Why Control Hole Deviation

Drilling practices have changed considerably through the years. When rotary drilling first started, drill collars (as we know them today) were unheard of. Bits were not designed for heavy loads and only the weight of the drill pipe with a crossover sub (called a collar) between the drill pipe and bit, supplied the drilling weight. The search for more hydrocarbons required penetration of deeper and harder formations and brought about the development of improved rotary bits with the need for additional weight to make the bit drill. In an effort to put more weight on the bit, additional drill pipe weight was slacked off putting more drill pipe in compression. This resulted in an increased number of drill pipe failures. It was discovered that when drill pipe is run in compression for bit weight, it buckles and is subject to severe bending fatigue resulting in these failures. This is due to the stress reversals in the thin wall of the drill pipe created by rotating the pipe in compression while it is bent. Using this theory another person developed the idea of using heavy thick-walled pipe between the bit and drill pipe to furnish the necessary weight for the bit. These joints of heavy thick-walled pipe were called drill collars, named after the crossover sub that had been used in the same position in the string. Only a few collars were used initially, but the quantity increased rapidly with improved bit design and deeper drilling. Very few problems were encountered when only six to nine drill collars were used; but connection failures increased rapidly with the running of additional collars, because the drill collars buckled under the additional drill collar weight. Drill collars differ from drill pipe in that the highest points of stress are in the connection, due to the tube or body being much stiffer and stronger than the connection. The use of special bottomhole assemblies to centralize the collars and stiffen the connections was unheard of at this time. Initially, not much thought was given to deviation. It was believed that if the kelly were held straight up and down in starting the hole, it would continue straight. No one realized holes were being drilled crooked until the development of the Seminole Field in Oklahoma in about 1928 and 1929. People started to be suspicious when some wells required considerably more footage of casing to complete than others. Since the wells were assumed to be in the same producing horizon, the geologists were confused. It wasn"t until two offset drilling wells actually intersected one another, causing numerous fishing jobs, that people realized that crooked holes were possible. They began to be concerned about the cost of the additional tubing and casing to complete these crooked holes, and deviation from vertical became an important factor in the drilling industry. It was at this time that the acid bottle came into use as a means of measuring the hole inclination from vertical. A bottle of hydrofluoric acid was lowered into the well on a line and allowed to sit long enough for the acid to etch the inside of the bottle. This wasn"t very accurate, but the approximate deviation from vertical could be determined.