Yes, you can use negative inclination values. Inclination is the angle the borehole is oriented in degrees and varies from 0 to 180. 0 indicates vertical pointing down, 90 indicates horizontal, and 180 indicates vertical pointing up. Negative and positive inclination values are treated the same for depth calculations. Negative inclination values are treated differently for well path calculations in the cross section and map views. A negative inclination changes the direction (azimuth) to the opposite of the similar positive inclination. For example, the azimuth value of 90 and inclination of 45 describes the same orientation as the azimuth value of 270 and inclination of -45 degrees. Both combinations describe an eastward direction at 45 degrees down from the horizontal plane.
Updated July 17, 2018