July 2007

Technical limit thinking produces steep learning curve

Implementation reduced the number of days to drill a well from almost 100 to below 60 in Saih Rawl field, Oman.

Vol. 228 No. 7  


Technical limit thinking produces steep learning curve

 Days to drill a well went from almost 100 to below 60 days in Saih Rawl. 

Olutayo Ajimoko, Petroleum Development Oman*

Technical limit thinking is a conceptual feedback tool for maximizing drilling efficiency. It involves first determining the conditions and engineering practices that will lead to the minimum time and/or cost at which a given well can be drilled, and then employing motivational and training techniques to implement those practices. Shell and Petroleum Development Oman (PDO) produced a highly developed technical limit program for the challenging Saih Rawl gas field, which culminated in the landmark drilling of the field’s first sub-60-day well, Saih Rawl 228, in 57.89 days, against an AFE time of 75 days. Shell and PDO leveraged the methodology to further teamwork, knowledge management and bit optimization to progressively reduce well construction time.


Saih Rawl is an onshore deep gas field in southern Oman and the biggest gas field in the country. It is split into three geographically and tectonically separated blocks: SR Main to the north, SR Graben and SR South, Fig. 1.

Fig. 1

Fig. 1. A stratigraphic column showing the formation tops associated with the Saih Rawl gas field. 

The field is a large, elongated northeast-southwest-oriented anticline, with the anticlinal axis dipping to the southwest. A system of northwest-southeast-trending faults delineates and seals the structure’s northern boundary. The varying compartmentalization of the fault system accounts for the high anisotropy, and thus for the extreme drilling challenges encountered in the different lithological packages.

A typical well design in the Saih Rawl field starts with a 17½-in. hole from surface to the Naith limestone at about 950 m (3,100 ft) to set 13 3/8-in. casing. This is followed by a 12¼-in. section to about 3,200 m (10,500 ft), where the 9 5/8-in. casing is set in the Al Khlata/Hasira formation. Thereafter, a 2,050-m (6,730-ft), 8 3/8-in. section is drilled to section TD in the Amin formation. The 17½-in. section and 12¼-in. section are drilled on rotary.

At the beginning of field exploration in 1991 and of exploration on surrounding wells, the 8 3/8-in. section was initially drilled with roller-cone bits on rotary assemblies. In the intervening time, roller-cone bits have been gradually replaced by diamond-impregnated bits, so that by 2005, the entire 8 3/8-in. section from drillout to TD was drilled almost exclusively with turbine drive systems and diamond-impregnated bits.


The technical limit is defined as a measure of perfect performance limited only by technology available and/or by the laws of physics. In well drilling, this performance measure is the minimum possible time and/or cost to drill a typical well in a given field. In Saih Rawl, lessons learned in previous wells helped the team to identify the optimization practices that would lead to a technical-limit well, Fig. 2. Among these:

  • A Le Fleur circulation packer could be used, and downtime could be minimized to reduce the flat spot between the 17½- and 12¼-in. sections.
  • A higher-capacity centrifuge could optimize hydraulics to speed up the 8 3/8-in. section.
  • A wireline formation tester could be run to eliminate three logging trips and a wiper trip.
Fig. 2

Fig. 2. The drilled depth is plotted against time for the technical limit well, along with practices that would be used to achieve the limit. 

In addition, the technical limit well would maximize trip efficiencies, optimize hole cleaning and mud conditioning and increase bit life. The team determined the Saih Rawl technical limit well time to be 32.7 days.

Removing technological limitations from the equation yields the “perfect well time,” a theoretical construct that can be useful for defining drilling targets. The perfect well time is the minimum time in which a well could be drilled limited only by the physics of the drilling practice, calculated from clearly defined physical factors that constrain the drilling time: the rock strength, operational limits, number of casing strings, hole size, etc. The perfect well time in Saih Rawl is 18.4 days, Fig. 3.

Fig. 3

Fig. 3. The performance curve of the perfect well is shown.


Historically, about 70% of the total well construction time in Saih Rawl, as in most deep gas wells, is spent drilling ahead and tripping. The bit penetration rate and bit life, therefore, both have a major impact on well construction time and cost. Hypothetically, if both penetration rate and bit life could be doubled, an average 30% total reduction in drilling time and cost would result.

A dynamic drilling optimization process has evolved for drilling the Saih Rawl field. The drilling program employed on Saih Rawl 228 involved drilling the 17½-in. tophole section with a roller-cone bit to a TVD of 959 m (3,150 ft) in the Naith-A formation and casing off with 13 3/8-in. casing. The casing was set about 15 m (49 ft) into the Naith-A limestone to minimize the exposure time of the Shargi shales and to reduce casing costs.

Next, two PDC bits on rotary drilled the 12¼-in. surface-hole section to a TVD of 3,049 m (10,000 ft) in the Al Khlata sandstone, and it was cased off with a 9 5/8-in. casing. It was critical in this hole section to case off the potentially hydrocarbon-bearing Shuaiba and Mafraq formations above the Al Khlata, and to isolate the troublesome and time-dependent Gharif shales.

The 8 3/8-in. section was drilled to a TVD of 5,287 m (17,350 ft) in the Amin formation with four diamond-impregnated bits on turbine. The 8 3/8-in. section served as both the intermediate and production sections. A tapered completion string of 4½ x 5 x 5½-in. chrome tubular was then run and cemented in place. The chrome tubular was selected to mitigate corrosion.

The 8 3/8-in. section presents the greatest challenge to successfully drilling useable holes in Saih Rawl, because the rock strength is very high, with unconfined compressive strength sometimes on the order of 60,000 psi. In addition, local lithological variation, compounded by diverse anisotropies, makes drilling optimization difficult. It follows that this is the most crucial hole section, accounting for about half of total well construction time. Moreover, it penetrates the Miqrat and Barik objective reservoirs. Drilling the section with diamond-impregnated bits is a lesson learned following several uneconomic and failed attempts to drill the section with PDC bits on rotary and with motors.


Approaching the technical limit requires optimizing all the factors that enter into the drilling operation. Optimizing the staff includes both increasing their productivity and encouraging and using crew members’ ideas for optimizing other factors, such as knowledge management and drillbit performance. Shell and PDO used these strategies in Saih Rawl to cut well time by over 40%.

Teamwork. The foundations of any successful team effort are open, free communication and a shared goal. The rig team is put on an incentive program at the beginning of the contract, and the performance incentive goal bonds the rig team together. The rig team consists of the drilling contractor, the service vendors and the drilling optimization engineer. The cohesiveness established early in the campaign has sustained the continuous performance improvement of the rig. The initial motivation for performance was the reward incentive, but this was largely superseded by the operational excellence principles and technical limit philosophies that the drilling office team incorporated into the modus operandi. The following key points are worth mentioning:

  • Continuity. The rig has maintained most of the crew members who began the contract in 2003. As a result, rig efficiency and effectiveness have been excellent.
  • Competition. Desire to outperform other rigs in the field forged a stronger bond between the rig team and the office team, perhaps overriding the team incentive alone.
  • Communication. Goals were clearly communicated and understood across the team.
  • Operational Excellence. OE is a process-oriented enterprise program that drives improvements in the way a work group or corporation delivers products and services to its customers. Operational excellence calls for more than subject matter expertise and a talented internal team.

The OE philosophy in PDO’s Exploration and Gas department aims to incorporate the following principles:

Ideas from field staff drive success. The Saih Rawl well engineering and rig team members are motivated to improve operations. Team members are respected and their suggestions are encouraged. A constant feedback and feed-forward cycle ensures continual improvement.

Analysis paralysis breeds crisis. Rather than focusing on analyzing potential problems or improvement opportunities, which may allow them to become much more troublesome and costly, team members are empowered to make decisions and solve problems at the lowest possible level. This has minimized operational failures while drilling in Saih Rawl. Every team member is empowered to stop work not only for safety, but also to increase operational excellence.

Ideas need champions. Leadership in the department aims to encourage originators of ideas to champion them by spearheading the efforts to develop and implement them. Team leaders also sometimes discretionally select champions to serve as focal points, enabling an all-inclusive performance improvement environment.

Knowledge management. The Saih Rawl experience illustrates that the time value of knowledge supersedes the time value of money, especially when drilling for deep gas. The primary objective of the team’s knowledge management strategy was to drill each hole section correctly the first time. Recycling of knowledge and lessons learned were de-emphasized in favor of reinforcing practices worth replicating or best practices, and this expedited decision making.

Total Quality Well Delivery (TQWD) was achieved in Saih Rawl field by adequately managing lessons learned from past wells and applying them to subsequent wells, Fig. 4. Team motivation was emphasized over complex logic to achieve TQWD.

Fig. 4

Fig. 4. Lessons learned while drilling the Saih Rawl 204 well helped operators progressively reduce drilling time to ultimately achieve a sub-60-day well.

Benchmarking also played an important role. Historically, three KCA Deutag rigs operated in the field since 2003, and they all drilled the same well types. Therefore, data exchange, information sharing and benchmark practices worth replicating were easily established, and most of the unpleasant lessons did not have to be learned twice.

Staff continuity also contributed to maintaining a useful knowledge base. Staff members moving up or moving on had replacements waiting within the crew and being mentored. This practice and long tenures (at least 2 yr) of the incumbents have facilitated a downward trend of the performance learning curve. Extensive lessons were learned on Saih Rawl 204, which took more than 130 days to drill. The well was especially problematic due to the difficult directional work carried out, and the lessons learned were passed on.

Knowledge management in the Saih Rawl field has had to be dynamic since 2003, because at any point in time there are at least three rigs drilling different hole sections or performing different phases of well construction. The drilling team was given the rare opportunity of simultaneously learning-before-doing, learning-while-doing and learning-after-doing. The end result is the progressive downward trend in learning curve time.

Bit optimization. The biggest factor to reduce well construction cost and time is drilling useable boreholes more quickly. Bit optimization is a continuously evolving process in Saih Rawl field. Drilling optimization engineers, supplied by the vendors who worked in the PDO office as part of the rig team, have sped up bit technological advancement and optimized the bit-application process.

To date, the biggest improvement has been recorded in the 12¼-in. section with innovative developments in PDC bit technology. The typical formations penetrated in the 12¼-in. section include shale, sandstone, limestone, clays, dolomites and sometimes streaks and beds of pyrite, the latter having been penetrated in the dolomitic limestone of the Khuff formation.

To improve drilling efficiency and meet the challenges posed by the formation vagaries, the bit design has undergone several iterations and revisions in cutter size, number of blades, diamond hardness and bit profile. The PDC bit durability, stability and aggressiveness also have undergone several tunings and re-tuning.

The Hycalog DSX290 drillbit has proven to be a game changer, as the bit has helped reduce construction time in the 12¼-in. section by an average of 41%, Fig. 5. The bit has a part-ring design that ensures total lateral stability and a proprietary “steering wheel” to ensure good borehole quality.

Fig. 5

Fig. 5. The 12 ¼-in. Hycalog DSX290A2 PDC drillbit.

Notable progress has also been recorded in the 83/8-in. section typically drilled with a diamond-impregnated bit on a turbine drive. The hole section is very tough due to the uncompromising mix of lithologies, and reliability is the prime objective. The Smith K507BCTPXX has proven to be the most reliable bit from the stock on offer; the choice was reached after several wells, Fig. 6.

Fig. 6

Fig. 6. The 8 3/8-in. Smith K507BCTPXX diamond-impregnated bit.

There are two scenarios for performance opportunity in this section. The first is improving the durability of diamond-impregnated bits and making them stay in the hole longer. This strategy gives a longer section interval but at a slower drilling rate. The second scenario is sacrificing the durability to achieve faster ROP. The downside to this strategy is that it will require more bits to drill the section. So far, the second scenario has given the best result and is the preferred strategy. The most critical success factor is pulling the bit at the right time, and this was determined by employing Minimum Specific Energy (MSE) and Economics Gradient Curves (EGC).


Technical limit is a means to an end, not an end in itself. It is only a vehicle to stimulate creative thinking and shift paradigms from the school of “business as usual” and “this is the way we do it,” to a “what if?” or “can do” mentality. If an organization or business does what it has always done, the same results will be achieved.

Living the limit raises the bar of operational excellence and safety by two notches. First, it involves innovative thinking, employing synergies and taking creative ownership of the results. Creative ownership of results involves turning failures into launching pads to future success and perpetually pursuing business improvements. Implementation of technical limit thinking in the Saih Rawl has contributed to well cost and time reduction of over 40% since its implementation. WO  

*Since writing this article, Mr. Ajimoko has changed employment. He now works for BP. 



Olutayo Ajimoko worked for Shell E&P as a well engineer in charge of two heavy land rigs drilling deep gas in the Saih Rawl field. He provided single-point well engineering, operations and logistics support to the combined drilling budget. He joined Petroleum Development Oman as a well delivery consultant and was actively involved in the implementation of the technical limit philosophy. He cross-trained as a production technologist with extensive deepwater experience working for Star Deep Water Petroleum Ltd. in Nigeria and the US. Mr. Ajimoko previously worked for Texaco and Chevron. He is currently employed by BP.


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