Minimum Angle : The lowest angle for easy control of azimuth in a directional well, almost universally agreed to be about 18o, not less than 14 o, and preferably 20 o. (Bul D20)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}