

What is the TVD?

The depth was measured along the length of the inclined hole. That length becomes the hypotenuse of a right triangle.

Using the trigonometric cosine function;

Cosine of angle theta = Adjacent (TVD) /Hypotenuse

The cosine of 10 degrees is .984

Adjacent (TVD) = cosine of angle theta x hypotenuse

TVD = .984 x 1000 = 984 feet

As the angle of deviation increases, the less True Vertical Depth is.

Example 2:

The deviation of a 1000 ft well is 20 degrees,

The cosine of 20 degrees = .940

TVD = .940 x 1000 = 940 feet.

WHY IS TRUE VERTICAL DEPTH IMPORTANT?

The importance of TVD is shown in a 10,000 ft oil well having 20 degree deviation drilled to a target at TVD of 9,500 feet.

The calculated TVD of this well is 9,400 feet. Conclusion: The well has not been drilled to the target depth.

When mapping a given area to establish stratigraphy of geologic formations from well data, geologists and hydrologists need

to know the true vertical depth of the formations that are to be mapped.