Abstract

Mechanical Specific Energy (MSE) is now a well-known concept to quantify the cutting efficiency of the rock. Thanks to its simplicity, its utilization has significantly increased over the last few years with electronic drilling recorders, especially in unconventional wells to optimize the drilling process and eventually reduce cost. A typical use is to compare the MSE to the rock strength to see whether the right amount of energy is utilized at the bit and not wasted or dispersed somewhere else. However, MSE alone cannot tell if drilling inefficiency is due to a change in the rock hardness, or due to vibrations, or bit wear or bit balling. This paper presents a new methodology that enables to fill the gap, in combining the MSE to the drilling strength (DS) to detect dysfunctions, such as vibrations or bit wear.

As MSE is mainly affected by the level of downhole torque (TOB), the effect of WOB is often neglected and is not taken into account in standard MSE analysis. In re-introducing the concept of drilling strength (DS) which is a function of WOB, and using the ratio of MSE over DS, a simple methodology can be derived to not only detect drilling inefficiencies but also determine the type of dysfunctions, such as vibrations or bit wear. This paper shows how the new methodology has been successfully used and validated in unconventional wells.

MSE has become a common way to analyze drilling efficiency post-run and, in some cases, make corrections in realtime to improve rate of penetration (ROP)1. Much can be learned from studying what has been done in past wells and applying lessons learned to the next well. To be the most effective however, MSE must be combined with other data and field knowledge to determine the root cause of MSE changes which are generally due to a formation change, bit wear, bit vibrations or bit balling just to name a few. A new methodology proposed in this paper reveals an effective way to dig deeper into MSE data and conclude why it could be changing and what can be done to improve drilling efficiency. When used in realtime, this methodology has the potential to save time, improve drilling efficiency, reduce vibration and therefore wear and tear on tools, and make the decision to trip for bit change easier by providing a way to analyze bit-wear at surface.