Recorded drilling times may show significant variations from well to well even for the same total depth in the same field. Apart from the formation characteristics, engineers' technical ability plays an important role in determining drilling time.
A drilling performance is usually compared with respect to the performances of previously drilled wells. This approach has two major drawbacks. One, in newly developed fields, there may not be sufficient number of drilled wells to make a healthy comparison. Two, past drilling practices may not represent good engineering practices.
In this paper, a different approach is introduced in assessing drilling performance. The new approach suggests a comparison of drilling performance with respect to what is called 'technical limit of drilling rate', a maximum achievable drilling rate without risking drilling safety.
The rate of drilling can be improved for a given field until it reaches its technical limit. Several variables affect drilling rate (1–4). Some of these variables originate from formations, and nothing can be done to change them practically. Formation properties such as; pore pressure, compaction, in-situ stresses and mineral content are among the uncontrollable variables. On the other hand, several drilling variables can be selected carefully and drilling rate can be improved significantly. Mud weight (MW), weight on bit (WOB), rotary speed or rotation per minute (RPM), bit type and hydraulics are among the controllable drilling variables.
It has long been observed that the drilling rate generally increases with increasing circulation rate (5), weight on bit (6), rotary speed (6) and bit tooth height. On the other hand, it decreases with increasing drilling fluid viscosity and density (7–11). Some of these variables may have a significant effect on drilling rate whereas others may have a marginal effect.
Several authors have proposed mathematical relationships of drilling rate with major controllable and uncontrollable drilling variables for rolling cutter bits. (12–15) Among them, perhaps the most complete mathematical drilling model being used is Bourgoyne and Young's model (21,22).
Equation (1) (Available in full paper)
The functions f1 through f3 represent the effect of uncontrollable drilling variables on drilling rate. For example, f1 represents formation strength on penetration rate. The functions f2 and f3 model the effect of compaction on penetration rate. For example, the function f2 accounts for the rock strength increase due to normal compaction with depth, and the function f3 models the effect of undercompaction experienced in abnormally pressured formations (14). The functions f4 through f8represent the effect of controllable drilling variables on the drilling rate.
The function f4 models the effect of overbalance on the drilling rate.
Equation (2) (Available in full paper)
To graphically demonstrate the effect of mud weight on drilling rate, equation 2 was substituted in equation 1, and DR in equation 1 was solved for the data given in table 1. Note that all the other controllable variables are kept at constant values while MW was changed.
