Optimizing the Drill Bit / Bottom Hole Assembly Pull Out of Hole Decision-Making Process Through the Use of Realtime, Downhole Drilling Data and Downhole Mechanical Specific Energy

Abstract

This paper seeks to demonstrate how the use of real-time downhole drilling data can help facilitate the timely, well-informed decision to pull out of hole (POOH) if the rate of penetration (ROP) reduces beyond expectation. The traditional decision process starts with offset ROP comparison. If the ROP is lower than expected, then typically there are two scenarios: either the drill bit has become dull or the formation is harder to drill than expected. If the first scenario is suspected then a cost-benefit analysis is performed whereby the remaining footage is assessed and the time to drill to section TD with the current ROP is calculated and compared to the cost of a new drill bit, rig time and the cost of round tripping to change the bit. However, when the bit is on surface and seen to still have good cutting structure then the round trip has been a waste of time and money and more patience should have been exercised in keeping the drill bit on bottom. With real-time, downhole drilling data it is possible to assess the drillability of the formation and estimate the dull condition of the bit. Primary parameters that are used for this assessment are downhole weight on bit and downhole torque, and their relationship to one another. In addition, the trend of the downhole mechanical specific energy is evaluated. The first case study under discussion was drilled in 12¼-in. section, offshore UAE field. For the initial 4,500ft the average ROP was 80ft/hr, at which point a dramatic decrease in ROP was observed. A reduction in ROP was expected at this point because of formation change, but the magnitude of the reduction was much greater. Because the ROP was expected to reduce, the drilling continued with some short-lived increases in ROP. After a sustained time period of low ROP the decision to POOH was made, when the drill bit was on surface the dull grade was 1-6-HC-S-X-I-RO-PR. Upon evaluation of downhole drilling data with the aid of hindsight regarding eventual dull grade it was seen that there were clear indications that the bit was no longer effective and the decision to POOH should have been made. The time period that elapsed between the drill bit becoming damaged and POOH was 30 hours. This base case lesson learned was utilized on a subsequent well in the drilling campaign on an analogous application. By using this downhole signature a quicker, more effective POOH decision was made. The POOH decision in this case was made after just 2.5 hours compared to the 30 hours on the previous well. Once on surface, the dull grade of the bit was 2-7-WT-S-X-I-RO-PR.