Drill-Off Tests for Drilling Optimization: Pitfalls and How to Avoid Them

Abstract

A drill-off test is a special drilling test where the drawworks is locked and the string elasticity causes the bit to penetrate the formation at a decreasing weight on bit (WOB). It is often used as a tool for finding how the rate of penetration (ROP) varies as a function of WOB and for finding optimal drilling parameters. This paper sheds a critical light on the use of drill-off test by discussing various errors and pitfalls usually ignored today. The paper starts by presenting a simple but widely used method for converting surface hookload data into an ROP vs. WOB curve, commonly known as a drilling performance curve. Next, it discusses and tries to quantify various error sources, such as hookload sensor errors, residual wellbore drag, and formation inhomogeneities. The paper uses real field data and a comprehensive simulation model to illustrate the magnitude of the various errors and how they affect the drilling performance curve. Finally, the paper suggests ways to check the accuracy and validity of the drill-off tests. The analysis shows that the variable wellbore friction is an error source that depends on many parameters, such as string length, wellbore inclination, rotational string speed, and ROP. It also shows that it is possible to model the errors and apply correction formulas to improve the quality and accuracy of WOB and ROP. Although friction-related errors can be reduced by applying proper corrections to the estimated WOB and ROP, there is still a need to check the validity of the drill-off test results with other types of drilling tests. The paper presents several proposals for alternative and complementary drilling tests, such as drill-on test (here defined as a test where WOB is continuously increased), steady state (constant WOB) drilling test, combo test, and sinusoidal perturbations test.