INTRODUCTION

For drilling a horizontal well, it is essential to decide on tolerance limits for well elevation. In other words, one has to decide how much deviation from a vertical elevation is tolerable. For small tolerance limits ( 5 ft), several measurements and surveys are required as the horizontal well is being drilled. Well drilling time is proportional to the number of directional surveys required. In the short-radius drilling technique, one may pull out of the hole with a drilling assembly and insertaluminumdrill pipe for directional surveys. After surveying, aluminum drill pipe will have to be pulled out of the hole and drill pipe with flexible (wiggly) collars is reinserted to continue drilling. Thus, in the short-radius technique, surveying may involve several trips and can be expensive (note that by late 1990 to mid-1991, MWD (measurement while drilling) tools may be available for the short-radius drilling technique!).

For a medium-radius well, MWD tools are employed for directional control. Many MWD tools are actuated using pressure pulsing or some other activation technique. In general, the drilling rate slows down when MWD is activated. Thus, the number of surveys has a direct influence on drilling time and costs. Additionally, MWD tools for directional surveys are presently about 50 to 90 ft behind the bit. Thus, the exact bit location may not be known at all times. This results in slight inaccuracies in drilling and results in a need for estimating tolerance limits on drilling plans.

The type of reservoir determines the drilling...