**WELLOG****
TRUE VERTICAL DEPTH**

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**REVISED ****01-05-2008**

**© ****WELLOG**** 2006 - 2008**

**All Rights Reserved**

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**TRUE VERTICAL DEPTH:**

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**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.**

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**HOW TRUE VERTICAL DEPTH IS CALCULATED:**

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

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**Example 1:**

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

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**What is the TVD?**

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

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**Using the trigonometric cosine function;**

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**
Cosine of angle theta = Adjacent (TVD) /Hypotenuse**

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**The cosine of 10 degrees is .984**

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**
Adjacent
(TVD) = cosine of angle theta x hypotenuse**

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TVD = .984 x
1000 = 984 feet**

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

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**Example 2:**

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**The deviation of a 1000
ft well is 20 degrees, **

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**The cosine of 20
degrees = .940**

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**TVD = .940 x 1000 = 940 feet.**

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**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.**

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

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**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.**

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**For more information on True Vertical Depth contact info@wellog.com.**