12.25 in Hard Rock Drilling in Oman Performance Drilling
Authors: Vasquez Bautista Ramiro Oswaldo , Andreasen Greg , Thani Yousuf A. , Sanderson Martin , Vos Arjan , Hajje Mohammed , Hood Brashdi 
Published: April 2015

Petroleum Development Oman (PDO) “M” gas field provides an unique set of challenges while drilling the 12.25 in intermediate hole section which include severe lateral vibrations, stick & slip, impact damage, limited bit life and low penetration rates. As a result, 3 BHA's were historically needed to complete the section totaling 16 days on average. A collaborated effort between Schlumberger and PDO was initiated, with the ultimate goal to reduce the number of days to drill the section. To achieve this goal a Petro Technical Engineering Center (PTEC) process was established to develop a system engineered BHA solution. This process was introduced to the field over several wells and resulted in drilling the entire section in one run and reducing the drilling time from 16 days to 7 days.

The well profile is a 12.25in vertical hole typically drilled from +/− 1000m to +/−3400m MD, through highly interbedded formations ranging from 5-30 kpsi UCS and varying levels of abrasiveness. In a hard rock environment a system engineered approach is valuable to reduce the number of design iterations required compared to a trial and error method, due to the significant amount of time a well takes to drill. To complete this, a three phased approach was executed.

Phase 1 –To identify the most stable bit and BHA combination to use in the 1st trial test, a time domain integrated FEA model1 was used determine the best suited bit combination. From this analysis it was concluded that a 5 blade 16 mm bit driven by a motorized rotary steerable system would provide the most stable option to pursue in the trial test. Along with the modeling a detailed parameter roadmap was generated from the collaborative efforts of both PDO's and Schlumberger's drilling teams. The roadmap was coupled with a ROP optimization application2 which provided real-time RPM and WOB drilling parameter recommendations to maximize ROP within the operational envelope defined on a per formation basis.

Phase 2 – Building on the lessons learnt from Phase 1, a time domain Integrated FEA Model2 was used to fine tune the BHA and recommeneded removing the motor from the system would provide a more stable BHA in this application as well a switching to a less aggressive cutting structure in a 6 Blade 16 mm design. The model was also used to optimize the BHA design so that it generated limited vibrations enabling an aggressive strategy to reduce cost by removing the MWD, surface system and personnel. Thanks to its capability to keep a precise vertical control in automatic mode without the need to over-regulate the drilling parameters. This was implemented and resulted in the first shoe to shoe run in the field.

Phase 3 – A new bit technology incorporating a conical diamond element3 was strategically placed in the center of the baseline PDC bit. It was then modeled using the same time domain integrated FEA model used in Phase 2 and selected for a trial test. The drilling system was run again and resulted in another successful shoe to shoe run with a further increase in penetration rate of 14.4%, and more improvements on following wells.

The engineered solutions supported by the team effort and simulations made significant improvement in subsequent wells and are ongoing.